"Every newly established fact and every new point of view, however insignificant, is sooner or later to be of use. In a year or in several decades they are bound to be of use to someone else." Aleksandr Kitaigorodsky III IV I would like to express my deepest gratitude to Dr. Peter Erk for his invaluable support and guidance during the past three years. I feel truly grateful to him for giving me the freedom to realize my ideas and for nurturing my scientific thinking. I sincerely appreciate all that I have learned from him, both professionally and personally. My deepest thanks go to Prof. Thomas Ebbesen for taking on the responsibility of being my PhD director, for the scientific discussions we had, and for welcoming me to his lab in Strasbourg.

I extend my heartfelt thanks to my colleagues Thomas Musiol, Thorsten Schwotzer, Sandra Gaube, and Sebastian Raths for generously investing their time in teaching me new lab techniques, for the insightful discussions, and for sharing their invaluable experimental knowledge with me. I would like to acknowledge Prof. Paolo Samori and Prof. Norbert Koch for hosting me and welcoming me during my secondments at the Supramolecular Science and Engineering Institute and Humboldt University of Berlin. I extend my gratitude to Dr. Guillaume Schweicher for his invaluable support and fruitful exchange of ideas. I am deeply grateful to Dr. Nicholas Turetta and Christos Gastios for their ability to teach me state-of-the-art measurements with conductive AFM and UPS, and more than that, for their company and the time we spent together. I would also like to express my thanks to Dr. Kripa Joseph for her dedication and patience in teaching me the secrets of optical cavities and vibrational strong coupling. I would like to thank all the principal investigators involved in the UHMob project for the fruitful discussions during the periodic meetings. I extend a special thank you to all my colleagues from the UHMob project: Martina, Rahul, Nemo, Marco, Mindaugas, Ann Maria, Lucia, Alessandro, Lamiaa, Priya, and Ines. They have not only been outstanding colleagues, but also friends and companions on this journey. I thank all of them for their great attitude towards science and for their willingness to collaborate both in and outside of the lab. I would also like to extend my personal thanks to some individuals who have been with me throughout these years. I thank Frieda for her warm welcome and for sharing our lovely apartment in Mannheim. I would like to thank Cecilia for being a great friend and for being there for me whenever I needed her. I express my deepest thanks to Luigi and Cecilia for sharing this German adventure with me and for their unwavering support when I experienced experimental setbacks. I would like to express my gratitude to Giulia for her immense support and for bringing out the best in me during the most challenging moments. I am grateful to her for consistently instilling in me a sense of curiosity and the appreciation for the beauty of discovery. Finally, I would like to express my deepest gratitude to my family, Mamma, Papà, and Francesca, even though they were far away, I always felt their strong support.

To all the other people I have met during these last three years, if I have not thanked you personally, know that each one of you has contributed to making this journey special. 3.9: Out-of-plane mobility (µout) obtained from J-V characteristics. 124 Table 3.10: Extracted transfer lengths (LT) and charge injection areas (Ainj) for TC TFTs with 25 3.12: Electrical performances and injection barriers of TC TFTs. ... 136 Table 3.

X

List of Figures

List of Tables

Preface

The field of organic semiconductors (OSCs) has undergone extensive research and development over the past few decades, driven by the technological applications in printed electronics. OSC molecules in the solid-state are held together by weak intermolecular interactions, which influence their charge transport properties. Thin-film transistors (TFTs) based on OSCs are highly promising for electronic applications due to their flexibility, low processingtemperature fabrication, solution-processing for cost-effective large area applications, and tailor-made molecular design. However, the electrical performance of OSC-based TFTs is hampered by static and dynamic disorder. Rational design of OSCs is required to ensure a well-defined crystal packing along with suitable molecular properties to maximize their potential electronic application.

This thesis aims to investigate the structure-properties relations of smallmolecule organic semiconductors, outlining guidelines for molecular design of high-performance organic TFTs. The first part of this work investigates the electrical performance of new classes of thienoacenes OSCs with different molecular cores and substitutions in TFTs. Structure-properties relations are established, unveiling the critical role of the thin film morphology, crystalline packing, and charge injection. In the second part, this work investigates the impact of molecular structures of thienoacenes on the contact resistance in TFTs. The study highlights the minor impact of molecular core substituents in both coplanar and staggered device geometry, while underlying the significance of maximizing in-plane charge carrier mobility to minimize contact resistance. In the third part, this thesis explores the effect of lightmatter coupling on perylene crystal growth and polymorphism, revealing the limitations of performing recrystallization experiments in an optical cavity and providing guidance for future experimental setups.

Chapter 1 Introduction

Introduction to organic semiconductors

The research field of organic electronics has sparked interest among the scientific community starting from the mid of 1970s, when the first organic field-effect transistor (OFET), [1] organic light emitting diode (OLED) [START_REF] Tang | Two-layer organic photovoltaic cell[END_REF] and organic photovoltaic (OPV) cell [START_REF] Merritt | Organic solar cells of hydroxy squarylium[END_REF] have been reported. Since then, this research field has been extensively studied and developed. One of the main advantages of using organic materials for optoelectronic applications, is the tunability of chemical and physical properties of organic semiconductors (OSCs) through molecular, [START_REF] Henson | Design strategies for organic semiconductors beyond the molecular formula[END_REF][START_REF] Kunkel | Active discovery of organic semiconductors[END_REF] supramolecular [START_REF] Operamolla | Molecular and Supramolecular Architectures of Organic Semiconductors for Field-Effect Transistor Devices and Sensors: A Synthetic Chemical Perspective[END_REF] and crystal design. [START_REF] Yu | Crystal Engineering of Organic Optoelectronic Materials[END_REF] In addition, OSCs are characterized by flexibility (depending on the utilised substrate), [START_REF] Zschieschang | Organic transistors on paper: a brief review[END_REF] low processing-temperature fabrication [START_REF] Ward | Versatile Organic Transistors by Solution Processing[END_REF] and solutionprocessing for cost-effective large area applications [START_REF] Ward | Versatile Organic Transistors by Solution Processing[END_REF] . All these features enable the synthesis of organic materials that can be designed for a tailormade need. On the other hand, the main drawbacks of OSCs are the lower stability under ambient conditions and the lower electrical performances if compared to the inorganic counterpart.

The concept of utilizing conjugated organic molecules for electronic devices can be traced back to the 1940s. [START_REF] Klauk | Organic Electronics: Materials, Manufacturing and Applications[END_REF] However, significant attention towards practical applications only emerged after the discovery and advancement of conductive polyacetylene, [START_REF] Chiang | Electrical Conductivity in Doped Polyacetylene[END_REF] thanks to the pioneering studied of Heeger, MacDiarmid and Shirakawa, who have been awarded with the Nobel Prize in chemistry in 2000.

The physical origin of electrical conductivity in OSCs relies in charge carrier delocalization through conjugated π-system. The formation of this system can be explained considering the electronic configuration of carbon atoms constituting the backbone of OSCs. In OSCs, carbon atoms are connected via alternated single and doubled bonds with sp 2 hybridised atomic orbitals. Considering the molecular orbital theory, adjacent carbon atoms exhibit overlapping sp 2 hybridized atomic orbitals, resulting in the formation of bonding and antibonding σ and σ* molecular orbitals (MOs). In contrast, the remaining atomic pz orbitals overlap to a lesser extent, leading to the creation of bonding π and antibonding π* MOs.

Benzene is one of the simplest aromatic compounds and consists of six carbon atoms. The combination of the 2pz orbitals leads to energetic splitting into three bonding π MOs and three antibonding π* MOs (Figure 1.1). The π molecular orbital with the highest energy is defined as the highest occupied molecular orbital (HOMO), while the π* molecular orbital with lowest energy is defined as the lowest unoccupied molecular orbitals (LUMO). HOMO and LUMO are identified as frontiers orbitals and at first approximation can be viewed as the valance and conduction band, respectively. Shape of the HOMO and LUMO of benzene. [START_REF] Kim | Relief of excited-state antiaromaticity enables the smallest red emitter[END_REF] The electrons within these π bonds, namely π-electrons, are delocalized among all six 2pz orbitals. As a result, the π electrons can be envisioned as an electron cloud that is distributed across the planes both above and below the atomic plane. In the solid state, OSC molecules are held together by Van der Waals forces and coulombic interactions and the overlap of π-molecular orbitals within neighbouring molecules provides charge conduction pathways. When charge carriers are introduced in the OSC by electrical injection of photoinduction, the energy of the HOMO and LUMO as well as their extension and overlap within neighbouring molecules determine the efficiency of charge carrier transport. Intermolecular charge transport across the OSC is enabled by the combined effects of charge delocalization, achieved through the π conjugated system, and the overlap of MOs within adjacent molecules.

The energy separation between the HOMO and the LUMO corresponds to the band gap (Egap) observed in inorganic semiconductors. In benzene, the band gap is approximately 5.9 eV, [START_REF] Kim | Relief of excited-state antiaromaticity enables the smallest red emitter[END_REF] causing it to possess insulating properties. In acenes, a group of compounds characterized by an increasing number of annealed benzene rings, the number of overlapping 2pz orbitals increases, changing the energetic splitting between bonding and antibonding molecular orbitals. This leads to a reduction in the HOMO-LUMO energy difference. For instance, pentacene, one of the most exploited OSCs, consists of five fused benzene rings and exhibits an energy gap of approximately 2.2 eV. [START_REF] Troisi | Band Structure of the Four Pentacene Polymorphs and Effect on the Hole Mobility at Low Temperature[END_REF] 1.2 Organic semiconductor materials

Organic polymers

OSCs can be divided in two classes: polymers and small molecules. Organic semiconductor polymers exhibit poorer film crystallinity and higher molecular disorder [START_REF] Griggs | n-Type organic semiconducting polymers: stability limitations, design considerations and applications[END_REF] compared to small molecule OSCs. OSC polymers are mostly semi-crystalline, characterized by alternating crystalline and amorphous domains. Charge transport is efficient through the crystalline regions but is hampered throughout the disordered regions. Thus, the latter in considered the bottleneck of electrical performances in devices based on OSC polymers, due to the high energetic barrier that charges must overcome to pass from one ordered domain to the other. [START_REF] Noriega | A general relationship between disorder, aggregation and charge transport in conjugated polymers[END_REF] Charge carrier mobility is defined as the drift velocity of the charge carrier per unit of the electric field, and it is the reference figure of merit for electrical performances of OSCs. Charge carrier mobility can be enhanced by ensuring that polymer chains are of sufficient length to assist charge transport within crystalline regions, acting as a tie-chain. [START_REF] Gu | The Role of Tie Chains on the Mechano-Electrical Properties of Semiconducting Polymer Films[END_REF][START_REF] Fratini | Charge transport in high-mobility conjugated polymers and molecular semiconductors[END_REF] . P3HT is one of the most exploited OSC polymer, showing charge carrier mobility of about 0.1 cm 2 V -1 s -1 . [START_REF] Sirringhaus | Two-dimensional charge transport in self-organized, high-mobility conjugated polymers[END_REF] P3HT has an additional hexyl side group if compared to polythiophene. The structural modification has shown to enhance the electrical properties of the P3HT thanks to the head-to-tail interaction of the hexyl side chains, leading to the formation of semicrystalline lamellar structures. [START_REF] Mccullough | Self-orienting head-to-tail poly(3-alkylthiophenes): new insights on structure-property relationships in conducting polymers[END_REF] This solid-state ordering enabled by the structural modification, highlights how structure-properties relations play a major role in the development and design of OSCs. OFETs devices based on PBTTT has revealed charge carrier mobility up to 1 cm 2 V - 1 s -1 . [START_REF] Hamadani | Undoped polythiophene field-effect transistors with mobility of 1cm2V-1s-1[END_REF] The semicrystalline structure is induced by sidechains interdigitation.

Small molecules

Compared to OSC polymers, small molecules are characterized by a higher degree of crystallinity resulting in more reproducible and reliable charge carrier mobility values in the range of 1 and 10 cm 2 V -1 s -1 for state-of-art molecular semiconductors. [START_REF] Schweicher | What Currently Limits Charge Carrier Mobility in Crystals of Molecular Semiconductors?[END_REF] As drawback, small-molecule OSCs have typically lower solubility in organic solvent compared to OSC polymers, preventing the deposition through solution-processing techniques. Nevertheless, solubility can be increased by the functionalization of the molecular cores with side groups (generally with long alkyl chains). Typical small molecule OSCs are shown in Scheme 1.2.

Pentacene has been extensively employed as benchmark for device physics studies. Due to the low solubility of pentacene, field-effect transistors are typically fabricated by vacuum deposition, giving charge carrier mobility of about 0.5 cm 2 V -1 s -1 . [START_REF] Gundlach | Pentacene organic thin-film transistors-molecular ordering and mobility[END_REF][START_REF] Günther | Hole mobility in thermally evaporated pentacene: Morphological and directional dependence[END_REF] TIPS-PEN has been obtained through the functionalization with triisopropyl groups to the 6 and 13 positions of pentacene, enabling the fabrication of transistors through solution processing. While pentacene crystallizes with a herringbone packing, TIPS-PEN packs with a "bricklayer" crystalline motif. [START_REF] Sheraw | Functionalized Pentacene Active Layer Organic Thin-Film Transistors[END_REF] This enhances the π-π overlapping of neighbouring molecules, leading to mobility up to 1.5 cm 2 V -1 s - 1 (see the next section regarding the packing motif of OSC small molecules). [START_REF] Giri | High-Mobility, Aligned Crystalline Domains of TIPS-Pentacene with Metastable Polymorphs Through Lateral Confinement of Crystal Growth[END_REF] Among acenes, rubrene has shown the highest charge carrier mobility yet reported at room temperature, in the range of 20 cm 2 V -1 s -1 in single crystal field effect transistor (SCFET). [START_REF] Podzorov | Intrinsic Charge Transport on the Surface of Organic Semiconductors[END_REF] The introduction of thiophene rings into the acene structures results in ladder-type molecular structures, namely thienoacenes. [START_REF] Takimiya | Thienoacene-Based Organic Semiconductors[END_REF] In this class of molecules, charge carrier mobility is boosted by an effective contribution of the sulphur atoms to the π-π overlapping of adjacent molecules. [START_REF] Takimiya | Organic Semiconductors Based on[END_REF] BTBT and DNTT and their derivatives are undoubtedly the most exploited OSCs among thienoacenes. The main challenges for the use of BTBT are the high ionization energy of about 5.8 eV which hinders charge injection from metal electrodes and, in addition, the poor film homogeneity when vacuum deposited. [START_REF] Takimiya | Organic Semiconductors Based on[END_REF] The introduction of alkyl chains to the position 2 and 7 of BTBT enables solution-processing and facilitate charge carrier injection from electrodes due to the decrease of ionization energy to values of about 5.0 eV. [START_REF] Ebata | Highly Soluble [1]Benzothieno[3,2-b ]benzothiophene (BTBT) Derivatives for High-Performance, Solution-Processed Organic Field-Effect Transistors[END_REF] Specifically C8-BTBT unveils charge carrier mobility of about 5 cm 2 V -1 s -1 . [START_REF] Darmawan | Optimal Structure for High-Performance and Low-Contact-Resistance Organic Field-Effect Transistors Using Contact-Doped Coplanar and Pseudo-Staggered Device Architectures[END_REF] Higher values of mobility have been claimed for C8-BTBT, but most likely these values were overestimated since extracted from devices with strongly gated contacts. [START_REF] Bittle | Mobility overestimation due to gated contacts in organic field-effect transistors[END_REF][START_REF] Minemawari | Inkjet printing of single-crystal films[END_REF][START_REF] Yuan | Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method[END_REF] DNTT has two additional benzene rings annealed to the BTBT core. The extended conjugation results in lower ionization energy (compared to BTBT) of 5.4 eV, which facilitates charge carrier injection in OFETs. [START_REF] Yamamoto | Facile Synthesis of Highly π-Extended Heteroarenes[END_REF] Moreover, the pronounced electronic density on the sulphur atoms increases the π-orbital overlap between neighbouring molecules giving rise to high charge carrier mobility up to 3 cm 2 V -1 s -1 . [START_REF] Takimiya | Thienoacene-Based Organic Semiconductors[END_REF] DNTT is one of the most exploited OSCs and it has been used for several applications, i.e., sensors for healthcare applications, [START_REF] Sugiyama | An ultraflexible organic differential amplifier for recording electrocardiograms[END_REF][START_REF] Kuribara | Organic transistors with high thermal stability for medical applications[END_REF][START_REF] Yuvaraja | Organic field-effect transistor-based flexible sensors[END_REF] flexible low voltage complementary circuits [START_REF] Kraft | Low-Voltage, High-Frequency Organic Transistors and Unipolar and Complementary Ring Oscillators on Paper[END_REF] and neuromorphic devices. [START_REF] Kim | Synaptic organic transistors with a vacuumdeposited charge-trapping nanosheet[END_REF][START_REF] Hosseini | An organic synaptic circuit: toward flexible and biocompatible organic neuromorphic processing[END_REF] DNTT derivatives functionalized in 2 and 7 positions such as C8-DNTT, C10-DNTT and diPh-DNTT exhibit even higher charge carrier mobility compared to the parent molecule, with values exceeding 10 cm 2 V -1 s -1 for alkylated DNTTs in solution-processed OFETs [START_REF] Hofmockel | High-mobility organic thin-film transistors based on a small-molecule semiconductor deposited in vacuum and by solution shearing[END_REF][START_REF] Peng | Ultrahigh On-Current Density of Organic Field-Effect Transistors Facilitated by Molecular Monolayer Crystals[END_REF] and values of about 5 cm 2 V -1 s -1 for diPh-DNTT in vacuum-deposited thinfilm transistors. [START_REF] Borchert | On the Minimization of Contact Resistance in Organic Thin-Film Transistors[END_REF][START_REF] Geiger | Subthreshold Swing of 59 mV decade -1 in Nanoscale Flexible Ultralow-Voltage Organic Transistors[END_REF] C10-DNTT has revealed record-breaking electrical performances in terms of lowest contact resistance (10 Ωcm) and highest on/off current ratio of (10 10 ) in OFETs. [START_REF] Borchert | Flexible low-voltage highfrequency organic thin-film transistors[END_REF][START_REF] Zeng | Ultralow contact resistance in organic transistors via orbital hybridization[END_REF] Scheme 1.2

Molecular packing

The understanding of structure-properties relations is essential for the design of new high performance OSCs, considering that the molecular structure as well as the crystalline packing strongly influences charge transport properties of OSCs. Molecular packing features are mainly driven by intermolecular interactions. [START_REF] Kitaigorodsky | Molecular Crystals and Molecules[END_REF] Typically, the molecular packing motif should exhibit a large number and long-range intermolecular interactions to ensure optimal charge transport properties. [START_REF] Wang | Organic semiconductor crystals[END_REF] The structural ordering of OSC molecular crystals can be described by the packing motif. This defines the pattern of orientation of the OSC molecular cores with respect to each other, reflecting the intermolecular interactions. The extensive research on the packing motif of molecular OSCs has led to the identification of four major packing motifs, [START_REF] Wang | Semiconducting π-Conjugated Systems in Field-Effect Transistors: A Material Odyssey of Organic Electronics[END_REF] depicted in Figure 1.2.

The herringbone packing motif is characterized by C-H•••π interactions, without π-π overlap within adjacent molecules which pack with an edge-to-face orientation. The slipped herringbone packing resembles the typical herringbone packing motif, but it is governed by both C-H•••π interactions and π-π overlap, resulting in face-to-face orientation of adjacent molecules. The π-π overlap is predominant both in slipped stack packing and brick-wall packing motifs. The former is characterized by columnar face-to-face molecular arrangement with columns tilted relative to one other. In the latter molecules are oriented face-to-face with respect to each other, forming alternated parallel columns. From a theoretical point of view, the brick-wall packing motif is the most optimal to ensure high charge carrier mobility, since it maximize the π-π overlap within neighbouring molecules. [START_REF] Wang | Dibenzothiophene Derivatives: From Herringbone to Lamellar Packing Motif[END_REF] Nevertheless, this crystalline arrangement is not a prerequisite to obtain high performance OSCs. For instance, rubrene (which exhibits one of the highest charge carrier mobility among OSCs) crystallizes with a slipped herringbone packing motif. In addition, many OSCs with excellent electrical performances such as DNTT and its alkylated derivatives, are characterized by a herringbone packing motif. Most generally, dense molecular packing and an effective overlap of the π-molecular orbitals are crucial for achieving efficient charge transport in molecular crystals. [START_REF] Zhou | High performance n-type and ambipolar small organic semiconductors for organic thin film transistors[END_REF]

Charge transport

The charge transport mechanism taking place in OSCs has been debated for many years, due to the complexity to establish a comprehensive theoretical model for a wide variety of organic materials. [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF] The intermolecular charge transport in OSCs can be influenced by both intrinsic and extrinsic factors. In the solid state OSC molecules are held together by Van der Waals forces. Hence, the overlap and the strength of interaction of electronic orbitals results to be susceptible to molecular orientation, temperature, static and dynamic disorder (see next sections), but also to the process and the materials used for the device fabrication. Yet, some fundamental principles are well established.

In crystalline OSCs, the interaction between the HOMO and the LUMO levels results in the formation of conduction and valence bands as in the case of inorganic semiconductors. It is worth noting that these transport bands are narrower in OSCs (on the order of 100 meV), if compared to the inorganic counterparts (on the order of few eV). In addition, in case of amorphous or disordered OSCs, static and dynamic disorder leads to a distribution of discrete occupied and unoccupied states rather than a continuous transport band (Figure 1.3). Charge carrier mobility depends on the width of the transport bands, which in turns reflects the strength of intermolecular interaction. The ionization energy (IE) and the electron affinity (EA) are defined as the energy difference between the vacuum energy level and the energy of HOMO and LUMO levels respectively:

𝐼𝐸 = 𝐸 𝑣𝑎𝑐𝑢𝑢𝑚 -𝐸 𝐻𝑂𝑀𝑂 (1.1) 𝐸𝐴 = 𝐸 𝑣𝑎𝑐𝑢𝑢𝑚 -𝐸 𝐿𝑈𝑀𝑂 (1.2)
As the inorganic counterpart, OSCs can be grouped depending on the transported charge carrier. This is mainly determined by the electronic structure of the OSC, which defines the HOMO and LUMO levels, and by the work function (WF) of the material used for the electrical contacts in devices. The work function is defined as the energy required to extract one electron from the Fermi level (EF) to the vacuum level (Figure 1.3):

𝑊𝐹 = 𝐸 𝑣𝑎𝑐𝑢𝑢𝑚 -𝐸 𝐹 (1.3)
Consequently, an OSC is defined as p-type (n-type) when the electrodes work function matches the ionization energy (electron affinity) of the OSC, leading to holes (electrons) transport. In some cases, OSCs can transport both electrons and holes and are defined as ambipolar.

Excess of charge carriers has to be accumulated in the conduction or valence bands to induce charge transport. This is accomplished by injecting charges from a metal electrode, in the case of OFETs and OLEDs, or by photoexcitation, in the case of OPVs.

Charge carrier mobility

Charge carrier mobility (µ) relates the drift velocity (v) of free carriers (electrons or holes) to the driving force of an applied electrical field (E):

𝑣 = µ 𝐸 (1.4)
Therefore, the current density (J) generated by the charge transport is describes as follow:

𝐽 = 𝑞 𝑛 𝑣 = 𝑞 𝑛 µ 𝐸 (1.5)
where q is the elementary charge and n is the charge carrier density. Charge carrier mobility is considered one of the most important parameters when comparing OSCs electrical performances.

For instance, in OFETs mobility determines the maximum current trough the device (at a given applied voltage), the maximum achievable switching speed and hence the maximum transient frequency. [START_REF] Paterson | Enabling thin-film transistor technologies and the device metrics that matter[END_REF][START_REF] Klauk | Organic thin-film transistors[END_REF] Charge carrier mobility can be extracted from OFETs, as explained in Paragraph 1.4.. To date, the highest reliable and reproducible mobility in OSCs has been achieved for rubrene at room temperature, giving values of about 20 cm 2 V -1 s -1 . Higher charge carrier mobilities have been claimed, but these values either have been extracted from non-ideal devices or have not yet been reproduced by other research groups. A literature survey has raised the problem of overestimated reported mobility, showing that more than the half of the articles reviewed in the survey either do not report the data from which mobility was extracted or the reported data set has some sort of nonideality. [START_REF] Paterson | Recent Progress in High-Mobility Organic Transistors: A Reality Check[END_REF] Thus, state-of-art OSCs typically exhibit reliable and reproducible charge carrier mobilities between 1 and 10 cm 2 V -1 s -1 , with few exceptions exceeding 10 cm 2 V -1 s -1 . [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF]

Static and dynamic disorder

The achievement of high charge carrier mobility in OSCs is hampered by static and dynamic disorder. Static disorder arises from impurities, defects, electrostatic effects (i.e., local dipoles) and in some cases from the presence of polymorphs. [START_REF] Rivnay | Quantitative Determination of Organic Semiconductor Microstructure from the Molecular to Device Scale[END_REF] In polycrystalline OSC thin films, static disorder is accentuated by grain boundaries which limit the charge transport between different crystalline domains. [START_REF] Kalb | Trap density of states in small-molecule organic semiconductors: A quantitative comparison of thinfilm transistors with single crystals[END_REF] The direct consequence of static disorder is the formation of trapping states, that are electronic defects where charge is trapped and spatially localized. Consequently, any defect and inhomogeneity in the crystal structure impacts the molecular orbitals overlap and locally induce changes of the electronic structure.

Dynamic disorder arises from the thermal motion of molecules in the crystalline arrangement. Considering the van der Waals intermolecular interactions, the phonon modes (vibrational motions of the lattice) tend to exhibit large vibration amplitudes, on the order of 0.1 Ǻ. [START_REF] Illig | Reducing dynamic disorder in small-molecule organic semiconductors by suppressing large-amplitude thermal motions[END_REF][START_REF] Eggeman | Measurement of molecular motion in organic semiconductors by thermal diffuse electron scattering[END_REF] The phonon modes can interact with the charge motion, leading to the so-called electronphonon coupling. This causes fluctuations of the molecular orbitals overlap and site energies across the molecular lattice.

In summary, impurities and defects results in static disorder, whereas thermal vibration of the molecules in the crystalline lattice results in dynamic disorder. Both give rise to trap states, which severely hinder charge transport in OSCs. While static disorder can be contained through the optimization of OSCs processing and device fabrication, there are currently no effective methods to prevent electron-phonon coupling. Nonetheless, large amplitude vibrations can be suppressed in OSCs with a stiff molecular structure. [START_REF] Schweicher | Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors[END_REF]

Transfer integrals and reorganization energy

At microscopic scale, charge transport is generally described by the transfer integrals (JT) and the reorganization energy (λ). Considering two adjacent molecules, JT expresses the overlap of the HOMO (or LUMO) wavefunctions. Specifically, JT represents the strength of the interaction between the wavefunctions associated to the electronic configurations when charge is localized on the first molecule and when is localized on the second molecule.

The reorganization energy is composed by the inner (λin) and the outer (λout) parts. [START_REF] Schweicher | What Currently Limits Charge Carrier Mobility in Crystals of Molecular Semiconductors?[END_REF] The former is associated to the sum of the energies required for the geometric relaxation from the neutral state geometry to the charged state geometry and vice versa. In other words, λin reflects the energy needed for the conformational change upon charge transfer. λout accounts for the modification of the surrounding media due to polarization. Generally, λout is neglected because it is considerably lower compared to λin, reducing λ mainly to the sole contribution of λin. [START_REF] Mcmahon | Evaluation of the External Reorganization Energy of Polyacenes[END_REF] JT can be estimated within a fragment orbital approach only when the crystal structure of the investigated compounds is determined (electronic couplings are computed among pairs of close neighbours). On the contrary λin can be calculated theoretically prior to the synthesis of the OSC molecule, since it depends just on the molecular structure. [START_REF] Purushotham | Conjugate acene fused buckybowls: evaluating their suitability for p-type, ambipolar and n-type air stable organic semiconductors[END_REF] When JT << λ, the charge wavefunction tends to be localized on a single molecule and charge transport occurs because of incoherent hopping events within the molecules (hopping model mechanism). In contrast, when JT >> λ, charge is delocalized over several molecules and charge transport occurs through diffusion (band model mechanism). [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF] OSC materials typically exhibit transfer integrals and reorganization energies in the range of 10-100 meV and and 50-500 meV respectively, [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF][START_REF] Coropceanu | Charge Transport in Organic Semiconductors[END_REF][START_REF] Ostroverkhova | Organic Optoelectronic Materials: Mechanisms and Applications[END_REF] hence JT ≈ λ. In this intermediate regime neither the hopping model nor the band model are solely appropriate to explain the charge transport mechanism. In the recent years, the transient localization model has been developed to describe charge transport of OSCs. [START_REF] Fratini | The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials[END_REF] In this model charges show both localized and extended characters.

Hopping model

In the hopping model of transport charges are localized at singles sites and charge transport occurs through incoherent hopping events (Figure 1.4). To a first approximation, the rate of charge transfer (KET) can be expressed by using the semiclassical Marcus theory, [START_REF] Marcus | Electron Transfer Reactions in Chemistry: Theory and Experiment (Nobel Lecture)[END_REF] firstly developed to describe the electron transfer rate in donor-acceptor complex:

𝐾 𝐸𝑇 = 2𝜋 ħ 𝐽 𝑇 2 1 √4𝜋𝐾 𝑏 𝑇 𝑒𝑥𝑝 ( -λ 4𝐾 𝑏 𝑇 ) (1.6)
where ħ is the reduced Planck constant, Kb is the Boltzmann constant, and T is the temperature. Assuming that charge transport occurs through hopping between individual sites placed on a 1D periodic array, charge carrier mobility can be defined as follow:

µ = 𝛥𝑥 𝐸 𝛥𝑡 = 𝐾 𝐸𝑇 𝛥𝑥 𝐸 (1.7)
where Δx and Δt are the intermolecular distance and the time interval of charge transfer, respectively. Considering equations 1.6 and 1.7 and assuming that JT and λ do not change with temperature, µ follows and Arrhenius-type temperature dependence, indicating a thermally activated process. [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF] In addition, from equations 1.6 and 1.7, it is clear that JT has to be maximized and λ has to be minimized to enhance charge carrier mobility.

Generally, the hopping model is considered an appropriate description for charge transfer occurring in materials with low mobility (µ << 1 cm 2 V -1 s -1 ), as in the case of disordered and amorphous OSC films.

Band model

In band like transport, the large overlap and strong interaction of molecular orbitals of neighbouring molecules results in the formation of narrow transport bands. Hence, charges are delocalized over the molecular units, leading to diffusive charge transport (Figure 1.4). This occurs when the energy of interaction between the molecular orbitals is stronger than dynamic and static disorder. However, the delocalized charge wavefunction may be scattered due to static and dynamic disorder.

In the Drude model, charge carrier mobility can be expressed as follow:

µ = 𝑞𝜏 𝑚 * (1.8)
where τ is the mean scattering time and m* is the effective mass of charge carrier. Band transport occurs when the mean free path of charge carrier is considerably larger than the lattice constant. Thus, the reorganization energy does not affect the charge carrier mobility, since the residence time of charge at a single molecular site is shorter than the time required for the geometrical relaxation upon charge transfer. Considering that m* is inversely proportional to JT, [START_REF] Fratini | The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials[END_REF] larger transfer integrals increase charge carrier mobility. In contrast to the hopping regime, in the band model mobility decreases by increasing the temperature, due to the larger lattice vibrations which lead to higher number of scattering sites.

This model is considered appropriate to describe charge transport in highquality single crystal OSCs with low density of charge traps (µ >> 10 cm 2 V -1 s - 1 ).

Transient localization model

Most of the state-of-art OSCs exhibit mobility in the range of 1-10 cm 2 V -1 s -1 , with transfer integrals and reorganization energies on the same order of magnitude. In this regime neither hopping model nor band model are appropriate to describe charge transport mechanism. As already stated, the rather weak van der Waals forces which hold together OSC molecules in the crystal lattice, result in highly disordered energetic landscape through which charges are transported. This is caused by the large transfer integrals fluctuations arising from electron-phonon coupling, which limits charge carrier diffusion. The transient localization model describes charge transport as a combination of localized and extended characters of charges. [START_REF] Fratini | A map of highmobility molecular semiconductors[END_REF] The model supposes that on timescale shorter than the timescale of intermolecular oscillations, charges are localized on molecular sites, while on longer timescale charge transport takes place through diffusion, governed by the molecular lattice fluctuations. [START_REF] Fratini | The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials[END_REF] In this model, the charge carrier mobility depends on the temperature-dependent localization length (Lν) and on the timescale of intermolecular vibrations (νvib):

µ = 𝑞 2𝐾 𝑏 𝑇 𝐿 ν 2 ν vib (1.9)
The localization length defines the length over which charges are localized and transport occurs through diffusion of localized charges across the crystalline lattice driven by intermolecular vibrations (Figure 1.4).

Considering equation 1.9, the strategies to improve mobilities rely in the decreasing of intermolecular vibration frequency and the increasing of the localization length. The latter can be achieved by increasing the molecular orbitals overlap (i.e., larger transfer integrals) and minimizing the coupling with the intermolecular motions. At last, it has been shown that isotropic transfer integrals in the different crystal directions results in higher resilience of the OSC to dynamic disorder due to the reduced localization phenomena. [START_REF] Schweicher | Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors[END_REF][START_REF] Fratini | A map of highmobility molecular semiconductors[END_REF]

Organic field-effect transistors

Transistors are semiconductor-based devices largely used to amplify or switch an electrical signal. The word transistor comes from the words "transfer" and "resistor" since in this device the electrical current is transferred across a resistor, formed by two-terminal electrical components. Field-effect transistors (FET) are three-terminal components type of transistors which exploits an electric field to control the flow of current in the device. The first FET based on an organic semiconductor was developed in 1986 by Tsumura, showing a hole mobility of ≈ 10 -5 cm 2 V -1 s -1 . [1] Thereafter, the field of organic semiconductors for logic operations has experienced an enormous development, and nowadays the highest reproducible value of hole mobility reported is in the range of 20 cm 2 V -1 s -1 . [START_REF] Podzorov | Intrinsic Charge Transport on the Surface of Organic Semiconductors[END_REF] This is the actual limit imposed by the dynamic disorder result of intermolecular interactions in organic semiconductor crystals, which entail significant thermal lattice fluctuations.

Albeit the organic material plays a dominant role on the final device performances; the dielectric layer, the contacts, the device geometry and the control of the materials properties at the interfaces are crucial to maximize the potential of this technology and obtain ideal and reliable devices. Since a thin-layer (ca. 20-30 nm) of OSC is generally deposited during the fabrication of OFET, these devices are also called thin-film transistors (TFTs). TFTs consist in the overlap of different layers, deposited via solution or vaporprocessing, which can typically be assembled as depicted in Figure 1.5. In top gate devices (TG), the gate contact is placed on the top of the device structure, while for the bottom-gate (BG) counterpart, the gate contact is located on the bottom. Considering the position of the drain and source contacts, which can be deposited on the top (TC) or on the bottom (BC) of the organic thin-films, four structures are obtained by combining the gate, drain and source contacts positions: top-gate top-contact (TGTC), top-gate bottomcontact (TGTC), bottom-gate top contact (BFTC) and bottom-gate bottomcontacts (BGBC). BGTC and TGBC geometries are named staggered since the source and the drain contacts are on one side while the gate contact is on the opposite side of the OSC. In contrast, BGBC and TGTC geometries are named coplanar since the source, drain and gate contacts are placed on the same side of the OSC.

In this section the static operation of an organic TFT is exposed, considering a standard BGTC configuration and a hole (p-type) charge transport. However, the same considerations can be extended to devices with a different structure and to electron (n-type) charge transport, upon considering the change in polarity.

Operating principles

In this section the charge transport in OFETs is explained according to the charge-sheet model, which treats the OSC/dielectric/gate electrode system as a parallel plate capacitor (i.e., all the mobile charge carriers are exclusively transported within the organic layer at the interface with the dielectric). [START_REF] Brews | A charge-sheet model of the MOSFET[END_REF][START_REF] Horowitz | Organic Field-Effect Transistors[END_REF] In OFETs, the OSC is coupled capacitively with a dielectric layer, as depicted in Figure 1.6. The application of a negative bias voltage between the source and the gate electrodes, namely the gate voltage (Vg), results in the accumulation of holes at the OSC/dielectric interface, forming an accumulation channel. The gate voltage modulates the charge carrier density in the accumulation channel and hence its conductivity. By applying a bias voltage between the source and the drain contacts (drain voltage, Vd), charges are injected into the OSCs and flow from the source to the drain electrodes across the accumulation channel. The measured current between the drain and the source contacts is defined as drain current (Id) and, according to the Drude model, can be expressed as follow:

𝐼 𝑑 ∝ 𝜎 𝑐ℎ 𝑉 𝑑 = 𝑝 𝑞 µ 𝑉 𝑑 (1.10)
where σch is the channel conductivity and p is the charge carrier density in the channel.

For a better visualization of the role of the gate voltage modulation on the channel conductivity, we consider idealized and simplified energy level diagrams (Figure 1.6). No mobile charges are accumulated at the OSC/dielectric interfaces if no gate voltage is applied. Thus, the only mobile carriers in the channel are those that are thermally activated or induced by doping. In this case, upon the application of a drain voltage, the collected drain current is referred as off-state current.

When a negative Vg is applied, the HOMO (and LUMO) energy levels of the OSC bend upwards to match with the Fermi energy of the source contact, leading to the accumulation of holes at the OSC/dielectric interface. Upon the application of a negative Vd, holes are injected (electrons are ejected) from the source contact and travel across the accumulation channel to the drain contact. It is important to underline that these energy level diagrams are useful to understand the operating mechanism of OFETs, but they are simplistic representations and do not account for several phenomena occurring at the contact/OSC and dielectric/OSC interface, such as the presence of traps, charge transfers, interface dipole, band banding, Fermi level pinning, formation of a depleted region, etc. The charge carrier density at the OSC/dielectric interface is regulated by both the drain voltage and the gate voltage. Specifically, the charge carrier density per unit area (Qind) at a given position x across the accumulation channel varies depending on the distance from the source contact edge and can be expressed as follow:

𝑄 𝑖𝑛𝑑 (𝑥) = 𝐶 𝑖 (𝑉 𝑔 -𝑉(𝑥))

(1.11)
where Ci is the dielectric capacitance per unit area, and V(x) is potential along the accumulation channel at a given position x. In real devices, generally mobile charge carriers are not induced into the OSC as soon as Vg < 0. The mismatch between the HOMO level of the OSC and the EF of the electrodes leads to charge transfer between the two materials which in turns results in the formation of interfacial dipoles and band banding. Hence, the application of Vg ≠ 0 is necessary to achieve the flat-band condition (VFB).

Moreover, deep trap states arising from dynamic and static disorder must be filled, prior the formation of the accumulation channel. Thus, the gate voltage required to induce mobile charge carriers is defined as threshold voltage (Vth) and equation 1.11 transforms into:

𝑄 𝑖𝑛𝑑 (𝑥) = 𝐶 𝑖 (𝑉 𝑔 -𝑉 𝑡ℎ -𝑉(𝑥))

(1.12)

Please note that the difference between Vg and Vth (Vg -Vth) is referred as gate-overdrive voltage. For Vg > Vth, the current flowing from the source to the drain electrode is expressed as follow:

𝐼 𝑑 = 𝑊 𝐿 µ 0 ∫ 𝑄 𝑖𝑛𝑑 (𝑥) 𝑑𝑉(𝑥) 𝑉 𝑑 0 (1.13)
where µ0 is the intrinsic field-independent charge carrier mobility, while W and L are the channel width and the channel length respectively.

Assuming that Vth does not change with x, at a given Vg the charge carrier density across the accumulation channel is constant if Vd << Vg -Vth (linear regime). When Vd is nonzero and lower than Vg-Vth, the accumulation channel is characterized by a linear gradient of charge carrier concentration (Figure 1.7).

Drain current in linear and saturation regimes

If the gradual channel approximation (GCA) is valid, * the charge carrier concentration is uniform along the channel and the voltage drop across the channel is linear (Vd < Vg -Vth), the integration of equation 1. 13

𝐼 𝑑 = 𝑊 2𝐿 µ 0 𝐶 𝑖 (𝑉 𝑔 -𝑉 𝑡ℎ ) 2 (1.15)
This implies that in saturation regime, the drain current is linearly proportional to the square of Vg.

Subthreshold regime

Considering the transfer characteristic of an OEFT, the subthreshold regime occurs when Vth < Vg < VFB. In this regime, the drain current increases exponentially with the increasing of the gate voltage. The subthreshold swing (SS) is defined as the inverse slope of the logarithmic Id as a function of Vg:

𝑆𝑆 = ( 𝜕 log 𝐼 𝑑 𝜕𝑉 𝑔 ) -1 (1.16)
The SS measures how fast the OEFT switch from the off-state to the on-state. This is directly connected to the density of trap states according to the following equation: (1.17)

*

The GCA assumes that the electric field perpendicular to the OSC/dielectric interface generated upon the application of Vg is much larger than the electric field parallel to the OSC/dielectric interface, generated upon the application of Vd.

where Nit is the interfacial trap density per unit area and unit energy at the OSC/dielectric interface, ε is the relative dielectric constant of the OSC and Nbulk is the bulk trap density per unit volume and unit energy. By neglecting the contribution of Nbulk (i.e., in the case of highly ordered OSC or in polycrystalline TFTs with a coplanar geometry), equation 1.17 can be used to estimate the interfacial trap density at the OSC/dielectric interface in OFETs.

Field-effect mobility

The derivatives of equation 1. 14 It is worth noting that up to now the charge carrier mobility used for the description of the drain current in OFETs was assumed to be an intrinsic field-independent mobility of the OSC layer (µ0). On the contrary, the extraction of the field-effect mobilities (µ,lin and µ,sat) from transfer characteristics by using equations 1.20 and 1.21, introduces the dependence on the applied electric field and hence on the charge carrier concentration. The Vg-dependence of charge carrier mobility extracted from OFETs is complex and difficult to predict. Many factors play a role in determining the gate voltage dependence of mobility, such as the charge transport mechanism, the presence of defect and trap states, the dielectric surface roughness and the contact resistance. [START_REF] Xu | Precise Extraction of Charge Carrier Mobility for Organic Transistors[END_REF] Most important, unreliable mobility values (and specifically overestimated values) are a major problem in the development of the organic transistors field. The overestimation of mobility extracted from OFETs arises mostly due to gated contacts, i.e., when contact resistance is larger than the channel resistance, with the former exhibiting a strong dependence on the applied gate voltage. [START_REF] Paterson | Recent Progress in High-Mobility Organic Transistors: A Reality Check[END_REF][START_REF] Xu | Precise Extraction of Charge Carrier Mobility for Organic Transistors[END_REF] It has been reported that charge carrier mobility extracted from OFETs can be overestimated by an order of magnitude when the contact resistance is strongly modulated by Vg and charge carrier injection controls the OFET characteristics. [START_REF] Bittle | Mobility overestimation due to gated contacts in organic field-effect transistors[END_REF] Therefore, the extraction of charge carrier mobility from OFETs characteristics should be performed on ideal devices (devices showing textbook like characteristics and ohmic contacts), to avoid over or underestimation. The respect of the following conditions, should result in the extraction of reliable mobilities: 1) the output and transfer characteristic should not exhibit sign of severe hysteresis and trapping, 2) the contribution of the contact resistance to the total device resistance should be negligible compared to the channel resistance contribution, 3) threshold voltage should be as near to 0 V as possible, to confirm the low concentration of traps, 4) the extracted mobility should not show a strong dependence on Vg and should be constant over the whole range of applied gate bias, 5) the extracted mobilities in linear and saturation regime should be equivalent. [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF]

On/off current ratio and hysteresis

Other important parameters to account in OFETs are the on/off current ratio and the hysteresis. The on/off current ratio is defined as the ratio between the drain current in the on-state (maximum Vg) at the drain current in the offstate (Vg = 0 V). The maximization of the on/off current ratio is critical for digital logic operation since it controls the difference between the logic levels. The hysteresis arises from the difference in drain current between the forward and backward sweep. Hysteresis should be minimized in OEFTs for logic circuits since severe hysteresis affects the reproducibility and reliability of OFET characteristics.

Contact resistance

Contact resistance (RC) originates from the voltage required to transfer the charges across the source contact/OSC interface and across the OSC/drain contact interface. Thus, the applied drain voltage can be expressed as:

𝑉 𝑑 = 𝛥𝑉 𝑠 + 𝛥𝑉 𝑐ℎ + 𝛥𝑉 𝑑 = 𝐼 𝑑 (𝑅 𝑆 + 𝑅 𝑐ℎ + 𝑅 𝐷 ) = 𝐼 𝑑 (𝑅 𝐶 + 𝑅 𝑐ℎ ) (1.22) where ΔVS, ΔVD and ΔVch represent the voltage drop at the source contact, at the drain contact and across the OSC channel respectively, while RS, RD and Rch are the resistances associated to the source contact/interface, to the drain contact/OSC interface and to the OSC channel, respectively (Figure 1.9). In equation 1.22 and hereinafter, the contacts are considered Ohmic, i.e., the voltage drop at the contacts is negligible compared to the voltage drop associated to the channel. This is not always the case, depending on the OSC material, the electrodes material and device geometry. For instance, a small Rch obtained either by increasing the OSC mobility or by scaling down the channel length, may lead to devices dominated by the contact resistance. [START_REF] Klauk | Will We See Gigahertz Organic Transistors?[END_REF] In this case, the GCA is not valid anymore and the use of equations 1.13 and 1.14 may lead to mischaracterization of device properties, including over or underestimation of charge carrier mobility. [START_REF] Waldrip | Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier[END_REF] The contact resistance is affected by several factors, such as the OSC material, the electrodes material, the OSC/contact interface, and device related parameters. Depending on the interfaces and on the device being investigated, the parameters influencing the contact resistance may be interdependent, making complex the disentangling of the different contributions to the final device performance.

Influence of the device geometry

The adopted OFET geometry affects contact resistance depending on the position of the drain and source contact with respect to the accumulation channel. In staggered devices charges injected from the source contact must travel perpendicular to the OSC/dielectric interface to achieve the accumulation channel. Similarly, after being transported across the channel, charges must travel perpendicular to the OSC/dielectric interface to reach the drain contact. Consequently, the contact resistance is given by two components: the resistance associated to the charge injection across the contact/OSC interface (Rint) and the one associated to the charge transport through the thickness of the OSC (Rbulk). [START_REF] Waldrip | Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier[END_REF] This introduces a RC dependence on the OSC thickness, i.e., thicker OSC layer results in larger Rbulk. [START_REF] Yamamura | Wafer-scale, layer-controlled organic single crystals for high-speed circuit operation[END_REF][START_REF] Matsumoto | Study of contact resistance of high-mobility organic transistors through comparisons[END_REF][START_REF] Boudinet | Influence of the semiconductor layer thickness on electrical performance of staggered n-and pchannel organic thin-film transistors[END_REF] In contrast, in coplanar geometry charges are injected from the source contact into the OSC channel straightaway, removing the contribution of Rbulk (Figure 1

.10).

Staggered devices typically exhibit larger injection area compared to the coplanar counterpart. This has been generally ascribed as the main reason of smaller contact resistance compared to the coplanar counterparts, for devices consisting of the same material and OSC thickness. [START_REF] Gupta | An analysis of the difference in behavior of top and bottom contact organic thin film transistors using device simulation[END_REF][START_REF] Kim | Fundamental Benefits of the Staggered Geometry for Organic Field-Effect Transistors[END_REF][START_REF] Kim | Compact DC Modeling of Organic Field-Effect Transistors: Review and Perspectives[END_REF][START_REF] Xu | Understanding Thickness-Dependent Charge Transport in Pentacene Transistors by Low-Frequency Noise[END_REF] In coplanar devices the injection area is limited by the height of the contact edge. In contrast, in staggered devices the larger injection area is provided by the extended overlap between the contacts and the channel region of gateinduced charge carriers underneath the contacts. In this case the charge carrier injection is regulated by the current crowding formalism. [START_REF] Chiang | Top-Gate Staggered Amorphous Silicon Thin-Film Transistors: Series Resistance and Nitride Thickness Effects[END_REF][START_REF] Richards | Analysis of the contact resistance in staggered, top-gate organic field-effect transistors[END_REF] This considers the area underneath the contacts as a network of resistors. The charge injection area in staggered devices is not simply given by the geometric overlap of the contact with the gate (Lov), whereas it is determined by the balance of contact resistance, which includes the contribution of Rint and Rbulk, and sheet resistance of the OSC in the channel region (Rsh) † . In fact, in staggered devices the channel region of gate-induced charge carriers extends under the source and drain contacts (Figure 1 Thus, charges injected at the far edge of the source contact must firstly pass through the OSC thickness to reach the channel region of gate-induced charge carriers under the contacts and afterward must travel parallel to the OSC/dielectric interface to reach the active channel (here referred as the accumulation channel between the source and drain contacts, neglecting the regions underneath the contacts).

The characteristic length over which the 63% of charges are injected across the contact/OSC interface is defined as transfer length (LT) and it is determined by the balance of Rsh, Rbulk and Rint. Thus, when the contribution of contact resistance to the total device resistance is negligible compared to the contribution of channel resistance (Ohmic contacts), charge injection/ejection occurs over a narrow area close to the edge of the source/drain contacts. Best performing OSCs have shown LT in the range of 1.5 and 15 µm. [START_REF] Sawada | Correlation between the static and dynamic responses of organic single-crystal field-effect transistors[END_REF][START_REF] Kraft | Detailed analysis and contact properties of low-voltage organic thin-film transistors based on dinaphtho[2,3-b:2′,3′-f]thieno[3,2b]thiophene (DNTT) and its didecyl and diphenyl derivatives[END_REF][START_REF] Chen | The Origin of Low Contact Resistance in Monolayer Organic Field-Effect Transistors with van der Waals Electrodes[END_REF][START_REF] Borchert | Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors[END_REF] At last, the dielectric thickness is another geometric parameter which influences contact resistance. In field-effect transistors the application of the drain voltage and of the gate voltage results in two electric field components, that are perpendicular (𝐸 ⟂ ) and parallel (𝐸 // ) to the OSC/dielectric interface, respectively. The sum of these two components gives the total effective applied electric field at the contact (𝐸 𝑇𝑂𝑇 ). The perpendicular component primarily contributes to charge carrier injection in staggered devices, while in coplanar devices 𝐸 ⟂ has no impact on charge carrier injection, in first approximation. Conversely, the parallel component plays a key role in charge carrier injection in coplanar devices where the injected current (Jint) flows in the same direction of the channel current (Jch), parallel to the OSC/dielectric interface (Figure 1.10). Thus, the contribution of 𝐸 // on charge carrier injection in coplanar devices would depend on the dielectric thickness. This has been firstly confirmed by drift diffusion simulations, which have shown that a reduced dielectric thickness facilitates charge injection in a BGBC organic thin-film transistors. [START_REF] Gruber | Impact of Materials versus Geometric Parameters on the Contact Resistance in Organic Thin-Film Transistors[END_REF][START_REF] Zojer | Impact of the Capacitance of the Dielectric on the Contact Resistance of Organic Thin-Film Transistors[END_REF] In fact, the larger 𝐸 // component due to the thinner dielectric layer, gave rise to higher charge carrier concentration in the accumulation channel and smaller potential drop at the source contact. These theoretical findings have been corroborated by the fabrication of TFTs with different dielectric thickness. The TFTs analysis showed that contact resistance in coplanar devices based on diPh-DNTT is lower than the one of the staggered counterpart if the dielectric layer is thinner than 30 nm and if the OSC morphology is similar across the contact-to-channel interface. [START_REF] Borchert | Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors[END_REF]

Measuring contact resistance: the gated transmission line method

The gated transmission line method (TLM), referred also as gated transfer line method, is one of the most employed methods to extract contact resistance from current-voltage characteristics of OFETs with varying channel length. This method was first developed for FETs based on inorganic semiconductors and further applied to OEFTs. [START_REF] Luan | An experimental study of the source/drain parasitic resistance effects in amorphous silicon thin film transistors[END_REF] The major assumptions of TLM are: 1) the GCA is valid, 2) the voltage drop at the contacts is negligible compared to the voltage drop across the channel, 3) the charge carrier concentration is homogeneous along the channel and 4) channel resistance scales linearly with the channel length, while the contact resistance is not affected by the channel length variation. [START_REF] Waldrip | Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier[END_REF][START_REF] Borchert | A Critical Outlook for the Pursuit of Lower Contact Resistance in Organic Transistors[END_REF] The total resistance (R) of a FET is given by the sum of the contact resistance (RC) and the channel resistance (Rch), where RC accounts for the contribution of both the source (RS) and drain (RD) contacts: 𝑅 = 𝑅 𝐷 + 𝑅 𝑆 + 𝑅 𝑐ℎ = 𝑅 𝐶 + 𝑅 𝑐ℎ (1.23) The channel resistance varies linearly with the channel length (L) and can be expressed as follow: (1.25) Thus, by measuring the transfer characteristics in the linear regime of fieldeffect transistors with varying channel length, the with-normalized contact resistance (RCW) and the intrinsic channel mobility (µ0) are obtained from the linear fit to the RW vs L data points. Specifically, the intercept of the linear fit with the Y-axis (L = 0) gives RCW, while µ0 is obtained from the slope at each defined gate-overdrive voltage (Figure 1.12).

It is worth noting that by using the TLM it is not possible to separate the source and drain contributions since they are both encompasses in RCW. Other methods can be used to access the contribution of RS and RD to the contact resistance, such as Kelvin probe force microscopy, [START_REF] De Tournadre | High voltage surface potential measurements in ambient conditions: Application to organic thin-film transistor injection and transport characterization[END_REF] gated four probe method [START_REF] Pesavento | Film and contact resistance in pentacene thin-film transistors: Dependence on film thickness, electrode geometry, and correlation with hole mobility[END_REF] and gated van der Pauw method. [START_REF] Rolin | Charge carrier mobility in thin films of organic semiconductors by the gated van der Pauw method[END_REF] The major drawbacks of these methods relies in the requirement of special measurement equipment, the long measurement and analysis time, and the special structure of the analysed devices.

The organic semiconductor-contact interface

The contact resistance associated to charge injection across the OSC/contact interface (Rint) is mainly related to the potential energy barrier that forms at the interface also known as charge injection barrier. In the easiest approximation (Schottky-Mott limit), the charge injection barrier, namely Schottky barrier (ΦB), is given by the energy difference between the work function of the contact (WFC) and the ionization energy of the OSC for hole injection in p-type OSC (ΦB,p) and by the difference between the work function of the contact and the electron affinity of the OSC for electron injection in ntype OSC (ΦB,n): [START_REF] Kahn | Electronic structure and electrical properties of interfaces between metals and pi-conjugated molecular films[END_REF] Φ 𝐵,𝑝 = WF 𝑐 -𝐼𝐸 (1.26) Φ 𝐵,𝑛 = WF 𝑐 -𝐸𝐴 (1.27) In this model, the WF of the contact and the IE (or EA) of the OSC are those of isolated materials.

However, the Schottky-Mott rule can only provide an estimate of the charge injection barrier at the interface. In reality, charge injection barriers are influenced by a number of factors, such as electrochemical processes, interface morphology, electrostatic interactions and interfacial trap states. From thermodynamic considerations when two different materials are brought into contact the resultant system must reach thermodynamic equilibrium. This is typically achieved by charge carrier transfer across the interface until their electrochemical potentials (Fermi levels) establish a common Fermi level (Fermi level pinning). [START_REF] Schroder | Semiconductor Material and Device Characterization[END_REF] The charge redistribution creates an electric field across the interface, commonly referred as the interface dipole (Δ). The presence of the electric field can basically shift the vacuum level and bend the electronic energy levels. In such a case the charge injection barriers will be given by equations 1.28 and 1.29.

Φ 𝐵,𝑝 = WF 𝑐 -Δ -𝐼𝐸 (1.28) Φ 𝐵,𝑛 = WF 𝑐 -Δ -𝐸𝐴 (1.29)
The origin of the interface dipole is attributed to different factors, such as charge transfer across the OSC/contact interface (resulting in anions or cations formation), potential chemical reactions at the interface, changes of OSC molecular orientation at the interface, intrinsic dipole moment of the OSC molecules and push-back effect. [START_REF] Borchert | On the Minimization of Contact Resistance in Organic Thin-Film Transistors[END_REF] The latter arises from the electron density of the OSC molecules in proximity to the interface which suppresses the electron cloud extending away from the contact surface. [START_REF] Ishii | Energy Level Alignment and Interfacial Electronic Structures at Organic/Metal and Organic/Organic Interfaces[END_REF] In the region close to the interface, due to the electric field between the OSC and the contact caused by charge transfer, the transport bands edges of the OSC are shifted, namely band bending (Figure 1.13). This leads to the formation of a region depleted of charge carriers at the interface, which extends towards the OSC bulk. The width of this depleted region (WD) depends on the interfacial trap density, which in turns is influenced by the OSC morphology across the interface. [START_REF] Zhang | Band Bending in Semiconductors: Chemical and Physical Consequences at Surfaces and Interfaces[END_REF] All these complex phenomena occurring at the OSC/contact interface contributes to determining the mechanism of charge carrier injection in a device and ultimately affect contact resistance. [START_REF] Waldrip | Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier[END_REF] In this context, ultraviolet photoelectron spectroscopy (UPS) is a powerful experimental technique used to study the electronic structure of solid surfaces, thin films, and interfaces. By irradiating the sample with ultraviolet photons, UPS can measure the energies of photoemitted electrons and provide information about the electronic levels of the material. In the case of organic semiconductors, UPS is particularly useful for determining the energy of the HOMO, the injection barrier, work function, and interfacial dipole at organic semiconductor interfaces.

Coupling with the vacuum field

In the last decade, strong coupling between light and matter has been explored to modify the physical and chemical properties of molecular systems. Strong light matter interaction between a molecular transition and a resonant optical mode can lead to the formation of new hybridized lightmatter states, called as polaritonic states. Such coupling is possible when the exchange of energy is faster than any relaxation process. This interaction has been successfully employed to alter molecular properties. The first part of this section is focused on the theoretical model that have been developed in the framework of light-matter strong coupling, while in the second part a brief overview of the effect of light-matter strong coupling on the chemical and physical properties of materials is presented.

Theoretical description

In order to have a better understanding of the basic light-matter interaction, it is advisable to start with the classical analogy of two coupled harmonic oscillators. When two harmonic oscillators are not coupled, they act as independent systems, preserving their parameters, i.e., the oscillation frequency. But, when the oscillators are coupled and their interaction is strong enough (the energy exchange is faster than any dissipation processes), they start to act as a singular system. Considering two independent oscillators with masses M1 and M2 and spring constants k1 and k2, the system can be described through the Newton's second law (introducing k3 as the spring constant when the oscillators are coupled, see Figure 1.14): [START_REF] Hertzog | Strong light-matter interactions: a new direction within chemistry[END_REF] 𝑀 1

𝑑 𝑥 1 ′′ 𝑑𝑥 + 𝑘 1 𝑥 1 + 𝑘 3 (𝑥 1 -𝑥 2 ) = 0 (1.30) 𝑀 2 𝑑 𝑥 2 ′′ 𝑑𝑥 + 𝑘 2 𝑥 2 + 𝑘 3 (𝑥 1 -𝑥 2 ) = 0 (1.31)
Solving the differential equations, we obtain:

𝜔 ± = 1 2 (𝜔 1 + 𝜔 2 + √4𝛺 2 + (𝜔 1 -𝜔 2 ) 2 ) (1.32)
where ω1 and ω2 are the frequencies of the two oscillators, Ω is the frequency of splitting and ω± are the two new frequencies obtained due to the coupling. It is worth noting that in this description no damping or dissipation effects are accounted, which should be introduced with a frictional term. The system is defined as strongly coupled when the dissipation is smaller than the coupling strength. The description of the coupled system using a classical theory, does not sufficiently explain the strong coupling phenomenon obtained by the interaction of molecular transitions with vacuum field fluctuations. Vacuum field fluctuations can be described as the ground state energy of the quantized electromagnetic field (EM). This was firstly described by Plank and Einstein in 1911 and 1913, respectively, using a quantum description. [START_REF] Hertzog | Strong light-matter interactions: a new direction within chemistry[END_REF] Dirac established the fundamental principles of quantum electrodynamics (QED) [START_REF] Dirac | The quantum theory of the emission and absorption of radiation[END_REF] and elucidated the mechanism behind the interaction between an excited emitter and the vacuum fields, which facilitates energy transfer via spontaneous photon emission. [START_REF] Nagarajan | Chemistry under Vibrational Strong Coupling[END_REF] In the free space, the electromagnetic field has a continuous spectrum of modes that tend to interact weakly with the emitter. To augment this interaction, a cavity can be utilized to confine the emitter. For example, a cavity constructed using two parallel metallic mirrors can be employed for this purpose. In such a constrained arrangement, the EM field manifests a discrete spectrum of modes. [START_REF] Garcia-Vidal | Manipulating matter by strong coupling to vacuum fields[END_REF] The strong coupling regime in light-matter interaction can be described using QED. In the easiest approximation we consider an atom formed by a two-level system interacting with a single mode of quantized electromagnetic field, by using the Jaynes-Cummings model. [START_REF] Jaynes | Comparison of quantum and semiclassical radiation theories with application to the beam maser[END_REF] Other models have been developed, such as the Tavis-Cummings model which describes the interaction of a single mode of the electromagnetic field with N identical atoms. [START_REF] Tavis | Exact Solution for an N -Molecule-Radiation-Field Hamiltonian[END_REF] In the following, by using the Jaynes-Cummings formalism, a simple model of light-matter coupling is presented. The complete description of model can be found in ref. [START_REF] Jaynes | Comparison of quantum and semiclassical radiation theories with application to the beam maser[END_REF][START_REF] Wang | Strong Light-Molecule Coupling: Routes to New Hybrid Material[END_REF] .

𝐻 𝑎 ̂= 1 2 ħ 𝝎 0 𝜎 𝑧 ̂ (1.34)
where 𝝎 0 is the transition frequency of the atom.

Likewise, defining 𝑎 ̂ and 𝑎 ̂ † as the creation and annihilation operators of the electromagnetic field mode and 𝜔 as the frequency of the field mode, 𝐻 𝑓 ̂ can be expressed as follow:

𝐻 𝑓 ̂= ħ 𝝎( 𝑎 ̂ † 𝑎 ̂) (1.35)
At last, the atomic-field interaction Hamiltonian describes the interaction between the transition dipole moment (𝑑 ̂) of the atom and the electric field (𝜀) operators, i.e.: 𝐻 𝑖 ̂= -𝑑 ̂• 𝜀. Within the rotating wave approximation (RWA), [START_REF] Merritt | Organic solar cells of hydroxy squarylium[END_REF] the atomic-field interaction Hamiltonian is expressed as:

𝐻 𝑖 ̂= -𝑖 ħ𝑔( 𝑎 ̂ † 𝜎 ̂+ 𝜎 ̂ † 𝑎 ̂) (1.36)
where 𝑔 accounts for the magnitude of light-matter coupling strength. Thus, the final expression of the Jaynes-Cummings Hamiltonian is given by:

𝐻 𝐽𝐶 ̂= 1 2 ħ 𝝎 0 𝜎 𝑧 ̂+ ħ 𝝎( 𝑎 ̂ † 𝑎 ̂) -𝑖 ħ 𝑔( 𝑎 ̂ † 𝜎 ̂+ 𝜎 ̂ † 𝑎 ̂) (1.37)
The diagonalization of the Jaynes-Cummings Hamiltonian yields to the description of these new two eigenstates, as a linear combination of light and matter states, namely polaritonic states or polaritons (P + and P -):

{ | 𝑃 + ⟩ = cos 𝜃 | 𝑒, 0⟩ + 𝑖 sin 𝜃 | 𝑔, 1⟩ | 𝑃 -⟩ = sin 𝜃 | 𝑒, 0⟩ -𝑖 cos 𝜃 | 𝑔, 1⟩ (1.38)
where 𝜃 is defined as:

tan 2𝜃 = - 2𝑔 𝝎-𝝎 0 (1.39)
When the transition frequency of the atom is resonant with the electromagnetic mode, the polaritons are hybrid state with half-matter and half-light character. The energy gap between P + and P -is called Rabi-splitting (ħΩR) and in absence of dissipation is described as:

ħ𝛺 𝑅 = 2 ħ 𝑔 = 2 𝑑 √ ħ𝜔 2𝜀 0 𝑣 𝑒 √ 𝑛 𝑝ℎ + 1 (1.40)
where d is the transition dipole, ħ𝜔 is the resonant energy, 𝜀 0 the vacuum permittivity, ve is the volume of the electromagnetic mode and 𝑛 𝑝ℎ the number of photons involved in the coupling. As can be seen in equation 1.40, in the absence of photons, there is a still a residual coupling to the vacuum electromagnetic field, the quantum EM fluctuations of the optical resonator.

In other words, strong coupling occurs even in the dark.

The light-matter strong coupling can be compared to a molecular analogy., i.e., the formation of J-and H-aggregates shown by π-conjugated organic molecules. [START_REF] Ebbesen | Hybrid Light-Matter States in a Molecular and Material Science Perspective[END_REF] In higher concentrated dye solutions, the self-association of molecules is frequently encountered due to strong intermolecular attractive forces between molecules. According to exciton theory, the molecule is described as a dipole which can interact with another molecule/dipole, leading to the formation of two new eigenstates. The molecules may aggregate in a parallel way (plane-to-plane stacking) to form a sandwich type arrangement (H-aggregates) or in a head-to-tail arrangement to form J-aggregates. In the first case (H-aggregates) the transition dipole moments are (anti-)parallel, while in the second case (J-aggregates) the transition dipole moments are perpendicular. Therefore, due to the coupling between molecular transition dipole moments, two new states are obtained (Figure 1.15). Schematic illustration of (a) the electronic coupling between transition dipole moments resulting in the formation of J-and H-aggregates and (b) the analogy with the interaction between a molecular transition and an optical mode resulting in the formation of two new hybrid light-matter states (P + and P -).

In order to complete the analogy, we can imagine light matter coupling as the previous described interaction, replacing a molecule transition dipole with a mode of the electromagnetic field.

Molecular and material properties under lightmatter strong coupling

From experimental point of view, light-matter strong coupling is typically achieved by confining the investigated compound in an optical or plasmonic cavity. The role of the cavity is to enhance the interaction between its optical modes (in case of an Fabry-Perot cavity) or its surface plasmon polaritons (in case of a plasmonic structures) and the transition dipoles of the molecules, though a confinement of the electromagnetic field.

The material properties can be altered by coupling either electronic transition, namely electronic strong coupling (ESC) and vibrational modes, namely vibrational strong coupling (VSC). Most important, chemical and physical properties of molecules can be modified by vibrational strong coupling and cooperative vibrational strong coupling. [START_REF] Lather | Cavity Catalysis by Cooperative Vibrational Strong Coupling of Reactant and Solvent Molecules[END_REF] The latter is required when the investigated compound is dissolved in a solvent and the direct coupling between the optical mode of the cavity and the molecular vibration is not possible (due to the low concentration of the solute). Thus, cooperative VCS consists in coupling the optical mode with a vibrational mode of the solvent that is resonant with a vibrational mode of the solute. The effect of ESC, VSC and cooperative VSC on material properties has started to be extensively investigated in last decade.

Hutchison et al. made the first observation of the impact of strong coupling in chemistry, [START_REF] Hutchison | Modifying Chemical Landscapes by Coupling to Vacuum Fields[END_REF] showing how ESC could change the energy landscape of reactants in case of spiropyran-merocyanine photoisomerization. Thereafter, the modification of work function, [START_REF] Hutchison | Tuning the Work-Function Via Strong Coupling[END_REF] the enhancement of energy transfer [108- 111] and the boost of conductivity in OSC have been reported. [START_REF] Orgiu | Conductivity in organic semiconductors hybridized with the vacuum field[END_REF][START_REF] Nagarajan | Conductivity and Photoconductivity of a p-Type Organic Semiconductor under Ultrastrong Coupling[END_REF] VSC and cooperative VSC have shown to greatly modify the chemical reactivity and the outcome of various reactions, such as cyclization, [START_REF] Hirai | Modulation of Prins Cyclization by Vibrational Strong Coupling[END_REF] charge transfer, [START_REF] Pang | On the Role of Symmetry in Vibrational Strong Coupling: The Case of Charge-Transfer Complexation[END_REF] solvolysis [START_REF] Lather | Cavity Catalysis by Cooperative Vibrational Strong Coupling of Reactant and Solvent Molecules[END_REF] and enzymatic [START_REF] Vergauwe | Modification of Enzyme Activity by Vibrational Strong Coupling of Water[END_REF][START_REF] Lather | Improving Enzyme Catalytic Efficiency by Co-operative Vibrational Strong Coupling of Water[END_REF] reactions. Recently, cooperative VSC has been revealed as an effective tool to influence the supramolecular assembly and ordering of materials. Specifically, the selective crystallization of a pseudo-polymorphic form of zeolitic imidazolate frameworks [START_REF] Hirai | Selective crystallization via vibrational strong coupling[END_REF] and the modification of the morphology of supramolecular assemblies [START_REF] Joseph | Supramolecular Assembly of Conjugated Polymers under Vibrational Strong Coupling[END_REF][START_REF] Sandeep | Manipulating the Self-Assembly of Phenyleneethynylenes under Vibrational Strong Coupling[END_REF] have been reported. This opens to new possibilities in the field of crystallography and supramolecular chemistry, where the molecular ordering can be modified via matter-light interaction. Specifically, crystal packing and polymorphism strongly affected charge carrier mobility in OSCs.

The development of methods to induce selective crystallization, preventing the concomitant growth of polymorphic forms, may offer new opportunities to maximize charge carrier mobility in OSCs.

Very recently, a 6 order of magnitude boost in the conductivity of polystyrene was reported when a subset of vibrations of the polystyrene were coupled to a Fabry-Perot cavity. [START_REF] Kumar | Extraordinary Electrical Conductance of Non-conducting Polymers Under Vibrational Strong Coupling[END_REF] This surprising result shows the promise of VSC for OSCs.

At last, it is worth noting that there are still many aspects of strong coupling that are unclear. Yet, in order to properly comprehend the mechanism in coupled systems and be able to forecast the result of strong coupling on a specific molecular property, more comprehensive theoretical explanations are required.

Chapter 2 Charge transport and charge injection properties of new thienoacene derivatives in thin-film transistors 2.1 Introduction

Due to their excellent hole mobility exceeding 10 cm 2 V -1 s -1 , [START_REF] Takimiya | Thienoacene-Based Organic Semiconductors[END_REF][START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF] chemical stability and reproducibility, thienoacenes have been extensively studied and serve as benchmarks in small-molecule organic TFTs device physics. BTBT was firstly reported by Takimya et al., [START_REF] Takimiya | Organic Semiconductors Based on[END_REF] who recognized its potential application in electronic devices due to an effective contribution of sulphur atoms to the intermolecular HOMO overlap which favours charge transport within adjacent molecules. BTBT has two main disadvantages: high ionization energy (IE) and poor film formation. Typically, IE of BTBT is ca. 5.8 eV, which severely limits charge carrier injection from electrodes into this OSC. The often-reported poor homogeneity of vacuum deposited thin-films prevents the preparation of functioning BTBT thin-film transistors.

The extension of the π-conjugated molecular core and the molecular shape have been demonstrated to affect electronic properties of OSCs. In the first place, the π-extension results in better delocalization of molecular orbitals, which in turns is reflected in reduced structural difference between the neutral and charged state (smaller reorganization energy). [START_REF] Watanabe | The synthesis, crystal structure and charge-transport properties of hexacene[END_REF][START_REF] Gao | Development of n-type organic semiconductors for thin film transistors: a viewpoint of molecular design[END_REF][START_REF] Takimiya | Design and elaboration of organic molecules for high field-effect-mobility semiconductors[END_REF] . Thanks to the π-extension, also the overlap of molecular orbitals within adjacent molecules can be enlarged, wherein the intermolecular distance and crystalline packing is preserved. This is expected to boost charge carrier mobility. The design of structurally modified BTBT derivatives has led to OSCs with high mobility and facile charge injection. Particularly, DNTT, [START_REF] Yamamoto | Facile Synthesis of Highly π-Extended Heteroarenes[END_REF][START_REF] Acharya | Stability of organic thin-film transistors based on ultrathin films of dinaphtho[2,3b :2′,3′-f ]thieno[3,2-b ]thiophene (DNTT)[END_REF][START_REF] Zschieschang | Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) thin-film transistors with improved performance and stability[END_REF][START_REF] Bestelink | The Secret Ingredient for Exceptional Contact-Controlled Transistors[END_REF] and DBTTT [START_REF] Park | Dibenzothiopheno[6,5-b :6′,5′-f ]thieno[3,2-b ]thiophene (DBTTT): High-Performance Small-Molecule Organic Semiconductor for Field-Effect Transistors[END_REF] have been synthesized by annealing thienyl and benzyl rings to the BTBT core, achieving mobilities > 3 cm 2 V -1 s -1 along with low contact resistance.

In contrast L-DBTTA, despite the extended π-core consisting of a central motive comprising two annealed thienyl groups between two naphthyl moieties, has shown lower mobility, with values of 0.15 cm 2 V -1 s -1 . [START_REF] Huang | Dibenzoannelated Tetrathienoacene: Synthesis, Characterization, and Applications in Organic Field-Effect Transistors[END_REF] The poorer electrical performance compared to DNTT and DBTTT has been attributed to the stacked crystalline motif, [START_REF] Jouclas | Dinaphthotetrathienoacenes: Synthesis, Characterization, and Applications in Organic Field-Effect Transistors[END_REF] which disfavours charge transport due to severe anisotropy of electronic interactions within adjacent molecules in the crystalline arrangement. Also, the molecular shape of thienoacenes plays a role in the design of new OSCs since it affects translational motions of molecules in crystal structures. Particularly, in case of OSCs that pack with an herringbone (HB) motif, motions along the longest molecular axis are suspected to weaken the HOMO wavefunctions overlap. [START_REF] Illig | Reducing dynamic disorder in small-molecule organic semiconductors by suppressing large-amplitude thermal motions[END_REF][START_REF] Schweicher | Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors[END_REF] N-shape molecular semiconductors which deviate from a linear molecular shape, even slightly such as DNTT, can decrease such detrimental motions. [START_REF] Abe | Thermally, Operationally, and Environmentally Stable Organic Thin-Film Transistors Based on Bis[1]benzothieno[2,3-d :2′,3′-d ′]naphtho[2,3-b :6,7-b ′]dithiophene Derivatives: Effective Synthesis, Electronic Structures, and Structure-Property Relationship[END_REF][START_REF] Mori | Consecutive Thiophene-Annulation Approach to π-Extended Thienoacene-Based Organic Semiconductors with[END_REF] With this view, the charge transport properties of two new isomeric thienoacene-based semiconductors DN4T and isoDN4T (Scheme 2.1) are investigated in TFTs. From a structural point of view, DN4T and isoDN4T have two additional thienyl rings in the central motif compared to DNTT and isoDNTT. DN4T and L-DBTTA share the same central motif of four thienyl rings, annealed by terminal naphthene or benzene moieties, respectively. The aim is to see whether the extended π-system, comprising four fused thienyl rings between two naphthyl moieties, and the more accentuated N-shape compared to DNTT leads to high charge carrier mobility while preserving HB packing. Moreover, TFTs of octyl-and decyl-substituted derivatives C8-DN4T and C10-DN4T are investigated, and their electrical performances are compared with the parent compound (DN4T) and the octyl-substituted derivative of DNTT, namely C8-DNTT. The functionalization of DNTT and BTBT with alkyl chains, besides enabling the device fabrication through solution processing, has shown to lead to more isotropic and balanced transfer integrals as well as to intermediate thermal disorder , boosting charge carrier mobility compared to the parent compound. [START_REF] Illig | Reducing dynamic disorder in small-molecule organic semiconductors by suppressing large-amplitude thermal motions[END_REF][START_REF] Schweicher | Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors[END_REF] Thus, a similar effect is expected for alkylated DN4Ts.

A significant enhancement of charge carrier mobility in BTBT derivatives has been obtained by introducing tert-butyl groups on the 2,7-positions (tBu-BTBT). This enhancement is related to the increased dimensionality of charge transport, made possible by making slight modifications to the tilt angles and distances between molecules in the crystalline structure. These adjustments were achieved by introducing an appropriate bulky side group, which resulted in significantly larger and more evenly distributed isotropic transfer integrals. [START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF] Specifically, kinetic Monte Carlo simulations revealed an almost perfect isotropic charge transport in the herringbone layer planes. Charge carrier mobilities of 7.1 and 2.4 cm 2 V -1 s -1 have been reported for tBu-BTBT in single crystal (SCFET) and thin-film field-effect transistors, respectively. [START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF][START_REF] Chung | Rotator side chains trigger cooperative transition for shape and function memory effect in organic semiconductors[END_REF] However, the alkyl substitution has just a limited effect on the ionization energy of 5.7 eV for tBu-BTBT. This value is comparable to BTBT and hinders charge carrier injection from metal contacts. In fact, the reported OFETs exhibit high threshold voltage, limiting the range of gate voltage for charge carrier extraction to Vg between -80 V and -100 V (20% of the total applied gate voltage) (see Figure 2.1).

Scheme 2.1

Moreover, the extracted mobilities are not constant over the whole range of investigated Vg and they are reported only in saturation regime by applying high drain voltage (Vd = -100 V). Only in ref. [START_REF] Chung | Rotator side chains trigger cooperative transition for shape and function memory effect in organic semiconductors[END_REF] mobility is reported also in linear regime (Vd = -2 V), exhibiting values up to 3 times lower compared to the values reported in saturation regime. In addition, only the backward characteristics are shown, preventing the detection of potential hysteresis. At last contact resistance has not been reported. It should be considered that the extraction of OFETs parameters is based on equations which model the behaviour of ideal devices. Hence, relevant deviation from ideal textbook-like characteristics (i.e., non-ideal behaviour such as high threshold voltage, gate voltage dependent mobility, discrepancy in mobility values extracted from linear and saturation regime, relevant hysteresis and high contact resistance), as in the case of the reported OFETs based on tBu-BTBT, could lead to severe overestimation of the claimed mobility. (a) Transfer characteristics of (a) SCFET exhibiting µ of 7.1 cm 2 V -1 s -1 [START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF] (b) TFT fabricated through solution shearing technique exhibiting µ of 2.4 cm 2 V -1 s -1 [START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF] (c) TFT fabricated through solution shearing technique exhibiting µ of 1.9 cm 2 V -1 s -1 . [START_REF] Chung | Rotator side chains trigger cooperative transition for shape and function memory effect in organic semiconductors[END_REF] All the curves are collected in saturation regime (Vd = -100 V) and exhibit Vth of ≈ -70 V (over an applied Vg of -100 V for (a) and (c) and of -120 V for (b)). (d) Calculated field-effect anisotropy as a function of the crystallographic orientation (bc plane corresponds to the herringbone layer plane). [START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF] As mentioned above, the extension of the π-conjugated molecular core influence both the reorganization energy and the transfer integrals, due to the larger charge delocalization. In addition, given that the ionization energy is the energy required to remove one electron from the HOMO of molecules, the increased charge delocalization due to the π-extension, generally results in high-lying HOMO energy levels. [START_REF] Takimiya | Organic Semiconductors Based on[END_REF][START_REF] Watanabe | The synthesis, crystal structure and charge-transport properties of hexacene[END_REF][START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF][START_REF] Yamamoto | Zigzag-Elongated Fused π-Electronic Core: A Molecular Design Strategy to Maximize Charge-Carrier Mobility[END_REF] Even the manner of the πextension play a key role in determining the electronic properties of OSCs. Annealed systems generally show smaller reorganization energies compared to oligomeric systems. Specifically, due to the presence of single bonds connecting the aromatic rings, oligomeric systems have more molecular motions comprising rotation around the single bonds. In contrast, annealed molecular structure would result is lower molecular geometric variation upon charge transfer. [START_REF] Takimiya | Design and elaboration of organic molecules for high field-effect-mobility semiconductors[END_REF] With this view, the influence of the π-extension and of the manner of πextension on the ionization energy and charge transport characteristics in new tert-butyl derivatives are investigated, by mean of photoelectron yield spectroscopy and fabrication of thin-film transistors. Specifically, the targeted compounds are tert-butyl derivatives of DNTT and DBTTT (tBu-DNTT and tBu-DBTTT), along with tert-butyl-thienyl derivatives of BTBT (tBu-BTBT-1 and tBu-BTBT-2), depicted in Scheme 2.2 Scheme 2.2

The extension of the π-system of the molecular core is obtained through an annealed approach (tBu-DNTT and tBu-DBTTT) and through an oligomer approach (tBu-BTBT-1 and tBu-BTBT-2). The aim is to improve charge carrier injection in thin film transistors, with the view to obtain ideal and reliable behaviours, while preserving high charge carrier mobility. At last, the charge transport properties of the analysed compounds are related to quantum chemical calculations, enabling structure-properties relations derivation.

Crystal structures

Crystal structures of the analysed compounds have been resolved and provided by Dr. Jiu Liu (Université libre de Bruxelles) and Dr. Alan Kennedy (University of Strathclyde).

The crystal structures of C8-DN4T and C10- DN4T have not yet been solved. Hence, in this section the alkylated DN4Ts are not considered.

The compounds investigated in this chapter adopt a standard layer-by-layer herringbone (HB) packing motif (Figure 2.2), with a HB angle between two adjacent molecules ranging from 46.4° to 53.8°, in line with previously reported thienoacenes with a similar packing arrangement. [START_REF] Mitsui | High-Performance Solution-Processable N-Shaped Organic Semiconducting Materials with Stabilized Crystal Phase[END_REF][START_REF] Okamoto | V-Shaped Organic Semiconductors With Solution Processability, High Mobility, and High Thermal Durability[END_REF] In herringbone arrangement, charge transport is facilitated by edge-to-face interactions. Among the most encountered packing motif in organic semiconductor crystals, herringbone packing has shown to give rise to high mobility, along with brick-wall packing, if compared to slipped π-stacking and slipped-stack packing motifs. [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF] DN4T belongs to monoclinic space group P 21, while tBu-DBTTT belongs to the triclinic P-1, both with one unique molecule in the asymmetric unit (Z' = 1). All the other compounds belong to monoclinic space group P 21/c, with half molecule in the asymmetric unit (Z' = 0.5), as reported in Tables 2.1.

If we consider the short contacts (which are defined as any intermolecular contact shorter than the sum of the van der Walls radii of the involved atoms), all the compounds exhibit C-H•••π, S•••C and S•••S interactions, except isoDN4T. The latter exhibits only C-H•••π interactions. This discrepancy in intermolecular interactions may play a role for the charge transport properties in isoDN4T. Sulphur atoms have relevant electron density compared to carbon and hydrogen atoms. Crystal arrangements which unveil S•••C and S•••S short contacts may show higher electron density of the HOMO that leads to a more efficient charge transfer within adjacent molecules. Considering a transient localization scenario, charges are delocalized over several molecules instead to be confined on one. Therefore, more efficient pathways for electron transfer enhance charge delocalization, which increases charge carrier mobility.

At last, it is worth mentioning that no polymorphic forms have been reported for the investigated compounds.

Thin-film transistors fabrication

TFTs were fabricated through thermal evaporation in high vacuum. Typically, OSCs were deposited at a base pressure of ≈10 -6 -10 -7 mbar onto substrates consisting of highly doped silicon wafer, which serves as global gate electrode, overgrown by atomic layer deposition with a 30 nm thick dielectric layer of Al2O3. The substrate was immersed in a 1.5 mM solution of n-tetradecylphosphonic acid (TDPA) in isopropanol prior to the deposition of the organic semiconductors, resulting in the formation of a self-assembled monolayer (SAM) on the dielectric surface. Phosphonic acids have been widely used for Al2O3 dielectric functionalization due to their ability to form stable and highly-order SAM. The TDPA/Al2O3 dielectric surface results to be highly hydrophobic if compared to the bare Al2O3 surface (Figure 2.3) minimizing the unintentionally contamination of the dielectric surface with water molecules and preventing electrochemical reactions between the charge carrier in the accumulation layer and the moisture at the dielectric interface. [START_REF] Casalini | Self-assembled monolayers in organic electronics[END_REF] The gate-dielectric capacitance (Cdiel) was calculated as follow:

𝐶 𝑑𝑖𝑒𝑙 = 𝜀 0 ( 𝑡 𝑜𝑥 𝜀 𝑜𝑥 + 𝑡 𝑆𝐴𝑀 𝜀 𝑆𝐴𝑀 ) -1 (2.1)
where ε0 is the vacuum permittivity, tox is the Al2O3 thickness, tSAM is the TDPA-SAM thickness, εox is the relative permittivity of Al2O3 and εSAM is the relative permittivity of TDPA-SAM. The calculated unit-area capacitance is 185 nF cm -2 . OSCs were deposited at 0.3 Ås -1 and with nominal thickness (monitored with a quartz crystal microbalance) of 25 nm. In case of alkylated DN4Ts, the deposition rate was set at 0.1 Ǻs -1 , as previously reported for other alkylated thienoacenes. [START_REF] Schweicher | Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors[END_REF][START_REF] Rolin | Charge carrier mobility in thin films of organic semiconductors by the gated van der Pauw method[END_REF] Semiconductor thickness between 20 and 30 nm is usually employed in TFTs, since it ensures a sufficient coverage of the active channel. Using thicker semiconductor layer may result in larger contact resistance in TC devices, due to the poor vertical charge transport through the semiconductor thickness (in case of small-molecule OSCs). Moreover, thicker semiconductor layer exhibit larger trap-state density in small-molecule p-type OSC. [START_REF] Matsumoto | Study of contact resistance of high-mobility organic transistors through comparisons[END_REF][START_REF] Fischer | Nonlinear Contact Effects in Staggered Thin-Film Transistors[END_REF] Both, bottom-gate top-contact (BGTC, hereinafter shortened to TC) and bottom-gate bottom-contact (BGBC, hereinafter shortened to BC) configurations were fabricated by thermal evaporation of gold electrodes with a typical thickness of ca. 50 nm by using shadow masks at a base pressure of 10 -5 -10 -6 mbar. In case of BC devices, gold contacts were deposited before the OSCs and their surfaces were functionalized with pentafluorobenzenethiol (PFBT), by immersing the substrate in a 10 mM solution of PFBT in isopropanol. The treatment results in the formation of a PFBT-SAM, which promotes a more uniform semiconductor morphology across the interface between the contacts and the active channel. Moreover, PFBT exhibits a large dipole moment created by the high density of fluorine atoms which points downward with respect to the gold surface. This large dipole increases the work function of gold contacts of ≈ 0.4 eV, [START_REF] Borchert | Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors[END_REF] which can improve charge injection in p-type OSCs by reducing the height of the nominal chargeinjection barrier. Also, BC devices with bare gold contacts were fabricated and compared to BC devices with PFBT treated gold contacts. During the OSCs deposition, the substates were kept at temperatures ranging from 40°C to 140°C with 20°C steps, for DN4T and isoDN4T, and from 40°C to 130°C with 30°C steps for the other compounds. § The substrate temperature during OSCs deposition (Ts) is one of the most important parameters to control in order to achieve the best charge carrier mobility. By increasing the substrate temperature, the surface diffusion of the adsorbed molecules onto the substrate increases as well, allowing the deposited molecules to find the lowest energy sites and inducing a long-range structural ordering within the OSC film. The heating of the substrate beyond a certain temperature leads to a decrease of the sticking coefficient of the organic molecules which is reflected in lack of inter-grain connectivity and partial dewetting of the OSC. [START_REF] Virkar | Organic Semiconductor Growth and Morphology Considerations for Organic Thin-Film Transistors[END_REF] § We acknowledge Michael Eustachi for his contribution to the development of the controlled substrate temperature feature integrated into the OSC evaporator (UNIVEX 300).

A schematic representation of the TC and BC devices is presented in

Thin-film characterization 2.4.1 Topography

Deposition onto TDPA/Al2O3

The morphology of the thin films was evaluated through atomic force microscopy (AFM) in tapping mode performed on the active channel of fabricated transistors with Ts of 40, 100 and 140 °C. All the compounds, except for the alkylated DN4Ts, exhibit a terrace structure topography characterized by the diffuse presence of grain boundaries which define the micro-(or nano) crystalline domains (Figure 2.5). In contrast, the alkylated DN4Ts are characterized by small crystallites exhibiting a slightly elongated shape (Figure 2.6). Hence, in this section the analysis of thin film topography of non-alkylated parent compounds is discussed at first, due to common terraced topography, followed by the analysis of alkylated DN4Ts.

By analysing the AFM images, the averaged grain size and height of the terrace steps of the parent compounds were extracted. The grain diameters are in the range of 200-1300 nm, depending both on the substrate temperature and on the compound. The average terraces height steps exhibit values between ≈ 18 Å to ≈ 24 Å.

For all the compounds the grain size increases from Ts of 40 °C to Ts of 100 °C and eventually decreases for Ts of 140 °C (Table 2.2). The largest grain sizes were identified for tBu-DNTT, with averaged diameter of ≈1340 nm, followed by for tBu-DBTTT, with values of ≈ 800 nm and isoDN4T, with values of ≈ 460 nm. In these compounds the grain size increases up to 2.5 times passing from Ts of 40 °C to Ts of 100 °C and eventually slightly decreases at Ts of 130 °C. At last, in case of DN4T, tBu-BTBT-1 and tBu-BTBT-2, the grain size increases by increasing the substrate temperature, but showing a weaker variation with respect to different Ts, resulting in values of ≈ 250 nm, ≈ 350 nm and ≈ 200 nm for DN4T, tBu-BTBT-1 and tBu-BTBT-2 respectively. In polycrystalline TFTs, charge carrier mobility can be significantly limited by the charge transport from one grain to another. Increasing the grain size (and therefore reducing the number of grain boundaries that charges must overcome) results in enhancement of the electrical performances, especially in term of charge carrier mobility. Hence, higher charge carrier mobility is expected in TFTs where the OSCs were deposited at Ts of 100 °C, compared to those with Ts of 40 and 140 °C. In contrast to the grain size, the heights of the terraces' steps did not change significantly upon the variation of the substrate temperature. The extracted heights correspond to the longest axis of the unit cell of each respective compound (see Table 2.2), suggesting that in the thin films, molecules adopt a polycrystalline layer by layer structure with an upright orientation with respect to the dielectric layer.

tBu-DBTTT is characterized by a smoother and flatter topography compared to the other compounds, which results to be ideal for AFM analysis. In contrast, the thin films formed by the other compounds show a more disordered topography, with the presence of numerous single crystals aggregates that stand out from the terraces surface by tens of nm.

Particularly, the massive presence of crystals aggregates in tBu-BTBT-2 thin films hinders the clear observation of the underneath terraces layers and makes it difficult the grains size estimation. It is still unclear the role of these crystalline aggregates on the final device performances. [START_REF] Rolin | Charge carrier mobility in thin films of organic semiconductors by the gated van der Pauw method[END_REF] Table 2 In contrast with the non-alkylated compounds, thin films based on alkylated DN4Ts do not show a terraced topography but are characterized by small crystallites of elongated shape with an average size lower than 100 nm (Figure 2.6).

These thin films were found to be mechanically delicate compared to similar compounds, such as C8-DNTT or C12-DBTTT (see Chapter 3). A nondestructive AFM imaging is unexpectedly very critical even by reducing the tip-surface interaction force to the minimum value that allowed to resolve the surface profile. Alkylated DN4Ts thin films show a particularly high roughness (between 7 and 24 nm) with respect to their absolute thickness (30 nm). These roughness values suggest that the growth of OSC crystalline domains does not occur in a layer-by-layer mode. OSC grains are not well connected, especially for films grown at higher temperatures, where large dips (at the substrate level or a few layers above) and other features (as high as 80 nm) appear on the surface. Despite the pitted thin-film morphology, the charge carrier percolation paths in the films are still sufficiently interconnected to give operating transistors.

It is worth noting that the high roughness of the OSC thin films alongside with the large number of grain boundaries may limit the achievement of higher mobility values in polycrystalline TFTs.

Deposition onto Au and PFBT/Au

In BC devices, OSCs are deposited onto drain and source gold contacts. An important requirement for efficient charge carrier injection is to have a similar morphology within the OSC deposited onto the contacts and the OSC deposited onto the dielectric. [START_REF] Niazi | Contact-Induced Nucleation in High-Performance Bottom-Contact Organic Thin Film Transistors Manufactured by Large-Area Compatible Solution Processing[END_REF][START_REF] Gundlach | Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits[END_REF] DN4T and isoDN4T were used to exemplify the change of the thin film morphology when deposited onto gold contacts and PFBT-treated gold contacts. AFM measurements were performed on the contact region covered by the OSCs.

In case of DN4T, thin film deposited on Au exhibits a large presence of needleshaped single crystals aggregates which hinders the observation of the underneath layers (Figure 2.7). On the contrary, DN4T thin film deposited onto PFBT/Au shows a more uniform morphology unveiling small grain crystallites with grain diameter of ≈ 95 nm.

In case of isoDN4T, thin film deposited onto Au shows small grain crystallites (grain diameter of ≈ 85 nm) covered by numerous crystals aggregates (Figure 2.4). On the contrary, thin film deposited onto PFBT/Au resembles the morphology of thin film deposited onto TDPA/Al2O3 but exhibiting smaller grain diameter (≈ 225 nm).

In conclusion, the functionalization of gold contacts with PFBT showed to promote a similar semiconductor morphology across the interface between the metal gold contacts and the active channel (where the OSCs are deposited onto TDPA/Al2O3), as previously reported. [START_REF] Borchert | Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors[END_REF][START_REF] Niazi | Contact-Induced Nucleation in High-Performance Bottom-Contact Organic Thin Film Transistors Manufactured by Large-Area Compatible Solution Processing[END_REF][START_REF] Gundlach | Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits[END_REF]

Crystallinity

To further investigate the film orientations, XRD patterns of the thin films were collected. Due to the larger spot size needed for the measurement, XRD was performed on thin films deposited through vacuum sublimation onto TDPA/Al2O3 substrate hold at 100°C during the deposition (Figure 2.8), and not on the active channel as for AFM measurements. The deposition parameters were the same used for TFTs fabrication.

Bragg peaks along the out-of-plane direction were indexed except for the alkylated DN4Ts, since for the latter it was not possible to solve the crystal structures. The peaks were assigned to the family planes {001} and {100}, depending on the compound. These planes can be defined as herringbone layer planes and are those enclosed between the two shorter axis of the unit cell of each respective compound. In other words, the d-spacing associated to these planes corresponds to the longest axis of the unit cell (see Figure 2.8).

The averaged d-spacing for each compound was calculated by applying the Bragg law, showing values between 15.7 Ǻ and 25.6 Ǻ (see Table 2.3). These values correspond to the height of the terrace steps extracted by AFM and as well to the unit cells long axis of each respective compound. Consequently, all the thin films adopt a layer-by-layer structures with the molecules standing approximately upright on their long axis respect to the dielectric surface, with the herringbone layer plane parallel to the dielectric.

Considering the position of the Bragg peaks for both C8and C10-DN4T and their analogy with other classes of alkylated thienoacenes like C8-DNTT, C10-DNTT and C10-DNBDT, [START_REF] Rolin | Charge carrier mobility in thin films of organic semiconductors by the gated van der Pauw method[END_REF] we can attribute the peaks at 2θ values of 7.02° and 6.36° (highlighted by vertical lines in Figure 2.8) to the crystal plane (003), assuming the c-axis of the unit cell as the longest axis. The d-spacing related to the peaks was calculated, giving values of 37.8 Ǻ and 41.7 Ǻ for C8-DN4T and C10-DN4T respectively. These values should correspond to the length of the alkylated DN4Ts and are in fact similar to the molecular lengths of the aforementioned alkylated thienoacenes. The difference between these lengths is related to the different length of the alkyl chains. Taking into account the crystallinity of the films and their morphology, we suppose that alkylated compounds adopt a 3D growth (Volmer-Weber type) [START_REF] Virkar | Organic Semiconductor Growth and Morphology Considerations for Organic Thin-Film Transistors[END_REF] when deposited onto TDPA/Al2O3 substrates, instead of a typical 2D layer-by-layer growth. 3D growth in organic semiconductor thin films has shown to limit the electrical performance in term of charge carrier mobility due to the formation of voids into the film and the presence of many grain boundaries. [START_REF] Virkar | The Role of OTS Density on Pentacene and C 60 Nucleation, Thin Film Growth, and Transistor Performance[END_REF][START_REF] Ruiz | Pentacene Thin Film Growth[END_REF]

Ionization energy

The ionization energy of the compounds was measured by photoelectron yield spectroscopy (PYS). The measurements were performed at ambient conditions on the same thin films used for XRD analysis. This technique enables the collection of the number of emitted photoelectrons per photon absorbed (quantum yield of photoelectron, Y) as a function of the incident photon energy (hν). Once the photon energy exceeds the ionization energy of the sample (threshold ionization energy, Ith), photoemission occurs. In the case of organic materials, the threshold region of the spectrum follows Y ∝ (hν -Ith) n , with n = 3. [START_REF] Ishii | Compendium of Surface and Interface Analysis[END_REF] Ionization energy is obtained by the onset of the yield curve (see Figure 2.9). At least 3 measurements for each film were recorded, giving standard deviation lower than 0.05 eV for all the samples. The compounds exhibit ionization energy between 4.94 eV and 5.37 eV.

Remarkably, alkylated DN4Ts show IE of ≈ 4.9 eV, that is about 0.3 eV lower compared to IE of DN4T and isoDN4T. The lowering of IE due to the introduction of alkylated substituents has been already reported for thienoacenes, as in the case of BTBT (5.8 eV) [START_REF] Takimiya | Organic Semiconductors Based on[END_REF] and its decyl derivative C8-BTBT (5.0 eV), [START_REF] Babuji | Effect of the Organic Semiconductor Side Groups on the Structural and Electronic Properties of Their Interface with Dopants[END_REF] as well as for DNTT (5.4 eV) and its octyl derivative, namely C8-DNTT (4.9 eV, Figure A1.6 in the Appendix). The reason behind the lower IE in alkylated derivatives is discussed later, in Paragraph 2.6.4.

Notably, the tert-butyl derivatives exhibit IE in the range of 5.23 and 5.37 eV, confirming the lowering of IE compared to tBu-BTBT due to the π-core extension. This is expected to ease charge carrier injection in TFTs, eventually leading to low threshold voltage and low contact resistance.

Electrical performances

TFTs were measured in dark at ambient air and at room temperature, by using a manual probe station and an Agilent 4156C Semiconductor Parameter Analyzer. The transfer characteristics were measured at a drainsource voltage (Vd) of -0.1 V for the linear regime and -4 V for the saturation regime and applying a source-gate voltage (Vg) from 1 to -4 V, sweeping Vg in steps of -0.225 V. The output characteristics were recorded at constant sourcegate voltage ranging from -2 V to -4 V with -0.5 V steps and varying the source-drain voltage from 0.5 V to -4 V, with steps of -0.225 V.

Charge carrier mobility (µ), threshold voltage (Vth) and on/off current ratio (ION/OFF) data reported in the main text of the thesis are always refereed to the linear regime of operation (Vd = -0.1 V) and to TFTs with channel length of 215 µm and channel width of 480 µm, unless otherwise stated.

Thin film transistors based on DN4T and isoDN4T

Impact of the substrate temperature

The key role of the substrate temperature in term of charge carrier mobility enhancement was confirmed by analysing the electrical characteristics of TC devices based on DN4T and isoDN4. For both the compounds, mobility increases by increasing the substrate temperature from 40 °C to 100 °C and eventually decreases for substrate temperature of 120 °C and 140 °C (Figure

2.10).

Particularly, for DN4T the mobility passes from 0.9 cm 2 V -1 s -1 at Ts of 40 °C to 2.1 cm 2 V -1 s -1 at Ts of 100 °C, incrementing more than 2 times. For substrate temperature of 120 and 140 °C, mobility slightly decreases to values of 1.9 cm 2 V -1 s -1 . TFTs based on isoDN4T shows a lower modulation of mobility upon the change of substrate temperature, passing from 0.0036 at Ts of 40 °C to 0.0042 cm 2 V -1 s -1 at Ts of 100 °C. Remarkably, the highest mobility values for both the compounds were found at substrate temperature of 100 °C, that corresponds to thin films which exhibit the longest grain diameter, as displayed in Figure 2.5 and Table 2.2. Threshold voltage and on/off current ratio were not notably affected by the variation of the substrate temperature (Figure 2.

11). TFTs based on DN4T

shows Vth between -1.9 and -2.4 V along with ION/OFF of ≈ 5 × 10 5 / 2 × 10 6 and isoDN4T based devices shows Vth between -1.9 V and -2.2 V, along with ION/OFF of ≈ 3 × 10 3 / 6 × 10 3 .

In summary, the variation of substrate temperature showed to have a major role in the enhancement of charge carrier mobility, as predicted by topography images obtained through AFM. No significant trend was found for threshold voltage and on/off current ratio upon the variation of Ts.

In Figure 2.12 the transfer and output characteristics of DN4T and isoDN4T based TC at Ts of 100 °C are displayed. Remarkably, a negligible hysteresis between the forward and backward scan is observed for all the devices, suggesting a minor presence of traps.

Values of µ, Vth and ION/OFF in the saturation regime are consistent with those extracted in linear regime for different Ts, confirming the reliability of the fabricated devices. Moreover, devices built on common substrate shows very low variation of mobility values, underling the high reproducibility of the fabricated TFTs.

Impact of the device geometry

To investigate the impact of device geometry on the electrical performance of DN4T and isoDN4T based TFTs, BC devices were fabricated and compared with the TC counterparts. As already stated above, the functionalization of source and drain gold contacts with PFBT is a necessary step to obtain operating devices. Bottom-gate bottom-contact devices with bare gold contacts were fabricated at Ts of 100 °C for both the isomers. All the measured devices did not show drain current modulation. The morphology of thin films of OSCs deposited onto bare gold contacts differs significantly from those deposited onto TDPA/Al2O3, as shown in For DN4T based BC devices, mobility goes from 1.3 cm 2 V -1 s -1 at Ts of 40 °C to 2.0 cm 2 V -1 s -1 at Ts of 100 °C and eventually decreases to 1.3 cm 2 V -1 s -1 at Ts of 140 °C. Likewise, for isoDN4T based BC devices, mobility goes from 0.0029 cm 2 V -1 s -1 at Ts of 40 °C to 0.0033 cm 2 V -1 s -1 at Ts of 100 °C and afterward decreases to 0.0032 cm 2 V -1 s -1 at Ts of 140 °C. On/off current ratio is not significantly affected by the change of device geometry, giving comparable values with those of TC devices (Figure 2.13).

While µ and ION/OFF did not change greatly between the TC and BC geometries, threshold voltage was found to be always lower in BC devices compared to TC counterparts for both the compounds (Figure 2 In Figure 2.15, transfer and output characteristics of DN4T and isoDN4T based BC TFTs at Ts of 100 °C are displayed. The same considerations reported above for TC devices apply here. The high reproducibility and reliability of the devices is confirmed by negligible hysteresis, low Vth, similar mobility values extracted in linear and saturation regime and low standard deviation (SD) of mobility values obtained from devices fabricated on the same substrate (maximum value of SD in DN4T based TFTs is 0.22 cm 2 V -1 s - 1 with averaged mobility of 1.9 cm 2 V -1 s -1 , while maximum value of SD in isoDN4T based TFTs is 0.00024 cm 2 V -1 s -1 with averaged mobility of 0.0032 cm 2 V -1 s -1 ). All the values of µ, Vth and ION/OFF for linear and saturation regime are reported in the Appendix (Table A1.1).

Thin film transistors based on C8-DN4T and C10-DN4T

To access charge transport properties of alkylated DN4T derivatives, polycrystalline BC and TC TFTs based on octyl-(C8-DN4T) and decylsubstituted (C10-DN4T) derivatives were fabricated. As previously shown for DN4T and isoDN4T based devices, BC TFTs show better performances in terms of lower threshold voltage along with similar mobility and on/off current ratio compared to the TC counterparts. Therefore, in the following section, only BC devices are reported in order to draw the attention to the electrical properties of C8and C10-DN4T rather than to device geometry. All the data extracted from TC devices are presented in the Appendix (Figure A1.1 and Table A1.2). In this case, the substrates were hold at temperature of 40, 70, 100 and 130 °C during the OSCs deposition. Considering the results obtained for DN4T and isoDN4T devices along with already reported TFTs based on other classes of thienoacenes obtained through vacuum deposition [START_REF] Abe | Thermally, Operationally, and Environmentally Stable Organic Thin-Film Transistors Based on Bis[1]benzothieno[2,3-d :2′,3′-d ′]naphtho[2,3-b :6,7-b ′]dithiophene Derivatives: Effective Synthesis, Electronic Structures, and Structure-Property Relationship[END_REF][START_REF] Yamaguchi | Solution-Processable Organic Semiconductors Featuring S-Shaped Dinaphthothienothiophene (S-DNTT): Effects of Alkyl Chain Length on Self-Organization and Carrier Transport Properties[END_REF] , 30 °C steps can be sufficient to find the optimal range of substrate temperature which gives the best electrical performances.

Comparing the substrate temperature dependent mobilities for C8-DN4T and C10-DN4T based devices, we observed a similar trend that was already exhibited by DN4T and isoDN4T devices. In case of C8-DN4T based TFTs, mobility increases more than 2 times upon increasing Ts from 40 to 100 °C with values of 1.1 and 2.3 cm 2 V -1 s -1 respectively, and eventually decreases to 1.9 cm 2 V -1 s -1 at Ts of 130 °C. In case of C10-DN4T based TFT, a stronger effect on µ was noticed. Mobility increases more than 12 times upon increasing Ts from 40 to 100 °C with values of 0.2 and 2.5 cm 2 V -1 s -1 respectively, and eventually decreases to 1.0 cm 2 V -1 s -1 at Ts of 130 °C (Figure 2.16).

Considering the 3D crystalline growth of C8and C10-DN4T thin films observed by AFM (Figure 2.6), we cannot derive relations between charge carrier mobility and thin-films morphology. The films do not show any significant difference which may be related to charge carrier mobility enhancement. The reason behind the increase of mobility upon substrate heating is linked to crystallinity and morphology of the first deposited layers onto the dielectric, which are not reflected in the crystal growth of the upper layers. Unfortunately, the diffuse presence of individual grains in the 100-nm height range severely limits the AFM imaging to the surface morphology of the grains with poor information about the underneath layers. Threshold voltage and on/off current ratio were not significantly affected by Ts ( Transfer and output curves (Figure 2.17) further confirm the ideal behaviour of the devices due to the absence of hysteresis and a constant mobility over the applied gate voltage range, (both in linear and saturation regime). The high reproducibility is corroborated by the low mobility values variation within TFTs fabricated on one substrate. The highest value of SD in C8-DN4T based TFTs is 0.21 cm 2 V -1 s -1 over an averaged mobility of 2.3 cm 2 V -1 s - 1 , while the highest value of SD in C10-DN4T based TFTs is 0.05 cm 2 V -1 s -1 over an averaged mobility of 2.5 cm 2 V -1 s -1 for C10-DN4T, respectively. All the values of µ, Vth and ION/OFF both in linear and saturation regime at different Ts are reported in the Appendix (Table A1.2).

Thin film transistors based on tert-butyl thienoacene derivatives 2.5.3.1 Thin-film transistors based on tBu-BTBT

The reported charge transport properties of tBu-BTBT were obtained from thin-film transistors fabricated through solution shearing processing or from single crystal devices. [START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF][START_REF] Chung | Rotator side chains trigger cooperative transition for shape and function memory effect in organic semiconductors[END_REF] . The reported devices were fabricated with a TC geometry, with contacts made through the deposition of a thin layer of MoO3 (7-8 nm) and silver or gold (35-40 nm). The insertion of MoO3 layer between the OSC and the metal contacts is known to alleviate large injection barriers, by increasing the contact work function. [START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF][START_REF] Gwinner | Doping of Organic Semiconductors Using Molybdenum Trioxide: a Quantitative Time-Dependent Electrical and Spectroscopic Study[END_REF] Nevertheless the reported device highlight a non-ideal behaviour. To have a fair comparison between tBu-BTBT and the new tert-butyl-substituted thienoacene derivatives with extended π-core reported in this work, BC and TC TFTs based on tBu-BTBT were fabricated, by varying Ts from 40 °C to 130 °C with 30 °C steps.

In this case, 40-nm-thick OSC films were deposited, since the deposition of 25 nm of tBu-BTBT resulted in discontinuous films with dewetted areas along the TFTs active channel (see optical micrographs in the Appendix, Figure A1.5). This incomplete coverage resulted in no field effect response both in case of TC and BC devices at all substrate temperature. In contrast, 40-nmthick OSC film results in a sufficient coverage of the active channel at all substrate temperature. At first sight, it is evident that tBu-BTBT exhibits poor performances highlighted by moderate hysteresis, high threshold voltage (-3.1 V) and low on/off current ratio (≈ 2 × 10 2 ). At last, the extracted mobility gives values up to 0.027 cm 2 V -1 s -1 in linear regime. As already mentioned, the deep ionization energy of tBu-BTBT (5.7 eV) strongly affects the electrical performances in TFTs, due to inefficient charge carrier injection which in turn is reflected in high Vth.

It is worth noting that TC devices were fabricated using the same process used for BC devices (except for the contacts fabrication). Therefore, the thin film across the active channel exhibits a similar morphology at given substrate temperature. Nonetheless, TC devices deposited at Ts of 40 °C did not show field-effect response, in spite of BC devices deposited at Ts of 40 °C. The reason behind this different device response may be linked to the film thickness and the ionization energy of tBu-BTBT. 40-nm-thick films may result in large contact resistance in TC devices, due to the poor charge transport properties of small-molecule OSCs along the film thickness. Moreover, in BC devices the gold electrodes are functionalized with a PFBT-SAM which increases the work function of gold contacts from 4.7 eV to 5.2 eV (see Chapter 3) reducing the height of the nominal charge-injection barrier.

Thin-film transistors based on tert-butyl thienoacene derivatives with extended π-system

As in the case of TFTs based on alkylated DN4Ts, in this section only TFTs with a BC geometry are reported, while the electrical performances of TFTs with a TC geometry are shown in the Appendix (Figure A1. 3 Specifically, considering devices with Ts of 100 °C, tBu-DNTT exhibits the highest charge carrier mobility, with values exceeding 1.9 cm 2 V -1 s -1 , followed by tBu-DBTTT, with µ up to 1.1 cm 2 V -1 s -1 , tBu-BTBT-1, with µ up to 0.25 cm 2 V -1 s -1 and tBu-BTBT-2 with µ up to 0.10 cm 2 V -1 s -1 . Also in this case, the augmented charge transport mobility obtained through the optimization of the substrate temperature is reflected in the thin-films topography investigated by AFM, which highlights the presence of bigger grain size for samples with Ts of 100 °C. As shown for TFTs based on DN4T, isoDN4T and alkylated DN4Ts, also for tert-butyl thienoacene derivatives the highest mobilities were achieved at Ts of 100 °C, confirming this temperature as the most optimal for thienoacenes-based devices examined in this work.

Contact resistance evaluation

The channel-width-normalized contact resistance (RCW) was calculated with the transmission line method (TLM) by using TFTs with channel length of Notably the alkylated DN4Ts exhibit the highest charge carrier mobility, while isoDN4T exhibits the lowest charge carrier mobility along with the largest contact resistance among the investigated OSCs. Contact resistance in BC devices can be split in two components, the injection at the metal/OSC interface and the charge transport through the depleted region formed at the metal/OSC interface. [START_REF] Hu | Effect of Alkyl-Chain Length on Charge Transport Properties of Organic Semiconductors and Organic Field-Effect Transistors[END_REF][START_REF] Hamadani | Temperature-dependent contact resistances in high-quality polymer field-effect transistors[END_REF][START_REF] Natali | Charge Injection in Solution-Processed Organic Field-Effect Transistors: Physics, Models and Characterization Methods[END_REF] Considering that the injection barrier is similar for the isoDN4T and alkylated DN4Ts due to a comparable HOMO energy (≈ 5.2 eV and ≈ 5.0 eV respectively), μ plays a crucial role in determining the contact resistance. The resistance associated to the depleted region is proportional to 1/μ, [START_REF] Gruber | Impact of Materials versus Geometric Parameters on the Contact Resistance in Organic Thin-Film Transistors[END_REF] resulting in more facile charge injection into to accumulation channel for high mobility OSCs.

It is worth noting that in case of DN4T and isoDN4T, TFTs exhibit RCW variation depending on the device geometry. DN4T TFTs shows RCW of 3.8 and 1.8 kΩcm for TC and BC devices respectively, while isoDN4T TFTs shows RCW of 2373 and 351 kΩcm for TC and BC devices (Figure 2.21).

These experimental results agree with already reported experimental observations and device simulations, which evidence that BC organic thinfilm transistors can show lower contact resistance compared to TC organic TFTs, if conditions are met. It has been shown that BC TFTs fabricated with a small organic molecule have lower contact resistance than a TC TFTs if the dielectric is sufficiently thin (< 30 nm) and if the if the energy barrier at contact-semiconductor interface is minimized (by treating the drain/source contacts with PFBT in case of BC devices). [START_REF] Borchert | Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors[END_REF] An extensive study of the role of mobility and molecular structure of thienoacenes organic semiconductors on the contact resistance in TFTs is presented in Chapter 3.

Discussion

The discussion of this chapter is divided in four parts: the first part is focused on the quantum chemical calculations of transfer integrals and reorganization energies of the investigated compounds, except for the alkylated DN4Ts due to the lack of their crystal structures. The second part concerns the electrical properties of DN4T and isoDN4T and their comparison with the already reported materials. The third part refers to the effect of the functionalization of the DN4T molecular core with alkyl chains (C8-DN4T and C10-DN4T). At last, the fourth part is focused on the effect of the π-system extension in tert-butyl derivatives.

Transfer integrals and reorganization energies

Quantum chemical calculations of transfer integrals and reorganization energies were provided by Dr. Vincent Lamaur and Marco Bardini (both Université de Mons). Internal reorganization energies were evaluated at the Density Functional Theory (DFT) level using the B3LYP functional and a 6-31G** basis set with the Gaussian16 package. [START_REF] Lemaur | Charge Transport Properties in Discotic Liquid Crystals: A Quantum-Chemical Insight into Structure-Property Relationships[END_REF] HOMO transfer integrals were computationally estimated in a fragment approach at the DFT level (B3LYP/DZ) with the Amsterdam Density Functional (ADF) package. [START_REF] Valeev | Effect of Electronic Polarization on Charge-Transport Parameters in Molecular Organic Semiconductors[END_REF][START_REF] Viani | Theoretical Characterization of Charge Transport in One-Dimensional Collinear Arrays of Organic Conjugated Molecules[END_REF] The reorganization energy (λ) reflects the geometric changes of the molecules involved in the charge transport upon charge transfer. [START_REF] Gruhn | The Vibrational Reorganization Energy in Pentacene: Molecular Influences on Charge Transport[END_REF] The reorganization energies of the investigated compounds were found to be in the range of 143-309 meV (Table 2.5), in line with already reported thienoacenes. [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF] DN4T, tBu-DNTT and tBu-DBTTT unveil the lowest reorganization energy of 152, 143 and 145 meV, respectively, along with the highest mobility in TFTs (2.0, 1.9 and 1.0 cm 2 V -1 s -1 , respectively). These values are almost the half compared to tert-butyl-thienyl derivatives of BTBT, which exhibit λ of 308 meV for tBu-BTBT-1 and 263 meV for tBu-BTBT-2. Larger reorganization energy should disfavour hole transport considering a pure hopping regime. This is the case of the tert-butyl-thienyl derivatives of BTBT, which exhibit charge carrier mobilities of 0.25 cm 2 V -1 s -1 and 0.10 cm 2 V -1 s -1 for tBu-BTBT-1 and tBu-BTBT-2, respectively, that are 1 order of magnitude lower compared to tBu-DNTT and tBu-DBTTT. It is worth noting that in the case of tert-butyl-substituted derivatives, tBu-DNTT and tBu-DBTTT show lower reorganization energy compared to thienyl substituted BTBTs. This suggests that annealed π-systems tend to have lower λ compared to oligomeric π-systems. The latter is characterised by single bonds connecting aromatic rings, which results in more molecular motions comprising rotation around the single bonds. On the contrary, stiffer molecular structure would result is lower molecular geometric variation upon charge transfer. [START_REF] Takimiya | Design and elaboration of organic molecules for high field-effect-mobility semiconductors[END_REF] At last, isoDN4T exhibits λ of 213 meV, that is larger than the one of DN4T and smaller than those of tert-butyl-thienyl derivatives of BTBT. Nevertheless, isoDN4T reveals mobility 3 orders of magnitude lower compared to DN4T and 2 orders of magnitude lower compared to tert-butylthienyl derivatives of BTBT. This aspect is discussed later (Paragraph 2.6.2.2).

In TFTs the molecules tend to crystallize by orienting the longest molecular axis of the π-system parallel to the dielectric surface. With this orientation the charge transport takes place within the ab plane, named herringbone layer plane if the OSC adopt a HB packing motif (see Figure 2.22). The reported transfer integrals values are referred to HOMO wavefunctions overlap between adjacent molecules along the ab plane, as depicted in Figure 22-c. Severe anisotropy of the electronic interaction along the ab plane together with poor overlap of the HOMO wavefunctions generally limit the charge transport leading to lower charge carrier mobility.

The HOMO transfer integrals were found to be in the range of 10 1 -10 2 meV (Table 2.5) in agreement with the class of thienoacenes. [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF] Particularly, DN4T, tBu-DNTT and tBu-DBTTT exhibit the largest transfer integrals, with values of (61,99,33) meV, (63,26,26) meV and (52,30,57) meV, respectively.

The computed transfer integrals reflect a moderate anisotropy of electronic interactions within adjacent molecules, with a variance percentage between 47% and 57%. The other compounds show smaller transfer integrals with values between 8 and 33 meV, with variance percentage of 48% for isoDN4T and 69% for tBu-BTBT-2. Only tBu-BTBT-1 reveals an almost perfect isotropy of electronic interactions, giving a variance percentage of 5% of the computed transfer integrals. Notably, the highest charge carrier mobilities extracted in TFTs, are exhibited by that compounds which show the lowest reorganization energies together with large values of transfer integrals. These results confirm that an extended HOMO wavefunctions overlap, along with small molecular geometry variation upon charge transfer, are necessary to boost charge carrier mobility in OSCs. [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF]

DN4T and isoDN4T vs DNTT, isoDNTT and L-DBTTA

In order to define structure-properties relationships for DN4T and isoDN4T, their crystalline packing and electronic parameters, were compared to those of DNTT, isoDNTT and L-DBTTA. These values, taken both from the literature and from the present work, are reported in

Packing motif

DN4T and isoDN4T show the same herringbone packing motif as DNTT and isoDNTT (sandwich herringbone for the latter). On the contrary, by comparing DN4T and isoDN4T with L-DBTTA, we observe that the introduction of an additional external naphthalene ring to the same central core leads to a herringbone motif packing, instead of a stacked arrangement.

In herringbone arrangement, charge transport is facilitated by edge-to-face interactions leading to efficient charge transport. Among the most encountered packing motif in organic semiconductor crystals, herringbone packing has shown to give rise to high mobility, along with brick-wall packing, if compared to slipped π-stacking and slipped-stack packing motifs.

Mobility of DN4T and isoDN4T

The electronic structures were calculated by Marco Bardini (Université de Mons) using density functional theory (DFT) to optimize geometries in gas phase at the B3LYP 6-311G* level.

Notably, isoDN4T TFTs show mobility 3 orders of magnitude lower compared to DN4T TFTs. This big difference may be related to different interactions within the molecules in the crystalline arrangement. If we consider the short contacts within neighbouring molecules (which are defined as any intermolecular contact shorter than the sum of the van der Walls radii of the involved atoms), DN4T is characterized by C-H•••π, S•••C and S•••S interactions. On the contrary, isoDN4T exhibits only C-H•••π interactions, as displayed in Figure 2.23, reflecting a larger distance between the sulphur atoms of adjacent molecules if compared to DN4T. This would result in less efficient charge transfer within neighbouring molecules. In addition, as already mentioned, sulphur atoms have higher electron density compared to carbon and hydrogen atoms. The larger electron densities on the sulphur atoms in the HOMO (i.e., larger HOMO coefficient on the sulphur atoms), increases the effective overlap of the HOMOs of neighbouring molecules. In contrast, nodal planes or small HOMO coefficients on the sulphur atoms would reduce intermolecular orbitals overlap, which would reduce the efficiency of charge transport. [START_REF] Takimiya | Thienoacene-Based Organic Semiconductors[END_REF] This is confirmed by the calculation of the coefficients of the HOMO orbitals of the two isomers, highlighting a larger electron density on the sulphur atoms of DN4T (Figure 2.23) compared to isoDN4T. Likewise, DNTT and L-DBTTA exhibit larger HOMO coefficients on the sulphur atoms compared to isoDNTT. [START_REF] Jouclas | Dinaphthotetrathienoacenes: Synthesis, Characterization, and Applications in Organic Field-Effect Transistors[END_REF][START_REF] Yamamoto | Three Structural Isomers of Dinaphthothieno[3,2-b]thiophenes: Elucidation of Physicochemical Properties, Crystal Structures, and Field-Effect Transistor Characteristics[END_REF] Notably, the same decrease of ≈2-3 orders of magnitude has been observed in DNTT and isoDNTT, [START_REF] Yamamoto | Three Structural Isomers of Dinaphthothieno[3,2-b]thiophenes: Elucidation of Physicochemical Properties, Crystal Structures, and Field-Effect Transistor Characteristics[END_REF] suggesting the smaller coefficients of HOMO on the sulphur atoms as one of the main reasons of lower mobility in isoDN4T and isoDNTT. In case of L-DBTTA, the lower mobility (0.15 cm 2 V -1 s -1 ) compared to DN4T and DNTT has been attributed to the stacked packing motif which results in 1D transfer integrals distribution, detrimental to charge transport. [START_REF] Jouclas | Dinaphthotetrathienoacenes: Synthesis, Characterization, and Applications in Organic Field-Effect Transistors[END_REF] The dynamic disorder in the OSC layer (i.e., electron-phonon coupling) plays a central role in charge transport. Thus, the low mobility of isoDN4T may be related also to a detrimental influence of the coupling of crystal vibrational modes with charge motion. This would also help to explain the lower mobility of isoDN4T compared to tBu-BTBT-1 and tBU-BTBT-2. The latter show small transfer integrals and large reorganization energy, as isoDN4T.

Nonetheless, the tert-butyl-thienyl derivatives exhibit carrier mobility of 2 orders of magnitude higher than isoDN4T. The morphological characterization of the thin films unveiled no substantial difference to justify this large mobility difference between the compounds. In the case of tBu-BTBT-1 the higher mobility can be linked to the isotropic transfer integrals. It has been shown that OSCs with balanced transfer integrals in the different crystalline directions are more resilient to dynamic disorder. [START_REF] Illig | Reducing dynamic disorder in small-molecule organic semiconductors by suppressing large-amplitude thermal motions[END_REF] In the case of tBu-BTBT-2 and in order to deeply understand the reason behind the low mobility of isoDN4T, an experimental and computational study of the electron phonon-coupling of the investigated compounds would be required.

In summary, the annealing of terminal naphthene moieties (DN4T and isoDN4T) instead of benzene moieties to the same central motif of four thienyl rings enables a favourable HB packing which promotes balanced 2D electronic interactions within adjacent molecules. This is reflected in higher mobility in DN4T, while in isoDN4T is characterized by lower mobility attributed to the lower electronic density of the HOMO. On the other hand, the two additional thienyl rings in the central motif of DN4T compared to DNTT, do not lead to significant variation of crystal packing motif, transfer integrals distribution and electronic density of the HOMO, resulting in similar charge carrier mobility of ≈ 2 cm 2 V -1 s -1 .

Charge carrier injection

DN4T and isoDN4T show IE values in line with the class of thienoacenes (4.9 eV < IE < 5.8 eV). [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF] Particularly, DN4T and isoDN4T show IE of 5.3 eV which suggest rather facile charge injection from the OSCs to the gold contacts. The contacts work function was determined by using UPS, giving values of 4.7 eV for bare Au and of 5.2 eV for PFBT/Au (see Chapter 3).

The ratio between the contact resistance and the total resistance of the devices (Rc/Rtot) equals to the ratio of the voltage drop at the contact and the applied source-drain voltage (Vc/Vd) [START_REF] Gruber | Impact of Materials versus Geometric Parameters on the Contact Resistance in Organic Thin-Film Transistors[END_REF] , revealing if the voltage drop at the contact is negligible compared to the voltage drop over the channel. For DN4T and isoDN4T, the voltage drop at the contacts is lower than 10% of the applied drain voltage for both TC and BC devices, confirming a facile charge injection from the gold electrodes to the OSCs. It is worth to underline that the energy barrier which a charge must overcame on injection from the metal contact to the OSC is not controlled just by the energy difference between the metal's work function and the IE of the organic semiconductor. Moreover, at the interface several factors like charge carrier's transfer, interface dipole, band banding, depletion region, etc., [START_REF] Natali | Charge Injection in Solution-Processed Organic Field-Effect Transistors: Physics, Models and Characterization Methods[END_REF] must be considered to have a precise and reliable energy value.

C8-DN4T and C10-DN4T vs C8-DNTT and DN4T

Charge carrier mobility, threshold voltage (extracted from the best performing devices) and ionization energy of C8-DN4T, C10-DN4T, C8-DNTT (see Figure A1.6 in the Appendix) and DN4T are compared (Table 2.7), in order to understand the effect of the functionalization of DN4T molecular core with alkyl side chains. In this case, all the devices are referred to this work and were produced using the same fabrication parameters, allowing a fair comparison between the compounds. Due to the inability to solve the crystal structure of C8-DN4T and C10-DN4T, the discussion is focused on the electrical performances of the devices only. Transfer and output characteristics of C8-DNTT are reported in Chapter 3.

Charge carrier mobility and thin-films morphology

The main differences between the alkylated DN4Ts and C8-DNTT is the higher charge carrier mobility of the latter (4.8 cm 2 V -1 s -1 ), approximately twice compared to the alkylated DN4Ts (2.3-2.5 cm 2 V -1 s -1 ). This difference may be linked to the dissimilar thin-film morphology. The growth process of the alkylated DN4T molecules is characterized by a 3D growth mode (Volmer-Weber type), which has shown to be a reason of poor charge transport due to the formation of severe grain boundaries acting as deep energetic traps. [START_REF] Virkar | The Role of OTS Density on Pentacene and C 60 Nucleation, Thin Film Growth, and Transistor Performance[END_REF] On the contrary, C8-DNTT shows a 2D growth mode, forming a typical terrace structure (covered by small single crystals) with terrace steps height that matches the unit cell long axis. The competition between moleculemolecule and molecule-substrate interaction determines the growth mode for the first layers onto the substrate. Therefore, the thin-film morphology of C8-DNTT (see Figure 2.24) reflects a stronger interaction between the molecule and the TDPA/Al2O3 substrate, while for C8-DN4T and C10-DN4T, moleculemolecule interaction dominates, leading to a 3D thin-film morphology which disfavours charge carrier transport. Nonetheless, the alkylation of DN4T leads to increased charge carrier mobilities compared to the parent molecule. The increment of charge carrier mobility from 2.0 cm 2 V -1 s -1 in DN4T up to 2.3-2.5 cm 2 V -1 s -1 in C8and C10-DN4T may be attributed to the decrease of thermal motions due to weak interaction within the alkyl chains of adjacent molecules. As already reported for BTBT, DNTT and their alkylated derivatives, the introduction of alkyl chains attached along the long axis of their conjugated core suppresses the amplitude of vibrations along the unit cell long axis thanks to a large number of weak interactions between the alkyl chains of neighbouring molecules. [START_REF] Illig | Reducing dynamic disorder in small-molecule organic semiconductors by suppressing large-amplitude thermal motions[END_REF][START_REF] Tsutsui | Unraveling Unprecedented Charge Carrier Mobility through Structure Property Relationship of Four Isomers of Didodecyl[END_REF] .

We suppose that also for C8-DN4T and C10-DN4T, the alkylation of the aromatic ring results in lower dynamic disorder of the crystal lattice, enabling a more extended charge delocalization.

This would lead to two main expected effects: increase of the charge carrier mobility and reduction of the IE in crystalline OSCs compared to the parent and non-alkylated compounds. The solution of the crystalline structure of C8-DN4T and C10-DN4T would be needed to confirm the hypothesis of a closer packing of the aromatic core which can reduces the detrimental effect of dynamic disorder.

Charge carrier injection

The ionization energy is almost identical for all the alkylated derivatives (4.9 eV), highlighting a reduction of IE if compared to the parent compounds (5.3 eV for DN4T and 5.4 eV for DNTT). Considering that IE is the energy required to remove one electron from the HOMO of a molecules, its value strongly depends on crystal packing that influences electronic delocalization. [START_REF] Tsutsui | Unraveling Unprecedented Charge Carrier Mobility through Structure Property Relationship of Four Isomers of Didodecyl[END_REF] Hence, IE of an isolated molecule in vacuum may differ from its value in the solid state, where intermolecular delocalization and electrostatic interaction within neighbouring molecules can lead to energy variation of the energy levels. As mentioned above, the introduction of alkyl chains may suppress the amplitude of lattice vibrations (thanks to weak interactions within adjacent molecules), enhancing charge carrier delocalization and leading to IE reduction. In the first-place alkyl substituents are weak electron donors. Lower IE in alkylated DN4T compared to the parent molecule is reflected in better charge carrier injection. This is confirmed by the lower Vth (close to 0 V) and RCW (< 1.2 kΩcm) in alkylated DN4T, compared to the parent molecule which shows Vth 0.8 V higher and RCW up to 1.0 kΩcm higher.

Tert-butyl substituted thienoacenes 2.6.4.1 Effect of π-extension on charge injection

In order to understand the effect of the π-core extension on charge injection properties in TFTs, ionization energy obtained by PYS, threshold voltage and contact resistance, both extracted from TFTs, of tert-butyl substituted thienoacenes are compared (Table 2.8). The extension of the π-core obtained by an annealed approach and an oligomeric approach led to IE reduction for all the synthesized compounds if compared to tBu-BTBT (IE = 5.7 eV).

Particularly, ionization energy is ≈ 0.3 eV lower for tBu-DNTT, ≈ 0.4 eV lower for tBu-DBTTT and tBu-BTBT-1, and ≈ 0.5 eV lower for tBu-BTBT-2. OSCs with ionization energy in the range of 5.1-5.4 eV are considered the most optimal for device applications since this IE range ensures a good compromise between facile charge injection and chemical stability against oxidation. [START_REF] Zschieschang | Flexible Low-Voltage Organic Transistors and Circuits Based on a High-Mobility Organic Semiconductor with Good Air Stability[END_REF] In the first place, this IE range minimize the Schottky barrier at the interface between gold electrodes and OSC. Lower IE may lead to easy oxidation of the compounds and to oxygen doping that disfavour charge transport in devices. [START_REF] Uttiya | Connecting molecule oxidation to single crystal structural and charge transport properties in rubrene derivatives[END_REF][START_REF] Barf | Compensation of Oxygen Doping in p-Type Organic Field-Effect Transistors Utilizing Immobilized n-Dopants[END_REF] Several examples of reduced ionization energy have been reported, both for extended π -units, obtained both from a annealed approach [START_REF] Takimiya | Thienoacene-Based Organic Semiconductors[END_REF][START_REF] Watanabe | The synthesis, crystal structure and charge-transport properties of hexacene[END_REF][START_REF] Takimiya | Organic Semiconductors Based on[END_REF] and oligomeric approach. [START_REF] Yamamoto | Zigzag-Elongated Fused π-Electronic Core: A Molecular Design Strategy to Maximize Charge-Carrier Mobility[END_REF][START_REF] Niebel | Thienoacene dimers based on the thieno[3,2-b]thiophene moiety: synthesis, characterization and electronic properties[END_REF] An example of the effect of the extended π-conjugated core has been already shown in for DNTT, DN4T Moreover, the improved charge carrier injection is furtherly confirmed by the low contact resistance extracted for the tert-butyl thienoacene derivatives with an extended π-system, which exhibit RCW lower than 5 kΩcm. On the contrary, it was not possible to extract RCW from devices based on tBu-BTBT, since TFTs with channel length smaller than 215 µm did not show drain current modulation upon the application of a gate bias -preventing the application of TLM. No field-effect response in small channel length devices may be related to high contact resistance, that is expected due to the deep ionization energy of tBu-BTBT.

In conclusion, the reduction of IE is reflected in a facile charge carrier injection, which in turns leads to the lowering of Vth for all the synthesized compounds. This allows the extraction of charge carrier mobility over a larger applied gate voltage, ensuring a high reliability of the extracted field-effect mobilities and hindering potential over-(or under-) estimations.

Calculated charge mobility anisotropy

Hole mobility anisotropy simulations were provided by Dr. Vincent Lamaur (Université de Mons). The simulations were performed by using a kinetic Monte Carlo approach (kMC) assuming a purely hopping regime.

The high mobility of tBu-BTBT (up to ≈ 7 cm 2 V -1 s -1 in single crystal FETs) has been attributed to the large and balanced transfer integrals, which are reflected in an almost perfect mobility isotropy obtained by Kinetic Monte Carlo (kMC) simulations with calculated mobility of ≈ 4 cm 2 V -1 s -1 . These simulations enable the calculation of charge carrier mobility anisotropy along the herringbone (HB) plane of charge transport (ab plane in Figure 2.

22).

By calculating the field-effect mobility anisotropy for the tert-butyl substituted thienoacenes (Figure 2.25), it is clear that solely tBu-BTBT-1 shows isotropic charge properties, but low simulated mobility (about 0.35 cm 2 V -1 s -1 ). On the contrary, tBu-DNTT and tBu-DBTTT show charge transport anisotropy along with higher simulated mobility (with maximum values of about 6 and 8 cm 2 V -1 s -1 respectively) while tBu-BTBT-2 exhibits both severe mobility anisotropy and low simulated mobility (maximum values of about 0.8 cm 2 V -1 s -1 ).

These results indicate that the functionalization of the aromatic cores with tert-butyl groups does not ensure isotropic electronic interaction within neighbouring molecules. Actually, the degree of anisotropy of charge transport depends on several factors, such as the π-core structure, the packing motif of OSC in solid state and the π-core substituents, all of which contribute to the type and strength of electronic interactions. The experimental mobility values extracted from TFTs, reflect the trend obtained from the computational calculations. Despite the significant large values of transfer integrals and simulated charge carrier mobility, tBu-DNTT and tBu-DBTTT reveal poor charge transport isotropy, limiting the achievement of higher mobility. On the contrary, tBu-BTBT-1 shows mobility isotropy, but better electrical performances are hindered by the reduced HOMO wavefunctions overlap. At last, tBu-BTBT-2 exhibits both severe mobility anisotropy along with small transfer integrals values, reflected in the lowest experimental charge carrier mobility extracted in TFTs. It has to be highlighted that, both DFT and kMC calculations were performed considering a pure hopping regime that can model charge transport properties for organic semiconductors with low mobility (μ << 1 cm 2 V -1 s -1 ).

Moreover, the two computational approaches were based on molecular crystals structure solved at low temperature, which may not consider the dynamic disorder present at higher temperature and the complexity of polycrystalline thin films, where the presence of different crystalline domains, grain boundaries and traps may further limit the comparison between the computational and experimental data. Notwithstanding, the mobility values extracted from polycrystalline thin-film devices are consistent with the trend shown in computational calculations, stressing the prominence to maximize the charge transport isotropy and the overlap of the HOMO levels to enhance charge carrier mobility.

Summary

I presented two isomers and the alkylated derivatives of a new class of thienoacenes bearing a central tetrathienyl core annealed with two external naphthalene rings that were employed in organic TFTs. The evaluation of the electrical characteristics revealed DN4T, C8-DN4T and C10-DN4T as high performance OSCs, with charge carrier mobility up to 2.1 cm 2 V -1 s -1 for the parent molecule and up to 2.5 cm 2 V -1 s -1 for the alkylated derivatives. In addition, I presented the electrical performances evaluation of four new tertbutyl substituted thienoacenes. The compounds have an extended π-system compared to tBu-BTBT, that ensures a better charge delocalization. This was confirmed by the lower ionization energy that in turns is reflected in lower Vth (< 1.7 V) in TFTs, along with small contact resistance (< 5 kΩcm). Among the compounds, tBu-DNTT was revealed as high performance OSC, with mobility exceeding 1.9 cm 2 V -1 s -1 . Structure-properties relationships were established, highlighting how several factors contribute to the final devices' performances. Herringbone packing motif, extended π-system, IE close to the electrodes work function, large and balanced transfer integrals and low reorganization energy are key parameters in OSCs design to obtain high performance materials for TFTs applications.

The impact of thin-film morphology on the device performances was evaluated through atomic force microscopy. All the compounds showed a typical 2D growth forming a terrace structure pattern, except for C8-DN4T and C10-DN4T that exhibited a 3D growth. This crystalline growth should disfavour charge transport along the active channel due to the presence of large number of grain boundaries. The crystal structure solution of C8-DN4T and C10-DN4T would be needed to confirm the proposed structure-properties relationship for the alkylated derivatives.

Moreover, considering the solubility enabled by the functionalization with alkyl side chains and with tert-butyl substitutions, solution processing could be employed to fabricate TFTs based on alkylated DN4Ts and tert-butyl substituted thienoacenes, with the view to optimize the thin-film morphology and induce a 2D crystalline growth in case of alkylated DN4Ts. For this purpose, solution-sharing technique could be used to fabricate TFTs. This technique allows the production of single crystalline domain within the first few active layer (removing the grain boundaries effects in the devices) that have shown charge carrier mobility higher than 10 cm 2 V -1 s -1 for alkylated thienoacenes. [START_REF] Peng | Crystallized Monolayer Semiconductor for Ohmic Contact Resistance, High Intrinsic Gain, and High Current Density[END_REF][START_REF] Peng | Solution-Processed Monolayer Organic Crystals for High-Performance Field-Effect Transistors and Ultrasensitive Gas Sensors[END_REF] Chapter 3 Elucidating the influence of the molecular structure of thienoacenes organic semiconductors on contact resistance

Introduction

The role of contact resistance in organic semiconductors-based devices has gained considerable attention within the scientific community. This is because the performance of the final device is influenced by both charge carrier injection and charge transport through the semiconductor layer. The effect of contact resistance on the development of OFETs is twofold. Firstly, a high contact resistance can lead to inaccuracies in the determination of device parameters, which can have significant consequences on material development by producing incorrect structure-property relationships or leading to the rejection of potentially valuable organic semiconductors. [START_REF] Waldrip | Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier[END_REF] Secondly, high contact resistance places limitations on the maximum transit frequency that can be achieved through device miniaturization. [START_REF] Borchert | A Critical Outlook for the Pursuit of Lower Contact Resistance in Organic Transistors[END_REF][START_REF] Zschieschang | Roadmap to Gigahertz Organic Transistors[END_REF] This limitation is particularly pertinent in the development of organic TFTs for low-power, high-frequency applications since reducing device size requires a corresponding increase in operating frequency. Therefore, it is essential to minimize contact resistance to enhance the performance of organic TFTs, establish accurate structure-property relationships, and maximize their potential in high-frequency applications. Several methodologies have been developed to address this issue, including the use of high charge carrier density in the electrolyte gate insulator, [START_REF] Braga | Ultralow contact resistance in electrolyte-gated organic thin film transistors[END_REF][START_REF] Lenz | Vertical, electrolyte-gated organic transistors show continuous operation in the MA cm-2 regime and artificial synaptic behaviour[END_REF] contact doping, [START_REF] Yamamura | Wafer-scale, layer-controlled organic single crystals for high-speed circuit operation[END_REF][START_REF] Matsumoto | Study of contact resistance of high-mobility organic transistors through comparisons[END_REF] mono-or bi-layer OSC films, [START_REF] Peng | Ultrahigh On-Current Density of Organic Field-Effect Transistors Facilitated by Molecular Monolayer Crystals[END_REF][START_REF] Sawada | Correlation between the static and dynamic responses of organic single-crystal field-effect transistors[END_REF] thin dielectric thickness, and transferred platinum contacts. [START_REF] Zeng | Ultralow contact resistance in organic transistors via orbital hybridization[END_REF] Although a few examples of width-normalized contact resistance (RCW) have been reported in OFETs to be below 100 Ωcm, [START_REF] Borchert | A Critical Outlook for the Pursuit of Lower Contact Resistance in Organic Transistors[END_REF] the majority of reported RCW values for state-of-the-art OSCs fall within the range of 100 Ωcm to 10 kΩcm. [START_REF] Waldrip | Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier[END_REF] This chapter aims to investigate the influence of molecular structure, specifically with regards to the molecular core substituents, on contact resistance in staggered and coplanar device geometries. While other factors that affect contact resistance have been extensively studied, little attention has been paid to this aspect. Therefore, we examined the charge transport

Crystal structures

The crystal structures of the parent and alkylated molecules of the DNTT and DBTTT families have been reported in the literature. [START_REF] Schweicher | Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors[END_REF][START_REF] Park | Dibenzothiopheno[6,5-b :6′,5′-f ]thieno[3,2-b ]thiophene (DBTTT): High-Performance Small-Molecule Organic Semiconductor for Field-Effect Transistors[END_REF][START_REF] Xie | Temperature-Independent Transport in High-Mobility Dinaphtho-Thieno-Thiophene (DNTT) Single Crystal Transistors[END_REF][START_REF] Chen | Crystal Engineering of Dibenzothiophenothieno[3,2-b ]thiophene (DBTTT) Isomers for Organic Field-Effect Transistors[END_REF] In Chapter 2, the crystal structures of tert-butyl derivatives were presented, and therefore, we focus here solely on the crystalline characteristics of the parent and alkylated compounds.

All the compounds display a characteristic layer-by-layer herringbone packing motif, where the HB angle between adjacent molecules ranges from 51.1° to 56.4°. The monoclinic space groups P 21, P 21/c and P 21/a accommodate DNTT, DBTTT, and C12-DBTTT, respectively, while C8-DNTT belongs to the triclinic space group P -1 (Figure 3.1). § § The parent molecules consist of one unique molecule in the asymmetric unit (Z' = 1), whereas the alkylated derivatives exhibit half a molecule in the asymmetric unit (Z' = 0.5). Notably, DBTTT displays four molecules in the unit cell (Z = 4) where the length of the c axis (longest axis) corresponds to the length of two DBTTT molecules (Table 3.1).

All the compounds exhibit C-H•••π, S•••C, and S•••S interactions

, with the latter found to enhance charge transfer within neighbouring molecules. The high efficiency of charge transport in these compounds is confirmed by reported charge carrier mobility exceeding 2 cm 2 V -1 s -1 in polycrystalline TFTs for all the compounds. [START_REF] Schweicher | Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors[END_REF][START_REF] Zschieschang | Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) thin-film transistors with improved performance and stability[END_REF][START_REF] Park | Dibenzothiopheno[6,5-b :6′,5′-f ]thieno[3,2-b ]thiophene (DBTTT): High-Performance Small-Molecule Organic Semiconductor for Field-Effect Transistors[END_REF][START_REF] Chen | Crystal Engineering of Dibenzothiophenothieno[3,2-b ]thiophene (DBTTT) Isomers for Organic Field-Effect Transistors[END_REF]

Thin-film transistor fabrication

Thin film transistors with bottom-gate top-contact (TC) and bottom-gate bottom-contact (BC) geometry were fabricated as already reported in Paragraph 2.1. In this case, the substrate temperature was kept at the optimal value which gives the best charge carrier mobility for each respective compound. The OSC deposition rate was set at 0.3 Ǻs -1 , except for the alkylated compounds that were deposited at rate of 0.1 Ǻs -1 (Table 3.2). The OSCs were deposited with nominal thickness of 25 nm and 80 nm, monitored by a crystal quartz microbalance and confirmed by and AFM measurements.

Thin-films characterization

Topography

Atomic force microscopy in tapping mode was employed to analyse the thinfilm topography of thienoacenes OSCs. Particularly AFM measurements were performed on the active layer and on the contact region of BC TFTs with 25nm-thick OSCs (Figure 3.2, left and right column). Across the active layer, OSCs are deposited onto TDPA/Al2O3 substrates. In this region the OSC morphology affects the charge carrier mobility, depending mainly on the OSC crystalline growth and on the formation of grain boundaries. While across the contacts, OSCs are deposited onto PFBT /Au surface. In this case the OSC morphology in this region influences charge carrier injection in BC devices. It is worth noting that the OSC morphology across the active layer of TC and BC devices are similar, since OSC is deposited onto TDPA/Al2O3 substrate either way. Moreover, considering that in TC devices gold is thermally evaporated through shadow masks onto the OSC, the effect of gold deposition on thin-film morphology of OSCs was analysed. Specifically, AFM measurements were carried out on samples which consists of 25-nm-thick OSC deposited onto TDPA/Al2O3 substrates, additionally coated with 10 nm of gold deposited at 0.5 Ǻs -1 (Figure 3.2, centre column).

Deposition onto TDPA/Al2O3

All the compounds deposited onto TDPA/Al2O3 substrates exhibit a terraced topography characterised by the diffuse presence of grain boundaries. By analysing the AFM images, the average height steps of the terrace structures were extracted, giving values that match the length of the long unit cell axis of each respective compound, except for DBTTT. The latter has four molecules in the unit cell and its unit cell long axis correlates to the length of two molecules. Hence, the terrace height steps extracted from AFM images of DBTTT coincide to half of the unit cell long axis, that in turns corresponds to the length of one molecule.

The calculated height steps are in the range of 1.5 nm and 3.6 nm, depending on the length of the respective molecule (Table 3.3). These results suggest that for all the compounds, molecules pack with an upright orientation respect to the substrates when vacuum deposited. The parent compounds and the tert-butyl derivatives exhibit relatively flatter topography compared to the alkylated derivatives. The latter are characterised by the presence of several needle-shaped crystal aggregates that stand on the top of the terraced surfaces. tBu-DNTT shows a significant reduction of these crystalline aggregates, while these aggregates are not exhibited for DNTT, DBTTT and tBu-DBTTT, that are characterised by smoother thin-film surfaces. It is still unclear the role of these crystalline aggregates on the final device performances, especially in the case of TC devices where the gold contacts are deposited on top of them.

In addition, when gold is deposited on top of OSCs to fabricate source and drain contacts in TC device geometry, gold nanoclusters may penetrate the semiconductor film. These clusters can modify the dipole barrier at Au/OSC interface and consequently affect contact resistance. [START_REF] Xu | Metal evaporation dependent charge injection in organic transistors[END_REF] Gold-cluster penetration into the OSC is generally denoted by the formation of craters and voids on the OSC thin film, with depth up to 30 nm. [START_REF] Wang | Damaging Effect of Hot Metal Atoms on Organic Semiconducting Films during Top Contact Formation[END_REF] From the AFM topography images, it is clear that no deep voids (neither a pitted morphology) are formed upon the deposition of 10-nm-thick gold layer on the top of the OSCs. For all the compounds, the thin-film morphology of OSCs coated with gold resembles the morphology of the uncoated OSCs. This suggests a minor penetration of gold clusters into the OSCs compared to those reported in the literature. [START_REF] Wang | Damaging Effect of Hot Metal Atoms on Organic Semiconducting Films during Top Contact Formation[END_REF] Moreover, no relevant differences were observed within the parent molecules, the tert-butyl and the alkylated derivatives upon the gold layer deposition, indicating a similar mechanical response to gold cluster penetration into the OSCs films.

Deposition onto PFBT/Au

The deposition of OSCs onto PFBT/Au surfaces led to a terrace structured topography for all the compounds, reflecting some features exhibited by the thin films deposited onto TDPA/Al2O3. Also in this case, the alkylated molecules unveil a large number of needle-shaped crystal aggregates on top of the films, in contrast with the parent compounds and the tert-butylsubstituted derivatives. Moreover, the average height steps of the terraced structures correspond to those obtained from AFM images of OSCs deposited on TDPA/Al2O3, suggesting that the molecules assume an upright orientation even on PFBT/Au substrates. Specifically, the height steps were found to be in the range of 1.5 nm and 3.6 nm, also in this case (Table 3.

3).

The main difference between OSCs deposited onto TDPA/Al2O3 substrates and onto PFBT/Au surfaces lies in the size of grain diameters. For all the compounds the grain diameter decreases passing from TDPA/Al2O3 to PFBT/Au substrates. Particularly, the grain diameter decreases from 3370 nm to 560 nm for DNTT, from 620 nm to 380 nm for tBu-DNTT-tBu, from 550 nm to 330 nm for C8-DNTT, from 610 nm to 390 nm for DBTTT, from 1900 nm to 540 nm for tBu-DBTTT-tBu and from 450 nm to 340 nm for C12-DBTTT (Table 3.3). This discrepancy in grain diameter values between TDPA/Al2O3 and PFBT/Au substrates, is linked to the different nucleation processes occurring on the surfaces.

Based on the larger grain diameters observed on the TDPA/Al2O3 substrate, it can be inferred that the TDPA-SAM promotes a lower nucleation density in comparison to the fluorinated surface provided by PFBT-SAM. [START_REF] Niazi | Contact-Induced Nucleation in High-Performance Bottom-Contact Organic Thin Film Transistors Manufactured by Large-Area Compatible Solution Processing[END_REF] As a consequence, this would lead to the creation of fewer grains with larger diameters on the TDPA-SAM when compared to the PFBT-SAM.

At last, these findings support the idea that gold contacts that have undergone PFBT treatment ensure a similar OSC morphology throughout the contact-to-channel region, which is expected to facilitate charge injection in the BC device described in this study.

Crystallinity

The crystallinity of OSCs thin films deposited onto TDPA/Al2O3 and PFBT/Au substates was investigated by XRD. Considering the larger spot size required for the measurements, OSCs were deposited onto 2.5 × 2.5 cm 2 TDPA/Al2O3 and PFBT/Au substrates using the same deposition parameter reported in Paragraph 3.3.

The XRD patterns collected from thin films deposited onto TDPA/Al2O3 and PFBT/Au exhibit Bragg peaks at the same 2θ values for each respective compound (Figure 3.3). The peaks were indexed and assigned to the family planes {001} and {100}, depending on the molecule. These planes correspond to the herringbone layer planes, defined as those planes enclosed between the two short unit cell axis. By employing the Bragg law, the d-spacing (interlayer

Thin-film transistors characteristics

The electrical characteristics were assessed by measuring TFTs, using the same measurement parameters mentioned in Chapter 2. Also in this chapter, all the reported transfer characteristics and extracted values are referred to the linear regime (Vd = -0.1 V) and to devices with W/L = 480/215 µm, unless otherwise stated. Particularly, the transmission lime method (TLM) used to extract contact resistance is valid only in the linear regime. TLM assumes a linear drop in voltage across the active channel, that is ensured by a uniform charge carrier density along the accumulation layer when operating the devices in linear regime. On the contrary, in saturation regime a step potential drop close to the drain contact (pinch-off) is present, invalidating the application of the method.

In the first part of this section the electrical characteristics of TFTs with 25nm-thick OSC layer are reported, while in the second part the impact of OSC thickness on the electrical performances and contact resistance is investigated by analysing TFTs with 80-nm-thick OSC layer. Please note that the recorded output characteristics and the plots showing the extracted RCW over the entire range of gate-overdrive voltage are reported in the Appendix.

TFTs with 25-nm-thick OSC layer

Electrical performances

By measuring the transfer characteristics, the electrical performances of TC TFTs with 25-nm-thick OSC layer were assessed. (Figure 3.4)

For both DNTT and DBTTT families, the parent molecules and the alkylated derivatives show high charge carrier mobility, with values of 2.3 cm 2 V -1 s -1 for DNTT, of 5.1 cm 2 V -1 s -1 for DBTTT, of 3.7 cm 2 V -1 s -1 for C8-DNTT and of 2.2 cm 2 V -1 s -1 for C12-DBTTT. Conversely, the tert-butyl derivatives exhibit lower mobility, with values of 0.8 cm 2 V -1 s -1 and 0.3 cm 2 V -1 s -1 for tBu-DNTT and tBu-DBTTT, respectively.

Likewise, TFTs based on parent molecules and alkylated derivatives show lower threshold voltage, ranging from -0.8 V to -1.2 V, than those based on tert-butyl derivatives, which show Vth ranging from -1.6 V to -1.8 V. The increased threshold voltage in tert-butyl derivative-based TFTs denotes a significant amount of trapping sites. A method to evaluate the trap density is by measuring the subthreshold swing (SS, defined as the inverse slope of the logarithmic Id as a function of Vg in the subthreshold regime) and use Equation 1.17. *** The Nbulk term (bulk trap density per unit volume and energy) cannot be ignored in the case of staggered devices since the injected charges must pass through the OSC's bulk in order to reach the accumulation channel. Thus, the subthreshold swing reflects both the contribution of the interfacial trap density and bulk trap density and thereby higher SS indicates a greater number of trap sites. As expected, the parent compounds and the alkylated derivatives are characterized by lower SS than tert-butyl derivatives. SS was found to be of 141 mV/dec and 163 mV/dec for tBu-DNTT and tBu-DBTTT, respectively.

In contrast DNTT and C8-DNTT show SS of 109 and 112 mV/dec, respectively, while DBTTT and C12-DBTTT show SS of 116 and 122 mV/dec respectively (Table 3.5). In summary, the analysis of TC TFTs characteristics highlights the inferior electrical performances of tert-butyl derivatives compared to the parent and alkylated molecules, due to the lower charge carrier mobility, higher threshold voltage and larger number of trap sites. Similarly, the electrical performances of BC TFTs were extracted from transfer characteristics (Figure 3.5). BC TFTs based on the parent compound unveil similar charge carrier mobility compared to the TC counterparts, with values of 2.3 and 4.7 cm 2 V -1 s -1 for DNTT and DBTTT respectively. On the contrary, BC TFTs based on tert-butyl and alkylated derivatives exhibit slightly higher mobility compared to TC TFTs, with values of 1.4 and 1.0 cm 2 V -1 s -1 for tBu-DNTT and tBu-DBTTT, respectively, and with values of 4.8 and 3.3 cm 2 V -1 s -1 for C8-DNTT and C12-DBTTT, respectively. As already shown in Chapter 2, also in this case the adoption of a BC device geometry results in the lowering of threshold voltage for all the compounds. Passing from TC to BC device, Vth decreases up to 0.3 V for DNTT, up to 0.4 V for C8-DNTT, up to 0.5 V for DBTTT and C12-DBTTT and up to 0.1 V for the tert-butyl derivatives. Both a more facile charge injection in BC devices (see the section on contact resistance) and a smaller number of trap sites can be linked to the lower threshold voltage.

Depending on the compound, the subthreshold swing of BC TFTs is between 9 and 18 meV/dec lower than the TC counterparts (Table 3.6). This highlights a minor presence of traps in BC devices. Given that each compound was deposited onto TDPA/Al2O3 dielectric using the same deposition parameters, TC and BC TFTs based on the same compound ought to exhibit a comparable nominal interfacial charge trap density. Device-to-device variation is expected. Yet, the systematic decreased subthreshold swing of BC TFTs raises the possibility that Nbulk is a contributing factor to the disparity in SS values. In TC TFTs trap sites may be present in the OSC volume between the contacts and the gate-induced carrier channel. In contrast, since charges are directly injected into the accumulation channel in BC TFT, this contribution is disregarded. Additionally, the parent compounds and the alkylated derivatives display lower SS than the tert-butyl derivatives, maintaining the trend seen in TC TFTs. This indicates a larger trap density of the latter. By setting Nbulk = 0, Nint were extracted in BC devices. The tert-butyl derivatives exhibit the largest Nit, with values of 1.35 × 10 12 and 1.72 × 10 12 eV -1 cm -2 for tBu-DNTT and tBu-DBTTT respectively. The parent compounds unveil the lowest Nit, with values of 6.19 × 10 11 and 7.62 × 10 11 eV -1 cm -2 for DNTT and DBTTT respectively, followed by the alkylated derivatives, which show Nit of 8.03 × 10 11 and 9.61 × 10 11 eV -1 cm -2 for C8-DNTT and C12-DBTTT respectively (Table 3.6). Through a linear fitting (all the fits exhibit adjusted R 2 ≥ 0.98), the widthnormalize contact resistance (RCW) as a function of the gate-overdrive voltage for BC and TC devices of each compound was obtained. As well, the intrinsic mobility (µ0) was obtained from the slope of the linear fits (Figure A2.12, in the Appendix).

In TC devices, tert-butyl derivatives exhibit considerably higher RCW compared to the respective parent molecules and alkylated derivatives over the entire range of gate-overdrive voltage (Figure 3.7). Specifically, for a gate-overdrive voltage of -2.0 V, tBu-DNTT and tBu-DBTTT show RCW of 24.5 kΩcm and of 85.0 kΩcm respectively. These values are one order of magnitude higher than those extracted for DNTT, C8-DNTT, DBTTT and C12-DBTTT which show RCW of 2.8, 3.1, 1.7 and 5.5 kΩcm, respectively. In addition, the RCW of tert-butyl derivatives of both families are more strongly modulated by the gate-overdrive voltage if compared to the respective parent and alkylated molecules. This can be primarily ascribed to the higher charge trap density of the former. Larger number of trap sites beneath the contact region delay charge carrier injection and as well prevent the extraction of charge carrier from the accumulation channel. [START_REF] Liang | Examination of Au, Cu, and Al contacts in organic field-effect transistors via displacement current measurements[END_REF][START_REF] Liu | Critical Impact of Gate Dielectric Interfaces on the Contact Resistance of High-Performance Organic Field-Effect Transistors[END_REF] As a result, the number of mobile charge carriers and charge carrier mobility beneath the contacts are reduced. The applied gate voltage modulates both the mobile charge carrier density and charge carrier mobility that subsequently affect contact resistance.

In BC devices, all the compounds exhibit lower width-normalized RCW compared to those extracted from TC devices over the whole range of gateoverdrive voltage. Specifically, for common fixed gate-overdrive voltage of -2.0 V, RCW is of 0.8 kΩcm for DNTT, of 3.7 kΩcm for tBu-DNTT, of 0.8 kΩcm for C8-DNTT, of 1.2 kΩcm for DBTTT, of 2.4 kΩcm for tBu-DBTTT and of 1.9 kΩcm for C12-DBTTT.

RCW in BC TFTs is lower than 4.3 kΩcm for all the compounds over the entire gate-overdrive voltage (Figure 3.8). It is worth to emphasize two aspects. First off, tert-butyl derivatives have a greater RCW than their respective parent molecules and alkylated derivatives. However, this difference in RCW is larger TC TFTs, while it is milder in BC TFTs. Specifically, in TC TFTs, RCW of tBu-DNTT is ≈ 8 times larger than DNTT and C8-DNTT, while and RCW of tBu-DBTTT is ≈ 50 times larger than DBTTT and ≈ 15 times larger than C12-DBTTT. In contrast in BC TFTs, RCW of tBu-DNTT is ≈ 5 times larger than DNTT and C8-DNTT, while and RCW of tBu-DBTTT is ≈ 2 times larger than DBTTT and almost similar to C12-DBTTT. Second, the RCW dependence on the applied gate-overdrive voltage of the tert-butyl derivatives is noticeably less pronounced, compared to the TC counterparts. This is explained by the lower charge trap density of BC TFTs, given that the contribution of the traps in the OSC bulk beneath the contact is neglected.

As already reported, [START_REF] Borchert | Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors[END_REF] BC devices can show lower contact resistance along with better electrical performances compared to TC devices if conditions are met. First, a dielectric layer with a thickness lower than 30 nm is required. For this purpose, materials with high dielectric constant must be employed as AlOx, TiOx or HfOx. Second, the formation of a PFBT-SAM on the surface of gold contacts is needed to ensure a uniform organic semiconductor morphology across the contact-channel interface and to minimize the energy barrier height between the contacts and the OSC. Particularly, it has been shown that TC TFTs based on DPh-DNTT exhibit RCW up to 2 times lower compared to the TC counterparts. This has been obtained by fabricating TFTs with 3-nm-thick dielectric layer of Al2O3, additionally functionalized with a TDPA-SAM, and by using PFBT/Au contacts. [START_REF] Borchert | Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors[END_REF] A similar trend is unveiled in our experiments for TFTs based on parent molecules and alkylated derivatives. The width-normalized contact resistance extracted in BC devices is 1. This significant variation in contact resistance between the two device geometries can be attributed to the involvement of the OSC resistance beneath the contacts, as well as the lower nominal injection barrier in BC TFTs due to the PFBT treatment of the gold contacts. In TC devices the contact resistance (RC) is given by the sum of the bulk resistance (Rbulk), that is the resistance associated with the charge transport through the OSC thickness, and the interface resistance (Rint) that is the resistance associated with the injection barrier between the metal contact and the OSC. In BC devices, charges are injected into the accumulation channel straightaway, removing Rbulk contribution.

When considering the direction of current density in TFTs, two charge transport directions may be defined: in-plane and out-of-plane (Figure 3.10).

The former is parallel to the OSC/dielectric interface, whereas the latter is perpendicular to the OSC/dielectric interface. The in-plane mobility (µin) is the mobility commonly extracted in OFETs by using the gradual channel approximation (please note that the term "charge carrier mobility" is always referred to the effective in-plane charge carrier mobility extracted from transfer characteristics using equation 1.20, † † † unless otherwise stated). Consequently, the sheet resistance (Rsh) is the resistance associated to charge transport through the gate-induced carrier channel, which can be limited by the presence of interfacial traps, grain boundaries and defects. by the larger distance between the aromatic cores. Moreover, the introduction of substituents as in the case of tert-butyl and alkylated derivatives, further increases the distance between the molecular cores, resulting in poorer outof-plane charge transport. At last, in TC TFTs charge transport along the outof-plane direction also depends on the semiconductor thickness (tOSC), wherein thicker OSC film leads to an increased Rbulk.

The results obtained from TC and BC devices based on the investigated compounds emphasize, first, the reduction in contact resistance when adopting a coplanar geometry. Second, they show that molecules with poor out-of-plane charge transport, such as tert-butyl and alkylated derivatives, are more influenced by device geometry than those with better out-of-plane charge transport (i.e., DNTT and DBTTT).

What remains unclear is the reason behind the significant difference in RCW between the tert-butyl derivatives and the alkylated derivatives in TC TFTs.

In fact, a higher Rbulk in alkylated derivatives than in tert-butyl derivatives is anticipated because the lengthy alkyl chains force a larger distance between the molecular cores in the out-of-plane direction. This would lead to high RCW, contrary to our experimental findings, which show that RCW in TC TFTs based on tert-butyl derivatives is one order of magnitude larger than in TC TFTs based on alkylated derivatives. The latter reveal RCW of the same order of magnitude of the parent molecules.

Contact resistance in TC TFTs does not depend only on Rbulk, but also on Rint, Rsh and on the extension of the charge injection area. The next paragraphs are focused on the study of these parameters to understand the reason behind the difference in RCW between the core molecules, the alkylated derivatives and the tert-butyl derivatives.

TFTs with 80-nm-thick OSC layer

Electrical performances

The electrical performances of TC and BC TFTs with 80-nm-thick OSC layer were evaluated by recording transfer characteristics, depicted in Figure 3.11 and 3.12.

For TC devices with 80-nm-thick OSCs, the parent molecules and the alkylated derivatives exhibit higher charge carrier mobility and lower threshold voltage than the tert-butyl derivatives, as in the case of 25-nm-thick OSC. Specifically, the extracted mobility values are of 1.8 cm 2 V -1 s -1 for DNTT, 4.0 cm 2 V -1 s -1 for DBTTT, 4.1 cm 2 V -1 s -1 for C8-DNTT and 1.5 cm 2 V -1 s -1 for C12-DBTTT. TC TFTs based on tBu-DNTT and tBu-DBTTT, on the other hand, show reduced charge carrier mobility of 0.6 cm 2 V -1 s -1 and 0.3 cm 2 V -1 s -1 , respectively. Threshold voltage is lower for the parent compound and alkylated derivatives, with values ranging from -0.8 V to -1.2 V, than for tertbutyl derivatives, with values ranging from -1.7 V to -2.0 V. (Table 3.7). The primary difference between TC TFTs with 80-nm-thick OSC layers and those with 25-nm-thick layers is the former's larger subthreshold swing. For all compounds except DNTT, SS was found to be between 10 and 27 meV/dec greater than the respective devices based on a 25-nm-thick OSC layer. (Table 3.7). This could be attributed to the increased number of trap sites formed as a result of the increased OSC thickness. In contrast, DNTT-based devices exhibit similar SS for the OSC thicknesses, suggesting that the OSC thickness has just a minor effect on the charge trap density for DNTT. The same trend of mobility and threshold voltage within the investigate compounds showed in TC TFTs, was observed in BC devices with 80-nm-thick OSCs. As compared to tert-butyl derivatives, the parent molecules and alkylated derivatives displayed higher charge carrier mobility and lower Vth.

Particularly, the parent molecules exhibit charge carrier mobility up to 2.4 cm 2 V -1 s -1 for DNTT, up to 2.9 cm 2 V -1 s -1 for DBTTT, while the alkylated derivatives show charge carrier mobility up to 4.9 cm 2 V -1 s -1 for C8-DNTT and up to 3.2 cm 2 V -1 s -1 for C12-DBTTT. BC TFTs based on tBu-DNTT-tBu and tBu-DBTTT-tBu show charge carrier mobility of 1.4 cm 2 V -1 s -1 and 0.9 cm 2 V - 1 s -1, respectively. Threshold voltage was found to be in the range of -0.5 and -1.2 V for the parent and alkylated molecules, and in the range of -1.6 and -1.7 V for the tert-butyl derivatives.

Interestingly, for each compound, the extracted SS from BC TFTs with 80nm-thick OSC layer exhibit comparable values to those from BC TFTs with a 25-nm-thick OSC layer. The calculated interfacial trap density values (Table 3.8) unveail no significant difference between the two OSC thicknesses. This is consistent with the fact that the greater SS in TC devices is caused by the OSC bulk volume through which charges must pass to reach the gate-induced accumulation channel. Because the contact edge of BC TFTs is on the same plane as the accumulation channel, the increase in OSC thickness has no effect on the charge trap density.

Contact resistance

The Similarly, the intrinsic mobility (µ0) was obtained from the slope of the linear fits (Figure A2.12, in the Appendix).

For all the compounds over the entire range of gate-overdrive voltage, RCW increased in TC TFTs with an 80-nm-thick OSC layer as compared to TC TFTs with a 25-nm-thick OSC layer. Particularly, for gate-overdrive voltage of -2.0 V, the extracted RcW is of 4. The dependence of contact resistance on OSC thickness in staggered geometry results from the extra space that charges must traverse following injection from the source contact to reach the accumulation layer. [START_REF] Peng | Ultrahigh On-Current Density of Organic Field-Effect Transistors Facilitated by Molecular Monolayer Crystals[END_REF][START_REF] Yamamura | Wafer-scale, layer-controlled organic single crystals for high-speed circuit operation[END_REF][START_REF] Matsumoto | Study of contact resistance of high-mobility organic transistors through comparisons[END_REF] Moreover, the augmented thickness of the OSC layer lead to higher charge-trap density which disfavour charge transport in the out-ofplane direction. [START_REF] Xu | Understanding Thickness-Dependent Charge Transport in Pentacene Transistors by Low-Frequency Noise[END_REF] As a result, an OSC layer thickness of 20 to 30 nm is commonly used in TC TFTs, as it ensures a minimal thickness to form a continuous film between the source and drain contacts while preventing a significant contribution of the OSC thickness to contact resistance. It is worth mentioning that when the OSC layer thickness increased from 25 nm to 80 nm (more than threefold), RCW was found to be only between 1.2 and 1.6 times higher -depending on the compound -in TC TFTs with the thickest OSC layer. This indicates that RCW depends nonlinearly on OSC thickness. [START_REF] Wang | Role of Schottky Barrier and Access Resistance in Organic Field-Effect Transistors[END_REF] In BC TFTs, the augmented OSC thickness is not reflected in an increased RCW. On the contrary, compared to BC TFTs made with 25-nm-thick OSC, all the compounds exhibit similar or slightly reduced width-normalized contact resistance, over the entire range of gate-overdrive voltage. Specifically, for gate-overdrive voltage of -2.0 V, the extracted RCW is 1. In case of coplanar geometry, charges are injected from the source contact into the accumulation layer straightaway, hindering the contribution of the OSC volume beneath the contacts to the contact resistance. Hence, the increase of semiconductor thickness does not result in higher contact resistance. On the contrary, this can lead to an improved OSC overlay along the contact-to-channel region and to an increased charge injection area, which may have a beneficial effect on the contact resistance. In coplanar geometry the charge injection area roughly corresponds to the product between the channel width and the OSC thickness. It is worth noting that OSC thickness larger than the thickness of the source contact does not lead to larger injection area, since (to first approximation) no charge exchange occurs above the contacts. [START_REF] Gruber | Impact of Materials versus Geometric Parameters on the Contact Resistance in Organic Thin-Film Transistors[END_REF][START_REF] Shim | Structural Analysis on Organic Thin-Film Transistor With Device Simulation[END_REF] At last, the fabrication of TC and BC TFTs with an 80-nm thick OSC layer confirmed the trend observed in devices with a 25-nm thick OSC layer (Figure 3.16). The contact resistance of the parent molecules and their alkylated derivatives for TC devices is similar, with values less than 9 kΩcm. Tert-butyl derivatives, on the other hand, unveail RCW values that are one or two orders of magnitude larger than the respective parent and alkylated molecules, with values of 38.5 kΩcm and 118.8 kΩcm for tBu-DNTT and tBu-DBTTT, respectively. For BC devices with 80-nm-thick OSC layer, all the compounds exhibit RCW lower than 4 kΩcm, as in the case of BC TFTs with 25-nm-thick OSC. In addition, the extracted RCW values always lower than those extracted for TC devices with 80-nm-thick OSC layer, over the entire range of gate-overdrive voltage.

Out-of-plane charge transport

Conductive atomic force microscopy measurements were performed in collaboration with Dr. Nicholas Turetta, at Supramolecular Science and Engineering Institute (ISIS) of Strasbourg, under the supervision of Prof. Paolo Samorì.

The out-of-plane charge transport properties of the investigated compounds were assessed by using conductive atomic force microscopy (C-AFM). C-AFM enables the simultaneous measurement of the topography and conductivity of a sample, by scanning the sample surface with a conductive tip which acts as a nanoscale electrical probe. Current density-voltage (J-V) characteristics are obtained via C-AFM on a single point of the material surface (with the extension of the probed area in the range of tens of nm 2 ) and to record material surface current mapping at given bias. Since the surface current is mapped at fixed bias, C-AFM can be considered as a local probe for vertical charge transport.

Current density-voltage characteristics obtained by C-AFM can be fitted by the space-charge limited current (SCLC) model to extract the out-of-plane mobility. In a typical configuration, the semiconductor is sandwiched between two electrodes, with one of them being an injection blocking electrode. [START_REF] Malliaras | Electrical characteristics and efficiency of single-layer organic light-emitting diodes[END_REF] SCLC implies unipolar injection of charge carriers from the other ohmic contact into the bulk of the semiconductor. At low bias voltage, the flow of current is characterized by an ohmic behaviour (J ∝ V). By increasing the applied bias voltage, charges accumulate around the electrode, leading to the formation of a space charge region, that is considered as a continuum distribution of charges rather than space localized charges. This "cloud" of charges is accumulated in proximity of the injection electrode, due to the inability of the material to transport the charges fast enough in the opposite direction. [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF][START_REF] Moiz | Conducting Polymers[END_REF] Therefore, at a given applied external electric field an equilibrium stage is reached, and the concentration of injected charges is comparable or higher than the free charge carrier concentration. At this stage, the current density exhibits a quadratic dependence on the bias voltage (J ∝ V 2 ). In absence of active traps, SCLC can be modelled by the Mott-Gurney charge model, as follow:

𝐽 = 9 8 𝜀𝜀 0 µ 𝑉 2 𝐿 3 (3.1)
where 𝜀 is the relative dielectric constant of the OSC, 𝜀 0 is the vacuum permittivity, V is the applied voltage bias and L is the channel length (thickness of the OSC). In this case, µ is referred to the charge carrier mobility along the thickness of the OSC and hence to the out-of-plane mobility. This expression was first derived for J-V curves extracted from diodes with parallel-plane geometry and eventually applied for C-AFM [START_REF] Moerman | Towards a unified description of the charge transport mechanisms in conductive atomic force microscopy studies of semiconducting polymers[END_REF][START_REF] Hourani | Anisotropic charge transport in large single crystals of π-conjugated organic molecules[END_REF][START_REF] Hu | Effect of Alkyl-Chain Length on Charge Transport Properties of Organic Semiconductors and Organic Field-Effect Transistors[END_REF][START_REF] Park | Large-Area Heteroepitaxial Nanostructuring of Molecular Semiconductor Films for Enhanced Optoelectronic Response in Flexible Electronics[END_REF] (Figure 3.17). More accurate expressions for J-V current collected by C-AFM with the view to extract a more reliable µ value have been presented. [START_REF] Reid | Space Charge Limited Current Measurements on Conjugated Polymer Films using Conductive Atomic Force Microscopy[END_REF][START_REF] Zubair | Thickness Dependence of Space-Charge-Limited Current in Spatially Disordered Organic Semiconductors[END_REF][START_REF] Button | High-resolution charge carrier mobility mapping of heterogeneous organic semiconductors[END_REF] Reid et al., [START_REF] Reid | Space Charge Limited Current Measurements on Conjugated Polymer Films using Conductive Atomic Force Microscopy[END_REF] proposed the introduction of empirical parameters and factors obtained from both finite element simulations and the comparison within J-V curves extracted from diodes and C-AFM. This model led to charge carrier mobility values 3 orders of magnitude lower compared to the ones obtained from Mott-Gurney model. However, the Reid model has been developed for thick semiconductor polymers and therefore could not be appropriate for nm-thick films, as in our case. In summary, considering that no accurate model has been yet established for nm-thick layers of small-molecule OSCs, the extracted values of mobility presented in this paragraph must be considered more qualitatively rather than quantitatively. Despite the selected model, the trend within analysed semiconductors is maintained, showing which one opposes the highest (or lowest) resistance along the out-of-plane direction.

Evaluation of the out-of-plane mobility

In a typical experimental set-up, Si/SiO2 wafer is coated with 30-nm-thin layer of gold and consequently immersed in a PFBT solution in isopropanol, which results in the formation of a PFBT-SAM. Eventually, a thin layer of OSC (≈ 10 nm) is vacuum-deposited onto the substrate (the extended procedure is reported in Chapter 5). The Au surface is electrically grounded to the sample holder of the instrument where the voltage bias is applied by Pt/Ir tip, as depicted in Figure 3.18. A set of at least 20 J-V curves were recorded in different region of the sample, by displacing the tip over a 5 × 5 array of points separated by 200 nm in a 1 × 1 µm 2 surface. The measurements were performed at controlled relative humidity (RH < 5%) to avoid potential chemical reactions, that may affect the electrical response of the materials. [START_REF] Mativetsky | Graphene Transistors via in Situ Voltage-Induced Reduction of Graphene-Oxide under Ambient Conditions[END_REF] The contact area between the C-AFM tip and the surface of the organic semiconductors was estimated to ≈ 78 nm 2 , by assuming ≈ 0.5 nm of tip indentation from the deflection error signal and a hemispherical shape at the apex of the C-AFM tip (with a nominal tip diameter of 25 nm). [START_REF] Reid | Space Charge Limited Current Measurements on Conjugated Polymer Films using Conductive Atomic Force Microscopy[END_REF] It is important to underline that the topography and the crystallinity of OSCs deposited onto Au/PFBT were analysed by AFM and XRD, confirming that all the compounds form crystalline films with the molecules standing approximately upright with respect to the substrate (see Paragraph 3.4). Therefore, the current-voltage characteristics collected via C-AFM refer to charge transport along the direction perpendicular to the π-staking direction of neighbouring molecules.

Except for C12-DBTTT, the current density acquired from the thin films exhibit a quadratic dependence on the bias voltage for V > 0.8 V, as depicted by the dashed lines and the yellow highlighted region in Figure 3.18. In this range, the Mott-Gurney model can be employed to extract the out-of-plane mobility of the compounds. Moreover, it is worth noting that the current density exhibits a linear dependence on the applied bias for V < 0.15 V, indicating an ohmic injection of charge carriers.

As expected, the parent molecules exhibit the highest µout, with value of 0.22 cm 2 V -1 s -1 , both for DNTT and DBTTT. The tert-butyl derivatives show µout 3 orders of magnitude lower compared to the parent molecules, with values of 2.8 × 10 -4 and 1.1 × 10 -4 cm 2 V -1 s -1 , for tBu-DNTT and tBu-DBTTT respectively. At last, the extracted µout for C8-DNTT is the lowest, giving value of 3.2 × 10 -5 cm 2 V -1 s -1 (µout values with relative standard deviations are displayed in Table 3.9). C12-DBTTT thin film did not show a detectable current response upon the application of bias voltage, most likely due to the poor charge transport that is hindered by the long alky chains. The current response shown by the sample was already close to the limit of the current amplifier sensitivity (0.1 pA) in C8-DNTT thin films. The spatial extension of the alkyl chains possessing four more carbon atoms may further reduce the out-of-plane mobility due to lower HOMO wavefunctions overlap. Therefore, we suppose that µout of C12-DBTTT is lower than µout of C8-DNTT (< 3.2 × 10 - 5 cm 2 V -1 s -1 ).

The out-of-plane mobilities extracted for DNTT and DBTTT are of the same order of magnitude of those reported for rubrene (0.19 cm 2 V -1 s -1 ) obtained by using admittance spectroscopy measurements in diode with planar-parallel geometry. [START_REF] Blülle | Charge transport perpendicular to the high mobility plane in organic crystals: Bandlike temperature dependence maintained despite hundredfold anisotropy[END_REF] Likewise, the out-of-plane mobility of C8-DNTT is similar to those reported for alkylated-NDIs (10 -5 /10 -6 cm 2 V -1 s -1 ), obtained by applying the SCLC model on J-V curves collected via C-AFM. [START_REF] Hu | Effect of Alkyl-Chain Length on Charge Transport Properties of Organic Semiconductors and Organic Field-Effect Transistors[END_REF] These results reflect the poorer charge transport along the thickness of OSC when the molecular core is functionalized with alkyl chains substituents.

In conclusion, we found the same trend for both DNTT and DBTTT families, confirming that the more efficient charge transport along the out-of-plane direction is exhibited by the parent molecules, followed by the tert-butyl and alkylated derivatives.

Table 3.9: Out-of-plane mobility (µout) obtained from J-V characteristics.

Out-of-plane mobility is extracted by using the Mott-Gurney model on averaged J-V curves in SCLC regime (bias voltage > 0.8 V).

Compound µout (cm

2 V -1 s -1 ) DNTT (2.2 ± 0.2) × 10 -1 tBu-DNTT (2.8 ± 0.1) × 10 -4 C8-DNTT (3.2 ± 0.2) × 10 -5 DBTTT
(2.2 ± 0.1) × 10 -1 tBu-DBTTT (1.1 ± 0.1) × 10 -4

Charge injection area

In first approximation, TC TFTs are characterized by larger contact injection area (Ainj) than the BC counterpart. In BC TFTs injection occurs from the contact edge, with a contact injection area that roughly corresponds to the product between the channel width and the thickness of the OSC. [START_REF] Shim | Structural Analysis on Organic Thin-Film Transistor With Device Simulation[END_REF] For TC TFTs, in principle, injection can involve all the contact area facing the gate. However, in the framework of current crowding model, not all the gate-tocontact overlap (Lov) contributes to the total injection area. A characteristic injection length has been defined, referred as transfer length (LT), which represents the effective length under the contacts through which 63% of charges are injected into the OSC (Figure 3.19). [START_REF] Richards | Analysis of the contact resistance in staggered, top-gate organic field-effect transistors[END_REF][START_REF] Schroder | Semiconductor Material and Device Characterization[END_REF] Therefore, in TC TFTs the contact injection area is expected to be about equal to the product of the channel width and the transfer length. Transfer length is obtained from TLM measurements by extrapolating the linear regression to RW = 0, considering that the point of intersection with the x-axis corresponds to -2LT. [START_REF] Schroder | Semiconductor Material and Device Characterization[END_REF] In case TC TFTs based on investigated compounds (with 25-nm-thick OSC layer), LT was extracted at gate-overdrive voltage of -2.0 V (Figure 3.20), giving values ranging from 13 to 101 µm. Eventually, the contact injection area was calculated as the product between LT and the channel width (Table 3.10). The parent molecules unveil the smallest LT of 13.7 µm and 17.9 µm for DNTT and DBTTT respectively, followed by the alkylated derivatives which exhibit LT of 27.3 µm and 23.4 µm for C8-DNTT and C12-DBTTT respectively. At last, the longest LT was extracted for tert-butyl derivatives, with values of 48.5 µm and 101.1 µm for tBu-DNTT and tBu-DBTTT respectively.

In order to compare the contact resistance independent of the contact area, the specific contact resistivity is defined (ρc) as follow: [START_REF] Schroder | Semiconductor Material and Device Characterization[END_REF][START_REF] Chan | The Motivation for and Challenges to Scaling Down Organic Field-Effect Transistors[END_REF] 𝜌 𝑐 = 𝑅 𝑐 𝑊 𝐿 𝑇

The above relationship is valid only if Lov ≥ 1.5 LT, that is the case for all the analysed devices. The contact resistivity as a function of the gate-overdrive voltage was obtained for each compound and displayed in Figure 3.20. Specifically, considering a gate-overdrive voltage of -2.0 V, DNTT, DBTTT and C8-DNTT and C12-DBTTT exhibit contact resistivity of 3.8 Ωcm 2 , 3.0 Ωcm 2 , 8.4 Ωcm 2 and 12.9 Ωcm 2 respectively, while the tert-butyl derivatives exhibit contact resistivity of 106.1 Ωcm 2 and 859.9 Ωcm 2 for tBu-DNTTT and tBu-DBTTT respectively.

By comparing the contact resistivity and the width-normalized contact resistance (the latter is not normalized to the transfer length), it is clear that the same trend is maintained. In fact, in both cases, the core and the alkylated molecules show values 1 or 2 orders of magnitude lower compared to the tertbutyl derivatives. These results suggest that, in TC TFTs presented in this work, the contact injection area does not play a major role in determining the contact resistance.

For BC TFTs, Ainj is estimated to be ≈ 12 µm 2 , that is 2 and 3 orders of magnitude lower compared to the contact injection areas extracted from TC TFTs. This difference is expected due to the small contact edge in BC devices limited by the 25-nn-thick OSC layer. It is worth noting that transfer length was not calculated for BC TFTs, since in first approximation charge injection does not occur underneath the contacts in a coplanar geometry. [START_REF] Gruber | Impact of Materials versus Geometric Parameters on the Contact Resistance in Organic Thin-Film Transistors[END_REF][START_REF] Shim | Structural Analysis on Organic Thin-Film Transistor With Device Simulation[END_REF] Thus, also in the case of BC TFTs, the achievement of lower contact resistance compared to the TC counterparts, is not limited by the reduced contact injection area.

Study of the OSC/contact interface using ultraviolet photoelectron spectroscopy

Ultraviolet photoelectron spectroscopy measurements were performed in collaboration with Christos Gatsios at the Humboldt University of Berlin, under the supervision of Prof. Norbert Koch.

In a typical ultraviolet photoelectron spectroscopy (UPS) measurement, the sample is irradiated with monochromatic light of energy hν. When the excitation energy is sufficient the electrons can obtain enough kinetic energy to be excited from their initial bound states to the vacuum level where they can be detected by an electron spectrometer. The electrons that have surpassed their characteristic initial binding energies (𝐸 𝐵 ) and work function of the material (𝑊𝐹) reach the vacuum level with kinetic energies (𝐸 𝑘 ). Since the total energy is conserved (neglecting inelastic scattering), the binding energies EB of the initial electron states can be determined using the following equation:

𝐸 𝑘 = ℎ𝜈 -𝐸 𝐵 -𝑊𝐹 (3.3)
The detector is calibrated by using a polycrystalline gold foil, in the way that the binding energy is referred to the fermi level (EF = 0 V) and not to the vacuum level, which may shift due to the presence of interfacial dipole. The distribution of bound states in the sample can be assessed by determining the kinetic energy distribution of the released photoelectrons. In a metal, the electrons with the highest kinetic energy originate the Fermi level. Conversely, in an organic semiconductor the electrons with the highest kinetic energy originate from the HOMO. It is commonly agreed that charge transport does not occur at the maximum of the density of states (DOS) of the HOMO, but rather at the low binding energy side that is characterized by a sufficient DOS at the onset to ensure charge transport. Consequently, the HOMO energy (EHOMO) can be extracted from UPS measurements, by considering the onset of the HOMO peak (that is the one at the lowest binding energy). [START_REF] Olthof | The Impact of UV Photoelectron Spectroscopy on the Field of Organic Optoelectronics-A Retrospective[END_REF] Furthermore, UPS measurements are distinguished by a background signal at low kinetic energies caused by inelastic scattering processes of electrons that lose some of their energy as they leave the sample. The secondary electron cutoff (SECO) is the point at low kinetic energies where this background abruptly ceases. This means that electrons of lower kinetic energy than the energy of the SECO cannot escape from the sample surface. Thus, the position of the SECO provides details regarding the material's work function:

𝑊𝐹 = 𝐸 𝑆𝐸𝐶𝑂 (3.4)
The interfacial dipole is calculated as the difference between the work function of the metal (WFm) and the work function of the metal coated with the OSC (WFm+OSC):

𝛥 = 𝑊𝐹 𝑚 -𝑊𝐹 𝑚+𝑂𝑆𝐶 (3.5)
The thickness of the OSC deposited onto the metal is the one at which subsequent OSC deposition does not result in a change in HOMO energy, indicating that the effect of the interfacial dipole saturates, causing no further bend bending of the HOMO. Hence, the ionization energy is given by the sum of EHOMO and the material work function:

𝐼𝐸 = 𝐸 𝐻𝑂𝑀𝑂 + 𝑊𝐹 𝑚+𝑂𝑆𝐶 (3.6)
In a typical experimental setup, two substrates were prepared for each investigated organic semiconductor compound. The first substrate consisted of a Si/SiO2 wafer coated with 30 nm of gold (hereinafter namely Au substrate), while the second substrate was the same except that it was immersed in a PFBT solution in isopropanol, resulting in the formation of a PFBT-SAM on the gold surface (hereinafter namely PFBT/Au surface). Each organic semiconductor compound was then thermally evaporated onto both substrates in an ultra-high vacuum, with different thicknesses monitored via a crystal quartz microbalance (the detailed preparation and measurement procedures are described in Chapter 5). As such, the OSC/Au interface reflects the OSC/contact interface of TC TFTs, while the OSC/PFBT-Au interface reflects the OSC/contact interface in BC TFTs.

Hole injection barrier and ionization energy

By depositing 20-Å-thick OSC layer onto the Au and PFBT/Au substrates, the hole injection barrier (ΦB,p) of each compound with respect to the relative substrate was extracted from the onset of the HOMO peak (Figure 3.

21).

Compounds deposited onto Au substrates unveail ΦB,p between 0.48 and 0.96 eV. Specifically, for both the family a similar trend is observed: the parent compounds and the tert-butyl derivatives exhibit larger injection barrier, with values in the range of 0.85-0.96 eV, while the alkylated derivatives exhibit smaller injection barrier, with values of 0.48 eV and 0.77 eV for C8-DNTT and C12-DBTTT respectively (Table 3.11). If the effect of the resistance associated to the charge transport beneath the contacts in TC TFTs is discarded, smaller injection barrier would result in reduction of contact resistance. The current density (Jint) transported across the metal/OSC interface is influenced by the injection barrier, in both thermionic field emission (Jint,TFE) and tunnelling (Jint,T) mechanisms of charge carrier injection: [START_REF] Waldrip | Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier[END_REF] ‡ ‡ ‡ 𝐽 𝑖𝑛𝑡,𝑇𝐹𝐸 ∝ exp(-Φ 𝐵 )

𝐽 𝑖𝑛𝑡,𝑇 ∝ exp(-√Φ 𝐵 )

Notably, the tert-butyl derivatives compounds demonstrate RCW values up to two orders of magnitude greater than their parent compounds, despite possessing comparable hole injection barriers (ΦB,p is of 0.96 eV for DNTT, 0.88 eV for tBu-DNTT, 0.85 eV for DBTTT and 0.87 eV for tBu-DBTTT). As such, in staggered TFTs, both the resistance associated to the OSC volume underneath the contacts and that at the metal/OSC interface contribute to the contact resistance, although the exact extent and relative significance of each component remain uncertain. This topic will be further elucidated in the subsequent section (Paragraph 3.9).

Upon deposition onto PFBT/Au substrates, all of the OSCs, with the exception of C8-DNTT and DBTTT, exhibit reduced values of ΦB,p when compared to those obtained from Au substrates. The hole injection barrier was observed to range between 0.15 and 0.84 eV, decreasing by up to 0.44 eV for DNTT, 0.53 eV for tBu-DNTT, 0.72 eV for tBu-DBTTT, and 0.27 eV for C12-DBTTT. In contrast, the injection barrier for DBTTT and C8-DNTT remains relatively unchanged (Table 3.11). Interestingly, tert-butyl derivatives display the largest reduction in the hole injection barriers, passing from Au to PFBT/Au substrates, along with the largest reduction of contact resistance, passing from TC to BC TFTs. However, this trend is observed across all compounds.

This highlights that the combination of a lower injection barrier at the metal/OSC interface, which minimizes interface resistance, and the fact that charges are directly injected into the accumulation channel collectively contribute to the reduced contact resistance of BC TFTs as compared to the TC counterpart. It should be noted, however, that the hole injection barrier is not the sole parameter governing the charge injection process and the resulting contact resistance in BC TFTs. If this were the case, tBu-DBTTTbased BC TFTs would display the lowest RCW, as tBu-DBTTT shows the lowest hole injection barrier when deposited onto PFBT/Au substrates, compared to the other compounds. However, the BC TFTs based on DNTT and C8-DNTT exhibit the lowest RCW values, indicating that additional ‡ ‡ ‡ Thermionic field emission occurs when the height of the injection barrier is above the thermal energy, while tunneling may occur when the width of depleted region at the metal/OSC interface is smaller than the free path of carriers in the OSC.

factors play a role in determining the charge injection process and ultimately, the contact resistance observed in BC TFTs. This aspect is further explored in the next section (Paragraph 3.9).

The lower injection barrier can be ascribed both to the higher work function of the PFBT/Au and to the increased interfacial dipole (both are outcomes of the Fermi level pinning) as compared to that of Au substrate. The work function of PFBT/Au substrates was determined to be approximately 0.5 eV higher than that of Au substrates. The former exhibits a work function of (5.16 ± 0.16) eV and the latter exhibits a work function of (4.67 ± 0.06) eV, obtained from the SECO onset of the respective substrates (Figure 3.22).

Furthermore, EHOMO was determined as the thickness of the OSC layer was increased, for both Au and PFBT/Au substrates, until it reached a saturation value. This indicates that the depleted region was surpassed. The EHOMO value did not exhibit any further variation beyond an OSC thickness of 100 Å for all the compounds.

Consequently, this specific OSC thickness was utilized to establish the interfacial dipole by extrapolating the work function of the metal coated with 100-Å-thick OSC layer (WFm+OSC) from the SECO (Figure 3.22) and using equation 3.5. Additionally, the ionization energy of each compound was determined by employing equation 3.6.

Effectively, the Schottky barriers, defined as the difference between the substrate work function and the ionization energy of the OSCs, were consistently lower in the case of OSCs deposited onto PFBT/Au substrates (ranging from 0.28 to 0.42 eV) in comparison to those deposited onto Au substrates (ranging from 0.34 to 0.92 eV). This finding confirms that minimizing the difference between the metal work function and the ionization energy of the OSC is a straightforward way to reduce contact resistance in organic TFTs. However, it should be noted that this method does not account for the bend bending at the metal/OSC interface. Therefore, it cannot be assumed that the Schottky barrier is equivalent to the injection barrier.

Interfacial dipole

The interfacial dipole created at the interface between the OSCs and the PFBT/Au substrates is consistently greater in magnitude than those formed with the Au substrates. More specifically, the interfacial dipole was determined to range from 0.19 to 0.73 eV for the former and from 0.10 to 0.20 eV for the latter, with the exception of DBTTT, which did not exhibit the formation of an interfacial dipole when deposited onto Au (Table 3.11). The black lines refer to Au substrates, the magenta lines refer to PFBT/Au substrates and the blue lines refer to the sample consisting of 100-Å-thick OSC layer deposited onto the respective substrate.

A higher interfacial dipole at the metal/OSC interface means that there is a greater separation of electrical charge across the interface. This dipole can have implications for device performance in organic electronics because it can affect the energy levels of the charge carriers, as well as their ability to move across the interface. The specific impact of a higher interfacial dipole depends on the materials and device architecture used. [START_REF] Cheng | Controlling Electron and Hole Charge Injection in Ambipolar Organic Field-Effect Transistors by Self-Assembled Monolayers[END_REF][START_REF] Wagner | Identification of different origins for s-shaped current voltage characteristics in planar heterojunction organic solar cells[END_REF][START_REF] Celle | Interface dipole: Effects on threshold voltage and mobility for both amorphous and poly-crystalline organic field effect transistors[END_REF][START_REF] Huang | Improving the charge injection in bottom contact organic transistors by carbon electrodes[END_REF] In our case, the higher interfacial dipole in combination with higher work function of the metal appears to improve charge injection in TFTs, by contributing to reducing the hole injection barrier at the OSC/PFBT-Au interface (Figure 3.23). The two exceptions to the observed trend are DBTTT and C8-DNTT, which exhibit a higher interfacial dipole compared to that of the Au substrate, but do not result in a significant variation of the injection barrier. Interestingly, considering the deposition onto PFBT/Au substrates, the interfacial dipole values extracted for DBTTT and C8-DNTT are the smallest and largest, respectively, among all the investigated compounds. This observation suggests that a proper balance between the metal work function, the ionization energy of the OSC and the interfacial dipole is necessary to reduce the hole injection barrier.

Discussion

The discussion of this chapter is divided into two parts: the first part focuses on the parameters that impact contact resistance in TC TFTs, and the second part focuses on the parameters affecting contact resistance in BC TFTs. A comprehensive outlook is presented, which considers both device geometries as well as the influence of the molecular structure of the OSCs.

Contact resistance in TC thin-film transistors

In the preceding sections, I presented the extraction and evaluation of various parameters that could potentially impact contact resistance in TC TFTs, including thin-film morphology, injection barrier, and out-of-plane charge transport (Table 3.12). We will now proceed to analyse how each parameter affects contact resistance, aiming to explain the lower contact resistance observed in alkylated derivatives (3.1 and 5.5 kΩcm for C8-DNTT and C12-DBTTT, respectively) in comparison to the tert-butyl derivatives (24.2 and 85.1 kΩcm for tBu-DNTT and tBu-DBTTT, respectively). In fact, it is expected that due to the lowest out-of-plane mobility of alkylated compounds compared to the analysed OSCs, they would exhibit a larger RCW than the tert-butyl derivatives. However, our experimental results contradict this expectation. Specifically, we found that the RCW in TC TFTs based on tertbutyl derivatives is one order of magnitude greater than in TC TFTs based on alkylated derivatives. Furthermore, the alkylated derivatives displayed RCW of the same order of magnitude as the parent molecules (2.8 and 1.7 kΩcm for DNTT and DBTTT, respectively).

Table 3.12: Electrical performances and injection barriers of TC TFTs. Contact resistance (RCW), in-plane mobility (µin), (in-plane) intrinsic mobility (µ0), out-of-plane mobility (µout) and hole injection barrier (ΦB,p). The investigation into thin-film morphology revealed no significant differences between the compounds that could explain the high contact resistance of the tert-butyl derivatives. All compounds exhibited a typical terraced structure morphology, which was characterized by the presence of grain boundaries. No direct correlation was found between grain size and contact resistance, and as well there was no correlation between the high presence of needle-shaped crystalline aggregates on top of the terraced structure (as observed in C8-DNTT, C12-DBTTT and tBu-DNTT) and contact resistance. Likewise, the calculated contact injection area was found to have negligible influence on determining contact resistance within the compounds. This was further emphasized by the fact that, despite the charge injection area being up to 2 orders of magnitude lower in BC TFTs compared to TC TFTs, the contact resistance is consistently lower in the former. We do not intend to imply that thin-film morphology and charge injection area can be disregarded when seeking to minimize contact resistance. However, in the case of our compounds and the selected device architecture, these parameters appear to have a minor impact on RCW.

Compound RCW (kΩcm) µin (cm 2 V -1 s -1 ) µ0 (cm 2 V -1 s -1 ) µout (cm 2 V -1 s -1 ) ΦB,p (eV)
Let us now consider the interplay between the hole injection barrier, which controls the contact resistance at the metal/OSC interface (Rint), and the outof-plane mobility, which governs the bulk resistance (Rbulk). The parent molecules and the tert-butyl derivatives have similar injection barriers, with DNTT exhibiting a slightly higher ΦB,p of 0.96 eV compared to DBTTT and the tert-butyl derivatives, which have ΦB,p in the range of 0.85-0.88 eV. Therefore, given a similar injection barrier, the higher contact resistance of the tert-butyl derivatives might be attributed to the three orders of magnitude lower out-of-plane mobility compared to the parent molecules. This hinders out-of-plane charge transport and contribute to the increased Rbulk. However, there are two unclear aspects. Firstly, increasing the OSC thickness from 25 nm to 80 nm (a three-fold increase) resulted in a 1.2 to 1.6 times increase in contact resistance, depending on the compound. Secondly, the alkylated derivatives exhibit even lower out-of-plane mobility than the tert-butyl derivatives but show contact resistance of the same order of magnitude as the parent compounds. Both of these factors indicate that while out-of-plane charge transport influence contact resistance, its impact is not predominant. [START_REF] Fischer | Nonlinear Contact Effects in Staggered Thin-Film Transistors[END_REF] Interestingly, we found a correlation between RCW and intrinsic mobility (µ0) extracted from TLM. It is worth noting that the extracted values of µ0 are slightly higher than the mobility values extracted from transfer characterisers (µin). § § § Considering both the families, RCW decreases as intrinsic mobility (and thus also µin) increases (Figure 3.24). This can be explained within the current crowding model. Considering the OSC volume beneath the contacts as a resistor network, charges are injected into the OSC and travel through the OSC thickness to reach the gate-induced charge accumulation channel. When charges are injected from the far contact edge, they encounter resistance when traveling along the OSC/dielectric interface (Rsh) to reach the TFT channel (Figure 3.24-b). [START_REF] Richards | Analysis of the contact resistance in staggered, top-gate organic field-effect transistors[END_REF][START_REF] Kraft | Low-Voltage Organic Transistors Based on Tetraceno[2,3-b ]thiophene: Contact Resistance and Air Stability[END_REF] This transport contributes to the device contact resistance. Lower mobility results in larger resistance to the charges traveling along this region (Rsh ∝ 1/µ0). Specifically, µ0 is of 2.7 and 5.7 cm 2 V -1 s -1 for DNTT and DBTTT, respectively, and of 1.2 and 0.6 cm 2 V -1 s -1 for tBu-DNTT and tBu-DBTTT, respectively. This results in larger Rsh of the latter. It is worth reminding that TFTs based on tert-butyl derivatives exhibit larger charge trap density than those based on the parent compounds and alkylated derivatives. Alongside lower in-plane mobility the higher charge trap density compared to the parent compounds may further contribute to increasing the resistance associated with charge § § § µ0 is extracted from TLM → 𝑅𝑊 = 𝑅 𝑐 𝑊 + 𝐿 𝑊µ 0 𝐶 𝑖 (𝑉 𝑔 -𝑉 𝑡ℎ )

, where transport along the OSC/dielectric interface underneath the contact. [START_REF] Liu | Critical Impact of Gate Dielectric Interfaces on the Contact Resistance of High-Performance Organic Field-Effect Transistors[END_REF] Therefore, the observed one-order-of-magnitude increase in RCW for tertbutyl derivatives compared to the parent compounds can be primarily attributed to the poorer charge transport properties along the in-plane direction, rather than the larger bulk resistance.

At last, it was found that C8-DNTT exhibits a significantly lower charge injection barrier (0.48 eV), while C12-DBTTT has a slightly lower injection barrier (0.77 eV) in comparison to their parent compounds and tert-butyl derivatives. While the reduction in injection barrier is beneficial in minimizing Rint contribution and contact resistance, it is not the sole reason for the observed lower RCW when compared to tert-butyl derivatives.

Although C8-DNTT has an injection barrier 0.40 eV lower than tBu-DNTT, the injection barrier of C12-DBTTT is only 0.10 eV lower than tBu-DBTTT Nevertheless, C12-DBTTT still displays RCW one order of magnitude lower than tBu-DBTTT, similar to the case of C8-DNTT and tBu-DNTT. Thus, also in this case, the lower RCW of alkylated derivatives compared to their respective tert-butyl derivatives may also be attributed to their considerably higher in-plane mobility (µ0 is of 5.7 and 2.5 cm 2 V -1 s -1 for C8-DNTT and C12-DBTTT, respectively). Clearly, the decrease in injection barrier assists in reducing the overall contact resistance, especially in case of C8-DNTT.

It is evident that the role of bulk charge transport cannot be dominant. If it were, the lower out-of-plane mobility of alkylated derivatives would result in larger RCW than that of tert-butyl derivatives. This supports the idea that RCW is primarily affected by in-plane charge transport along the OSC/dielectric interface beneath the contacts and the metal/OSC injection barrier, rather than the bulk charge transport.

In brief, these findings emphasize the significance of maximizing the in-plane mobility to decrease the resistance that hinders charge transport along the OSC/dielectric interface beneath the contacts, which in turn affect the overall contact resistance. The role of charge transport through the bulk of OSC is minor compared to the charge transport along the in-plane direction and the injection barrier. It is noteworthy that the bulk resistance resulting from outof-plane charge transport is influenced by the overlap of the HOMO wavefunction in the out-of-plane direction. In this context, the molecular structure plays a critical role as the functionalization of the core with tertbutyl or alkyl chains increases the distance between molecular cores, thereby decreasing the transfer integrals. Despite poorer out-of-plane charge transport caused by the core functionalization, high contact resistance is not necessarily a consequence if in-plane charge transport is efficient (high mobility and low charge trap density) and the injection barrier is reduced. However, when seeking to achieve the maximum reduction in contact resistance in TC TFTs, it is essential to minimize Rbulk as well. Albeit to a minor extent, Rbulk affects the contact resistance.

Contact resistance in BC thin-film transistors

The results of our study indicate that BC TFTs consistently exhibit lower contact resistance compared to the TC counterparts for all the analysed compounds and over the entire range of gate-overdrive voltage. This is mainly due to direct charge injection into accumulation channel in BC TFTs, removing the contribution of charge transport in the region underneath the contacts as in the case of TC TFTs. Consequently, contact resistance in BC TFTs is primarily regulated by the injection barrier at the metal/ OSC interface. Additionally, the use of PFBT-treated contacts in BC TFTs leads to an increased work function, up to 0.5 eV higher than bare gold, facilitating charge injection and contributing to the lowering of RCW.

It is important to note that the morphology of the OSC across the contact-tochannel region is crucial in BC TFTs. Keeping the OSC morphology as similar as possible can help prevent charge trapping and inefficient charge injection. [START_REF] Borchert | Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors[END_REF][START_REF] Niazi | Contact-Induced Nucleation in High-Performance Bottom-Contact Organic Thin Film Transistors Manufactured by Large-Area Compatible Solution Processing[END_REF][START_REF] Gundlach | Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits[END_REF] The AFM images show that the morphology of all investigated compounds resembles that of the OSC deposited onto the TDPA/Al2O3 substrate in the channel region.

We previously highlighted that tBu-DBTTT exhibits the lowest hole injection barrier (0.15 eV), followed by tBu-DNTT (0.35 eV). However, this does not necessarily correspond to the lowest contact reistance. RCW was found to be of 3.7 kΩcm for tBu-DNTT and 2.4 kΩcm for tBu-DBTTT. The parent compounds and alkylated derivatives show lower RCW, between 0.8 and 1.9 kΩcm despite exhibiting higher injection barriers, which range from 0.50 to 0.84 eV (see Table 3.13). This can be attributed to the formation of a region depleted of charges due to band bending at the metal/OSC interface, which contributes to the overall contact resistance.

The resistivity of the depleted region has been shown to be proportional to 1/µ in drift diffusion simulations of BC devices based on pentacene at a given injection barrier. [START_REF] Gruber | Impact of Materials versus Geometric Parameters on the Contact Resistance in Organic Thin-Film Transistors[END_REF] The relationship between RCW and 1/µ is not proportional in our case due to the compounds having different injection barriers and possibly different extension of the depleted region. Nonetheless, the lower RCW of the parent compounds and alkylated derivatives can be attributed to their higher mobility (between 2.3 and 4.8 cm 2 V -1 s -1 ) compared to the tert-butyl derivatives (mobility of 1.4 and 1.0 for tBu-DNTT and tBu-DBTTT, respectively). Hence, despite their lower injection barrier, an efficient charge carrier injection in tert-butyl derivatives is hindered by the higher resistivity of the depleted region caused by their lower mobility.

In conclusion, within the coplanar device geometry, the variation in molecular structures resulting from molecular core functionalization does not influence contact resistance, since charges are directly injected into the accumulation channel. Primarily, to achieve a lower contact resistance in BC TFTs, it is necessary to minimize the injection barrier and maximize the charge carrier mobility, which in turns reduces the resistance associated to the depleted region at the metal/OSC interface.

Summary

In this chapter, the impact of the molecular structure of DNTT and DBTTT families on the contact resistance of organic TFTs was examined, with a particular emphasis on the device geometry, injection barrier at the metal/OSC interface, and out-of-plane and in-plane charge transport properties.

In the context of BC TFTs, the lowest contact resistance is shown by DNTT and C8-DNTT, despite their larger injection barrier compared to the tertbutyl derivatives. The observed effect is attributed to the formation of a depleted region at the metal/OSC interface, which contributes to contact resistance. The resistance opposed by the depleted region is lowered when mobility increases. Thus, although a lower injection barrier is beneficial for minimizing contact resistance, it is also important to maximize the charge carrier mobility. As such, compounds with a good balance between a low injection barrier and high in-plane mobility exhibit the lowest RCW.

Considering that in BC TFTs charges are directly injected into the accumulation channel, the variation of the molecular structure due to the core functionalization appears to have no influence on the contact resistance.

In the case of TC TFTs, it was found that the bulk charge transport through the thickness of the OSC influences contact resistance, but its impact is not dominant. The alkylated compounds showed out-of-plane mobilities 4 orders of magnitude lower than the parent compound due to the larger distance between the molecular cores along the out-of-plane direction. Yet RCW of C8-DNTT and C12-DBTTT is comparable to those of the parent compounds. In contrast, tert-butyl derivatives showed RCW one order of magnitude higher than the alkylated derivatives, despite their higher out-of-plane mobility. The comparison of the various factors affecting contact resistance revealed that RCW is primarily determined by the resistance opposing charge transport along the OSC/dielectric interface beneath the contacts (governed by in-plane mobility and charge trap density) and the injection barrier at the metal/OSC interface. Thus, in TC TFTs, a larger distance between the molecular cores due to the introduction of substituents does not necessarily result in high contact resistance if the injection barrier is reduced and in-plane charge transport is efficient.

At last, the study found that BC TFTs consistently exhibit lower contact resistance compared to TC counterparts for all the analysed compounds. This is attributed to the direct injection of charges into the accumulation channel in BC TFTs. In contrast to TC TFTs, where the resistance to charge transport beneath the contacts contributes to the contact resistance, this contribution is discarded in BC TFTs. The reduction in contact resistance was mild for the parent compounds and alkylated derivatives, with RCW between 1.3 and 2.9 times lower in BC TFTs, compared to TC TFTs. However, BC TFTs based on tBu-DNTT showed up to 7 times lower RCW, and BC TFTs based on tBu-DBTTT showed up to 35 times lower RCW compared to the TC counterparts. This emphasizes the advantageous impact of employing a coplanar device geometry in terms of electrical performance and charge injection properties.

Chapter 4 Exploring the effect of vibrational

strong coupling on perylene crystallization in a Fabry-Perot cavity

Introduction

The coupling of molecular vibrations to optical cavity vacuum fields has shown to alter molecular and material properties i.e., by modifying the reactivity of organic reactions [START_REF] Lather | Cavity Catalysis by Cooperative Vibrational Strong Coupling of Reactant and Solvent Molecules[END_REF][START_REF] Hirai | Modulation of Prins Cyclization by Vibrational Strong Coupling[END_REF][START_REF] Thomas | Tilting a ground-state reactivity landscape by vibrational strong coupling[END_REF][START_REF] Thomas | Ground state chemistry under vibrational strong coupling: dependence of thermodynamic parameters on the Rabi splitting energy[END_REF] . The alteration of chemical reactivity has been attributed to a reshaping of the Morse potential of the coupled bond modified by vibrational strong coupling (VSC), predicting a shortening or strengthening of the coupled bond and hence a change of its energy. [START_REF] Thomas | Tilting a ground-state reactivity landscape by vibrational strong coupling[END_REF][START_REF] Flick | Atoms and molecules in cavities, from weak to strong coupling in quantum-electrodynamics (QED) chemistry[END_REF] Therefore, if the energy and the strength of a bond involved in intermolecular interactions would be altered by coupling with the vacuum field, VSC may influence crystal nucleation as well as crystal growth.

The modification of the supramolecular assembly of a conjugated polymer [START_REF] Joseph | Supramolecular Assembly of Conjugated Polymers under Vibrational Strong Coupling[END_REF] and of two structural isomers of phenyleneethynylene, [START_REF] Sandeep | Manipulating the Self-Assembly of Phenyleneethynylenes under Vibrational Strong Coupling[END_REF] as well as the selective crystallization of a pseudo-polymorphic form of zeolite imidazolate frameworks, [START_REF] Hirai | Selective crystallization via vibrational strong coupling[END_REF] serve as initial examples confirming the impact of VSC on supramolecular chemistry. It is worth noting that the reported supramolecular assembly modifications have been obtained through cooperative VSC (i.e., the optical mode couples with a vibrational mode of the solvent that resonates with a vibrational mode of the solute). The underlying mechanism of this phenomenon is not yet fully understood and may be related to changes in dispersive forces and/or the stabilization of one molecular assembly over another when the investigate system is strongly coupled with the vacuum field.

Controlling the crystal packing and polymorphism is crucial for optimizing the physical properties and electrical characteristics of OSC-based devices. Hence, the development of methods to induce selective crystallization, preventing the concomitant growth of polymorphic forms, may offer new opportunities to maximize the electrical performances of OSCs. In this context, perylene has been selected as a model case since it is readily available, well-known molecule and it represents the core of the perylenebased organic semiconductor molecules. Two crystals forms of perylene are reported, historically named alpha and beta (Figure 4.1) [START_REF] Botoshansky | Towards a Complete Description of a Polymorphic Crystal: The Example of Perylene[END_REF] . Crystallization techniques have been utilized using both solution-based and sublimation methods, typically resulting in the concomitant growth of both polymorphic forms. [START_REF] Botoshansky | Towards a Complete Description of a Polymorphic Crystal: The Example of Perylene[END_REF][START_REF] Urbelis | Phase-Selective Crystallization of Perylene on Monolayer Templates[END_REF] However, selective growth of each polymorph has also been reported. [START_REF] Yago | Growth of β-Perylene Crystal[END_REF][START_REF] Lei | Phase-and Shape-Controlled Synthesis of Single Crystalline Perylene Nanosheets and Its Optical Properties[END_REF][START_REF] Li | Controllable Synthesis of Organic Microcrystals with Tunable Emission Color and Morphology Based on Molecular Packing Mode[END_REF] Specifically, β-crystal-based OFETs exhibit hole mobility of 1.1 × 10 -1 cm 2 V -1 s -1 , up to 10-fold higher than those based on the mixed-phase crystals and α-phase crystal. [START_REF] Hsieh | Polymorphic Behavior of Perylene and Its Influences on OFET Performances[END_REF] This finding reinforces the significance of controlling the packing of OSC and polymorphism to enhance device performance.

Accordingly, in this chapter, we endeavour to investigate the potential impact of vibrational strong coupling on perylene crystal growth and polymorphism through crystallization experiments conducted in a Fabry-Perot (FP) cavity.

Perylene crystallization

Crystal structures

Extensive research has been conducted on the packing motifs of perylene polymorphs. [START_REF] Botoshansky | Towards a Complete Description of a Polymorphic Crystal: The Example of Perylene[END_REF] The α-form is characterized by a dimeric sandwichherringbone motif, whereas the β-form has a monomeric γ-herringbone structure (Figure 4.1). [START_REF] Desiraju | From molecular to crystal structure; polynuclear aromatic hydrocarbons[END_REF] Both polymorphs belong to monoclinic space groups (P 21/c for the α-form and P 21/a for the β-form), with a single unique molecule in the asymmetric unit (Table 4.1). The α-form is mainly characterized by π-π dimer interactions and C-H•••H interactions, whereas the β-form is mainly characterized by π-π in-stack interactions and C-H•••π interactions. Therefore, it is of interest to investigate whether vibrational strong coupling between cavity optical modes and the aromatic C-H stretching or C=C stretching of perylene could potentially influence crystal growth and polymorphism.

Crystallization experiments

Perylene polymorphs exhibit different macroscopic habits, with α-crystals appearing as rectangular shapes β-crystals appearing as rhombic shapes. This renders them readily distinguishable under an optical microscope. Crystallization experiments were conducted to verify the correlation between crystal habit and the underlying crystalline structure. Both polymorphic forms were selectively grown using a solution-dropping method in accordance with established protocols. [START_REF] Li | Controllable Synthesis of Organic Microcrystals with Tunable Emission Color and Morphology Based on Molecular Packing Mode[END_REF] Perylene was dissolved in chlorobenzene (CLB) with a concentration of 4 mg/mL and dropped onto a quartz substrate to obtain square-shaped crystals, upon solvent evaporation at room temperature. The rectangular crystals exhibit edge lengths ranging from 10 to 100 µm. Likewise, rhomboidalshaped crystals were obtained by the same procedure but using a 4 mg/mL perylene solution in tetrahydrofuran (THF). The rhomboidal crystals exhibit edge length ranging from 20 to 80 µm. Also, a small number of rectangularshaped crystals were observed, indicating the simultaneous crystallization of the α-form. However, the amount of the rectangular-shaped crystals was considerably lower compared to those of rhomboidal shape (as illustrated in Figure 4.2).

X-ray diffraction was performed on the collected crystals and compared to the simulated powder XRD patterns of the α-and β-forms (XRD patterns were simulated using Mercury distributed by Cambridge Crystallographic Data Center, referencing PERLEN04 for the α-form and PERLEN01 for the βform). The diffraction peaks from the rectangular perylene crystals obtained from chlorobenzene corresponds to the calculated XRD pattern of the α-form (highlighted with red asterisks in Figure 4.2). The diffraction pattern from the rhombohedral perylene crystals from THF is consistent with the simulated β-phase (highlighted with black asterisks in Figure 4.

2).

Nonetheless, peaks of low intensity were detected at 2θ = 8.7° and 2θ = 11.4°, which are attributed to the minor presence of the rectangular-shaped crystals (α-form) observed from the optical micrographs. This confirms that the rectangular-shaped crystals correspond to the α-form, whereas the rhomboidal-shaped crystals correspond to the β-form.

Both polymorphs exhibit strong preferred orientations along (100). The two phases are easily distinguishable, as the α-(100) and β-( 100) reflections appeared at 2θ = 8.7° and 9.1°, respectively, without overlap. Furthermore, the α-form exhibit (110) and (11-1) reflections at 2θ = 11.4 and 13.5, respectively, where no reflections of the β-form are unveiled.

It is worth noting that THF has a boiling point of 66 °C, whereas chlorobenzene has a boiling point of 132 °C. Consequently, when solutions are deposited onto quartz substrates, THF evaporates at a faster rate than chlorobenzene (i.e., 50 µL of perylene solution in THF evaporates in approximately 3 minutes, while 50 µL of perylene solution in chlorobenzene evaporates in approximately 40 minutes at room temperature). The formation of β-crystals from a THF solution during rapid solvent evaporation suggests that β-crystal formation is kinetically driven rather than thermodynamically. This is consistent with the findings of Yago et al., [START_REF] Yago | Growth of β-Perylene Crystal[END_REF] who reported that fast cooling of a saturated solution of perylene in toluene (30 °C/min) selectively precipitates the β-polymorph, while slow cooling (1 °C/min) favours the growth of the α-polymorph. In addition, recrystallization experiments were conducted, whereby three solutions were prepared: (a) perylene was dissolved in THF at a concentration of 10 mg/mL through stirring and heating at 50 °C for 30 min (namely R-THF), (b) perylene was dissolved in toluene at a concentration of 10 mg/mL through stirring and heating at 70 °C for 30 min (namely R-TOL), and (c) perylene was dissolved in benzonitrile at a concentration of 10 mg/mL through stirring and heating at 70 °C for 30 min (namely R-BNT). The solutions were cooled at a rate of ≈ 1 °C/min until they reached room temperature. The resulting crystals at the bottom of the vials were collected and subjected to XRD analysis (see Figure 4.3). All recrystallization experiments yielded both polymorphic forms, as indicated by the presence of peaks at 2θ = 8.7° and 9.1°, corresponding to the α-and β-form, respectively. At last, slurry experiments were conducted by preparing a 10 mg/mL chlorobenzene solution of perylene, which was stirred and maintained at room temperature. After 4 days, XRD analysis was performed on the slurry, which exhibited peaks corresponding to the α-form. Considering the thermal properties, the β-form undergoes an irreversible phase transition to the α-form, with the latter remaining stable up to the melting point. Differential scanning calorimetry has been reported by Hsieh et al., [START_REF] Hsieh | Polymorphic Behavior of Perylene and Its Influences on OFET Performances[END_REF] determining the phase transition of the α-and β-form. During heating, the α-crystals displayed a single endothermic phase transition corresponding to their melting point of 277 °C. On the other hand, the βcrystals displayed two endothermic transitions during heating: one corresponding to the melting process at 277 °C and the other occurring between 100 and 140 °C, indicating a phase transition towards a more stable α-phase. X-ray diffraction study of the β-to-α phase transition by Botoshansky et al. [START_REF] Botoshansky | Towards a Complete Description of a Polymorphic Crystal: The Example of Perylene[END_REF] also showed that the β-to-α phase transition occurs between 100 and 140 °C. Exothermic transition between 100 and 140 °C has not been observed during the subsequent cooling process, indicating that the α-crystal does not change back to the β-crystal during cooling.

To summarize, the results of the crystallization experiments indicate that the solution-dropping method using a high boiling point solvent (chlorobenzene) favours the formation of the α-form, whereas the solution-dropping method using a lower boiling point solvent (THF) leads to the preferential formation of the β-form. Recrystallization experiments carried out by cooling saturated solutions at a rate of 1°C/min resulted in the formation of both polymorphs, while prolonged slurry maturation yielded the selective formation of the αform. These findings, in conjunction with the reported thermal properties, suggest that the α-form is the thermodynamically stable form, whereas the βform is considered to be metastable.

Crystallization in Fabry-Perot cavity 4.3.1 Fabry-Perot cavity fabrication

In order to investigate the impact of VSC on perylene crystallization, a microfluidic optical Fabry-Perot (FP) cavity was utilized. The FP cavity comprises two parallel CaF2 windows coated with 10 nm of Au and a 100 nm thick layer of spin-coated polyvinyl alcohol (PVA) for protection. The two mirrors were assembled into a microfluidic cell with a 25 μm thick Mylar spacer between them. Within this setup, a solution can be injected between the two mirrors, and the distance between the mirrors can be precisely adjusted by tightening or loosening the four screws that hold the cell (Figure 4.4). This allows for the achievement of VSC between FP optical modes and molecular vibrations. A comprehensive description of the FP cavity is provided in Chapter 5.

It should be noted that in the following section non-cavity experiments were conducted in the same microfluidic cell as described previously, but without the gold coating of the CaF2 windows. In the non-cavity condition, the experiment is performed in a micro-confined space (the volume between the CaF2 windows), but without the possibility to strongly couple molecular vibrations with the vacuum field. Due to the limited solubility of perylene in the selected solvents (THF, chlorobenzene and benzonitrile) it was not possible to directly achieve VSC between optical cavity modes and molecular vibrations of the solute. To overcome this limitation, we employed cooperative VSC, which involves coupling the solute through the solvent if there are overlapping vibrational bands between them. In particular, benzonitrile and perylene have an overlapping vibrational band at 1492 cm -1 , which corresponds to the aromatic C=C stretching (Figure 4.5). On the right: perylene and benzonitrile spectra in the region between 1710 and 1350 cm -1 , highlighting the overlap of the vibrational bands at 1492 cm -1 (marked with a blue line).

In typical experiments, a 10 mg/mL solution of perylene in benzonitrile was prepared by stirring and heating the solution at 70 °C for 30 min. Subsequently, the solution was introduced into the pre-heated FP cavity at 70 °C by coupling a heating jacket to the assembled microfluidic cell. By modulating the distance between the mirrors, two different scenarios under two different VSC conditions were achieved (Figure 4.6):

1. VSC of the solvent band at 2228 cm -1 (C≡N stretching) with the 18 th mode of the cavity, along with cooperative VSC of the solvent (and perylene) band at 1492 cm -1 (aromatic C=C stretching) with the 12 th mode of the cavity, namely VSC1 hereinafter, 2. VSC of the solvent band at 1445 cm -1 (aromatic C=C stretching) with the 12 th mode of the cavity, namely VSC2 hereinafter.

VSC was confirmed by the formation of polaritonic states separated by an energy called Rabi splitting (ħΩR), which is higher than the full width at half maximum of both the coupled optical mode (νo) and the vibrational band (νs).

In VSC1, VSC of the vibrational mode at 2228 cm -1 resulted in Rabi splitting of 51 cm -1 , with νo and νs being 19 cm -1 and 13 cm -1 , respectively; while VSC of the vibrational mode at 1492 cm -1 resulted in Rabi splitting of 28 cm -1 , with νo and νs being 19 cm -1 and 11 cm -1 , respectively. In VSC2, Rabi splitting was found to be 26 cm -1 , with νo and νs being 14 cm -1 and 6 cm -1 , respectively.

In VSC1, selective coupling of only the vibrational band at 1492 cm -1 was not possible without coupling other vibrational modes of the solvent, due to the small free spectral range (FRS) of 124.2 cm -1 . FSR is the difference in frequency between two adjacent resonance modes of the system. FSR is related to the refractive index of the medium and the distance between the mirrors using the following equation: [START_REF] Thomas | Tilting a ground-state reactivity landscape by vibrational strong coupling[END_REF] 𝐹𝑆𝑅 = where m is the mode order, n is the refractive index of the medium and Lm is the distance between the mirrors. However, attempting to increase the free spectral range (FSR) by using a thinner spacer to achieve selective coupling conditions resulted in a decrease in the volume of solution injected into the cavity. This ultimately led to the formation of few or no crystals during the crystallization experiment.

As a control, non-cavity experiments were conducted using the same recrystallization method described above, but without the gold coating on the CaF2 windows. The experimental setup involved injecting a 10 mg/mL solution of perylene in benzonitrile at 70°C into the microfluidic cell, which was pre-heated at 70°C.

After establishing the VSC conditions as described in VSC1 and VSC2, as well as in the case the non-cavity experiment, the microfluidic cell was cooled gradually from 70°C to 25°C at a rate of 1 °C/min. In all the recrystallization experiments (VSC1, VSC2, and non-cavity condition), rectangular and rhomboidal shaped platelet crystals were obtained, referred to as α and β form, respectively (Figure 4.6). This is consistent with the recrystallization results presented in Paragraph 4.2.2, where a 10 mg/mL solution of perylene in benzonitrile was cooled from 70°C to room temperature in a vial, resulting in the formation of both polymorphs.

The polymorphic ratio was determined by categorizing the formed crystals according to their crystal habit and counting them in three independent replicates. The results indicated that the α-form constituted (27±6) % of the crystals in VSC1, (20±5) % in VSC2, and (15±2) % in the non-cavity condition.

There was no significant difference in the edge length of the crystals, which ranged from 10 to 150 µm. A slight increase in the ratio of the α-form to the β-form was observed when transitioning from off-cavity conditions to VSC2 and VSC1. However, since the difference between the polymorphic ratio values obtained from VSC1 and VSC2 compared to non-cavity conditions was small (α-form increase of 5 % in VSC1 and of 12 % in VSC2, compared to non-cavity condition), we cannot draw any straightforward conclusions.

In addition, the higher standard deviation of α-form to the β-form ratio in VSC1 and VSC2 compared to the non-cavity condition indicates a greater variability in the polymorphic ratio across different experimental replicates in the former. This variability may be linked to variations in the free spectral range (FSR) of the FP cavity, which can be affected by changes in the refractive index due to crystal formation and the thermal expansion of the device system holding the gold coated CaF2 windows. To investigate this, we calculated the FSR of an empty FP cavity (filled with air) with a 25 µm Mylar spacer between the mirrors. The FSR increased from 189.2 to 190.6 cm -1 as the cavity was cooled from 70 °C to 25 °C, indicating that the reason of the increased FSR is linked to the variation of the distance between the cavity mirrors (Figure 4.7a).

Changes in the FSR resulted in modifications to the resonance conditions of polaritonic states in both VSC1 and VSC2. The energy levels of these states were found to change as the FP cavity was cooled. Specifically, in VSC1, where polaritonic states were formed through the coupling of the C≡N stretching and the 18 th optical mode, P + and P -energies increased by 9 cm -1 when the temperature was lowered from 70 °C to 25 °C. Similarly, the polaritonic states formed through the coupling of the C=C stretching at 1492 cm -1 and the 12 th optical mode, P + and P -, respectively, increased by 5 and 4 cm -1 as the temperature decreased. Finally, in the case of VSC2, P + and P - shifted to higher wavenumbers by up to 10 and 9 cm -1 respectively, as the FP cavity was cooled to 25 °C (Figure 4.7).

In conclusion, the recrystallization experiments showed a slight increase in the ratio of the α-form to the β-form under VSC conditions compared to noncavity condition. Additionally, there was a higher variability in the polymorphic ratio observed during crystallization experiments under VSC. These findings suggest that VSC may have an effect on the crystallization outcome. However, the effect of VSC is likely weakened and variable due to the change of the FRS which in turns shifts the energy of polaritonic states during crystallization. It is important to note that previous studies reporting modifications in supramolecular assemblies induced by VSC were conducted at a fixed room temperature. Therefore, to confirm the influence of VSC on the polymorphic ratio in a systematic and reliable manner, crystallization experiments should be conducted at a constant temperature of the FP cavity to prevent variations in the energy of polaritonic states throughout the experiment.

Summary

In this chapter the impact of VSC on the crystallization of perylene was investigated. First, the formation of the two polymorphic forms of perylene was studied using conventional crystallization methods such as solutiondropping, recrystallization and slurry techniques. The results indicate that both polymorphs grew concurrently in recrystallization experiments, whereas selective crystallization of either the α-form or β-form was achieved through solution-dropping method. The α-form was identified as the more stable thermodynamic form, which was obtained through slurry crystallization.

To examine the impact of VSC, recrystallization experiments were conducted in a FP cavity using two different conditions: cooperative VSC of the aromatic C=C stretching of benzonitrile that is resonant with perylene vibrational band and VSC of the aromatic C=C stretching solvent vibrational band that is not resonant with perylene vibrational modes. The experiments under VSC resulted in a slightly higher ratio of the α-form to the β-form compared to the non-cavity condition. However, the FSR changed during the experiment due to the change in temperature of the microfluidic cell, causing a variation of the resonance conditions of the polaritonic states. This could potentially result in instability of the system and compromise the reproducibility of the results.

The study attempted to recrystallize perylene in a FP cavity but highlighted the limitation of the experimental setup due to the temperature changes of the FP cavity microfluidic cell. Therefore, alternative crystallization methods that can be conducted at room temperature, such as antisolvent crystallization, should be used in future experiments to fully explore the potential of VSC on crystal growth while avoiding issues with FSR changes.

Chapter 5 Materials and methods

OSC materials

DNTT (> 99% sublimed grade) and perylene (> 99.5 % sublimed grade) were purchased from Sigma-Aldrich, C8-DNTT (> 99% sublimed grade) was purchased from Lumtec. DN4T, [START_REF] Jouclas | Dinaphthotetrathienoacenes: Synthesis, Characterization, and Applications in Organic Field-Effect Transistors[END_REF] isoDN4T, [START_REF] Jouclas | Dinaphthotetrathienoacenes: Synthesis, Characterization, and Applications in Organic Field-Effect Transistors[END_REF] , tBu-BTBT-1, [START_REF] Richard | Conception, Synthèse et Caractérisation de Semi-Conducteurs Moléculaires à Dimensionnalité Élevée[END_REF] tBu-BTBT-2, [START_REF] Richard | Conception, Synthèse et Caractérisation de Semi-Conducteurs Moléculaires à Dimensionnalité Élevée[END_REF] tBu-BTBT [START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF] and C12-DBTTT [START_REF] Chen | Crystal Engineering of Dibenzothiophenothieno[3,2-b ]thiophene (DBTTT) Isomers for Organic Field-Effect Transistors[END_REF] were provided by Prof. Yves Geerts (Université Libre de Bruxelles), and their synthesis and purification are reported elsewhere.

The synthetic routes and purification procedures for C8-DN4T, C10-DN4T, tBu-DNTT and tBu-DBTTT have not been published yet (manuscripts in preparation) and are reported in the following sections.

The synthesis and purification of C8-DN4T and C10-DN4T were performed by Dr. Remy Jouclas, while the synthesis and purification of tBu-DNTT and tBu-DBTTT were performed by Dr. Antoine Leliege, in the laboratory of Prof. Yves Geerts (Université Libre de Bruxelles).

Synthesis of C8-DN4T and C10-DN4T

The synthesis followed the synthetic scheme previously published by Niimi et. al. [207] for the synthesis of dialkyl-DNTT. Introduction of the side alkyl chains was performed at the first step through a Kumada cross-coupling between the commercially available 6-bromo-2-methoxynaphthalene 1 and the corresponding commercial Grignard reagents to afford intermediates 2ab. Introduction of the thiomethyl group dedicated to the final thiopheneforming cyclization was performed by selective ortho-lithiation directed by the methoxy group. Subsequent conversion of the latter to triflate was carried out by demethylation in presence of boron tribromide followed by triflation with triflic anhydride to afford triflates 5a-b in good yields for the 2 steps. The end of the synthesis was performed following the previously published synthetic scheme of DN4T [START_REF] Jouclas | Dinaphthotetrathienoacenes: Synthesis, Characterization, and Applications in Organic Field-Effect Transistors[END_REF] involving the use of commercial 2,5-bistrimethylstannylthieno [3,2-b]thiophene and triflates 5a-b in order to place the center of the tetrathienyl core. Cyclization of the resulting disulfides 6ab was performed upon oxidation into disulfoxydes in presence of m-CPBA with quantitative yields, prior to 2-steps cyclization in triflic acid in presence of phosphorus pentoxide followed by demethylation of the resulting triflate sublimation, to tBu-DNTT in 82 % yield. Starting from thieno [3,2b]thiophene the yield of tBu-DNTT is 47 %. The crude product was purified by sublimation (330 °C under a pressure of 10 -6 mbar), a yellow solid was obtained. Scheme 5.2: i) HNO3, H2SO4 (96% yield); ii) Na2S2O4, 2-methoxyethanol/water (66% yield); iii) NaNO2, HCl then KI, water (79% yield); iv) iPrMgCl then DMF, THF (76% yield).

Thin-film transistors fabrication and electrical measurements

All the thin film transistors were fabricated on highly doped silicon wafers with 30 nm of Al2O3 (Christian-Albrecht University of Kiel, Institute for Electrical Engineering and Information Technology). The Al2O3 substrates were exposed to oxygen plasma (Diener Electronic; oxygen flow rate 20 sccm, pressure 0.50 mbar, plasma power 100 W, duration 2 min) and then immersed overnight in 1.5 mM solution of n-tetradecylphosphonic acid (TDPA, Sigma-Aldrich) in 2-propanol (Acros Organics) to obtain a 1.5-nm-thick selfassembled monolayer (SAM). Subsequently, the substrates were rinsed first in 2-propanol then in deionized water and finally in 2-propanol again and dried on a hot plate at 100 °C for 10 min. Thus, the Al2O3/SAM dielectric has a capacitance of 185.5 nF cm -2 . In the case of BC devices, gold bottom contacts were deposited by vacuum sublimation (UNIVEX 300, Leybold GmbH; pressure of ~10 -5 mbar, deposition rate of 0.5 Ǻ s -1 and nominal thickness of ca. 50 nm monitored by a quartz crystal microbalance) through a shadow mask onto the gate-dielectric substrates at room temperature. Afterwards, the substrates were immersed in a 10 mM solution of pentafluorobenzenethiol (PFBT, Alfa Aesar) in 2-propanol for 30 min, obtaining a SAM on the gold bottom contacts, then rinsed with 2-propanol and dried. OSCs were evaporated in vacuum (pressure of ~10 -6 mbar) through a shadow mask onto the substrates which were held at desired temperatures, obtaining the final devices with a channel width of 480 μm and channel length of 65, 115, 165 and 215 μm. In case of TC devices, OSCs were deposited by vacuum sublimation (pressure of ~10 -6 mbar) on the Al2O3/SAM dielectric substrates, heated at desired temperatures, before the gold contacts deposition through a shadow mask (pressure of ~10 -5 mbar, deposition rate of 0.5 Ǻ s -1 , nominal thickness of ca. 50 nm), obtaining the final devices with a channel width of 480 μm and channel length of 65, 115, 165 and 215 μm. The electrical measurements (Agilent 4155C Semiconductor Parameter Analyzer) were carried out in ambient air and at room temperature.

Ultraviolet photoelectron spectroscopy measurements

Ultraviolet photoelectron spectroscopy measurements were conducted using a UHV (ultra-high vacuum) system with a pressure of 10 -10 mbar. A Helium discharge lamp was used to produce the monochromatized He Ia line (21.22 eV) for all measurements. The spectra were recorded at room temperature using a hemispherical SPECS Phoibos 100 analyzer. Prior to the measurements, a polycrystalline gold foil was utilized to calibrate the detector and determine the energy resolution. The pass energy was set at 5 eV, providing an energy resolution of 120 meV for the valence region spectra. The work function values were determined from the secondary electron cutoff (SECO) spectra, measured with a sample bias of -10 V and a pass energy of 2 eV, which allowed for an energy resolution of 78 meV.

Before the UPS measurements, the molecules were deposited onto Au and PFBT/Au substrates by thermal evaporation under ultra-high vacuum conditions (10 -8 mbar). The Au substrates were obtained by thermal evaporation of 30 nm layer of Au under high vacuum conditions (10 -6 mbar) onto Si/SiO2 substrates at a rate of 0.5 Ǻ s -1 . The PFBT/Au substrates were obtained by immersing the Au substrate in a 10 mM solution of PFBT (Alfa Aesar) in 2-propanol for 30 minutes, followed by rinsing with 2-propanol and drying. OSC films were grown onto the substrates, with the nominal thickness being monitored via a quartz crystal microbalance.

Atomic force microscopy

Topography imaging

The thin films morphologies were evaluated through atomic force microscopy (AFM, Brucker Dimension Icon) in tapping mode with Olympus OMCL-AC240TS-R3 tips.

Conductive atomic force microscopy

C-AFM measurements were performed on thin films of OSCs that were deposited on PFBT/Au substrates via vacuum sublimation (UNIVEX 300, Leybold GmbH; pressure of ~10 -6 mbar, nominal thickness of approximately 10 nm monitored by a quartz crystal microbalance). The PFBT/Au substrates were prepared as described earlier for UPS measurements. The Au surface was grounded to the instrument's sample holder for electrical conductivity. C-AFM measurements were conducted using a Bruker Dimension Icon setup in contact mode, in air with a relative humidity level below 5%. Pt/Ir coated SCM-PIT-V2 tips were utilized for the measurements (tip stiffness: k = 3 N/m).

Thin-films X-Ray Diffraction

Thin film for X-Ray diffraction analysis were obtained by depositing OSCs onto TDPA/Al2O3 substrates and onto PFBT/Au substrates with a nominal thickness of ≈ 25 nm, as described in section 5.2 and 5.3 respectively. X-ray diffraction was performed by using Rigaku SmartLab. The diffractometer is equipped with a Cu Kα source. The measurements were carried out at a tube voltage of 40 kV (tube current of 50 mA), with scanning steps of 0.04° at scanning speed of 1.5°/min.

Photoelectron Yield Spectroscopy

Thin films used for X-ray diffraction measurements were employed for PYS measurements as well. Photoelectron yield curves were acquired utilizing a Riken Keiki spectrophotometer (model AC-2) with an energy increment of 0.05 eV and UV spot intensity of 10 nW.

Optical Microscopy

Optical microscope images were taken with a Zeiss Axiotron equipped with Zeiss Mikroscope Objektiv Epiplan-Neofluar lenses and with AxioCam MRc. Polarized optical microscope images were taken with Leica DM4500 P, equipped with N PLAN EPI POL lenses.

Powder X-Ray diffraction

The XRD patterns were obtained in Bragg-Brentano geometry, over the 2θ range of 3-40°, with a step size of 0.01° and a speed of 10.0°/min, using a Rigaku MiniFlex 600 diffractometer with Cu Kα radiation generated from a copper sealed tube with 40 kV and 15 mA.

Fabry-Perot cavity fabrication

CaF2 windows and demountable microfluidic cells were purchased from Specac. The CaF2 windows were sputtered with 10 nm of Au using Emitech K575X metal sputterer, followed by spin-coating a 100 nm thick insulating layer with a solution of PVA in water (4% w/w) at 4000 rpm. The Au mirrors were separated by a 25 μm Mylar spacer (Specac) and assembled into the microfluidic cell. The temperature of the microfluidic cell was controlled by using a heating jacket (Specac). All spectroscopic data were acquired via a tuned area of the cavity using a 2 mm diameter aperture. For non-cavity experiments, CaF2 windows were prepared by spin-coating PVA onto them.

Fourier transformed infrared spectroscopy

The FT-IR spectroscopy data were recorded with a Bruker Vertex 70 FT-IR spectrometer in transmission mode. The spectra were acquired with 2 cm -1 resolution and averaged over at least 38 scans.

Quantum calculations

Transfer integrals

The optimization of geometry, computation of electronic structure, and estimation of reorganization energies of neutral isomers were carried out using the B3LYP functional and a 6-31G(d,p) basis set at the Density Functional Theory (DFT) level with the Gaussian 16-A03 package. [START_REF] Martinelli | Influence of Structural Dynamics on Polarization Energies in Anthracene Single Crystals[END_REF] The transfer integrals between closely situated neighbours in the frozen crystal structure were calculated using the reported experimental crystal structures as input. To estimate these parameters at the DFT B3LYP/DZ level of theory within a fragment orbital approach, the ADF package was utilized. [START_REF] Velde | Chemistry with ADF[END_REF]211] In practice, a 3×3×3 supercell was created for each molecule and electronic couplings were computed among all pairs of close neighbours, which included molecules in the unit cell at the center of the system. Any molecule B for which at least one atom was within a 5 Å range of any atom of molecule A was considered a close neighbour to a given molecule A.

Reorganization energies

The nuclear relaxation energy associated with the transition from neutral to charged geometry was evaluated using a displaced harmonic oscillator model. The individual contributions of each intramolecular vibrational mode were determined, while neglecting Duchinsky rotation effects [START_REF] Coropceanu | Charge Transport in Organic Semiconductors[END_REF] :

𝐸 𝑟𝑒𝑙 = ∑ 𝑔 2 (𝑖) ℏ𝜔 𝑖 = ∑ 𝑆 𝑖 ℏ𝜔 𝑖 𝑖 𝑖 (5.1)
with the index i running over all intramolecular vibrational modes of energy ħi. g(i) is the local electron-phonon coupling constant associated to the A.2 Appendix referred to Chapter 3: "Elucidating the influence of the molecular structure of thienoacenes organic semiconductors on contact resistance".

Chapter 6 Conclusions

New classes of thienoacenes OSCs were processed and successfully integrated in TFTs, allowing the determination of their electrical performances. The charge transport and charge injection properties were analysed in relation to the crystalline and molecular structures of the OSCs. Additionally, a novel approach to influencing the crystal growth of OSCs through vacuum field coupling was explored.

In Chapter 2, thienoacenes with various molecular cores and substitutions were used as active materials in TFTs. The structure-properties relations for high-performance OSCs were established, highlighting the importance of factors such as the herringbone packing motif, layer-by-layer growth of the thin films, extended π-system, ionization energy close to the electrodes' work function, large and balanced transfer integrals, and low reorganization energy. A novel class of thienoacenes was presented, with DN4T, C8-DN4T, and C10-DN4T showing charge carrier mobility up to 2.1 cm 2 V -1 s -1 for the parent molecule and up to 2.5 cm 2 V -1 s -1 for the alkylated derivatives. Furthermore, the electrical performance of four newly synthesized thienoacenes with tert-butyl substitutions, which have an extended π-system compared to tBu-BTBT, were evaluated. The lower ionization energy observed in these compounds was attributed to their extended charge delocalization, which resulted in a lower threshold voltage and smaller contact resistance in TFTs compared to those based on tBu-BTBT. Among the compounds, tBu-DNTT was identified as a high-performance OSC, with mobility exceeding 1.9 cm 2 V -1 s -1 . In Chapter 2, the contact resistance of the analysed compounds was also investigated, highlighting a significant discrepancy in RCW values between TC devices based on DN4T and isoDN4T. This was unexpected due to the almost identical injection barrier of the two isomers.

Driven by this finding, in Chapter 3, the impact of the molecular structure of DNTT and DBTTT families on the contact resistance of organic TFTs was explored, with a focus on the device geometry, injection barrier at the metal/OSC interface, and out-of-plane and in-plane charge transport properties. Our findings indicate that in BC devices the molecular structure do not significantly influence contact resistance. Efficient charge carrier injection is achieved when the injection barrier at the contact/OSC interface is minimized and the OSC exhibits high in-plane charge carrier mobility, thus reducing the resistance opposed to charge transport at the depleted region formed at the contact/OSC interface. Among the analysed compounds, DNTT and C8-DNTT exhibited the lowest contact resistance (0.8 kΩcm) due to their good balance between low injection barrier (≈ 0.5 eV) and high in-plane mobility (2.3 and 4.8 cm 2 V -1 s -1 , respectively). In TC TFTs, where charges must travel through the OSC thickness to achieve the accumulation channel, it was found that the resistance associated with bulk charge transport is not a dominant factor for determining the overall contact resistance. Instead, the primary factors determining the contact resistance are the resistance opposing charge transport along the OSC/dielectric interface beneath the contacts (which is influenced by in-plane mobility and charge trap density) and the injection barrier at the metal/OSC interface. Therefore, in TC TFTs, if the injection barrier is reduced and in-plane charge transport is efficient, a larger distance between the molecular cores due to the introduction of substituents does not necessarily lead to high contact resistance. At last, the study determined that BC TFTs consistently exhibit lower contact resistance than TC TFTs for all the compounds analysed. BC TFTs showed a modest reduction in contact resistance for the parent compounds and alkylated derivatives, with RCW between 1.3 and 2.9 times lower compared to TC TFTs. However, BC TFTs based on tBu-DNTT showed up to 7 times lower RCW, and BC TFTs based on tBu-DBTTT showed up to 35 times lower RCW compared to the TC counterparts. This underlines the advantages of using a coplanar device geometry, which improves electrical performance and charge injection properties.

In the Chapter 4, we explored the effect of light-matter coupling on perylene crystal growth and polymorphism. Recrystallization experiments were carried out in an FP cavity under two different conditions: cooperative VSC of the aromatic C=C stretching of benzonitrile that is resonant with perylene vibrational band, and VSC of the aromatic C=C stretching solvent vibrational band that is not resonant with perylene vibrational modes. The results indicated a slightly higher ratio of the α-form to the β-form compared to the non-cavity condition. However, the FSR varied during the experiment due to the temperature changes in the microfluidic cell, potentially resulting in an unstable system and reducing the reproducibility of the results. The study showed that the experimental setup using an FP cavity was limited by temperature changes in the microfluidic cell. Thus, to fully explore the potential of VSC on crystal growth and avoid FSR changes, alternative crystallization technique that can be conducted at room temperature (such as antisolvent crystallization) should be considered for future experiments.

In conclusion, this thesis sheds light on the molecular design of thienoacenes OSCs, while offering new perspectives on contact resistance of organic thin-film transistors and the manipulation of OSC crystallization. The potential of organic semiconductors in logic operators hinges on their ability to overcome the dynamic and static disorder that frequently hinders their electrical performance, making them less efficient than their inorganic counterparts. The various experimental chapters presented in this thesis are aimed at addressing significant challenges in organic electronics. To achieve efficient charge transport, a rational molecular design is essential. Therefore, it is crucial to establish a correlation between the chemical and crystalline structure of OSCs and their electrical properties, which would be beneficial for the future growth of this field.

Emerging fields such as coupling with the vacuum field, chiral induced spin selectivity, and bioelectronics can benefit from the wealth of knowledge accumulated by the scientific community in the past decades regarding OSCs for logic operators. The ability to fine-tune molecular properties through synthetic design is still crucial for creating custom OSCs for specific applications. In this framework, this thesis contributes to further understanding of structure-properties relations, minimization of contact resistance, and exploration of new methods for manipulating OSC crystallization, providing valuable insights for scientists developing novel OSCs toward the progress of organic electronics.

Résume

Introduction

Le domaine des semi-conducteurs organiques (Organic SemiConductors -OSC) a été largement étudié et développé au cours des dernières décennies, en raison de ses potentielles applications technologiques en électronique imprimée. [START_REF] Wang | Organic crystalline materials in flexible electronics[END_REF] L'origine des propriétés semi-conductrices de ces matériaux provient de leur conjugaison moléculaire par le biais d'orbitales hybrides sp 2 , formant à la fois des liaisons σ-et π. Alors que dans les premières, les électrons sont localisés ; les électrons sont délocalisés dans les secondes, permettant des voies de conduction aux charges électriques. À l'état solide, les molécules d'OSC sont maintenues ensemble par des interactions intermoléculaires faibles de type Van der Waals (VdW), de sorte que le recouvrement des orbitales moléculaires de molécules voisines permet le transport de charges dans le cristal (ou le film polycristallin). Par conséquent, les propriétés électriques des OSC dépendent fortement de l'organisation supramoléculaire à l'état solide. Les OSC sont notamment employés dans les transistors à effet de champ (Field-Effect Transistors -FET), qui sont des dispositifs à trois électrodes où le flux de courant est modulé par un champ électrique. Les FET sont largement utilisés dans l'industrie électronique pour fabriquer des circuits intégrés, à la base des appareils et des dispositifs de l'électronique moderne. A l'heure actuelle, la technologie des FET de l'industrie électronique est principalement basée sur des semi-conducteurs inorganiques en raison de leurs performances électriques supérieures. Les OSC offrent toutefois une technologie alternative de semi-conducteurs présentant des avantages tels que la flexibilité, [START_REF] Zschieschang | Organic transistors on paper: a brief review[END_REF] une mise en oeuvre à températures plus faibles que celles requises par leurs homologues inorganiques, [START_REF] Ward | Versatile Organic Transistors by Solution Processing[END_REF] le dépôt en solution via des solvants organiques classiques offrant la possibilité d'applications rentables sur de grandes surfaces [START_REF] Ward | Versatile Organic Transistors by Solution Processing[END_REF] et une conception moléculaire sur mesure (chimie de synthèse). [START_REF] Mei | Integrated Materials Design of Organic Semiconductors for Field-Effect Transistors[END_REF] Cependant, la mobilité des porteurs de charge dans les OSC est entravée par un désordre dynamique empêchant une délocalisation complète de la fonction d'onde des charges. Une conception rationnelle des OSC est donc nécessaire pour assurer un arrangement bien défini des molécules au sein du cristal et d'ainsi permettre de maximiser le recouvrement des orbitales moléculaires de molécules voisines (larges intégrales de transfert) tout en minimisant les réarrangements géométriques de molécules impliquées dans (faible énergie de réorganisation) le transport de charge. [START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF] L'objectif de cette thèse est d'étudier les relations structure-propriétés de semi-conducteurs organiques basés sur le thiénoacène afin d'esquisser des lignes directrices pour améliorer leur conception moléculaire. Le couplage lumière-matière pour influencer la croissance cristalline et le polymorphisme des OSC a également été exploré.

Résultats et discussion

Cette thèse de doctorat commence par une brève introduction sur les semiconducteurs organiques, mettant l'accent sur les aspects structurels, les transistors à effet de champ organiques et l'effet du couplage lumière-matière sur le transport de charge dans les OSC. Trois chapitres se rapportant au travail expérimental effectué pendant le doctorat sont ensuite présentés. Le résumé de chaque chapitre est présenté ci-dessous.

• Evaluation des propriétés de transport et d'injection de charges de nouveaux dérivés de thiénoacènes dans des transistors en films minces

Les propriétés de transport de charge de deux nouveaux semi-conducteurs isomères à base de thiénoacène, DN4T et isoDN4T, sont présentées (Figure 1). L'objectif est de comprendre si l'intégration d'un motif central de quatre cycles thiényles fusionnés entre deux fragments naphtyles conduit à une mobilité élevée des porteurs de charge dans les transistors à couche mince (Thin-Film Tranistors, TFT), tout en préservant un arrangement « herringbone » des molécules à l'état cristallin. Le désordre dynamique, résultant des modes de vibration moléculaire, limite fortement l'obtention d'une mobilité élevée dans les semi-conducteurs organiques. Dans les OSC présentant une structure « herringbone », les modes de vibration ayant un mouvement de translation des molécules le long de leur axe long, sont majoritairement responsables des variations du recouvrement orbitalaire et donc de la réduction de mobilité. [START_REF] Illig | Reducing dynamic disorder in small-molecule organic semiconductors by suppressing large-amplitude thermal motions[END_REF][START_REF] Schweicher | Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors[END_REF] Toutefois, ces modes de vibration nuisibles peuvent être mitigés dans les OSC présentant des structures moléculaires qui s'écartent de la linéarité. [START_REF] Okamoto | Bent-Shaped p -Type Small-Molecule Organic Semiconductors: A Molecular Design Strategy for Next-Generation Practical Applications[END_REF] Dans le cas du DN4T et de l'isoDN4T, l'intégration de quatre cycles thiényles fusionnés entre deux fragments naphtyles accentue la non-linéarité de la forme moléculaire par rapport au DNTT (Figure R.1). Ce dernier a été choisi comme référence pour les études de physique des dispositifs électroniques en raison de sa grande mobilité des porteurs de charge (> 2 cm 2 V -1 s -1 ). [START_REF] Zschieschang | Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) thin-film transistors with improved performance and stability[END_REF] Dans cette optique, les performances électriques du DN4T et de l'isoDN4T sont étudiées dans des transistors à couche mince et comparées à celles des OSC les plus performants du domaine. Les TFT sont fabriqués par dépôt en phase vapeur sous vide, en optimisant la température du substrat et la géométrie du dispositif. Les meilleures performances électriques sont obtenues pour les dispositifs qui présentent une architecture « bottom contact » (BC) des électrodes des transistors et une température de substrat de 100 °C durant l'évaporation. Plus précisément, les TFT à base de DN4T présentent une mobilité des porteurs de charge allant jusqu'à 2.1 cm 2 V -1 s -1 alors que ceux basés sur l'isoDN4T présentent seulement une mobilité des porteurs de charge 3 ordres de grandeur plus faible jusqu'à 0.003 cm 2 V - Ces derniers sont supposés avoir une mobilité des porteurs de charge plus élevée par rapport au coeur aromatique de base, le DN4T. En effet, l'introduction de chaînes alkyles latérales permet de réduire l'impact de certains modes de vibration. [START_REF] Illig | Reducing dynamic disorder in small-molecule organic semiconductors by suppressing large-amplitude thermal motions[END_REF][START_REF] Schweicher | Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors[END_REF] Les TFT basés sur C8-DN4T et C10-DN4T présentent une mobilité allant jusqu'à 2.4 cm 2 V -1 s -1 et 2.5 cm 2 V -1 s -1 respectivement, légèrement supérieures à celle du DN4T. L'obtention de meilleures performances est entravée par la mauvaise morphologie des films minces, caractérisée par un mode de croissance 3D. Ce mode de croissance s'est avéré être une raison du mauvais transport de charge en raison de la formation de nombreux joints de grains agissant comme des pièges énergétiques profonds. [START_REF] Virkar | The Role of OTS Density on Pentacene and C 60 Nucleation, Thin Film Growth, and Transistor Performance[END_REF] Enfin, les performances électriques ainsi que l'injection de charges dans les TFTs basés sur des dérivés de thiénoacènes substitués par des groupes périphériques stériques sont étudiées. Les composés étudiés sont des dérivés tert-butyl de DNTT et DBTTT, à savoir tBu-DNTTT et tBu-DBTTT, et des dérivés tert-butyl-thiényl de BTBT, à savoir tBu--BTBT-1 et tBu-BTBT-2 (Figure R.2). Ces composés ont été sélectionnés en raison de leur coeur aromatique π étendu par rapport au tBu-BTBT. En effet, ce dernier a montré une mobilité élevée des porteurs de charge dans les TFT (jusqu'à 2.4 cm 2 V -1 s - 1 ), qui a été attribuée au transport de charge presque isotrope au sein du plan des couches « herringbone ». [START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF] Malgré sa mobilité intrinsèque élevée, le tBu-BTBT présente une énergie d'ionisation profonde (5.7 eV) qui défavorise l'injection de porteurs de charge à partir de contacts métalliques en or dans les transistors. Cela se traduit par une tension de seuil élevée et des nonidéalités des dispositifs électroniques fabriqués à partir de ce dernier. Le coeur aromatique π étendu des composés étudiés dans ce chapitre devrait réduire l'énergie d'ionisation, en raison d'une meilleure délocalisation des porteurs de charge. [START_REF] Takimiya | Design and elaboration of organic molecules for high field-effect-mobility semiconductors[END_REF] Ceci est confirmé par mesures spectroscopiques (spectroscopie de photoémission -photoelectron spectroscopy in air) réalisées sur films minces des composés étudiés, mettant en évidence des énergies d'ionisation comprises entre 5.23 et 5.37 eV. Ces valeurs se traduisent par une tension de seuil modérée dans les TFT, ce qui garantit des dispositifs plus fiables et idéaux. Le tBu-DNTT présente la plus grande mobilité parmi les composés étudiés, avec des valeurs dépassant 1.9 cm 2 V -1 s -1 , suivi par tBu-DBTTT (1.0 cm 2 V -1 s -1 ), tBu-BTBT-1 (0.25 cm 2 V -1 s -1 ) et enfin tBu-BTBT-2 (0.10 cm 2 V -1 s -1 ). Les calculs de chimie quantique confirment les meilleures propriétés de transport de charge des dérivés tert-butyl de DNTT et DBTTT, présentant des intégrales de transfert plus grandes ainsi que des énergies de réorganisation plus faibles par rapport aux dérivés tert-butyl-thiényl de BTBT. Additionnellement, les simulations de Monte Carlo mettent en évidence une anisotropie du transport de charge pour tous les composés, à l'exception de tBu-BTBT-1. En conclusion, l'élargissement du coeur aromatique π assure une injection plus facile des porteurs de charge dans les TFT par rapport à ceux basés sur le tBu-BTBT. Plus particulièrement, le tBu-DNTT se révèle être un OSC de haute performance, avec une mobilité dépassant 1.9 cm 2 V -1 s -1 . • Résistance de contact dans les transistors en couches minces à base de dérivés de thiénoacène

Ces dernières années, la résistance de contact a fait l'objet d'un intérêt accru dans le domaine de la recherche sur les OSC. En effet, les performances des dispositifs sont souvent limitées par l'injection de porteurs de charge plutôt que par le transport de charge le long de la couche active. [START_REF] Waldrip | Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier[END_REF] Une résistance de contact élevée peut conduire à une nonidéalité des dispositifs et ainsi affecter l'extraction des paramètres, conduisant à des sur-(ou sous-)estimations des performances électriques. Cela peut ainsi mener à une interprétation incorrecte des relations structure-propriétés, qui à son tour affecte la conception moléculaire de futurs OSC. Il est également à noter qu'une résistance de contact élevée limite la fréquence de transit dans les TFT organiques, en particulier pour les dispositifs miniaturisés dont la longueur de canal est inférieure à quelques dizaines de micromètres. [START_REF] Zschieschang | Roadmap to Gigahertz Organic Transistors[END_REF] Ce chapitre se concentre sur le rôle de la structure moléculaire des OSC et en particulier sur l'impact des substituants du coeur moléculaire sur la résistance de contact dans des dispositifs à géométrie top contact (TC) et bottom contact (BC) afin de les comparer. En effet, cet aspect a été peu étudié par rapport aux autres La fabrication de TFT basés sur ces composés, permet l'extraction de la résistance de contact normalisée en largeur (RcW). Nous avons observé que les dispositifs présentant une géométrie coplanaire (BC) ne sont pas significativement affectés par les substituants du coeur moléculaire, montrant pour tous les composés des RcW inférieures à 5 kΩcm. Au contraire, dans les dispositifs TC, les dérivés tert-butyle présentent des RcW supérieures de 2 ordres de grandeur par rapport aux molécules de base et à leurs dérivés alkylés. Cet écart important de RcW est attendu entre les molécules de base et les dérivés tert-butyle, car la présence de substituants sur les coeurs moléculaires augmente la distance entre les coeurs moléculaires selon l'axe long des molécules, ce qui entrave la mobilité hors du plan. Par conséquent, même dans le cas des dérivés alkylés, une faible mobilité hors du plan est attendue en raison du faible recouvrement orbitalaire des molécules voisines (selon l'axe z) causé par l'extension spatiale des chaînes alkyles. En revanche, les dérivés alkylés présentent une faible résistance de contact, similaire à celle des molécules de base. La combinaison des résultats expérimentaux obtenus par plusieurs techniques (microscopie à force atomique pour la conductivité électrique, spectrométrie photoélectronique UV, diffraction des Rayons-X, et études physiques de dispositifs électroniques) nous a permis de mettre en évidence que la plus faible résistance de contact observée pour les composés alkylés (comparé à leurs homologues tert-butyl) pouvait être le résultat de leur plus grande mobilité des porteurs de charge "dans le plan" dans la région sous les contacts. La densité de porteurs de charge plus élevée compense la faible mobilité hors du plan, ce qui conduit à une faible résistance de contact dans les dispositifs TC pour les dérivés alkylés. Ce travail met en évidence la contribution mutuelle de la mobilité hors du plan (qui dépend fortement de la structure moléculaire de l'OSC) et de la densité de porteurs de charge dans la détermination de la résistance de contact dans les TFT à configuration TC.

• Exploration de l'effet du couplage fort vibratoire sur la cristallisation du pérylène dans une cavité Fabry-Perot

Le couplage des vibrations moléculaires ou des transitions électroniques aux niveaux énergétiques du champ de vide des cavités optiques s'est avéré capable de modifier de multiples propriétés de molécules et matériaux. On peut citer par exemple la modification de la réactivité de réactions organiques [START_REF] Thomas | Tilting a ground-state reactivity landscape by vibrational strong coupling[END_REF] ainsi que l'augmentation du courant électrique dans des semiconducteurs organiques [START_REF] Orgiu | Conductivity in organic semiconductors hybridized with the vacuum field[END_REF] . L'effet sur la réactivité chimique a été attribué à un remodelage du potentiel de Morse de la liaison couplée modifiée par le couplage fort vibrationnel (Vibrational Strong Coupling, VSC), prédisant un raccourcissement ou un renforcement de la liaison couplée et donc un changement de son énergie. Par conséquent, si l'énergie et la force d'une liaison impliquée dans des interactions intermoléculaires sont modifiées par le couplage avec le champ du vide, le VSC peut influencer la germination des cristaux ainsi que leur croissance. Comme premiers exemples, la modification de l'agrégation supramoléculaire d'un polymère conjugué [START_REF] Joseph | Supramolecular Assembly of Conjugated Polymers under Vibrational Strong Coupling[END_REF] et l'influence sur le pseudo-polymorphisme des structures imidazolate de zéolite [START_REF] Hirai | Selective crystallization via vibrational strong coupling[END_REF] confirment l'influence des VSC dans la chimie supramoléculaire. De plus, l'arrangement cristallin et le polymorphisme ont fortement affecté la mobilité des porteurs de charge dans les OSC. Le développement de méthodes pour induire une cristallisation sélective, empêchant la croissance concomitante de plusieurs polymorphes peut donc offrir de nouvelles opportunités pour maximiser la mobilité des porteurs de charge dans les OSC.

Conclusions

Les relations entre la structure moléculaire, l'arrangement cristallin à l'état solide et les propriétés électriques des OSC sont explorées au cours de ce travail. Grâce à la fabrication de transistors en couches minces, aux calculs de chimie quantique et à une approche multi-technique, des relations structure-propriétés sont mises en évidence. Une attention particulière est accordée à l'effet de la structure moléculaire et cristalline sur les propriétés de transport de charge, l'injection de charges et la résistance de contact. Enfin, une approche innovante pour influencer la croissance cristalline et le polymorphisme des OSC est présentée. Cette dernière met en évidence les limites de l'expérience de recristallisation dans une cavité Fabry-Perot et expose des lignes directrices pour de futurs montages expérimentaux. En conclusion, cette thèse tente de répondre à certains défis dans le développement de nouveaux OSC à haute performance, tout en apportant de nouvelles perspectives sur la conception moléculaire, leur utilisation dans les transistors à couche mince et une manière innovante d'influencer la croissance cristalline.

Federico MODESTI

Relations structurespropriétés de semiconducteurs organiques thiénoacènes

Résumé

Le domaine de recherche des semi-conducteurs organiques a gagné en importance grâce à leur potentiel d'applications dans l'électronique imprimée. Cependant, les faibles interactions intermoléculaires provoquent un désordre statique et dynamique dans les transistors à film mince organiques, limitant leurs performances. Cette thèse étudie les relations structure-propriétés des semiconducteurs organiques de petites molécules pour fournir des lignes directrices de conception pour des transistors à film mince organiques performants. L'étude établit l'impact de la morphologie du film mince, de l'arrangement à l'état cristallin et de l'injection de charge sur les performances électriques des thiénocènes. Elle souligne également l'importance de maximiser la mobilité du porteur de charge dans le plan du transport de charge pour minimiser la résistance de contact dans les transistors à film mince. Enfin, ce travail explore l'effet du couplage lumièrematière sur la croissance cristalline et le polymorphisme des cristaux de perylène, en fournissant des orientations pour de futures expériences.

Résumé en anglais

The research field of organic semiconductors has gained traction due to their potential applications in printed electronics. However, weak intermolecular interactions cause static and dynamic disorder in organic thin-film transistors, limiting their performance. This thesis investigates the structure-properties relations of small-molecule organic semiconductors to provide design guidelines for high-performance organic thin-film transistors. The study establishes the impact of thin film morphology, crystalline packing, and charge injection on the electrical performance of thienoacenes. It also highlights the significance of maximizing in-plane charge carrier mobility to minimize contact resistance in thinfilm transistors. Finally, this work explores the effect of light-matter coupling on perylene crystal growth and polymorphism, providing guidance for future experimental setups.

  Résume ......................................................................................................... References .................................................................................................... List of Publications ......................................................................................

Figure 1 .

 1 Figure 1.1: Charge delocalization in π-molecular orbitals of benzene. .......... 4 Figure 1.2: Typical crystalline packing motifs of molecular OSCs. ................ 9 Figure 1.3: Organic semiconductor and electrode energy band diagrams. .. 11 Figure 1.4: Illustration of charge transport model mechanisms. ................. 16 Figure 1.5: Schematic illustration of the four different OFET structures. .. 17Figure 1.6: Idealized representation of operating principle and energy band diagram of a p-type OFET. ............................................................................. 19 Figure 1.7: Charge carrier concentration profile of OFETs. ......................... 21 Figure 1.8: Transfer and output characteristics of a p-type OFET. ............. 21 Figure 1.9: Contact resistance in OFET. ....................................................... 25 Figure 1.10: Contact resistance in OFETs with staggered and coplanar geometries. ...................................................................................................... 27 Figure 1.11: Current crowding in staggered OFET....................................... 28 Figure 1.12: Contact resistance extraction by using the transmission line method. ............................................................................................................ 30 Figure 1.13: Organic semiconductor-contact interface. ................................. 32 Figure 1.14: Schematic illustration of two coupled harmonic oscillators. .... 34 Figure 1.15: Schematic energy diagrams of light-matter interaction. ......... 36 Figure 2.1: Charge transport properties of tBu-BTBT. ................................. 43 Figure 2.2: Crystalline packing. ..................................................................... 45 Figure 2.3: Contact angle of water droplets on bare Al2O3 and on TDPA/Al2O3. ......................................................................................................................... 49 Figure 2.4: Illustration of devices geometry. ................................................. 50 Figure 2.5: AFM topography images of thin films deposited on TDPA/Al2O3 at different substrate temperatures. .................................................................. 52 Figure 2.6: AFM topography images of C8-DN4T and C10-DN4T thin films deposited on TDPA/Al2O3. .............................................................................. 54 Figure 2.7: AFM topography images of DN4T and isoDN4T thin films deposited on PFBT/Au. ................................................................................... 55 Figure 2.8: XRD patterns of thin films. ......................................................... 57 Figure 2.9: PYS measurements performed on thin films of thienoacene derivatives. ...................................................................................................... 59 Figure 2.10: Mobility vs substrate temperature for DN4T and isoDN4T. ... 62 Figure 2.11: Threshold voltage and on/off current ratio vs substrate temperature for DN4T and isoDN4T. ............................................................ 63 Figure 2.12: Transfer and output characteristics of DN4T and isoDN4T topcontact devices. ............................................................................................... 64 Figure 2.13: Mobility and on/off current ratio vs substrate temperature for BC and TC TFTs based on DN4T and isoDN4T. ........................................... 65 Figure 2.14: Threshold voltage vs substrate temperature for BC and TC TFTs based on DN4T and isoDN4T. ........................................................................ 66

1 : 4 Figure 1 . 2 : 9 Figure 1 . 3 : 11 Figure 1 . 4 : 16 Figure 1 . 5 :

 141291311141615 Figure 1.1: Charge delocalization in π-molecular orbitals of benzene. .......... 4 Figure 1.2: Typical crystalline packing motifs of molecular OSCs. ................ 9 Figure 1.3: Organic semiconductor and electrode energy band diagrams. .. 11 Figure 1.4: Illustration of charge transport model mechanisms. ................. 16 Figure 1.5: Schematic illustration of the four different OFET structures. .. 17Figure 1.6: Idealized representation of operating principle and energy band diagram of a p-type OFET. ............................................................................. 19 Figure 1.7: Charge carrier concentration profile of OFETs. ......................... 21 Figure 1.8: Transfer and output characteristics of a p-type OFET. ............. 21 Figure 1.9: Contact resistance in OFET. ....................................................... 25 Figure 1.10: Contact resistance in OFETs with staggered and coplanar geometries. ...................................................................................................... 27 Figure 1.11: Current crowding in staggered OFET....................................... 28 Figure 1.12: Contact resistance extraction by using the transmission line method. ............................................................................................................ 30 Figure 1.13: Organic semiconductor-contact interface. ................................. 32 Figure 1.14: Schematic illustration of two coupled harmonic oscillators. .... 34 Figure 1.15: Schematic energy diagrams of light-matter interaction. ......... 36 Figure 2.1: Charge transport properties of tBu-BTBT. ................................. 43 Figure 2.2: Crystalline packing. ..................................................................... 45 Figure 2.3: Contact angle of water droplets on bare Al2O3 and on TDPA/Al2O3. ......................................................................................................................... 49 Figure 2.4: Illustration of devices geometry. ................................................. 50 Figure 2.5: AFM topography images of thin films deposited on TDPA/Al2O3 at different substrate temperatures. .................................................................. 52 Figure 2.6: AFM topography images of C8-DN4T and C10-DN4T thin films deposited on TDPA/Al2O3. .............................................................................. 54 Figure 2.7: AFM topography images of DN4T and isoDN4T thin films deposited on PFBT/Au. ................................................................................... 55 Figure 2.8: XRD patterns of thin films. ......................................................... 57 Figure 2.9: PYS measurements performed on thin films of thienoacene derivatives. ...................................................................................................... 59 Figure 2.10: Mobility vs substrate temperature for DN4T and isoDN4T. ... 62 Figure 2.11: Threshold voltage and on/off current ratio vs substrate temperature for DN4T and isoDN4T. ............................................................ 63 Figure 2.12: Transfer and output characteristics of DN4T and isoDN4T topcontact devices. ............................................................................................... 64 Figure 2.13: Mobility and on/off current ratio vs substrate temperature for BC and TC TFTs based on DN4T and isoDN4T. ........................................... 65 Figure 2.14: Threshold voltage vs substrate temperature for BC and TC TFTs based on DN4T and isoDN4T. ........................................................................ 66

Figure 1 . 6 : 19 Figure 1 . 7 : 21 Figure 1 . 8 : 21 Figure 1 . 9 : 30 Figure 1 . 13 : 32 Figure 1 . 14 : 34 Figure 1 . 15 : 36 Figure 2 . 1 : 43 Figure 2 . 2 : 45 Figure 2 . 3 : 49 Figure 2 . 4 : 50 Figure 2 . 5 : 55 Figure 2 . 8 : 57 Figure 2 . 9 : 59 Figure 2 . 10 : 62 Figure 2 . 11 : 63 Figure 2 . 12 :

 161917211821193011332114341153621432245234924502555285729592106221163212 Figure 1.1: Charge delocalization in π-molecular orbitals of benzene. .......... 4 Figure 1.2: Typical crystalline packing motifs of molecular OSCs. ................ 9 Figure 1.3: Organic semiconductor and electrode energy band diagrams. .. 11 Figure 1.4: Illustration of charge transport model mechanisms. ................. 16 Figure 1.5: Schematic illustration of the four different OFET structures. .. 17Figure 1.6: Idealized representation of operating principle and energy band diagram of a p-type OFET. ............................................................................. 19 Figure 1.7: Charge carrier concentration profile of OFETs. ......................... 21 Figure 1.8: Transfer and output characteristics of a p-type OFET. ............. 21 Figure 1.9: Contact resistance in OFET. ....................................................... 25 Figure 1.10: Contact resistance in OFETs with staggered and coplanar geometries. ...................................................................................................... 27 Figure 1.11: Current crowding in staggered OFET....................................... 28 Figure 1.12: Contact resistance extraction by using the transmission line method. ............................................................................................................ 30 Figure 1.13: Organic semiconductor-contact interface. ................................. 32 Figure 1.14: Schematic illustration of two coupled harmonic oscillators. .... 34 Figure 1.15: Schematic energy diagrams of light-matter interaction. ......... 36 Figure 2.1: Charge transport properties of tBu-BTBT. ................................. 43 Figure 2.2: Crystalline packing. ..................................................................... 45 Figure 2.3: Contact angle of water droplets on bare Al2O3 and on TDPA/Al2O3. ......................................................................................................................... 49 Figure 2.4: Illustration of devices geometry. ................................................. 50 Figure 2.5: AFM topography images of thin films deposited on TDPA/Al2O3 at different substrate temperatures. .................................................................. 52 Figure 2.6: AFM topography images of C8-DN4T and C10-DN4T thin films deposited on TDPA/Al2O3. .............................................................................. 54 Figure 2.7: AFM topography images of DN4T and isoDN4T thin films deposited on PFBT/Au. ................................................................................... 55 Figure 2.8: XRD patterns of thin films. ......................................................... 57 Figure 2.9: PYS measurements performed on thin films of thienoacene derivatives. ...................................................................................................... 59 Figure 2.10: Mobility vs substrate temperature for DN4T and isoDN4T. ... 62 Figure 2.11: Threshold voltage and on/off current ratio vs substrate temperature for DN4T and isoDN4T. ............................................................ 63 Figure 2.12: Transfer and output characteristics of DN4T and isoDN4T topcontact devices. ............................................................................................... 64 Figure 2.13: Mobility and on/off current ratio vs substrate temperature for BC and TC TFTs based on DN4T and isoDN4T. ........................................... 65 Figure 2.14: Threshold voltage vs substrate temperature for BC and TC TFTs based on DN4T and isoDN4T. ........................................................................ 66

Figure 2 . 15 : 67 Figure 2 . 16 : 69 Figure 2 . 17 : 70 Figure 2 . 18 : 71 Figure 2 . 19 : 74 Figure 2 . 21 : 84 Figure 2 . 25 : 88 Figure 3 . 1 : 95 Figure 3 . 2 : 98 Figure 3 . 3 : 101 Figure 3 . 4 : 104 Figure 3 . 5 : 105 Figure 3 . 6 : 109 Figure 3 . 9 : 110 Figure 3 . 10 : 111 Figure 3 . 11 : 113 Figure 3 . 12 : 114 Figure 3 . 13 : 117 Figure 3 . 15 : 119 Figure 3 . 17 : 122 Figure 3 . 19 : 125 Figure 3 . 20 : 130 Figure 3 . 22 : 149 Figure 4 . 3 : 150 Figure 4 . 4 : 152 Figure 4 . 5 : 153 Figure 4 . 6 : 155 Figure 4 . 7 :

 215672166921770218712197422184225883195329833101341043510536109391103101113111133121143131173151193171223191253201303221494315044152451534615547 Figure 2.15: Transfer and output characteristics of DN4T and isoDN4T bottom-contact devices. ................................................................................... 67 Figure 2.16: Mobility vs substrate temperature for C8-DN4T and C10-DN4T. ......................................................................................................................... 69 Figure 2.17: Transfer and output characteristics of C8-DN4T and C10-DN4T bottom-contact devices. ................................................................................... 70 Figure 2.18: Representative transfer and output characteristics of BC TFT based on tBu-BTBT......................................................................................... 71 Figure 2.19: Charge carrier mobility as a function of the substrate temperature for TC TFTs based on tert-butyl thienoacene derivatives. ...... 73 Figure 2.20: Transfer (left) and output (right) characteristics of the best performing BC TFTs based on tert-butyl-substituted thienoacene derivatives. ......................................................................................................................... 74 Figure 2.21: Transmission line method applied to TFTs. ............................. 75 Figure 2.22: Molecular arrangement with respect to a, b and c directions. . 79 Figure 2.23: Short contacts between adjacent molecules in the crystalline arrangement and calculated HOMO of DN4T and isoDN4T. ....................... 82 Figure 2.24: Thin-film growth mode. ............................................................. 84 Figure 2.25: Mobility anisotropy plots. .......................................................... 88 Figure 3.1: Crystalline packing of the parent and alkylated compounds. ... 95 Figure 3.2: AFM topography images of thin films. ........................................ 98 Figure 3.3: XRD patterns of OSCs deposited onto TDPA/Al2O3 and PFBT/Au substrates. ..................................................................................................... 101 Figure 3.4: Transfer characteristics of TC TFTs with 25-nm-thick OSC layer. ....................................................................................................................... 104 Figure 3.5: Transfer characteristics of BC TFTs with 25-nm-thick OSC layer. ....................................................................................................................... 105 Figure 3.6: Transmission line method applied to TC and BC TFTs with 25nm-thick OSC layer. ..................................................................................... 107 Figure 3.7: Width-normalized contact as a function of the gate-overdrive voltage of TC TFTs with 25-nm-thick OSC layer. ....................................... 108 Figure 3.8: Width-normalized contact as a function of the gate-overdrive voltage of BC TFTs with 25-nm-thick OSC layer. ....................................... 109 Figure 3.9: Width-normalized contact resistance of TC and BC TFTs. ...... 110 Figure 3.10: Schematic illustration of the in-plane and out-of-plane directions of charge transport in a staggered device. ................................................... 111 Figure 3.11: Transfer characteristics of TC TFTs with 80-nm-thick OSC layer. ....................................................................................................................... 113 Figure 3.12: Transfer characteristics of BC TFTs with 80-nm-thick OSC layer. ....................................................................................................................... 114 Figure 3.13: Transmission line method applied to TC and BC TFTs with 80nm-thick OSC layer. ..................................................................................... 116 Figure 3.14: Width-normalized contact resistance of TC TFTs with 80-nmthick OSC layer. ............................................................................................ 117

4 :

 4 Width-normalized contact resistance extracted from TC and BC TFTs. ............................................................................................................... 76 Table 2.5: Reorganization energy (λ), transfer integrals (JT) and charge carrier mobility (µ) of the investigated compounds. ...................................... 78 Table 2.6: Crystalline packing, ionization energy (IE), and mobility (µ) of DN4T, isoDN4T, DNTT, isoDNTT and L-DBTTA. ........................................ 80 Table 2.7: Ionization energy, threshold voltage and charge carrier mobility of C8-DN4T, C10-DN4T and C8-DNTT. ............................................................... 83 Table 2.8: Ionization energy (IE), threshold voltage (Vth) and mobility (µ) of tert-butyl substituted thienoacenes. .............................................................. 86 Table 3.1: Crystallographic data of DNTT, DBTTT, C8-DNTT and C12-DBTTT. ......................................................................................................................... 94 Table 3.2: Fabrication parameters used for TC and BC TFTs. ..................... 96 Table 3.3: Averaged grain size extracted from AFM images of thin films deposited onto TDPA/Al2O3 and PFBT/Au. ................................................. 100 Table 3.4: Comparisons of the d-spacing, terrace height steps and unit cell long axis......................................................................................................... 102 Table 3.5: Electrical performance of TC TFTs with 25-nm-thick OSC layer. ....................................................................................................................... 105 Table 3.6: Electrical performance of BC TFTs with 25-nm-thick OSC layer. ....................................................................................................................... 106 Table 3.7: Electrical performance of TC TFTs with 80-nm-thick OSC layer. ....................................................................................................................... 113 Table 3.8: Electrical performance of BC TFTs with 80-nm-thick OSC layer. ....................................................................................................................... 115 Table

  -nm-thick OSC layer. .................................................. 126 Table 3.11: Extracted values from UPS measurements. ............................ 134 Table

13 :

 13 Contact resistance, charge carrier mobility and hole injection barrier of BC TFTs. .......................................................................................

Figure 1 . 1 :

 11 Figure 1.1: Charge delocalization in π-molecular orbitals of benzene. (a) Molecular structure of benzene, (b) representation of the pz atomic orbitals, (c) diagram illustrating the energy-level splitting that occurs during bonding in a benzene molecule, along with the formation of HOMO and LUMO energy levels. (d)Shape of the HOMO and LUMO of benzene.[START_REF] Kim | Relief of excited-state antiaromaticity enables the smallest red emitter[END_REF]

  Scheme 1.1

Figure 1 . 2 :

 12 Figure 1.2: Typical crystalline packing motifs of molecular OSCs. (a) herringbone packing, (b) slipped herringbone packing, (c) slipped-stack packing and (d) brick-wall packing.

Figure 1 . 3 :

 13 Figure 1.3: Organic semiconductor and electrode energy band diagrams. Schematic illustration of (a) single crystal OSC where narrow conduction (in red) and valance (in blue) bands are formed, (b) disordered or amorphous OSC where distribution of states are formed and (c) metal electrode.

Figure 1 . 4 :

 14 Figure 1.4: Illustration of charge transport model mechanisms. The yellow areas exemplify the charge delocalization. The main parameters governing charge transport (JT, λ, Lν and νvib) for each transport model are shown.

Figure 1 . 5 :

 15 Figure 1.5: Schematic illustration of the four different OFET structures. Bottom-gate top-contact (BGTC), bottom-gate bottom-contact (BGBC), top-gate topcontact (TGTC) and top-gate bottom-contact (TGBC). The substrate is shown in white, the dielectric in light blue, the OSC in orange, the gate electrode in grey and the drain and source contacts in yellow.

Figure 1 . 6 :

 16 Figure 1.6: Idealized representation of operating principle and energy band diagram of a p-type OFET. (a) Schematic illustration of holes accumulation at the OSC/dielectric interface and transport when Vg < 0 V and Vd < 0 V. Idealized energy diagram for the case of (b) Vg = 0 V and Vd = 0 V, (c) Vg < 0 V and Vd = 0 V and (d) Vg < 0 V and Vd < 0 V.

Figure 1 . 7 :

 17 Figure 1.7: Charge carrier concentration profile of OFETs. (a) linear regime, (b) starting point of saturation regime and (c) full saturation regime.

Figure 1 . 8 :

 18 Figure 1.8: Transfer and output characteristics of a p-type OFET. (a) Transfer and (b) output characteristics. Depending on the values of Vg and Vd, the different regimes of operations are highlighted.

Figure 1 . 9 :

 19 Figure 1.9: Contact resistance in OFET. (a) Voltage drop profile of an ideal OEFT (with RC = 0), (b) voltage drop profile of a real OFET with voltage drops at the source and drain contacts (RC ≠ 0) and (c) schematic illustration of the resistance associated to the source contact (RS), to the channel (Rch) and to the drain contact (RD).

Figure 1 . 10 :

 110 Figure 1.10: Contact resistance in OFETs with staggered and coplanar geometries. (a) In device with staggered geometry RC is given by the contribution of Rint and Rbulk, while (b) in device with coplanar geometry Rbulk contribution is neglected. (c)(d) Directions of electric fields components (𝐸 ⟂ ,𝐸 // ), charge injection current (Jint) and channel current (Jch) in staggered and coplanar FETs. The schematic illustrations refer only to the source side of the OFETs.

Figure 1 . 11 :

 111 Figure 1.11: Current crowding in staggered OFET. Schematic illustration of charge injection in a staggered OFET, according to the current crowding effect. Charge injection occurs over a narrow area close to the edge of the contacts, defined by the transfer length LT. The blue region indicates the current density.

  𝑖 (𝑉 𝑔 -𝑉 𝑡ℎ ) (1.24) By combining equations 1.23 and 1.24, and normalizing by the channel width (W), the width-normalized contact resistance (RCW) is obtained: 𝑅𝑊 = 𝑅 𝑐 𝑊 + 𝐿 𝑊µ 0 𝐶 𝑖 (𝑉 𝑔 -𝑉 𝑡ℎ )

Figure 1 . 12 :

 112 Figure 1.12: Contact resistance extraction by using the transmission line method. (a) Transfer characteristics of TFTs with varying channel length, (b) linear fits to the total resistance vs channel length data points at varying gate-overdrive voltage, (c) width-normalize contact resistance and intrinsic mobility extracted from (b) as a function of the gate-overdrive voltage.

Figure 1 . 13 :

 113 Figure 1.13: Organic semiconductor-contact interface. Schematic energy levels diagram of the OSC/contact interface which accounts for the formation of an interfacial dipole (Δ), band bending and a region depleted of carriers in proximity of the interface (WD).

Figure 1 . 14 :

 114 Figure 1.14: Schematic illustration of two coupled harmonic oscillators.

Figure 1 . 15 :

 115 Figure 1.15: Schematic energy diagrams of light-matter interaction.Schematic illustration of (a) the electronic coupling between transition dipole moments resulting in the formation of J-and H-aggregates and (b) the analogy with the interaction between a molecular transition and an optical mode resulting in the formation of two new hybrid light-matter states (P + and P -).

Figure 2 . 1 :

 21 Figure 2.1: Charge transport properties of tBu-BTBT.(a) Transfer characteristics of (a) SCFET exhibiting µ of 7.1 cm 2 V -1 s -1[START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF] (b) TFT fabricated through solution shearing technique exhibiting µ of 2.4 cm 2 V -1 s -1[START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF] (c) TFT fabricated through solution shearing technique exhibiting µ of 1.9 cm 2 V -1 s -1 .[START_REF] Chung | Rotator side chains trigger cooperative transition for shape and function memory effect in organic semiconductors[END_REF] All the curves are collected in saturation regime (Vd = -100 V) and exhibit Vth of ≈ -70 V (over an applied Vg of -100 V for (a) and (c) and of -120 V for (b)). (d) Calculated field-effect anisotropy as a function of the crystallographic orientation (bc plane corresponds to the herringbone layer plane).[START_REF] Schweicher | Bulky End-Capped [1Benzothieno[3,2-b ]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF]

Figure 2 . 2 :

 22 Figure 2.2: Crystalline packing. (a) DN4T, (b) isoDN4T, (c) tBu-DNTT, (d) tBu-DBTTT, (e) tBu-BTBT-1 and (f) tBu-BTBT-2.

Figure 2 . 3 :

 23 Figure 2.3: Contact angle of water droplets on bare Al2O3 and on TDPA/Al2O3. On the left: 2 µm water droplet onto bare Al2O3 substrates, with contact angle of 65°. On the right: 2 µm water droplet onto TDPA/Al2O3 substrates, with contact angle of 110°.

Figure 2 . 4 .

 24 The fabricated TFTs have a channel length (L) of 65, 115, 165 and 215 μm and a channel width (W) of 480 μm. A more detailed procedure of TFTs fabrication is presented in Chapter 5.

Figure 2 . 4 :

 24 Figure 2.4: Illustration of devices geometry. (a) Schematic illustration of TC (on the left) and BC (on the right) TFTs, (b) optical micrographs of fabricated devices with channel length (L) of 65, 115, 165 and 215 μm and channel width (W) of 480 μm. Scale bar: 200 μm.

Figure 2 . 5 :

 25 Figure 2.5: AFM topography images of thin films deposited on TDPA/Al2O3 at different substrate temperatures. Nominal thickness of the films: 25 nm. Lateral scale bar size: 500 nm.

Figure 2 . 6 :

 26 Figure 2.6: AFM topography images of C8-DN4T and C10-DN4T thin films deposited on TDPA/Al2O3. Nominal thickness of the films: 25 nm. Lateral scale bar size: 1 μm. Root mean square roughness (RRMS) values are calculated on 1×1 μm 2 areas. Z-scales (Z) are expressed in nm for each temperature as follows: (a) C8-DN4T: 40 °C (Z = 95), 100 °C (Z = 175), 130 °C (Z = 175). (b) C10-DN4T: 40 °C (Z = 60), 100 °C (Z = 150), 130 °C (Z = 110).

Figure 2 . 7 :

 27 Figure 2.7: AFM topography images of DN4T and isoDN4T thin films deposited on PFBT/Au. Ts = 100 °C. Nominal thickness of the films: 25 nm. Lateral scale bar size: 500 nm. Z-scales (Z) are expressed in nm for each sample as follows: (a) DN4T on Au (Z = 80), DN4T on PFBT/Au (Z = 35), isoDN4T on Au (Z = 80), isoDN4T on PFBT/Au (Z = 40).

Figure 2 . 8 :

 28 Figure 2.8: XRD patterns of thin films. All the compounds were deposited onto TDPA/Al2O3 at substrate temperature of 100°C with a nominal thickness of 25 nm. On the right: illustration of the (100) and (200) planes, belonging to the family plane {100} of tBu-DNTT.

Figure 2 . 9 :

 29 Figure 2.9: PYS measurements performed on thin films of thienoacene derivatives. Thin films were obtained through vacuum deposition onto Al2O3/TDPA substrates at substrate temperature of 100°C.

Figure 2 . 10 :

 210 Figure 2.10: Mobility vs substrate temperature for DN4T and isoDN4T. Mobility as a function of the substrate temperature in (a) DN4T and (b) isoDN4T based TC TFTs. The values are referred to the linear regime and to devices with W/L = 480/215 µm.

Figure 2 . 11 :

 211 Figure 2.11: Threshold voltage and on/off current ratio vs substrate temperature for DN4T and isoDN4T. Threshold voltage and on/off current ratio as a function of the substrate temperature in (a),(c) DN4T and (b),(d) isoDN4T based TC TFTs. The values are referred to the linear regime and to devices with W/L = 480/215 µm. The dot lines indicate the averaged values of Vth and ION/OFF.

Figure 2 . 12 :

 212 Figure 2.12: Transfer and output characteristics of DN4T and isoDN4T topcontact devices. On the top: representative transfer (on the top) and output (on the bottom) characteristics of TC TFTs based on (a),(c) DN4T and (b),(d) isoDN4T at substrate temperature of 100 °C. In transfer characteristics, solid lines are referred to drain current and dot lines are referred to mobility. TFTs have W/L = 480/215 µm.

Figure 2 . 7 .

 27 This dissimilarity in morphology across the contact-to-channel interface may be responsible for the absence of field-effect response in bottom-gate bottom-contact TFTs fabricated with bare gold contacts.In case of BC TFTs fabricated with PFBT-treated gold contacts, OSCs were deposited at different Ts, from 40 to 140 °C with 20 °V steps. Both for DN4T and isoDN4T, charge carrier mobilities extracted from transfer characteristics follow the same trend observed in TC devices, along with similar values (Figure2.13).

Figure 2 . 13 :

 213 Figure 2.13: Mobility and on/off current ratio vs substrate temperature for BC and TC TFTs based on DN4T and isoDN4T. On the top: Mobility as function of the substrate temperature for BC and TC TFT based on (a) DN4T and (b) isoDN4T. On the bottom: on/off current ratio as function of the substrate temperature for BC and TC TFT based on (a) DN4T and (b) isoDN4T. The dot lines indicate the average values.

  .14). Particularly, for DN4T based TFTs, Vth was found to be up to 1.3 V (on average 1.2 V) lower in BC devices and for isoDN4T based TFTs, Vth was found to be up to 1.1 V (on average 0.8 V) lower in BC devices. This notable Vth decrease may be related to lower contact resistance observed in BC devices for both DN4T and isoDN4T based TFTs (see the Paragraph 2.5.4), which should result in a more facile charge carrier injection. Vth as function of the substrate temperature for BC and TC TFTs based on (a) DN4T and (b) isoDN4T. The dot lines indicate the average values.

Figure 2 . 14 :

 214 Figure 2.14: Threshold voltage vs substrate temperature for BC and TC TFTs based on DN4T and isoDN4T.

Figure 2 . 15 :

 215 Figure 2.15: Transfer and output characteristics of DN4T and isoDN4T bottom-contact devices. On the top: representative transfer (on the top) and output (on the bottom) characteristics of BC TFTs based on (a),(c) DN4T and (b),(d) isoDN4T at substrate temperature of 100 °C. In transfer characteristics, solid lines are referred to drain current and dot lines are referred to mobility. TFTs have W/L = 480/215 µm.

Figure A1. 2 ,

 2 in the Appendix). C8-DN4T based devices shows Vth between -0.2 and -0.4 V, along with ION/OFF of ≈ 8 × 10 4 / 2 × 10 5 , while C10-DN4T based devices shows Vth between -0.3 and -0.6 V, along with ION/OFF of ≈ 1 × 10 4 / 4 × 10 5 .

Figure 2 . 16 :

 216 Figure 2.16: Mobility vs substrate temperature for C8-DN4T and C10-DN4T. Mobility as a function of the substrate temperature in (a) C8-DN4T and (b) C10-DN4T based BC TFTs. The values are referred to the linear regime and to devices with W/L = 480/215 µm

Figure 2 . 17 :

 217 Figure 2.17: Transfer and output characteristics of C8-DN4T and C10-DN4T bottom-contact devices. On the top: representative transfer (on the top) and output (on the bottom) characteristics of BC TFTs based on (a),(c) C8-DN4T and (b),(d) C10-DN4T at substrate temperature of 100 °C. In transfer characteristics, solid lines are referred to drain current and dot lines are referred to mobility. TFTs have W/L = 480/215 µm.

  No field effect response was observed in TC devices at all substrate temperature as well as for BC devices with Ts of 70,100 and 130 °C. Drain current modulation upon the application of a gate bias was recorded for BC TFTs with Ts of 40 °C, allowing the collection of transfer and output characteristics (Figure 2.18).

Figure 2 . 18 :

 218 Figure 2.18: Representative transfer and output characteristics of BC TFT based on tBu-BTBT. (a) Transfer and (b) output characteristics. In transfer characteristics, solid lines and dashed lines are referred to drain current and mobility respectively. All TFTs have W/L = 480/215 μm.

Figure 2 . 19 : 2 . 2 . 20 ,

 2192220 Figure 2.19: Charge carrier mobility as a function of the substrate temperature for TC TFTs based on tert-butyl thienoacene derivatives. (a) tBu-DNTT, (b) tBu-DBTTT, (c) tBu-BTBT-1 and (d) tBu-BTBT-2.

Figure 2 . 20 :

 220 Figure 2.20: Transfer (left) and output (right) characteristics of the best performing BC TFTs based on tert-butyl-substituted thienoacene derivatives. (a, b) tBu-DNTT, (c, d) tBu-DBTTT, (e, f) tBu-BTBT-1 and (g, h) tBu-BTBT-2. In transfer characteristics, solid lines and dashed lines are referred to drain current and mobility respectively. All TFTs have W/L = 480/215 μm.

65 , 115 ,

 65115 165 and 215 µm (channel width of 480 µm) fabricated at Ts of 100 °C and for gate overdrive voltage of -2.0 V (Figure2.21). Measurements were conducted in linear regime (Vd = -0.1 V) to ensure a homogeneous charge carrier distribution in the channel.TFTs with a BC geometry exhibit RCW between 1.0 and 4.4 kΩcm, except for isoDN4T that shows RCW of 445 kΩcm (see Table2.4). Particularly, the lowest RCW is shown by the alkylated DN4Ts, with values of 1.0 kΩcm and 1.2 kΩcm for C10-DN4T and C8-DN4T respectively.

Figure 2 . 21 :

 221 Figure 2.21: Transmission line method applied to TFTs. Linear fits to the width-normalized total resistance (RW) as a function of the channel length (L) for TC and BC devices of (a) DN4T and (b) isoDN4T, BC devices of (c) alkylated DN4Ts and (d) tert-butyl substituted thienoacenes. The fits are referred to gate-overdrive voltage of -2.0 V. TFTs with channel length of 65, 115, 165 and 215 µm are fabricated on the same substrate hold at 100 °C during the OSCs deposition.

Figure 2 . 22 :

 222 Figure 2.22: Molecular arrangement with respect to a, b and c directions. a) schematic illustration of a TFT with the active layer along the ab plane. (b) Example of the molecular arrangement of DNTT onto the dielectric (in light blue), with the molecules standing approximately upright with respect to the substrate. (c) Schematic illustration of transfer integrals within adjacent molecules along the ab plane.

Figure 2 . 23 :

 223 Figure 2.23: Short contacts between adjacent molecules in the crystalline arrangement and calculated HOMO of DN4T and isoDN4T. Short contacts in (a) DN4T and (b) isoDN4T crystalline structures are displayed as distances less or equal to the sum of van der Waals radii (S•••S in blue, C•••S in purple, C-H•••π in green). Shape of the HOMO orbital of (c) DN4T and (d) isoDN4T.

Figure 2 . 24 :

 224 Figure 2.24: Thin-film growth mode. (a) Schematic illustration of 2D and 3D thin-film growth. (b) AFM topography image of C8-DNTT thin film deposited onto TDPA/Al2O3 through vacuum deposition at substrate temperature of 100 °C (deposition rate of 0.1 Ǻs -1 and nominal thickness of ca. 25 nm). Scale bar: 1 µm.

  isoDN4T. DN4T and isoDN4T possess two additional thienyl rings in the central motif compared to DNTT. The IE shifted from 5.42 eV in DNTT to 5.27 and 5.23 eV in DN4T and isoDN4T, respectively, confirming the IE reduction caused by the extended π-core.In this case, the IE reduction turns into lower Vth for all the synthesized compounds when employed in TFTs, compared to those fabricated with tBu-BTBT. Particularly, considering TFTs fabricated by vacuum deposition with a BC geometry, TFTs based on tBu-BTBT show Vth of -3.1 V. Because of the π-core extension, Vth is found to be 1.4 V lower in TFTs based on tBu-DNTT and on tBu-BTBT-1, 1.8 V lower in TFTs based on tBu-DBTTT, and 1.6 V lower in TFTs based on tBu-BTBT-2.

Figure 2 . 25 :

 225 Figure 2.25: Mobility anisotropy plots. (a) tBu-DNTT-tBu, (b) tBu-DBTTT-tBu, (c) tBu-th-BTBT-th-tBu and (d) tBu-thth-BTBT-th-th-tBu Plots are obtained from kMC simulations.

  Scheme 3.1

Figure 3 . 1 :

 31 Figure 3.1: Crystalline packing of the parent and alkylated compounds. a) DNTT, (b) DBTTT, (c) C8-DNTT, (d) C12-DBTTT.

Figure 3 . 2 :

 32 Figure 3.2: AFM topography images of thin films. OSCs are deposited onto TDPA/Al2O3 substrates (left column), onto TDPA/Al2O3 substrate and covered with 10 nm of Au (center column), onto PFBT/Au surfaces (left column). Nominal thickness of OSCs films = 25 nm. Lateral scale bar = 1 µm.

Figure 3 . 4 :

 34 Figure 3.4: Transfer characteristics of TC TFTs with 25-nm-thick OSC layer. Solid lines and dashed lines are referred to drain current and mobility respectively. All TFTs have W/L = 480/215 μm.

Figure 3 . 5 :

 35 Figure 3.5: Transfer characteristics of BC TFTs with 25-nm-thick OSC layer. Solid lines and dashed lines are referred to drain current and mobility respectively. All TFTs have W/L = 480/215 μm.

3. 5 . 1 . 2

 512 Contact resistance Contact resistance was evaluated by recording the trasnfer characteristics of TFTs with channel length of 65, 115, 165, 215 µm and channel width of 480 µm (transfer characteristics of devices with channel length of 65, 115 and 165 µm are reported in the Appendix -Figure A2.6). The width-normalized total resistance (RW) was calculated for each gate-overdrive voltage (Vg -Vth) and plotted as a function of the channel length (Figure 3.6).

Figure 3 . 6 :

 36 Figure 3.6: Transmission line method applied to TC and BC TFTs with 25nm-thick OSC layer. Linear fits to the width-normalized contact resistance as a function of the channel length at different values of gate-overdrive voltage.

Figure 3 . 7 :

 37 Figure 3.7: Width-normalized contact as a function of the gate-overdrive voltage of TC TFTs with 25-nm-thick OSC layer. (a) DNTT, tBu-DNTT and C8-DNTT, (b) DBTTT, tBu-DBTTT and C12-DBTTT.

Figure 3 . 8 :

 38 Figure 3.8: Width-normalized contact as a function of the gate-overdrive voltage of BC TFTs with 25-nm-thick OSC layer. (a) DNTT, tBu-DNTT and C8-DNTT, (b) DBTTT, tBu-DBTTT and C12-DBTTT.

  3 times lower for DNTT, 2.8 times lower for C8-DNTT, 1.4 times lower for DBTTT and 2.9 times lower for C12-DBTTT, than the TC devices. Surprisingly, a dramatic reduction of RCW in BC TFTs based on tert-butyl derivatives was observed, with values about 7and 35-fold lower than TC based on tBu-DNTT and tBu-DBTTT respectively (Figure 3.9).

Figure 3 . 9 :

 39 Figure 3.9: Width-normalized contact resistance of TC and BC TFTs. (a) DNTT, tBu-DNTT-tBu and C8-DNTT, (b) DBTTT, tBu-DBTTT-tBu and C12-DBTTT. Gate-overdrive voltage = -2.0 V

Figure 3 . 10 :

 310 Figure 3.10: Schematic illustration of the in-plane and out-of-plane directions of charge transport in a staggered device.

Figure 3 . 11 :

 311 Figure 3.11: Transfer characteristics of TC TFTs with 80-nm-thick OSC layer. Solid lines and dashed lines are referred to drain current and mobility respectively. All TFTs have W/L = 480/215 μm.

Figure 3 . 12 :

 312 Figure 3.12: Transfer characteristics of BC TFTs with 80-nm-thick OSC layer. Solid lines and dashed lines are referred to drain current and mobility respectively. All TFTs have W/L = 480/215 μm.

7 )

 7 width-normalized contact resistance of TC and BC TFTs with 80-nmthick OSC layer was evaluated with TLM by recording transfer characteristics for devices with channel length of 65, 115, 165 and 215 µm (transfer characteristics of devices with channel length of 65, 115 and 165 µm are reported in the Appendix -Figure A2.The width-normalized contact resistance was derived as a function of the gate-overdrive voltage, as in the case of devices with 25-nm-thick OSC afterwards (Figure 3.13).

Figure 3 . 13 :

 313 Figure 3.13: Transmission line method applied to TC and BC TFTs with 80nm-thick OSC layer. Linear fits to the width-normalized contact resistance as a function of the channel length at different values of gate-overdrive voltage.

  5 kΩcm for DNTT, 38.5 kΩcm for tBu-DNTT-tBu, 5.2 kΩcm for C8-DNTT, 2.1 kΩcm for DBTTT, 118.8 kΩcm for tBu-DBTTT-tBu and of 8.8 kΩcm for C12-DBTTT (RCW values are between 1.2 and 1.6 times higher than TC TFTs based on 25-nm-thick OSC layer, see Figure 3.14).

Figure 3 . 14 :

 314 Figure 3.14: Width-normalized contact resistance of TC TFTs with 80-nmthick OSC layer. (a) DNTT, tBu-DNTT and C8-DNTT, (b) DBTTT, tBu-DBTTT and C12-DBTTT. Gate-overdrive voltage = -2.0 V.

  1 kΩcm for DNTT, 3.0 kΩcm for tBu-DNTT-tBu, 0.3 kΩcm for C8-DNTT, 1.0 kΩcm for DBTTT, 1.4 kΩcm for tBu-DBTTT-tBu and of 0.9 kΩcm for C12-DBTTT (Figure 3.15).

Figure 3 . 15 :

 315 Figure 3.15: Width-normalized contact resistance of BC TFTs with 80-nmthick OSC layer. (a) DNTT, tBu-DNTT and C8-DNTT, (b) DBTTT, tBu-DBTTT and C12-DBTTT. Gate-overdrive voltage = -2.0 V

Figure 3 . 16 :

 316 Figure 3.16: Width-normalized contact as a function of the gate-overdrive voltage TFTs with 80-nm-thick OSC layer. RcW of DNTT, tBu-DNTT and C8-DNTT extracted from (a) TC TFTs and (b) BC TFTs. RcW of DBTTT, tBu-DBTTT and C12-DBTTT extracted from (c) TC TFTs and (d) BC TFTs.

Figure 3 . 17 :

 317 Figure 3.17: Sample geometry for SCLC measurements. Schematic representation of (a) C-AFM set-up presenting a tip-plane geometry and (b) diode parallel-plane geometry. The current density direction is shown by blue arrows.

Figure 3 .

 3 Figure 3.18: J-V characteristics measured using C-AFM on thin films plotted in logarithmic scale. (a) J-V characteristics of DNTT, tBu-DNTT and C8-DNTT thin films, (b) J-V characteristics of DBTTT and tBu-DBTTT thin films. The highlighted region (between 0.8 and 2.0 V) indicates the SCLC regime, while the dashed lines serve as references displaying lines with power coefficient (m) = 2. (c) J-V characteristics of DNTT, tBu-DNTT, C8-DNTT thin films and (d) J-V characteristics of DBTTT and tBu-DBTTT thin films for bias voltage < 0.15 V (here the dashed lines have a power coefficient m = 1). On the bottom: schematic representation of the experimental setup employed for J-V characteristics measurement via C-AFM.

Figure 3 . 19 :

 319 Figure 3.19: Charge carrier injection in TC and BC TFTs. Schematic illustration of (a) injection underneath the contact in TC geometry and (b) injection through the contact edge in BC geometry. The red arrows represent the flow of the injected charges. In TC devices the 63% of charges are injected over a characteristic length denoted as transfer length (LT).

Figure 3 . 20 :

 320 Figure 3.20: Transfer length extraction and contact resistivity in TC TFTs. Total device resistance as a function of the channel length for (a) DNTT family and (b) DBTTT family, at gate-overdrive voltage of -2.0 V. The intersection of the linear regression with the x-axis (RW = 0) corresponds to -2LT. Calculated contact resistivity as a function of the gate-overdrive voltage for (c) DNTT family and (d) DBTTT family.

Figure 3 . 21 :

 321 Figure 3.21: UPS spectra showing the valance region of thin film deposited onto Au and PFBT/Au substrates. The thin films were deposited with a nominal thickness of 20 Å. The black lines show the onset of the HOMO peaks.

Figure 3 . 22 :

 322 Figure 3.22: Secondary electrons cut-offs used to determine work functions. The black lines refer to Au substrates, the magenta lines refer to PFBT/Au substrates and the blue lines refer to the sample consisting of 100-Å-thick OSC layer deposited onto the respective substrate.

Figure 3 . 23 :

 323 Figure 3.23: Schematic energy-level diagram of the bands alignment at Au and PFBT/Au substrates. This schematic representation is applicable to all the analysed compounds, except for DBTTT and C8-DNTT. Passing from Au to PFBT/Au substrate, the values of Δ and WFm were observed to increase, leading to a decrease in the hole injection barrier.

Figure 3 . 24 :

 324 Figure 3.24: Contact resistance vs intrinsic mobility. (a) RCW as a function of the intrinsic mobility of TC TFTs. (b) Schematic illustration of the OSC volume beneath the source contact treated as a resistors network within the current crowding model.

1 µ 0

 0 𝐶 𝑖 (𝑉 𝑔 -𝑉 𝑡ℎ ) = 𝑅 𝑠ℎ µin is extracted from transfer characteristics → µ 𝑖𝑛 = 𝐿 𝑊 𝐶 𝑖 𝑉 𝑑 𝜕𝐼 𝑑 𝜕𝑉 𝑔 | 𝑉 𝑑

Figure 4 . 1 :

 41 Figure 4.1: Polymorphic forms of perylene. (a) Molecular structure of perylene, (b) crystalline arrangement of the α-form, (c) crystalline arrangement of the β-form.

Figure 4 . 2 :

 42 Figure 4.2: XRD patterns and optical micrographs of α-and β-form of perylene. On the left: XRD patterns obtained from solution-dropping method. α and β refer to the simulated patterns, THF refers to crystals obtained by solution-dropping method of 4 mg/mL solution in THF and CLB refers to crystals obtained by solution-dropping method of 4 mg/mL solution in THF. The black and red asterisks highlight the peaks attributed to the β-and α-form, respectively. On the right: optical micrograph (scale bar = 200 µm).

Figure 4 . 3 :

 43 Figure 4.3: Recrystallization and slurry experiments. On the left: XRD patterns obtained from recrystallization and slurry experiments.α and β refer to the simulated patterns, R-THF refers to recrystallization of 10 mg/mL solution in THF cooled from 50 °C to room temperature, R-TOL refers to recrystallization of 10 mg/mL solution in toluene cooled from 70 °C to room temperature, R-BNT refers to recrystallization of 10 mg/mL solution in benzonitrile cooled from 70 °C to room temperature. On the right: optical micrographs (scale bar = 200 µm).

Figure 4 . 4 :

 44 Figure 4.4: Fabry-Perot cavity. (a) Assembled Microfluidic cell, (b) schematic illustration of the FP cavity and of the microfluidic cell employed for off-cavity experiments.

Figure 4 . 5 :

 45 Figure 4.5: Fourier transformed infrared spectra of perylene and benzonitrile.On the right: perylene and benzonitrile spectra in the region between 1710 and 1350 cm -1 , highlighting the overlap of the vibrational bands at 1492 cm -1 (marked with a blue line).

Figure 4 . 6 :

 46 Figure 4.6: Recrystallization experiments conducted in the microfluidic cell. On the top: (a) On-resonance FP cavity where the vibrational mode at 2228 cm -1 of benzonitrile is coupled with the 18 th optical mode and the vibrational mode at 1492 cm -1 of benzonitrile and perylene are coupled with the 12 th optical mode (VSC1), and (b) the respective optical micrograph of formed crystals upon cavity cooling. (c) Onresonance FP cavity where the vibrational mode at 1445 cm -1 of benzonitrile is coupled with the 12 th optical mode (VSC2), and (d) the respective optical micrograph of formed crystals upon cavity cooling. (e) Optical micrograph of crystals obtained in a non-cavity condition. Scale bar = 200 µm.

Figure 4 . 7 :

 47 Figure 4.7: Shift of the optical cavity modes due to temperature change. (a) Evolution of the optical cavity modes of an empty FP cavity when cooled from 70 °C to 25 °C. (b),(c) Shifts of the polaritonic states obtained in VSC1, referring to the coupling between the benzonitrile vibration at 2228 cm -1 and the 18 th optical mode and to the cooperative VSC between the vibration at 1492 cm -1 and the 12 th optical mode. (d) Shifts of the polaritonic states obtained in VSC2, referring to the coupling between the benzonitrile vibration at 1445 cm -1 and the 12 th optical mode

Scheme 5 . 3 :

 53 Scheme 5.3: i) n-BuLi then 2-bromo-5-(tert-butyl)benzaldehyde (11) leading to the diol 12, THF; ii) NaBH3CN, ZnI2, 1,2-dichloroethane; iii) n-BuLi then DMF, THF; iv) Amberlyst 15 ®, chlorobenzene.

Figure A1. 2 :

 2 Figure A1.2: Threshold voltage and on/off current ratio vs substrate temperature for C8-DN4T and C10-DN4T. Threshold voltage and on/off current ratio as a function of the substrate temperature in (a),(c) C8-DN4T and (b),(d) C10-DN4T based BC TFTs. The values are referred to the linear regime and to devices with W/L = 480/215 µm. The dot lines indicate the averaged values of Vth and ION/OFF.

Figure A1. 3 :

 3 Figure A1.3: Transfer (left) and output (right) characteristics of the best performing TC TFTs based on tert-butyl theienoacene derivatives. (a, b) tBu-DNTT, (c, d) tBu-DBTTT, (e, f) tBu-BTBT-1 and (g, h) tBu-BTBT-2. In transfer characteristics, solid lines and dashed lines are referred to drain current and mobility respectively. All TFTs have W/L = 480/215 μm.

Figure A1. 4 :

 4 Figure A1.4: Threshold voltage and on/off current ratio vs the substrate temperature for BC TFTs based on tert-butyl thienoacene derivatives. (a) tBu-DNTT, (b) tBu-DBTTT, (c) tBu-BTBT-1 and (d) tBu-BTBT-2. The values are referred to the linear regime and to devices with W/L = 480/215 µm. The dot lines indicate the averaged values of Vth and ION/OFF.

Figure A1. 5 :

 5 Figure A1.5: Optical micrographs of 25-nm-thick (on the left) and 40-nm-thick (on the right) layer of tBu-BTBT deposited onto Al2O3/TDPA at substrate temperature of 70 °C. Scale bar = 20 µm.

Figure A1. 6 : 2 :

 62 Figure A1.6: PYS measurements performed on thin films of (a) DNTT and (b) C8-DNTT

Figure A2. 1 :

 1 Figure A2.1: Output characteristics of TC TFTs with 25-nm-thick OSC layer. All TFTs have W/L = 480/215 μm.

Figure A2. 2 :

 2 Figure A2.2: Output characteristics of BC TFTs with 25-nm-thick OSC layer. All TFTs have W/L = 480/215 μm.

Figure A2. 4 :

 4 Figure A2.4: Output characteristics of TC TFTs with 80-nm-thick OSC layer. All TFTs have W/L = 480/215 μm.

Figure A2. 5 :

 5 Figure A2.5: Output characteristics of BC TFTs with 80-nm-thick OSC layer. All TFTs have W/L = 480/215 μm.

Figure A2. 6 :

 6 Figure A2.6: Transfer characteristics of TC and BC TFTs with 25-nm-thick OSC layer at different channel length.

Figure A2. 7 :

 7 Figure A2.7: Transfer characteristics od TC and BC TFTs with 80-nm-thick OSC layer at different channel length.

Figure A2. 8 :

 8 Figure A2.8: Effective mobility extracted from TC and BC TFTs as a function of the channel length.

Figure A2. 9 :

 9 Figure A2.9: RCW vs gate-overdrive voltage of TC and BC TFTs with 25-nm thick OSC layer.

Figure A2. 10 :

 10 Figure A2.10: RCW vs gate-overdrive voltage of TC TFTs with 25-and 80-nm-thick OSC layer.

Figure A2. 11 :Figure A2. 13 :

 1113 Figure A2.11: RCW vs gate-overdrive voltage of BC TFTs with 25-and 80-nm-thick OSC layer.

1 s - 1 .

 1 Cette différence de mobilité est attribuée à plusieurs facteurs. Tout d'abord, la comparaison des structures cristallines des deux isomères révèle un plus petit nombre de contacts courts dans l'isoDN4T. De plus, les calculs de chimie quantique mettent en évidence des intégrales de transfert plus importantes ainsi qu'une énergie de réorganisation plus faible pour le DN4T, comparé à l'isoDN4T. Dans ce chapitre, nous présentons également l'évaluation des performances électriques de dérivés alkylés de DN4T, à savoir le C8-DN4T et le C10-DN4T (Figure R.1).

Figure R. 1 :

 1 Figure R.1: Structures moléculaires du DNTT, DN4T, isoDN4T et des dérivés alkylés du DN4T.

Figure R. 2 :

 2 Figure R.2: Structures moléculaires des dérivés de thiénoacènes substitués par des groupes périphériques stériques étudiés au chapitre 2 : tBu-DNTT, tBu-DBTTT, dBu-BTBT-1 et tBu-BTBT-2. Les structures du BTBT et du tBu-BTBT sont également présentées.

  facteurs ayant un lien direct avec la résistance de contact, tels que la barrière Schottky et le matériau utilisé pour l'électrode. Les composés choisis dans ce travail sont le DNTT et le DBTTT, ainsi que leurs dérivés tert-butyle et alkylés, à savoir tBu-DNTT, tBu-DBTTT, C8-DNTT et C12-DBTTT (Figure R.2 et R.3).

Figure R. 3 :

 3 Figure R.3: Structures moléculaires des composés utilisés dans le chapitre 3, à savoir le DNTT, le DBTTT et leurs dérivés alkylés, le C8-DNTT et le C12-DBTTT.

  Ce chapitre se concentre sur la cristallisation du pérylène dans une cavité Fabry-Perot (FP), afin d'explorer l'effet du VSC sur la croissance cristalline et le polymorphisme. Les conditions expérimentales pour obtenir la cristallisation du pérylène à l'intérieur d'une cavité FP ont été établies, montrant une formation concomitante des formes α et β (Figure R.4), dans les expériences de recristallisation. Le VSC coopératif est obtenu en couplant la vibration moléculaire du solvant (benzonitrile) avec les modes optiques de la cavité. Malheureusement, le rapport entre les deux polymorphes n'est pas modifié de manière significative par le VSC. Nous avons observé des décalages des modes optiques de la cavité qui peuvent être une raison d'instabilité et un problème pour la reproductibilité des expériences. En résumé, pour éviter ces décalages causés par le changement de température de la cavité, des expériences de cristallisation à température constante doivent être réalisées. Ce n'est que dans ce cas que l'on pourra explorer le potentiel du VSC à influencer la croissance cristalline et le polymorphisme des OSC.

Figure R. 4 :

 4 Figure R.4: (a) Structure moléculaire du pérylène, (b) arrangement moléculaire dans la phase α, (c) arrangement moléculaire dans la phase β. La direction d'observation est perpendiculaire au plan (bc).

Table 2 .

 2 1: Crystallographic data of the investigated compounds. ................ 47 Table 2.2: Averaged grain size extracted from AFM images. ....................... 53 Table 2.3: Values extracted from XRD and AFM measurements. ................ 58

Table 2 .

 2

and equation 1.15 refer

									to the
	transconductance (gm) in the linear and saturation regimes, respectively:
	𝑔 𝑚,𝑙𝑖𝑛 =	𝜕𝐼 𝑑 𝜕𝑉 𝑔	|	𝑉 𝑑	=	𝑊 𝐿	µ 0 𝐶 𝑖 𝑉 𝑑	(1.18)
	𝑔 𝑚,𝑠𝑎𝑡 =	𝜕𝐼 𝑑 𝜕𝑉 𝑑	|	𝑉 𝑔	=		𝑊 𝐿	µ 0 𝐶 𝑖 (𝑉 𝑔 -𝑉 𝑡ℎ )	(1.19)
	Thus, from the transfer characteristics, it is possible to extract the charge
	carrier mobility for the linear (µ,lin) and saturation (µ,sat) regimes:
		µ ,𝑙𝑖𝑛 =	𝐿 𝑊 𝐶 𝑖 𝑉 𝑑	𝜕𝐼 𝑑 𝜕𝑉 𝑔	|	𝑉 𝑑	(1.20)
		µ ,𝑠𝑎𝑡 =	2𝐿 𝑊 𝐶 𝑖	( 𝜕√𝐼 𝑑 𝜕𝑉 𝑔	|	𝑉 𝑑	2 )	(1.21)

Table 2 .1: Crystallographic data of the investigated compounds.

 2

	Compound	DN4T	isoDN4T	tBu-DNTT	tBu-DBTTT	tBu-BTBT-1	tBu-BTBT-2
	Formula	C26H12S4	C26H12S4	C30H28S2	C26H24S4	C30H28S4	C38H32S6
	Temperature (K)	172	100	123	255	100	123
	Crystal system	Monoclinic	Monoclinic	Monoclinic	Triclinic	Monoclinic	Monoclinic
	Space group	P 21	P 21/c	P21/c	P-1	P21/c	P21/c
	a (Å)	6.15993(17)	19.1163(18)	17.5612(4)	6.2944(2)	20.3894(12)	24.1489(6)
	b (Å)	7.5925(2)	7.6335(8)	6.1886(11)	11.9761(4)	5.9706(2)	5.87800(10)
	c (Å)	20.0487(6)	6.5402(5)	10.5700(2)	16.0005(4)	10.5460(5)	11.0355(2)
	α (deg)	90	90	90	94.024(2)	90	90
	β (deg)	91.695(3)	92.970(8)	99.974(2)	100.072(3)	104.531(5)	90.017(2)
	γ (deg)	90	90	90	100.828(3)	90	90
	Volume [Å 3 ]	937.25(5)	953.09(16)	1131.38	1159.78	1242.77	1566.46
	Z	2	2	2	2	2	2
	Z'	1	0.5	0.5	1	0.5	0.5

Table 2 .3: Values extracted from XRD and AFM measurements

 2 . d-spacing calculated from XRD patterns (d XRD), terrace height steps obtained from AFM (d AFM) and cell long axis from SCXRD (L SCXRD). a average values obtained from all the Bragg peaks referred to the same family plane {001} or {100}; b average values

	Compound	d XRD (Ǻ) a	d AFM (Ǻ) b	L SCXRD (Ǻ)
	DN4T	20.2 ± 0.5	20.0 ± 1.0	20.0
	isoDN4T	19.2 ± 0.1	18.0 ± 1.0	19.1
	tBu-DNTT	17.9 ± 0.5	17.5 ± 1.2	17.6
	tBu-DBTTT	15.7 ± 0.1	15.0 ± 1.4	16.0
	tBu-BTBT-1	19.8 ± 0.1	19.0 ± 0.9	20.4
	tBu-BTBT-2	25.6 ± 0.2	23.6 ± 2.0	24.2

  For all the compounds, charge carrier mobility improved by increasing the substrate temperature from 40 °C to 100 °C and consequently decreased for Ts of 130 °C (see Figure 2.19).

and Tables A1.3, A1.4, A2.5 and A1.6). In addition, even in this case, the substrate temperature was varied between 40 °C and 130 °C, with 30 °C steps.

The substrate temperature dependent electrical performances in terms of charge carrier mobility, threshold voltage and on/off current ratio were evaluated. Here again as in the case of TFTs based on DN4T, isoDN4T and alkylated DN4Ts, the substrate temperature shows to have a major role in the enhancement of charge carrier mobility, while its effect of Vth and ION/OFF can be considered negligible (Figure

A1

.4, in the Appendix).

Table 2 .4: Width-normalized contact resistance extracted from TC and BC TFTs.

 2 RCW is obtained by TLM at gate-overdrive voltage of -2.0 V.

	Compound Geometry	RCW (kΩcm)
	DN4T	BC	1.8 ± 0.4
		TC	3.8 ± 0.3
	isoDN4T	BC	351 ± 103
		TC	2373 ± 217
	C8-DN4T	BC	1.2 ± 0.1
	C10-DN4T	BC	1.0 ± 0.2
	tBu-DNTT	BC	4.4 ± 0.7
	tBu-DBTTT	BC	2.9 ± 0.3
	tBu-BTBT-1	BC	3.8 ± 0.2
	tBu-BTBT-1	BC	3.8 ± 0.4

Table 2 .5: Reorganization energy (λ), transfer integrals (JT) and charge carrier mobility (µ) of the investigated compounds

 2 . µ is referred to BC TFTs.

	Compound	λ (meV)	JT (meV)	µ (cm 2 V -1 s -1 )
	DN4T	152	61, 99, 33	2.0
	isoDN4T	213	17, 33, 33	3.3 × 10 -3
	tBu-DNTT	143	63, 26, 26	1.9
	tBu-DBTTT	145	52, 30, 57	1.0
	tBu-BTBT-1	309	19, 20, 20	0.25
	tBu-BTBT-1	263	8, 26, 26	0.10

Table 2 .6.

 2

Table 2 .6: Crystalline packing, ionization energy (IE), and mobility (µ) of DN4T, isoDN4T, DNTT, isoDNTT and L-DBTTA.

 2

	(*) = Measurements performed in this work
	HB = herringbone, S-HB = sandwich herringbone
	Compound	Packing	IE (eV) µ (cm 2 V -1 s -1 )	Ref.
	DN4T	HB	5.27 (*)	2.0 (*)	[130]
	isoDN4T	HB	5.23 (*)	3.3 × 10 -3 (*)	[130]
	DNTT	HB	5.42(*)	2.3 (*)	[28,157,158]
	isoDNTT	S-HB	5.58	(10 -3 -10 -2 )	[157]
	L-DBTTA	Stacked	5.16	0.15	[129,130]

Table 2 .7: Ionization energy, threshold voltage and charge carrier mobility of C8-DN4T, C10-DN4T and C8-DNTT.

 2 Vth and µ and RCW values are referred to devices fabricated on optimized substrate temperature.

	Compound	IE (eV)	Vth (V)	µ (cm 2 V -1 s -1 )	RCW (kΩcm)
	C8-DN4T	4.95	-0.4	2.3	1.2
	C10-DN4T	4.97	-0.4	2.5	0.8
	C8-DNTT	4.92	-0.4	4.8	0.8
	DN4T	5.27	-1.2	2.0	1.8

Table 2 .8: Ionization energy (IE), threshold voltage (Vth) and mobility (µ) of tert-butyl substituted thienoacenes.

 2

	(#) Measurements performed in this work.
	(*) Not reported
	( §) TLM not applicable
	(SD) Devices fabricated through solution shearing technique
	Compound	IE (eV)	Vth (V)	RcW (kΩcm)	Ref.
			≈ -70 (SD)	*	[133]
	tBu-BTBT	5.7	-69 (SD)	*	[134]
			-3.1	§	#
	tBu-DNTT	5.37	-1.7	4.4	#
	tBu-DBTTT	5.31	-1.3	2.9	#
	tBu -BTBT-1	5.34	-1.7	4.4	#
	tBu-BTBT-2	5.23	-1.5	4.8	#

Table 3 .

 3

	Compound	DNTT	C8-DNTT	DBTTT	C12-DBTTT
	Formula	C22 H12 S2	C38H44S2	C18 H8 S4	C42 H56 S4
	Crystal system	Monoclinic	Triclinic	Monoclinic	Monoclinic
	Space group	P 21	P-1	P 21/c	P 21/a
	a (Å)	6.187(4)	5.8734(4)	5.91290(10)	5.9874(4)
	b (Å)	7.662(6)	8.9824(5)	7.8545(2)	7.8611(4)
	c (Å)	16.208(11)	35.2165(19)	31.0943(8)	35.2165(19)
	α (deg)	90	92.810(5)	90	90
	β (deg)	92.49(2)	90.799(5)	91.5270(10)	99.860(7)
	γ (deg)	90	93.613(5)	100.828(3)	90
	Volume [Å 3 ]	767.611	1851.73	90	1579.71
	Z	2	2	4	2
	Z'	1	0.5	1	0.5

1: Crystallographic data of DNTT, DBTTT, C8-DNTT and C12-DBTTT. § § The accuracy of assigning a P 21 polar space group to DNTT and DN4T is currently ambiguous. This uncertainty arises from the fact that crystals belonging to polar space groups typically exhibit a growth pattern characterized by a single preferred direction, which is not observed in either DNTT or DN4T.

Table 3 .2: Fabrication parameters used for TC and BC TFTs.

 3

	Compound	Ts (°C)	Deposition rate (Ǻs -1 )
	DNTT	70	0.3
	tBu-DNTT	100	0.3
	C8-DNTT	100	0.1
	DBTTT	70	0.3
	tBu-DBTTT	100	0.3
	C12-DBTTT	100	0.1

Table 3 .3: Averaged grain size extracted from AFM images of thin films deposited onto TDPA/Al2O3 and PFBT/Au.

 3

	Compound	TDPA/Al2O3 (nm)	PFBT/Au (nm)
	DNTT	3370 ± 860	563 ± 80
	tBu-DNTT	615 ± 110	378 ± 102
	C8-DNTT	547 ± 115	327 ± 70
	DBTTT	612 ± 183	388 ± 96
	tBu-DBTTT	1983 ± 398	353 ± 160
	C12-DBTTT	445 ± 77	338 ± 60

Table 3 .4: Comparisons of the d-spacing, terrace height steps and unit cell long axis

 3 . d-spacing (d-XRD) is calculated from XRD patterns, terrace height steps (d-AFM) are obtained from AFM images, and unit cell long axis (L SCXRD) is obtained from the single crystal structure. In the table are shown the values extracted from TDPA/Al2O3 and PFBT/Au substrates. The unit cell long axes are taken from the reported crystalline structures.

	Compound	d XRD	d XRD	d AFM	d AFM	L
		TDPA/Al2O3	PFBT/Au	TDPA/Al2O3	PFBT/Au	SCXRD
		(Ǻ)	(Ǻ)	(Ǻ)	(Ǻ)	(Ǻ)
	DNTT	16.3	16.3	15.8 ± 1.8	15.7 ± 1.7	16.2
	tBu-DNTT	17.9 ± 0.1	17.8 ± 0.3	17.3 ± 1.0	16.8 ± 2.1	17.6
	C8-DNTT	33.5 ± 0.2	33.4 ± 0.2	33.5 ± 3.0	35.4 ± 4.5	34.1
	DBTTT	31.1 ± 0.1	31.3 ± 0.4	15.3 ± 0.8	15.2 ± 1.5	31.4
	tBu-DBTTT	15.8 ± 0.1	15.7 ± 0.3	16.3 ± 2.6	15.9 ± 2.4	16.0
	C12-DBTTT	34.6 ± 0.2	34.8 ± 0.2	35.7 ± 4.0	35.8 ± 3.8	35.2

Table 3 .5: Electrical performance of TC TFTs with 25-nm-thick OSC layer.

 3

	Compound	µ (cm 2 V -1 s -1 )	Vth (V)	SS (mV/dec)
	DNTT	2.3 ± 0.1	-1.0 ± 0.1	109 ± 5
	tBu-DNTT	0.8 ± 0.1	-1.8 ± 0.1	141 ± 4
	C8-DNTT	3.7 ± 0.2	-0.8 ± 0.1	112 ± 4
	DBTTT	5.1 ± 0.1	-1.2 ± 0.1	116 ± 3
	tBu-DBTTT	0.3 ± 0.1	-1.6 ± 0.2	163 ± 2
	C12-DBTTT	2.2 ± 0.1	-1.1 ± 0.1	122 ± 2

Table 3 .6: Electrical performance of BC TFTs with 25-nm-thick OSC layer.

 3

	Compound	µ (cm 2 V -1 s -1 )	Vth (V)	SS (mV/dec)	Nit (10 12 eV -1 cm -2 )
	DNTT	2.3 ± 0.1	-0.7 ± 0.1	100 ± 8	0.62 ± 0.04
	tBu-DNTT	1.4 ± 0.1	-1.7 ± 0.1	128 ± 4	1.34 ± 0.10
	C8-DNTT	4.8 ± 0.3	-0.4 ± 0.1	100 ± 3	0.80 ± 0.07
	DBTTT	4.7 ± 0.1	-0.7 ± 0.1	98 ± 5	0.76 ± 0.12
	tBu-DBTTT	1.0 ± 0.1	-1.5 ± 0.1	147 ± 3	1.72 ± 0.07
	C12-DBTTT	3.3 ± 0.2	-0.6 ± 0.1	108 ± 5	0.96 ± 0.13

Table 3 .7: Electrical performance of TC TFTs with 80-nm-thick OSC layer.

 3

	Compound	µ (cm 2 V -1 s -1 )	Vth (V)	SS (mV/dec)
	DNTT	1.8 ± 0.1	-0.7 ± 0.1	112 ± 2
	tBu-DNTT	0.7 ± 0.1	-2.0 ± 0.2	155 ± 7
	C8-DNTT	4.2 ± 0.3	-0.6 ± 0.2	122 ± 3
	DBTTT	4.0 ± 0.2	-1.3 ± 0.1	129 ± 4
	tBu-DBTTT	0.4 ± 0.1	-1.7 ± 0.1	190 ± 9
	C12-DBTTT	1.5 ± 0.2	-1.2 ± 0.1	143 ± 11

Table 3 .8: Electrical performance of BC TFTs with 80-nm-thick OSC layer.

 3

	Compound	µ (cm 2 V -1 s -1 )	Vth (V)	SS (mV/dec)	Nit (10 12 eV -1 cm -2 )
	DNTT	2.4 ± 0.1	-0.5 ± 0.1	97 ± 2	0.74 ± 0.05
	tBu-DNTT	1.4 ± 0.1	-1.7 ± 0.1	136 ± 11	1.51 ± 0.27
	C8-DNTT	4.9 ± 0.2	-0.4 ± 0.1	101 ± 6	0.82 ± 0.15
	DBTTT	2.9 ± 0.2	-0.9 ± 0.1	107 ± 3	0.94 ± 0.08
	tBu-DBTTT	0.9 ± 0.1	-1.6 ± 0.1	151 ± 4	1.81 ± 0.10
	C12-DBTTT	3.2 ± 0.2	-0.8 ± 0.1	114 ± 4	1.08 ± 0.11

Table 3 .10: Extracted transfer lengths (LT) and charge injection areas (Ainj) for TC TFTs with 25-nm-thick OSC layer.

 3

	Compound	LT (µm)	Ainj (µm 2 )
	DNTT	13.7	6.6 × 10 3
	tBu-DNTT	48.5	2.3 × 10 4
	C8-DNTT	27.3	1.3 × 10 4
	DBTTT	17.9	8.6 × 10 3
	tBu-DBTTT	101.1	4.9 × 10 4
	C12-DBTTT	23.4	1.1 × 10 4

Table 3 .11: Extracted values from UPS measurements

 3

	Compound	Au	PFBT/Au
	ΦB,p

.

Hole injection barrier (ΦB,p), interfacial dipole (Δ) and ionization energy (IE) of thin films deposited onto Au and PFBT/Au substrates. ΦB,p is determined from 20-Åthick OSC layer, while Δ and IE are determined from 100-Å-thick OSC layer.

(eV) Δ (eV) IE (eV) ΦB,p (eV) Δ (eV

						)	IE (eV)
	DNTT	0.96	0.18	5.42	0.52	0.29	5.44
	tBu-DNTT	0.88	0.10	5.45	0.35	0.31	5.56
	C8-DNTT	0.48	0.50	5.00	0.54	0.73	5.21
	DBTTT	0.85	≈ 0	5.58	0.84	0.19	5.58
	tBu-DBTTT	0.87	0.17	5.42	0.15	0.54	5.40
	C12-DBTTT	0.77	0.20	5.14	0.50	0.45	5.25

Table 3 .13: Contact resistance, charge carrier mobility and hole injection barrier of BC TFTs.

 3 Contact resistance (RCW), charge carrier mobility (µ) and hole injection barrier (ΦB,p).

	Compound	RCW (kΩcm)	µ (cm 2 V -1 s -1 )	ΦB,p (eV)
	DNTT	0.8	2.3	0.52
	tBu-DNTT	3.7	1.4	0.35
	C8-DNTT	0.8	4.8	0.54
	DBTTT	1.2	4.7	0.84
	tBu-DBTTT	2.4	1.0	0.15
	C12-DBTTT	1.9	3.3	0.50

Table 4 .1: Crystallographic data of perylene polymorphs.

 4

		α-form	β-form
	Formula	C20H12
	Crystal system	Monoclinic	Monoclinic
	Space group	P 21/c	P 21/a
	a (Å)	10.270	11.277
	b (Å)	10.839	10.826
	c (Å)	11.278	10.263
	α (deg)	90	90
	β (deg)	100.53	100.55
	γ (deg)	90	90
	Z	4	4
	Z'	1	1

Table A1 . 3 :

 A13 Electrical performances of BC and TC TFTs based on tBu-DNTT, in linear (Vd = -0.1 V) and saturation (Vd = -4.0 V) regime.

	Table A1.5: Electrical performances of BC and TC TFTs based on tBu-BTBT-1, in linear (Vd = -0.1 V) and saturation (Vd = -4.0 V) regime.
				μ (cm 2 V -1 s -1 )	Vth (V)	ION/OFF
	Compound Compound	40 T (°C) Geometry Substrate T (°C) Geometry Substrate	2 V -1 s -1 ) Saturatio 0.28 ± 0.10 Saturation μ (cm 2 V -1 s -1 ) Linear 0.22 ± 0.04 Linear Linear Saturation	Vth (V) Linear Saturatio -2.0 ± 0.12 -2.0 ± 0.06 Linear Saturation Vth (V) Linear Saturation	ION/OFF Linear 5 × 10 3 1 × Saturatio Linear ION/OFF Saturation Linear Saturation
			70 40	0.53 ± 0.14 0.04 ± 0.007	n 0.83 ± 0.20 0.05 ± 0.005	-2.0 ± 0.06 -2.3 ± 0.07	n -2.0 ± 0.03 -2.3 ± 0.1	2 × 10 4 9 × 10 2	n 7 × 1 ×
		TC	40 100 70	1.1 ± 0.09 0.87 ± 0.14 0.05 ± 0.004	1.2 ± 0.07 1.0 ± 0.11 0.08 ± 0.008	-0.2 ± 0.08 -2.0 ± 0.07 -2.1 ± 0.15	-0.2 ± 0.08 -2.0 ± 0.06 -2.1 ± 0.19	~ 8 × 10 4 2 × 10 4 1 × 10 3	~ 1 × 5 × 3 ×
	tBu-DNTT	TC	70 130 100	1.5 ± 0.09 0.44 ± 0.08 0.18 ± 0.019	1.4 ± 0.24 0.59 ± 0.14 0.26 ± 0.026	-0.3 ± 0.02 -2.0 ± 0.10 -2.1 ± 0.14	-0.3 ± 0.02 -2.0 ± 0.07 -2.1 ± 0.16	~ 1 × 10 5 1 × 10 4 5 × 10 3	~ 1 × 2 × 1 ×
	tBu-BTBT-1	BC	100 40 130	2.3 ± 0.21 1.4 ± 0.08 0.14 ± 0.018	2.2 ± 0.22 1.3 ± 0.07 0.18 ± 0.018	-0.4 ± 0.09 -1.7 ± 0.04 -2.2 ± 0.17	-0.3 ± 0.07 -1.7 ± 0.04 -2.2 ± 0.18	~ 2 × 10 5 5 × 10 4 4 × 10 3	~ 4 × 1 × 6 ×
			130 70 40	1.9 ± 0.04 1.7 ± 0.03 0.20 ± 0.005	1.9 ± 0.02 1.6 ± 0.02 0.18 ± 0.006	-0.3 ± 0.05 -1.7 ± 0.02 -1.9 ± 0.07	-0.8 ± 0.05 -1.7 ± 0.02 -1.9 ± 0.07	~ 2 × 10 5 5 × 10 4 1 × 10 4	~ 1 × 1 × 2 ×
		BC	40 100 70	1.3 ± 0.16 1.8 ± 0.07 0.23 ± 0.007	1.2 ± 0.15 1.7 ± 0.05 0.22 ± 0.005	-0.9 ± 0.02 -1.7 ± 0.01 -1.9 ± 0.06	-0.8 ± 0.02 -1.7 ± 0.01 -1.9 ± 0.09	~ 4 × 10 4 6 × 10 4 1 × 10 4	~ 6 × 4 × 1 ×
		BC	70 130 100	1.4 ± 0.01 1.8 ± 0.01 0.25 ± 0.008	1.4 ± 0.01 1.7 ± 0.01 0.22 ± 0.005	-0.9 ± 0.01 -1.7 ± 0.03 -1.7 ± 0.04	-0.8 ± 0.01 -1.7 ± 0.01 -1.7 ± 0.05	~ 5 × 10 4 7 × 10 4 1 × 10 4	~ 7 × 2 × 1 ×
		TC	100 130	2.0 ± 0.02 0.19 ± 0.033	2.0 ± 0.02 0.21 ± 0.002	-0.8 ± 0.07 -1.9 ± 0.07	-0.8 ± 0.06 -1.9 ± 0.11	~ 9 × 10 4 8 × 10 3	~ 1 × 3 ×
			130	1.6 ± 0.02	1.5 ± 0.02	-0.9 ± 0.04	-0.2 ± 0.04	~ 5 × 10 4	~ 7 ×
			40	0.2 ± 0.01	0.2 ± 0.01	-0.6 ± 0.02	-0.6 ± 0.02	~ 2 × 10 4	~ 3 ×
	C10-DN4T Compound tBu-DBTTT Compound tBu-BTBT-2	BC TC Geometry Substrate 70 100 130 40 70 100 130 Geometry Substrate T (°C) TC 40 T (°C) 70 40 100 70 130 TC 100 BC 40 130 70 40 100 70 130 BC 100	1.0 ± 0.03 2.5 ± 0.05 1.0 ± 0.03 0.1 ± 0.01 0.8 ± 0.07 1.8 ± 0.12 0.7 ± 0.01 μ (cm 2 V -1 s -1 ) 1.1 ± 0.03 2.5 ± 0.05 1.0 ± 0.03 0.1 ± 0.01 0.8 ± 0.07 1.8 ± 0.12 0.7 ± 0.01 μ (cm 2 V -1 s -1 ) Linear Saturation Linear Saturation 0.09 ± 0.03 0.19 ± 0.03 0.17 ± 0.01 0.24 ± 0.01 0.05 ± 0.005 0.05 ± 0.001 0.16 ± 0.02 0.17 ± 0.02 0.08 ± 0.002 0.07 ± 0.001 0.11 ± 0.03 0.19 ± 0.05 0.07 ± 0.003 0.09 ± 0.004 0.7 ± 0.03 0.2 ± 0.01 0.06 ± 0.007 0.06 ± 0.005 1.0 ± 0.01 1.1 ± 0.03 0.05 ± 0.001 0.05 ± 0.001 1.0 ± 0.03 2.5 ± 0.05 0.08 ± 0.002 0.08 ± 0.002 0.5 ± 0.13 1.0 ± 0.03 0.10 ± 0.003 0.09 ± 0.001	-0.3 ± 0.03 -0.4 ± 0.06 -0.5 ± 0.06 -1.6 ± 0.07 -0.9 ± 0.05 -0.8 ± 0.04 -1.1 ± 0.04 Vth (V) -0.3 ± 0.06 -0.4 ± 0.06 -0.5 ± 0.06 -1.4 ± 0.03 -0.8 ± 0.05 -0.8 ± 0.04 -1.0 ± 0.03 Vth (V) Linear Saturation Linear Saturation -1.5 ± 0.03 -1.5 ± 0.03 -1.5 ± 0.03 -1.4 ± 0.06 -1.8 ± 0.14 -1.8 ± 0.06 -1.5 ± 0.04 -1.3 ± 0.21 -1.8 ± 0.04 -1.7 ± 0.05 -1.5 ± 0.08 -1.3 ± 0.15 -1.8 ± 0.01 -1.8 ± 0.03 -1.4 ± 0.01 -0.6 ± 0.02 -1.8 ± 0.08 -1.8 ± 0.02 -1.5 ± 0.07 -0.3 ± 0.06 -1.7 ± 0.02 -1.6± 0.01 -1.3 ± 0.02 -0.4 ± 0.06 -1.4 ± 0.02 -1.4 ± 0.02 -1.4 ± 0.03 -0.5 ± 0.06 -1.5 ± 0.01 -1.4 ± 0.01	~ 1 × 10 5 ~ 4 × 10 5 ~ 2 × 10 4 ~ 2 × 10 3 ~ 2 × 10 4 ~ 7 × 10 4 ~ 2 × 10 4 ION/OFF ~ 4 × ~ 2 × ~ 4 × ~ 1 × ~ 3 × ~ 1 × ~ 2 × ION/OFF Linear Saturation Linear Saturation 2 × 10 4 1 × 3 × 10 4 3 × 10 3 1 × 9 × 1 × 10 4 2 × 10 3 2 × 1 × 2 × 10 4 3 × 10 3 2 × 2 × 6 × 10 4 3 × 10 3 2 × 9 × 6 × 10 4 7 × 10 3 1 × 1 × 6 × 10 4 2 × 10 4 9 × 1 × 6 × 10 4 9 × 1 × 10 4 7 ×
			130	0.09 ± 0.001	0.08 ± 0.001	-1.4 ± 0.02	-1.4 ± 0.02	1 × 10 4	2 ×

Table

A1

.4: Electrical performances of BC and TC TFTs based on tBu-DBTTT, in linear (Vd = -0.1 V) and saturation (Vd = -4.0 V) regime.

Table A1.6:

Electrical performances of BC and TC TFTs based on tBu-BTBT-2, in linear (Vd = -0.1 V) and saturation (Vd = -4.0 V) regime.

The product between 𝑑 ̂ and 𝜀̂ gives four terms. Two terms are related to the normal absorption (emission) of photon associated to a transition to the upper (lower) state, and two terms are related to anti-resonant processes. The latter terms are neglected within the RWA.

Acknowledgments

Results and discussion

VSC experiments were performed in collaboration with Dr. Kripa Joseph, at Supramolecular Science and Engineering Institute (ISIS) of Strasbourg, under the supervision of Prof. Thomas Ebbesen.

List of Abbreviations

AFM

Atomic Force Microscopy BGBC distance) associated to the family planes was calculated, giving values ranging from 1.5 to 3.6 nm (see Table 3.4). The d-spacing values match the length of the unit cell long axis and as well the terrace height steps extracted from AFM images, for each respective compound -except for DBTTT, where terrace height steps correspond to the half of the d-spacing and therefore to the half of unit cell long axis (as already explained). This confirm that the molecules adopt a layered structure both on TDPA/Al2O3 and PFBT/Au, standing approximately upright with respect to the substrates and orienting their unit cell long axis towards the direction perpendicular to the substrate's surface.

In the case of C8-DNTT thin films, the XRD pattern exhibit a peak at 2θ = 22.5°, that is associated to the reflection of (020) plane. This is likely attributed to the reflection of the 3D crystalline aggregates that are massively formed on the terraced structure of C8-DNTT thin films.

At last, the diffraction patterns collected from OSCs deposited onto PFBT/Au substrates show a lower relative intensity compared to the patterns collected from TDPA/Al2O3 substrates. This is caused likely by the rougher and higher X-ray reflectivity of the underneath gold layer. [START_REF] Borchert | On the Minimization of Contact Resistance in Organic Thin-Film Transistors[END_REF] salts in refluxed pyridine to afford C8-DN4T and C10-DN4T in moderate yields, with overall yields ranging from 22 to 28%. Purification of the final compounds was performed by recrystallization in 1,2,4-trichlorobenzene, giving NMR grade pure compounds as shiny gold powders composed of nanoto micrometric flake aggregates. Finally, analytical grade powders could be obtained by careful thermal sublimation of the obtained powders at 10 -6 mbar and 380°C.

Scheme 5.1

Synthesis of tBu-DNTT

Scheme 5.2 shows the synthesis of the benzaldehydes 11 required for preparation of tBu-DNTT. The tert-butyl compound 11 is prepared in four steps starting from the commercial 1-bromo-4-(tert-butyl)benzene. The three first steps leading to the iodo compound 10 are described in a literature procedure and have been repeated. [START_REF] Wegner | Oligoindenopyrenes: A New Class of Polycyclic Aromatics[END_REF] The iodo compound 10 has been obtained in 38% yield from the starting compound 1-bromo-4-(tertbutyl)benzene. The tert-butyl compound 11 has not been reported, yet. It has been synthesized, at -78°C in THF, by iodine-magnesium exchange with iPrMgCl followed by trapping with DMF, in 76% yield. tBu-DNTT has been obtained by the synthetic route shown in Scheme 5.3, inspired by the one developed by Park et al. for the DBTTT. [START_REF] Park | Dibenzothiopheno[6,5-b :6′,5′-f ]thieno[3,2-b ]thiophene (DBTTT): High-Performance Small-Molecule Organic Semiconductor for Field-Effect Transistors[END_REF] tBu-DNTT is synthesized in four steps from the thieno[3,2-b]thiophene and 11. The two first steps of Scheme 5.3 afford dibromo compounds 13 in an 88 % yield. The dialdehyde derivatives 14 has been obtained in 69 % yield. Finally, the cyclization, followed by a dehydration and an aromatization gives rise, after purification by

Synthesis of tBu-DBTTT

tBu-DBTTT was obtained by a similar synthetic route developed by Park et al. for the DBTTT. [START_REF] Park | Dibenzothiopheno[6,5-b :6′,5′-f ]thieno[3,2-b ]thiophene (DBTTT): High-Performance Small-Molecule Organic Semiconductor for Field-Effect Transistors[END_REF] Compound 16 was synthesized from 15 by a halogen dance reaction at low temperature using LDA. The carbaldehyde 16 was then obtained by condensation of DMF on the lithium derivative. The final compound tBu-DBTTT is synthesized with a good yield from the thieno[3,2b]thiophene (33% in four steps). Selective deprotonation at the α position of the thieno[3,2-b]thiophene with n-BuLi, followed by the condensation with carbaldehyde 16, gave diol 17. This compound was reduced without more purification in presence of NaBH3CN and ZnI2 for give the dibromo compound 18. Compound 19 was obtained from compound 11 after lithium/bromine exchange with n-BuLi at low temperatures, followed by the addition of DMF. tBu-DBTTT was obtained by using Amberlyst-15® as a cyclization and aromatization agent in a Bradsher reaction on dialdehyde 12. The final compound was purified by recrystallization to afford a white solid. 𝑔 2 (𝑖) = 𝑉 2 (𝑖)

with Mi the effective mass of mode i.

Mobility Anisotropy

Mobility anisotropy plots were computed using a hopping regime and a Kinetic Monte Carlo algorithm that employs the first reaction method. [START_REF] Troisi | Dynamics of the Intermolecular Transfer Integral in Crystalline Organic Semiconductors[END_REF] Initially, a single charge was localized at a specific site or molecular unit at t=0, and hopping rates were subsequently determined based on the transfer integrals and reorganization energies computed at the quantum-chemical level. Random waiting times were generated for each hopping process, and the process with the shortest waiting time was chosen. The simulation clock was then advanced by the chosen waiting time, and the distance travelled by the charge was incremented by the distance between the centers of mass of the involved molecules in the charge transfer. This cycle was repeated until the simulation clock reached a time limit of 10-9 s. Finally, the mobility was determined using the following equation:

Where E is the amplitude of the electric field (set to 1000 V/cm). Mobilities were computed for different directions of the electric with regard to the unit cell vector a, from θ=0° to θ=350°.

Appendix

List of Publications

Scientific papers

In relation to Chapter 2:

• R.