Structural and charge transporting properties of pure liquid crystalline organic semiconductors and composites for applications in organic electronics

Keywords: organic semiconductors, liquid crystals, photopolymers, doping, time of flight, organic field effect transistors Contents organic semiconductors, liquid crystals, photopolymers, doping, time of flight, organic field effect transistors

This thesis is dedicated to various aspects of liquid crystalline (LC) organic semiconductors (OSCs) in regard to their applications in the field of organic electronics.

The first part of this work deals with a well-known LC OSC based on phenylnaphthalene. Two major ways of performance improvement are proposed and investigated: stabilization of LC structure by in situ photo-polymerization and introduction of electron acceptor doping impurity. In the first case, the influence of polymer network on mesophase order and charge transport is investigated by conventional experimental techniques and Time-of-Flight (TOF) mobility measurements. For the doped materials, ab initio calculations are employed to predict their spectroscopic properties which is exhaustively compared with the experimental data obtained by optical and vibrational spectroscopy. The charge transport is studied by TOF method in the mesophase, while crystalline phase is investigated via conductive atomic force microscopy. A prototype of organic field effect transistor (OFET) is prepared to obtain an estimate of performance for a relevant real-world application.

The second part of this work includes design and synthesis of a novel LC semiconductor based on anthracene, additional attention is made to obtain an easy-to-make and low production cost material. Novel molecule is fully characterized: molecular structure is confirmed by relevant techniques; frontier molecular energy levels are studied by optical spectroscopy and cyclic voltammetry and confronted to values obtained via ab initio calculations; mesophase properties are investigated by optical microscopy and scanning calorimetry. Charge transporting properties are characterized by means of an OFET device: it is found that new anthracene-molecule exhibits significant improvement of field-effect hole mobility over previously studied phenyl naphthalene derivative. Finally, photoconductive properties of the novel material are addressed in order to investigate its potential applications to organic phototransistors.

This PhD thesis is a result of three-year-long journey, which I would not be able to complete alone. This section is dedicated to people who helped me on my way to becoming a Doctor of Philosophy in Physics.

First of all, I would like to thank Pr. Abdelhak Hadj Sahraoui, the director of Unité de Dynamique et Structure des Matériaux Moléculaires (UDSMM), where I have conducted my research. His help and support have allowed me to succeed in this endeavor.

Next, I would like to thank my thesis supervisor, Pr. Christian Legrand, for accepting my candidature for this PhD project, as well as his guidance and thorough help with experimental part, fruitful discussions and his wisdom which altogether benefited the present work.

I am sincerely thankful to my thesis co-supervisor, Pr. Abdelylah Daoudi. I learned a great deal from him. He is somebody who always listens and has an open mind for new ideas and discussions. In our work, he always supported and encouraged me to pursuit my project despite numerous difficulties. His professionalism and experience has helped me to bring out the best of my experimental results and compile this manuscript.

I am grateful to Dr. Yahia Boussoualem, with whom we worked side by side every day for over last three years. He is the person who first introduced me to the topic of liquid crystals, when I arrived at UDSMM for my Master 2 internship. He was my mentor during that time, and with his contribution I have completed my Master studies, which has allowed me to join this PhD project. He was further directly involved with my research, and his availability, help and support were indeed invaluable.

My gratitude is also extended to all committee members: rapporteurs Pr. Bernard Ratier (XLIM, Université de Limoges) and Pr. Jeroen Beeckman (Ghent University) and examiners Pr. Laurence Vignau (IMS Bordeaux, Université de Bordeaux), Dr. Marc Ternisien (LAPLACE, Université de Toulouse) and Pr. Kamal Lmimouni (IEMN, Université de Lille). I am sincerely thankful to them for their time and expertise put into the evaluation of this work, as well as their participation in my defense.

I would like to express my gratitude to our research engineer, Benoît Duponchel, for his hard work and availability. His great experience and deep knowledge of various experimental techniques have enabled me to learn by his side and obtain the results without which the present work would not be possible.

I am thankful to Pr. Stephan Longuemart, who taught me many things about development of experimental setups, instrument programming and signal conditioning. His pedagogical gift has also allowed me to extend my knowledge in condensed matter physics.

I am also thankful to Dr. Dharmendra Pratap Singh, whose great energy, curiosity and cheerful attitude helped to keep my morale up during these years. His ideas always inspired me and his perseverance showed me a great personal example.

I was delighted by the chance to work with my colleagues from UDSMM. I have shared many moments of my work and spare time with them: Michael Depriester, Sylvain Delenclos, Mathieau Bardoux, Fabrice Goutier, Philippe Hus, Abdelaziz Elass, Frederic Dubois, Freddy Krasinsky and others. I am particularly thankful to Veronique and Virginie, whose organizational and administrative talents are greatly appreciated. Also, their homemade cakes and cookies are a true delight.

It would be impossible to accomplish the multidisciplinary goals of my PhD project without the help from other people outside of UDSMM. In the beginning of this PhD, my co-supervisor, Pr. Daoudi, has introduced me to his collaborator, Pr. Nicolae Hurduc from Technical University «Gheorghe Asachi» in Iasi, Romania. His expertise as well as his open mind has helped us to establish the preliminary chemical structure of perspective materials based on 2-amino-anthracene. He has also granted us his support in establishing a collaboration with the Laboratory of Natural and Synthetic Polymers at his university. Professor Dan Scutaru, who was the head of that laboratory at the time, has generously accepted my candidature to visit his laboratory in the end of the second year of my PhD. During the one month stay (which was funded by Collège doctoral Lille Nord de France) I have had chance to work with his team and was thoroughly helped in the synthetic procedures which have resulted in the new material presented in the Chapter 4 of the present manuscript. I am especially thankful to Dr. Irina Carlescu and Dr. Simeon Aurel for their collaborative work and great help.

I express my sincere gratitude to Dr. Pierre-Edouard Danjou from UCEIV, ULCO for his great help with purification of my substances, as well as characterization of their chemical structure.

I am thankful to Pr. Arnaud Cuisset and Dr. Daniel Fontanari from LPCA, ULCO for their support and great advices on the ab initio simulations.

I would like to thank as well Dr. Jean-François Blach from Université d'Artois, who was my supervisor during the second year of my Master studies, where I was first introduced to Raman spectroscopy. He also helped me to carry out the experiments at his facilities in Lens during my second year of PhD.

I greatly appreciate the help received from Pierre Kulinski in preparation of facilities for the practical classes of physics and Amaury Kasprowiak for his help with optical spectroscopy.

I am also thankful to the PhD students and postdocs from ULCO for the good moments which will stay in my memory: Karim, Allen, Eliane, Elie, Nassima, Said, Yoan, Yaochen, Alla, Sarah, Asmita, Clémence, Tarek, Corentin, Somenath and others.

Finally, I would like to express my sincere gratitude to my family, friends and everybody else who supported we along the way. I consider myself lucky to be surrounded by these people during my lifetime. I hope that I did not forget to mention anybody in this section. If I did, please forgive me.

Kirill 17th of December 2019

Propriétés structurales et de transport de charge de semi-conducteurs organiques cristaux liquides purs et de composites pour applications en électronique organique Résumé Cette thèse est dédiée à divers aspects des semi-conducteurs organiques (SCO) cristaux liquides (CL) qui concernent leurs applications dans le domaine de l'électronique organique.

La découverte au début du XIX siècle du phénomène de photo-conductivité dans les cristaux d'anthracène a marqué l'apparition d'une nouvelle classe de matériaux organiques aux propriétés semi-conductrices. Il a été clairement démontré dans les années 1960, par la découverte du phénomène d'électroluminescence dans les matériaux organiques π-conjugué que les SCOs possèdent un réel intérêt pratique. La découverte des propriétés de transport de charge électrique dans le poly-acétylène (Figure 1) en 1977 [1] a apporté le prix Nobel de chimie à Heeger, MacDiarmid et Shirakawa en 2000 pour leur contribution au développement de l'électronique organique. Depuis les années 1980, plusieurs exemples d'applications pratiques (diodes luminescentes, dispositifs photovoltaïques, transistors à effet de champ) ont vu leur performances augmenter de façon remarquable. Aujourd'hui, l'une des applications les plus développés concerne les diodes luminescentes (OLEDs) qui sont en train de remplacer les dispositifs classiques à base des cristaux liquides dans le domaine de l'affichage.

iii

RÉSUMÉ

Bases théoriques

Premièrement, les concepts basiques concernant les matériaux SCO nécessitent une introduction détaillée. Il s'agit de matériaux organiques possédant des structures πconjugué très fortement développées à travers leur structure moléculaire, ce qui permet d'avoir un certain degré de délocalisation de densité des électrons. Les structures conjuguées sont majoritairement constituées d'atomes de carbone en hybridation de type sp 2 (comme dans la molécule d'éthylène, Figure 2,(a)). Cette délocalisation est une condition nécessaire afin de permettre aux porteurs de charge (les électrons et les trous) de se déplacer à travers la structure moléculaire ou une chaine polymérique, ou même sauter d'une molécule à l'autre (souvent appelée "hopping") [2]. Il faut bien souligner que le transport est effectué par les électrons situés dans le domaine énergétique des liaisons π, et les électrons impliqués dans les liaisons σ ne sont pas concernés par ce procédé.

Les notions des semi-conducteurs classiques, dit inorganiques (SCIs) sont bien applicables aux SCOs. Le concept d'une bande interdite (le "band gap", E g ) dans le cas des SCOs est basé sur les niveaux d'énergie des orbitales moléculaires frontières (Figure 2, (b)) : orbitale haute occupée -HO, souvent appelée HOMO (de highest occupied molecular orbital ) et basse vacante -BV ou LUMO (de lowest unoccupied molecular orbital ).

Cette considération est directement liée à la nature moléculaire des matériaux SCOs, qui sont des systèmes à couche d'électrons fermée, contrairement aux SCIs, où le matériau massif est à l'état solide, donc constitué d'atomes liés par des liaisons covalentes. En effet, la grande majorité des particularités des SCOs par rapport aux matériaux inorganiques est liée à leur nature moléculaire. Il s'agit de matériaux constitués de molécules ou de v chaines polymériques qui sont liées par des interactions de Van der Waals [3].

Les exemples typiques des SCOs incluent les molécules aromatiques polycycliques comme les oligoacènes (anthracène, tetracène, pentacène (Figure 3, a)) ou hétérocycles (comme plusieurs dérivés du thiophène). Un autre type des matériaux se compose de polymères conjugués (poly-para-phenylène, poly-fluorene, poly-thiophène (Figure 3,b)). La nature des interactions intermoléculaires intervient directement au niveau des mécanismes de transport de charge dans les matériaux SCOs. La densité de courant électrique j dans le cas d'un matériau massif peut être exprimée à l'aide de l'équation suivante :

j = enµE, (1)
où e représente la charge élémentaire, n la densité des porteurs de charge, µ leur mobilité et E le champ électrique appliqué au matériau. On peut remarquer que j dépend de deux paramètres intrinsèques au matériau : la densité des porteurs de charge n et leur mobilité µ. Ces deux paramètres font l'objet d'une grande partie de la recherche consacrée aux SCOs. La mobilité d'un porteur de charge est généralement très influencée par l'ordre du système, donc pour les matériaux fortement désordonnés (le cas des SCOs) ce paramètre peut atteindre quelques dizaines de cm 2 /(V s) dans le meilleur des cas. La densité des porteurs de charge est directement liée à la densité des états des électrons (ou des trous) disponibles dans un matériau, à la concentration des états "pièges" ainsi qu'à l'énergie de la bande interdite et la température.

Tenant compte de ces considérations, deux voies d'amélioration des propriétés de transport électrique pour les matériaux SCOs ont été proposées: amélioration de la mobilité par augmentation ou stabilisation d'ordre du système, ou accroissement du nombre de porteurs de charge par dopage.

Pour contrôler l'ordre d'un matériau massif d'une manière simple et efficace, on peut faire appel aux cristaux liquides. Ce type de substances possède des propriétés thermodynamiques et structurales très particulières. Généralement ils montrent un comportement dit mésogenique, c'est-à-dire intermédiaire entre l'état solide et liquide : une mésophase [4]. Il existe plusieurs types de mésophases selon la forme de la molécule du cristal liquide : calamitiques ou discotiques. Nous nous sommes intéressé aux phases calamitiques (Figure 4). Les mésophases calamitiques sont formées par des molécules en forme de bâtonnets et sont divisées en deux groupes : nématiques et smectiques. Les phases nématiques ne possèdent qu'un seul degré d'ordre intermoléculaire : les axes longs des molécules (dit directeurs) sont plus ou moins orientés dans une direction préférentielle, en revanche leurs centres de masse sont distribués de manière aléatoire (il n'y a pas d'ordre à courte distance). Les phases smectiques ont une structure plus élaborée : les molécules sont rangées sous forme de couches et leurs axes (directeurs) sont orientés généralement dans une même direction (sauf pour les phases torsadées). Il existe plusieurs types de mésophases smectiques selon l'ordre dans la couche. vii Les phases les plus simples sont (Figure 4) : smectique A (les axes directeurs sont normaux au plan de la couche, il n'y a pas d'ordre dans la couche), smectique C (les molécules ont un certain angle de tilt dans les couches) et smectique B (idem que smectique A, mais avec un ordre local pseudo-hexagonal).

Certains SCOs présentent des propriétés mésogeniques si la forme de la molécule favorise l'organisation en structures mésomorphes. Certains travaux ont également démontré un lien fort de la mobilité des porteurs de charge avec l'ordre de la mésophase.

L'avantage de combiner les propriétés semi-conductrices avec des propriétés mésomorphes est lié à la diversité de moyens qui permettent de contrôler l'ordre et l'alignement d'un matériau par des facteurs externes : champ magnétique ou électrique, photo-alignement ou mécaniquement par l'incorporation de différents types des couches d'alignement. Dans ce travail nous exploiterons l'utilisation de matériaux composites en stabilisant la structure de la mésophase par photo polymérisation in situ. La modification de la densité des porteurs de charge n par dopage constitue une deuxième voie d'amélioration de propriétés de transport de charge. Il est nécessaire de préciser quelques différences importantes entre le dopage d'un SCO et celui d'un matériau inorganique. Dans le cas des matériaux SCI, les porteurs de charge sont créés par l'introduction d'atomes d'éléments chimiques avec un excès (dans le cas de dopage de type n) ou un manque (dopage p) d'un électron, ce qui change la distribution des porteurs de charge libres dans un matériau massif. Cette approche n'est pas applicable aux matériaux SCOs à cause de leur nature moléculaire. Les premiers travaux qui ont abordé ce sujet démontrent l'instabilité et la non-reproductibilité des caractéristiques des viii RÉSUMÉ matériaux organiques dopés à cause de la diffusion et de la migration de sites de dopage.

La solution à ce problème inclut l'utilisation d'une autre molécule avec les niveaux HOMO et LUMO adaptés qui lui permettent d'accepter (dans le cas de dopage p) ou de donner (dopage n) un électron à la molécule SCO et en même temps de rester associé avec elle [5]. L'étude de dopage d'un SCO CL et ses effets sur la structure de mésophase ainsi que le transport des trous constitue une partie des travaux développés dans cette thèse.

Techniques de caractérisation

Les matériaux élaborés à base de cristaux liquides semi-conducteurs ont été caractérisés au moyen d'un ensemble de techniques expérimentales. On peut classer ces techniques selon les groupes suivants : caractérisation structurale et thermodynamique, techniques spectroscopiques, techniques de caractérisation de transport de charge électrique et techniques complémentaires. Le premièr groupe se compose de la microscopie optique sous lumière polarisée qui permet d'observer les textures de mésophases, de la microscopie électronique à balayage (MEB) et de la microscopie à force atomique (AFM) (équipé d'un module C-AFM pour mesurer la conductivité de l'échantillon) pour observer la structure des matériaux à l'échelle nanométrique, de la calorimétrie différentielle à balayage (DSC) pour étudier les transitions de phase et de la diffusion des rayons X (WAXS) afin de caractériser la structure des mésophases. Les techniques de mesure de transport de charge inclut la méthode de Temps de Vol (TOF de time of flight) et la mesure des caractéristiques d'un transistor à effet de champ. La technique TOF est basée sur la mesure du temps de transit d'un porteur de charge (électron ou trou) photo-généré à travers l'épaisseur de l'échantillon soumis à l'action d'un champ électrique continu [6]. Dans un échantillon d'épaisseur connue d, la mobilité µ peut être calculée en utilisant l'équation suivante:

µ = d 2 τ tr V , (2)
où V est la tension appliquée entre deux faces d'un échantillon et τ tr le temps de transit d'un porteur de charge.

Dans cette technique, les porteurs de charge sont générés par un pulse de lumière laser de l'ordre de quelques ns (donc négligeable par rapport à τ tr ) à l'interface entre l'échantillon et l'électrode, ce qui nécessite l'utilisation d'électrodes transparentes en ITO. Le montage expérimental (Figure 7) est constitué d'un laser pulsé à une longueur d'onde λ=355 nm et de durée de pulse de 5 ns, une source de tension continu, un four avec un contrôleur de type PID, une résistance de 1 kΩ et un amplificateur de tension.

Le signal de photo-courant est visualisé sur un oscilloscope qui nous permet d'extraire le temps de transit τ tr en traçant deux ajustements numériques des parties linéaires de la courbe de photo-courant (Figure 8)

La caractérisation des transistors à effet de champ organiques (OFETs) nous permet également de remonter à la mobilité d'un porteur de charge. Cette technique est très avantageuse car elle nous donne l'estimation de la performance d'un matériau dans les conditions réelles. Elle nous permet également de connaître l'influence du procédé de fabrication d'un dispositif OFET sur le transport de charge.

Un OFET est un dispositif actif qui permet de contrôler l'intensité de courant électrique en utilisant l'effet de champ. Ce phénomène décrit le comportement de la densité des porteurs de charge dans un semi-conducteur en fonction d'un champ électrique externe. Concernant les OFETs, on peut appliquer un champ électrique entre les électrodes de grille (G) et de source (S) (Figure 9 (a) et (b)) afin de contrôler le nombre de porteurs de charge disponibles situés près de l'interface SCO/couche d'isolant.

L'une des caractérisations typiques d'un OFET est la caractéristique de transfert (Figure 9 (c), en bleu) où la tension de grille V GS est balayée en fixant la tension entre les électrodes de source et drain V SD . En traçant la racine carrée du courant de canal I SD en fonction de V GS , on récupère la tension de seuil V th à laquelle la voie de conduction se forme (Figure 9 (c), en rouge). La courbe I SD en fonction de V SD nous donne la caractéristique de sortie courant-tension pour les différentes valeurs de V GS (Figure 9 (d)). Ce graphique contient deux régimes de courant I SD : linéaire et saturation. Dans le premier régime, I SD est proportionnel à V SD jusqu'au "point de pincement" où la densité xi de porteurs de charge dans le canal ne dépend plus de la tension V SD . Dans ce régime, I SD est décrit par l'équation suivante [7] :

I DS = W 2L µ F C i (V GS -V th ) 2 , (3)
où W est la largeur du canal, L la longueur du canal (encart de Figure 9 (c)), C i la capacité par unité de surface de la couche d'isolant de grille et µ F la mobilité sous effet de champ.

Matériaux à base de 2-dodecyloxy-6-(4-octylphényle)-naphthalène La première partie de ce travail concerne un SCO CL bien connu à base de phényle naphtalène, le 2-dodecyloxy-6-(4-octylphényle)-naphthalène ou 8-PNP-O12 (SYNTHON Chemicals GmbH & Co. KG (Chemie Park Bitterfeld Wolfen, Allemagne), pureté de 99%.

H 17 C 8 OC 12 H 25
).

Ce matériau possède les propriétés mésogeniques de caractère smectique: Smectique B (Sm B , de 66 • C à 101 • C) et Smectique A (Sm A , de 101 • C à 121 • C).

Le transport des porteurs de charge de ce matériau a été précédemment étudié par de nombreuses équipes de recherche [8,9,10,11,12,13]. La mobilité des trous atteint des valeurs de l'ordre de 10 -3 cm 2 /(V s) en Sm B et presque dix fois moins (10 -4 cm 2 /(V s)) en Sm A . Deux moyens principaux d'amélioration des performances de 8-PNP-O12 sont proposés et étudiés : la stabilisation in situ de la structure du CL par photo polymérisation et l'insertion d'une molécule dopante (accepteur d'électrons).

Dispersion d'un photo-polymère

Dans le premier cas, l'influence du réseau polymère sur l'ordre et le transport de charge est étudiée à l'aide de techniques expérimentales classiques et par la mesure de la mobilité par la technique du Temps de Vol.

Le polymère utilisé dans cette étude est une molécule disponible commercialement chez Merck (RM82) : le 2-methyl-1,4-phénylène bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate) [14], ayant une mésophase nématique de 86 • C à 116 • C. Ce monomère a été utilisé avec un photo-amorceur (Irgacure 369) et un stabilisant thermique 2,6-di-tert-butyl-4methylphenol (BHT).

Nous avons preparé 4 dispersions contenant 1 (g1), 3 (g2), 5 (g3) et 10 (g4) % wt. de RM82. Les échantillons ont été polymérisés en phase Sm B (90 • C), qui ont donnés Afin de caractériser le transport de charge dans les composites préparés, nous avons effectué les mesures de mobilité par la méthode de temps de vol. La mobilité des trous dans les matériaux composites est peu affectée par l'introduction d'un réseau de polymère: on a remarqué qu'elle diminue légèrement avec l'augmentation du taux de RM82 (Figure 13 L'étude des propriétés des matériaux dopés par un accepteur d'électrons constitue l'étape suivante de cette thèse. Dans cette partie nous avons utilisé un accepteur d'électrons bien connu, le 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimèthane (F 4 TCNQ, Figure 14) [15,16,17]. Cette molécule possède un niveau LUMO assez profond (de l'ordre de -5.3 eV) qui nous permet de prévoir un transfert de charge entre cette molécule et le SCO hôte (HOMO de 8-PNP-O12 est de -5.6 eV).

Les calculs ab initio (Gaussian 16, Rev. A03 [18], Figure 15) ont été utilisés pour prédire les propriétés spectroscopiques des matériaux dopés, qui ont été comparées de Cinq mélanges ont été préparés selon la concentration massique du dopant : 0.1 (c1), 0.5 (c2), 1 (c3), 3 (c4), 5 (c5) % wt. La spectroscopie optique nous montre que xv les matériaux composites ont des caractéristiques spectrales très particulières : on voit l'apparition de nouveaux pics d'absorption optique dans le domaine proche infrarouge (à partir de 1.16 eV, Figure 16).

L'intensité de cette réponse spectrale est directement proportionnelle au contenu de F 4 TCNQ. Nous avons également effectué des calculs TD-DFT (time dependent density functional theory) et les avons confrontés aux résultats expérimentaux pour confirmer la formation d'un complex avec transfert de charge (CTC) (Figure 16, marquées avec les étoiles rouges). La spectroscopie Raman nous a permis de confirmer les résultats observés par la spectroscopie d'absorption ainsi que de quantifier ce transfert de charge entre les deux composantes du CTC. Les vibrations moléculaires liées aux systèmes aromatiques de 8-PNP-O12 et quinoïdes de F 4 TCNQ sont clairement affectées par leur interaction lors de la formation du CTC (Figure 17,(a)). Le décalage des fréquences vibrationelles est lié au changement de la longueur des liaisons chimiques dans ces molécules, induit par le phénomène de transfert de charge.

Nous avons choisi le mode de vibration b 1u de la molécule F 4 TCNQ afin d'estimer le degré de transfert de charge δ. Ce mode est accessible uniquement par la spectroscopie FTIR dans la molécule neutre F 4 TCNQ et l'anion moléculaire F 4 TCNQ -à cause des effets de symétrie. Dans notre cas, les contraintes géométriques dans le CTC perturbent la symétrie, ce qui nous permet de sonder ce mode de vibration par la spectroscopie Raman (Figure 17,(b)). Le degré de transfert de charge a été estimé à 0.31, ce qui est en bon accord avec les données issues des calculs ab initio.

Les propriétés de transport de charge ont aussi été étudiées. Nous avons effectuées les mesures TOF pour tous les échantillons (Figure 18, (a)). Ces résultats montrent que la mobilité des trous augmente en Sm B et diminue en Sm A en fonction du taux de dopage. Un prototype de transistor à effet de champ organique (OFET) a été réalisé par spin-coating afin d'obtenir une estimation des performances en vue d'une application potentielle dans les dispositifs électroniques à base de matériaux organiques (Figure 19). Nous avons également découvert que cet effet de photo-conductivité a une nature persistante, c'est-à-dire que la conductivité ne revient pas à la valeur initiale dans l'obscurité après illumination. D'autre part, cet effet peut être géré par application de la tension de grille V GS : les tensions négatives augmentent la réponse de photoconductivité, les valeurs positives effacent l'effet persistant. Nous avons effectué une simple démonstration de la flexibilité des propriétés de 10-OPIA dans la Figure 27. Ces considérations nous suggèrent des applications photosensibles, ainsi que dans le domaine des dispositifs de mémoire ou des capteurs.

Conclusion

Dans cette thèse, nous avons exploré plusieurs aspects des matériaux semi-conducteurs organiques possédant des propriétés de cristaux liquides. Premièrement, 8-PNP-O12 étant un matériau connu, il nous a servi comme fondation solide afin d'aller plus loin pour explorer et améliorer ses propriétés de transport de charge. Les deux approches appliquées (stabilisation par un photo-polymère et dopage moléculaire) ont donné des résultats originaux et intéressants. La deuxième partie de ce travail a concerné la synthèse et la caractérisation d'une molécule nouvellement développée. Cette démarche nous a permis d'obtenir un matériau plus performant et de proposer quelques applications potentielles en relation avec ces propriétés photo-conductrices.

Mots

General Introduction

The This thesis in material science reunites different aspects of chemistry and physics, which means that we will have to address both in order to better explore our subject.

In this manuscript, we have made an attempt to explore the vast domain of organic semiconductors, which deals with complex physical mechanisms of charge transport in condensed matter. To look for the electronic charge transport in organic matter may look quite counter-intuitive, especially for a stranger to this field of research. Indeed, when we hear about products of organic chemistry, we usually think « plastic », which a normal person would think of as an isolating material. However, not all organic materials are created equal. First experimental evidences of charge transport in organic materials date back to the beginning of XX century (more on the history of this subject in the beginning of Chapter 1). Significant research efforts conducted on the frontier of millennia have resulted in an apparition of the whole new class of organic materials -the organic semiconductors, which regroup all the benefits the organic chemistry has to offer (that is, the unprecedented flexibility of chemical structure and synthetic procedure, as well as great production scalability) with charge transporting properties. To date, it is indeed hard to underestimate the significance of these materials -their use in light-emitting applications becomes more and more ubiquitous, especially in the handheld devices and computer and TV screens. Other applications, such as photovoltaics, are still in the stage of development and are yet to become widely commercialized. This brings us to another potential application which we have envisioned for this work -the field effect transistors.

These devices are believed to find their place in fully organic circuits, which will make them solution processable, as well as enable flexible devices which may be produced "en masse" at low cost and high surface area.

In this work, we try to exploit the remarkable dependency of charge transporting properties on structural order of the material. properties of these materials. The maturity of this research field benefits us in a sense of profound understanding of mesophase structure and the means to control their order and macroscopic orientation, which we are going to apply to study charge transporting properties of organic semiconductors.

In order to apply this expertise in liquid crystals to organic semiconductors, we need to focus our attention on a molecule which would combine these properties. It turns out that semiconducting, as well as mesogenic properties may be controlled by tuning the molecular structure. Large aromatic and π-conjugated structures are generally favorable for charge transport due to high delocalization degree of π-electrons. In turn, the overall shape of the molecule, as well as the presence of flexible lateral alkyl chains are believed to govern the mesogenic properties. It is then necessary to find a compromise between these two aspects, since large π-conjugated systems tend to have strong π-π intermolecular interactions, disrupting mesogenic properties and shifting melting temperatures higher.

This makes the creation of a suitable material candidate a non-trivial task.

The experimental work of this thesis would be impossible without the solid understanding of basic physical principals (Chapter 1) as well as a vast array of experimental techniques (Chapter 2). We have put significant attention into the presentation of the most important concepts, including charge transport, doping, photopolymers and different aspects of the field-effect transistors. The experimental techniques cover the necessary structural, energetic as well as electrical properties of the studied materials. We also present the developed experimental procedure for sample preparation.

The experimental part of this thesis is divided in two parts. First part (Chapter 3) deals with a well-known liquid crystalline organic semiconductor based on phenyl naphthalene (8-PNP-O12). We have made attempts to improve its charge transporting properties by preparing composite materials with it. The first approach is related to the liquid crystalline nature of 8-PNP-O12: we employ a photo-polymeric reactive mesogen, which has good physical compatibility with host material in order to stabilize the structure of host's mesophase. The second approach is closely connected to the semiconducting properties of 8-PNP-O12: we perform a process which is called P-doping by introducing an electron accepting impurity in the host material. This has allowed us to improve hole transport in the resulting composite as well as study the interconnection between the doping process and mesogenic nature of 8-PNP-O12. Finally, we have made an attempt to combine these approaches and study the resulting material.

In the second part of this work (Chapter 4), we have created a novel molecule which exhibits semiconducting as well as liquid crystalline properties. This time, we have tried to improve upon the experience which we have obtained in the first part of this work conducted on 8-PNP-O12. We fully characterize this molecule and propose several potential application for this new material.

To summarize, the principal aims of this work include:

• Understanding how the molecular structure influences mesogenic and semiconducting properties.

• Research of new ways to improve charge transport in liquid crystalline organic semiconductors.

• Investigation of multi-component systems in application to organic semiconductors (doping and polymer stabilization).

• Preparation of a novel mesogenic organic semiconductor, study of its potential applications.

Chapter1

Organic semiconductors: concepts and theoretical basics

Introduction

The main objective of the first chapter is to briefly introduce the theoretical material relevant for the present study. It is designed to be accessible for wider public without a specific background in the organic electronics.

Firstly, the organic semiconductors are presented as a material class. Different classes of semiconducting molecules are presented as a function of their chemical structure.

Further, the main theoretical aspects of the research subjects explored in the current thesis are covered, such as molecular doping and photo-activated polymers. The liquid crystals are also introduced, taking an important place in the present study. Finally, some information is given on the basic functioning of a field effect transistor, as well as recent progress in this field.

1.2 Organic semiconductors

History

The advent of semiconductor era has begun in the middle of last century, when the transistor has been invented. Solid state electronic devices based on inorganic semiconductor (ISC) materials such as silicon (Si) and germanium (Ge) have successively eliminated devices based on thermionic emission from almost all applications with rare exceptions.

With overwhelming success of monocrystalline silicon and all the research facilities focused on ISC field, electronic properties of organic substances were somewhat neglected.

While organic materials were known to have dark and photo-conductivity for more than a century (one of the first studies of photoelectric properties of anthracene crystals date back to 1913 [1]), very little effort was initially put into the research of this material class. Some works on optical excitation and charge carrier transport in organic crystals [2] were inspired by the discovery of electroluminescence in 1960s, however practically viable parameters of samples were never attained (poor current density, quantum yield and excessively high operating voltage, notably due to high thickness of prepared samples [3]).

It was not until 1977, when Heeger, MacDiarmid and Shirakawa (awarded with the Nobel Prize in Chemistry in 2000 for their contribution to organic electronics development) discovered conductivity in polyacetylene [4]. Synthesis and doping of π-conjugated polymers have lead to the production of conducting coatings and photo-detectors.

The demonstration of an efficient photovoltaic cell based on heterojunction of organic p-and n-type semiconductors [5], followed by working prototypes of thin film transistors based on π-conjugated polymers [6] [7], has revived the interest of scientific community in intrinsic (undoped) organic semiconductors (OSC).

However, the real breakthrough in application of OSC has been induced by fabrication of electroluminiscent diodes (OLED) based on evaporated thin films of OSC [8] [9] and π-conjugated polymers [10]. OLED field has made significant progress over last twenty years, owing to combined academical and industrial research facilities and nowadays represent a significant market with growth perspectives, being in process of eliminating liquid crystal displays (LCDs) from the market, with Active-Matrix OLED (AMOLED)

technology being one of the most widely employed. Organic photovoltaics (OPV) are on the brink of commercialization, induced by the development of deposition technologies (notably, OSC inks for printing). Logic circuits, based on OFETs are expected to follow in the nearest future.

Structure of organic semiconductors

By the term of organic semiconductors we usually mean solid substances constituted of π-conjugated organic molecules or polymer chains, presented in form of molecular crystals or thin amorphous films, depending on the method of processing. Their semiconducting properties show up upon chemical doping, charge injection or photoexcitation (in case of photovoltaics). A good example of large-scale π-conjugated system is polyacetylene [4] (Figure 1.2), where entire polymer chain consists of sp 2 -hybridized carbon atoms.

C H C H C H C H C H           n Figure 1.2 -Basic π-conjugated system, polyacetylene.
Conjugated π-system leads to band gap formation, which is the one of basic concepts in organic semiconductors physics. From the classic definition for ISCs, this is the energy range between the so-called valence and conduction bands where electron states do not exist. For semiconductors, and what comes from their definition, the band gap lies between 1.5 and 5 eV. Speaking of OSC, the terminology differs slightly. Their band gap energy range is between π (bonding) and π * (antibonding) orbitals while electrons in strong σ carbon-carbon (or other heteroatom) bonds typically do not take part in excitation process, explaining the OSC' structural stability but also the ability to allow its π electrons to participate in charge transport.

The notions of valence and conductivity bands (which in fact comes from ISC domain) also differ: band gap energy being the difference between highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) which are analogous to conduction and valence bands in regular inorganic semiconductors. This emphasizes the attention to molecular nature of OSC: as the molecule or a monomer (in case of polymeric chains) is considered a basic unit of semiconducting substance, which is not necessarily of crystalline constitution, but very often presented in a form of amorphous solid, in contrast to conventional ISCs.

The band gap energy value, as being relative, is particularly important for such applications as LEDs and photovoltaics, since the wavelength of emitted/absorbed light depends on the energy difference between HOMO and LUMO levels (Figure 1.3).

However, for other applications (OFET), values of HOMO and LUMO could be regarded as key parameters as well: for example, a high HOMO level can pose potential barrier for charge injection on metal -OSC interface. Also, lowering HOMO level makes OSC molecule more chemically stable and less prone to oxidation under ambient conditions. For OLEDs and OPVs the wavelength of emitted/absorbed light λ is reverse proportional to band gap energy E g :

λ ∝ 1 E g . (1.1)
Several methods may be used to determine HOMO and LUMO levels: X-ray and inverse photoelectron spectroscopy, optical spectroscopy with conjunction to cyclic voltammetry, with the latter being widely employed. The band gap energy depends on conjugation length, deviations from planarity, aromatic resonance energy (quinoid structure has lower band gap energy than aromatic), inductive * and mesomeric † effects and intermolecular or interchain interactions. All these factors are interrelated, which makes the band gap engineering not an easy task.

• Conjugation length is the size of double-single bond alternating backbone: increase of conjugation length reduces band gap energy level. Since the light absorption process is strongly related to band gap energy E g value (equation 1.1), it is a very intuitive way to compare E g by comparing light absorption characteristics. The family of oligoacenes (Table 1.1) illustrates this correlation well [11].

• Separate molecular orbitals do not exist for the material in bulk: the interactions between molecules smear HOMO and LUMO levels in more band-like structure (Figure 1.3), which once again lowers the band gap. This phenomena is related to * The inductive effect is related to the difference of electronegativity between the atoms of host structure and substituent group, which manifests as a change in electron density of the molecule.

† The mesomeric effect is related to the interactions between the π-conjugated bonds through resonance stabilization, and helps to describe electron density distribution in the molecule. energetic disorder which is induced by intrinsic (crystalline grain boundaries and structural disorder) and extrinsic (impurities) trap states and low dielectric constant of material. For polymers, the repetition of monomer unit also rapidly reduces the band gap energy until certain effective conjugation length [12].

• The delocalization degree has direct impact on the band gap energy: the more electrons are delocalized, the more OSC behave similar to metals. This allows to tune the band gap by either localizing electron density in the monomer or delocalizing it across the polymer chain.

Aromaticity lowers the delocalization degree across the chain and increases band gap [13]. Increase of torsion angle in the system decreases the interaction with neighboring π-orbitals with an angle of 90 • disrupting the delocalization completely.

This underlines the importance of steric factors for the performance of π-conjugated polymers.

• Various substituents of conjugated system induce their effects on HOMO/LUMO levels: electron acceptor groups lower HOMO and LUMO, simultaneously bringing LUMO level closer to HOMO, decreasing the band gap energy, contrary to electron donor groups. This defines the combination of functional groups as another way to tune the band gap energy.

Charge transport in organic semiconductors

The current which flows through the material on macroscopic scale may be expressed via following equation:

j = env = enµE, (1.2)
where j is the current density, n is the number density of charge carriers and v is carrier drift velocity which is defined as a product of mobility µ and applied field E. Two of these parameters (n and µ) are intrinsic to the particular OSC substance. To date, charge carrier mobility µ of OSC at their best is comparable only with amorphous silicon (tenths of cm 2 /(V s)). Carrier density n also deserves a profound discussion.

The electronic properties of OSC are governed by these two parameters, which in turn depend on numerous factors. Basically, these factors have intra-molecular (or intrachain in case of polymers) and intermolecular (or interchain) origins. Intra-molecular factors include the backbone structure, conjugation length, various functional groups, either providing or withdrawing electron density to the π-conjugated system, breaks in conjugation chain and other structural deviations. Intermolecular factors concerns larger scale -molecular packing (herringbone or lamellar pattern, see subsection 1.5.3) which influence π -π stacking efficiency, chain length for polymers, phase nature for liquid crystal, etc.

Even in case of semiconducting polymer, there is never any chain crossing entire sample.

But how in this case charge carrier is able to cross a long distance (compared to molecule length or chain size)? The possibility of effective charge transport between molecules and chains in the bulk is provided by π-π stacking interactions [14]: π-conjugated structures tend to aggregate in a sandwiched manner (Figure 1.4), bringing them close enough to allow charge hopping. Band transport model is valid mostly for monocrystalline structures of high purity and on low temperatures, where scattering due to lattice vibrations is minimized.

The interaction energy J (often called bandwidth or transfer integral ) between neighboring sites of lattice is sufficiently high to overcome the effects of static and dynamic disorder which results in formation of conduction band. It is only true if the band is wider than energetic uncertainty of charge carrier (in case of metals, for example, J ≈ 1 eV) [15]. Materials, studied in this work were not examined under conditions favorable for band transport.

Bässler's Gaussian disorder model explains the contrary case of band transport model.

It describes charge carrier transport in disordered or amorphous solids, where excitations sites are localized on separate molecules, polymer chains or chain segments. This model introduces temperature dependence of mobility, since the charge carrier moves by "hopping" in the broad density of states, thus requiring thermal activation in order to overcome energy difference in different lattice sites [16]. Behavior of carrier mobility is described by following equation:

µ = µ ∞ exp - 2σ 3kT 2 exp C 0 σ kT 2 -Γ 2 √ E , (1.3)
where E and T are applied field and temperature, respectively, µ ∞ is the value of mobility at infinite temperature and zero applied field, σ is energetic disorder, Γ is positional disorder (between neighboring "hopping" sites), C 0 is an empirical constant (which may be considered to be 2.9 × 10 -4 (cm/V) 1/2 for a system with cubic lattice constant of 6 Å) and k is the Boltzmann constant.

Holstein's small polaron model describes the coupling of electron excitation to intramolecular or intermolecular vibrations [17]. In this model the charge is represented by a localized polaron with a hole-electron binding energy E p , and the carrier mobility is described by the following equation:

µ = 1 (kT ) 3 2 ed 2 π 2E p 1 2 J 2 exp -E p 2kT , (1.4)
where d is the intermolecular distance, J is the transfer integral between neighboring sites and is the reduced Planck's constant. The importance of this theoretical approach emerges more clearly in comparison to previously presented Gaussian disorder model of transport: since the equation contains both exponential and preexponential factors of temperature which provided with certain values of binding energy and transfer integral (as a more consistent way to describe intermolecular interactions, which also may be accessed through ab initio calculations) parameters will lead to weak temperature dependency of mobility in limits of a given mesophase, often encountered in pure liquid-crystalline OSC [18] [19].

The density of charge carriers can be expressed as [16]:

n = N 0 exp - E g 2kT , (1.5)
where N 0 is the effective density of states * (DOS). We may use the parameters corresponding to the Si (E g =1.12 eV and N 0 = 10 19 /cm 3 ) to obtain intrinsic carrier density n Si = 3.35 × 10 9 /cm 3 at room temperature (298.15 K) and compare it to the value calculated for pentacene (E g =1.9 eV and N 0 = 7.5 × 10 16 /cm 3 [START_REF] Puigdollers | Density-of-states in pentacene from the electrical characteristics of thin-film transistors[END_REF]) which yields carrier density of about n P c = 6.35/cm 3 (this estimation provides lower border value, since any impurities will contribute to the resulting DOS). This estimation allows us to better understand the very low intrinsic conductivity of highly pure OSCs.

Doping in organic semiconductors

Since the intrinsic DOS of OSC is low, and their band gap energy is much larger than of their inorganic partners, intrinsic carrier density in organic substances is by orders of magnitude lower (equation 1.5). To obtain desirable charge transporting properties, it is often necessary to increase carrier density n by means of "doping". The conception of doping is very different for OSC (compared to traditional silicon-based materials) as it includes not only "classic" chemical doping, but any means of creating charge carriers in organic semiconducting system, for example, photoexcitation (photovoltaics and time-offlight experiment), field-effect or direct charge injection from contacts (transistors and photoluminiscent diodes) which are also considered as doping. Chemical, or more correct to say, molecular doping in case of OSC is an effective way to increase charge carrier density, but is considerably more complicated compared to conventional processing of inorganic semiconductors due to the nature of OSC. Elemental doping (iodine doping of polyacetylene [START_REF] Beniere | Doping of polyacetylene by diffusion of iodine[END_REF], alkaline metal doping [START_REF] Ding | Alkali metal doping and energy level shift in organic semiconductors[END_REF]) of organic materials does not give desirable results due to diffusion of dopant species: migration of doping sites prevents from obtaining stable and reproducible composites. Next logical step in this direction is to use organic molecules with desired properties. However such substances are not easy to obtain. P-doping could be achieved by transferring an electron from HOMO of OSC to the LUMO of dopant. During n-doping the electron is transfered from HOMO of the dopant to LUMO of the OSC (Figure 1.5). In order to be efficient, a p-type dopant should have its LUMO close to the HOMO of the host semiconductor. This gives rise to another problem: since most of OSC have quite a large band gap, hence their HOMO lies quite deep (about -5 eV to -6 eV), implying the p-dopant to have very high LUMO providing pronounced electrophilic properties. The opposite is true for a n-type dopant, which molecule typically should have its HOMO above -2.5 eV to -3.5 eV. In addition, this makes it unstable in atmospheric conditions, because it would be readily oxidized.

Doping also results in shift of HOMO and LUMO levels of composite material, as well as band gap reduction, which sometimes could be beneficial for applications in electronics, but is able to make final result hard to predict.

P-doping of the classic OSC materials such as polythiophenes [START_REF] Pingel | Effect of molecular p-doping on hole density and mobility in poly ( 3-hexylthiophene ) Effect of molecular p-doping on hole density and mobility[END_REF] and pentacene [START_REF] Heimel | On the fundamental processes in molecular electrical doping of organic semiconductors[END_REF]25] has been thoroughly explored by numerous groups: it has been confirmed that introduction of strong electron acceptors, including the various substituted derivatives of tetracyanoquinodimethane (TCNQ) [START_REF] Pfeiffer | Controlled p-doping of pigment layers by cosublimation: basic mechanisms and implications for their use in organic photovoltaic cells[END_REF][START_REF] Avilov | Quantum-Chemical Characterization of the Origin of Dipole Formation at Molecular Organic/Organic Interfaces[END_REF]28] and other molecules possesing deep LUMO levels [START_REF] Pfeiffer | Doped organic semiconductors: Physics and application in light emitting diodes[END_REF][START_REF] Salzmann | Intermolecular hybridization governs molecular electrical doping[END_REF] results in substantial increase of the conductivity (sometimes even of several order magnitude [START_REF] Kroon | Polar Side Chains Enhance Processability, Electrical Conductivity, and Thermal Stability of a Molecularly p-Doped Polythiophene[END_REF]). These results have given rise to the profound interest in the closer inspection of intermolecular charge transfer (which is not a new subject by itself [START_REF] Mulliken | DONOR-ACCEPTOR COMPLEXES[END_REF]) in application to organic electronics [START_REF] Lussem | Doped organic transistors operating in the inversion and depletion regime[END_REF][START_REF] Xu | Functional solid additive modified PEDOT:PSS as an anode buffer layer for enhanced photovoltaic performance and stability in polymer solar cells[END_REF][START_REF] Liu | Improving the charge carrier transport of organic solar cells by incorporating a deep energy level molecule[END_REF]36].

Doping of some OSCs results in the apparition of ionized states within the bulk of the OSC. However, it has been recently demonstrated that this concept does not apply to every OSC. When an intermolecular charge transfer takes place, two possible options are usually considered [37]: so-called integral charge transfer (ICT) and partial charge transfer (PCT). The concept of ICT has dominated the field, particularly for classic OSCs with relatively shallow band gap and high lying HOMO, with poly(3-hexylthiophene) (P3HT) [38]. However, not every OSC with an appropriate level of HOMO will forcibly undergo an integral charge transfer. In fact, Mendez et al. [39] has confronted the ICT in P3HT -2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) system with the observable PCT signatures for the quaterthiophene (4T) oligomer. The charge transfer degree for 4T was only of 0.24 [39] as compared with that of fully ionized P3HT.

To better understand the concept of PCT, it is of interest to take a closer look into the nature of molecular orbitals (MOs). A classic example for this is the hydrogen molecule (Figure 1.6, a): each hydrogen atom possess one electron on its 1s atomic orbital (AO).

The electronic interactions between two hydrogen atoms give rise to the splitting of two [START_REF] Heimel | On the fundamental processes in molecular electrical doping of organic semiconductors[END_REF].

The rudimentary example of He dimer allows to better understand the behavior of OSC molecules, since most of them are closed-shell systems (HOMO is doubly occupied).

In the case of good alignment between two adjacent molecules the MO splitting may arise, though the dimer is still held with only Van der Waals interactions (Figure 1.7a, a).

This where the concept of PCT between the host OSC and the dopant applies a more elaborated approach. As discussed earlier, in case of good frontier MO energy alignment between the two molecules (donor and acceptor), they form a so-called charge transfer complex (CTC) which is regarded from the supramolecular point of view: the electron pair from the donor's HOMO is not entirely transferred to the LUMO of the acceptor, but is situated on a new supramolecular orbital which is hybridized during the complex "bonding" [START_REF] Salzmann | Intermolecular hybridization governs molecular electrical doping[END_REF](Figure 1.7a, b). This MO has lower energy than initial HOMO of the donor and LUMO of the acceptor, which makes the complex state more beneficial and thermodynamically stable. Thus, the electron neither truly leaves the HOMO of the donor nor is fully transferred to the LUMO of acceptor, but is continuously "shared" between two molecular components [START_REF] Heimel | On the fundamental processes in molecular electrical doping of organic semiconductors[END_REF]. It has been widely demonstrated for doped systems to exhibit an increase of charge carriers density [START_REF] Walzer | Highly efficient organic devices based on electrically doped transport layers[END_REF][START_REF] Pingel | Effect of molecular p-doping on hole density and mobility in poly ( 3-hexylthiophene ) Effect of molecular p-doping on hole density and mobility[END_REF]37]. However, further consequences of this process are not sufficiently addressed. It is expected that a change in electron arrangement of two neighboring molecules of the donor and acceptor substances may lead to the change in their respective geometry, which may be further expressed as a change in their phase structure at higher scale [START_REF] Ouyang | Tuning morphologies and field-emission properties of CuTCNQF AgTCNQF nanostructures[END_REF][START_REF] Xiao | Growth, patterning, and one-dimensional electron -Transport properties of self-assembled Ag-TCNQF4 organic nanowires[END_REF]43,[START_REF] Thomas | Charge Transfer Doping Induced Conformational Ordering of a Non-Crystalline Conjugated Polymer[END_REF]45]. For the OSCs, the molecular geometry and order are believed to be the ones of the key parameters governing their performance, notably, the charge carrier mobility [46].

One of the ways to explore the influence of the doping on these parameters is to utilize an OSC which order can be effectively controlled. Liquid crystalline OSCs (LC OSCs) provide a viable tool to govern the large and small scale order of the system by passing from one phase state to another on changing the temperature. Indeed, many groups have previously employed LC OSCs to explore the link between the molecular order and the performance of the material since the discovery of the photoconductivity in LC OSC [START_REF] Closs | Liquid crystalline photoconductors[END_REF].

Applications and processing

To date, two widely commercialized applications of OSC exist: OLEDs and xerography.

Xerography was invented in 1939 [48] and utilizes photoconductive properties of OSC to pattern the image by light. OLEDs in turn are gaining popularity and effectively pushing out their inorganic counterparts out of market. Organic solar cells have also begun to enter the market. For example, danish startup InfinityPV (founded in 2014) offers flexible solar cells and mobile phone chargers. It is impossible for OPVs to compete with monocristalline Si or perovskite solar cells in terms of conversion rate, however they are significantly easier to produce. With the technology becoming more cost effective every year, it is possible for OPVs to become industrial standard for solar energy conversion.

Since one of the main advantages of OSC is their unique flexibility in relation to industrial processing, huge efforts are being made in order to optimize production process. There are numerous ways to produce an OSC device, which choice would largely depend on physical and chemical properties of semiconductor molecule:

• Drop casting The most plain way to obtain an OSC film. The substance is dissolved in conventional organic solvents and then casted on previously prepared substrate, following with spontaneous solvent evaporation. The thickness of obtained film is directly proportional to the OSC concentration in working solution. This method is very simple and cost-efficient: no solution is lost. However, it possesses severe disadvantages: the area of coverage is limited, which in turn imposes difficulties in controlling film thickness. It is very hard to control film morphology,which is only possible via solvent combination and/or alteration of speed of evaporation by changing ambient conditions.

• Spin coating This method utilizes centrifugal force to control the thickness (get the limits of thickness) of obtained film and allows to achieve good uniformity and reproducibility of results. Thickness of deposited film is inversely proportional to the angular speed of spinning substrate and also depends on solvent choice and OSC concentration.

While being much more advanced than simple drop casting, this method has some disadvantages related to its principle: the area of spinning substrate is limited due to high angular speed (speed limit) and significant quantity of solution is wasted during process while solvent on deposited part evaporates much faster, leaving less time for molecule ordering (some way to overcome this downside is to introduce post-deposition thermal annealing [START_REF] Mcculloch | Liquid-crystalline semiconducting polymers with high charge-carrier mobility[END_REF] and to use slower evaporating solvents [START_REF] Chang | Enhanced mobility of poly ( 3-hexylthiophene ) transistors by spin-coating from high boiling point solvents[END_REF].

This technique is going to be described in more details in the Chapter 2.

• Blading

The basic principle of blading (and closely related screen printing) technique is to spread the working solution with a micrometer-controlled blade on moving or stationary substrate. This helps to attain high uniformity of film on relatively large surface without wasting solution. However, the dimensions of blade and the fact that it is controlled mechanically impose limit on attainable film thickness Thermal evaporation methods are not the only ones for insoluble OSC deposition. It is possible to prepare an OSC film by using soluble precursor with any of solution processing techniques and then to transform the obtained film to the desired OSC by post-treatment (electropolymerization of pyrrole, for example).

(> 150 ÷ 200nm)

Classification of organic semiconductors

This section does not attempt to fully cover all organic semiconductor types, but serves to demonstrate significant diversity which exists in the field of OSC. These organic compounds are usually divided in two large sub-groups: semiconducting polymers and so-called small molecules. This classification presents two distinct set of requirements, approaches to characterize and improve charge-transporting properties of these materials.

Polymeric semiconductors are historically the first regarded type of OSC, and their

most

Polymeric OSCs

Members of this group are essentially polymers or co-polymers of various aromatic and π-conjugated structures, interconnected in a way which maintains π-π conjugation along the polymeric chain and may include different sidechain substituents, often alkyl chains with varying degree of ramification. These sidechains allow the polymeric structure to be readily solubilised in common solvents, effectively decreasing intermolecular Van der Waals interactions. Most widely employed monomer units include phenylene, fluorene, thiophene and other heterocycles (Figure 1.8). Notable examples include polyparaphenylene (PPP, Figure 1.8, a), paraphenylenevinylene (PPV) as well as their alkyl substituted versions (for example, MEH-PPV with improved solubility). They are relatively simple and mass-produced OSCs, which have found their use in OLED industry [START_REF] Grem | Realization of a blue-light-emitting device using poly(pphenylene)[END_REF] [53] [START_REF] Yang | Efficient blue polymer light-emitting diodes from a series of soluble poly(paraphenylene)s[END_REF] and exhibit low charge carrier mobility of order from 10 -7 cm 2 /(V s) to 10 -5 cm 2 /(V s) [START_REF] Campbell | Consistent time-of-flight mobility measurements and polymer light-emitting diode current-voltage characteristics[END_REF]. (BBL), a so-called ladder polymer, which is reported to have n-channel OFET mobility of 0.1 cm 2 /(V s) [START_REF] Babel | High Electron Mobility in Ladder Polymer Field-Effect Transistors[END_REF]. Other examples are some fluorinated copolymers,naphthalene dicarboximide, pyridinium phenylene, etc.

R           n (a) R           n (b) S R           n (c)

Small molecule OSCs

The concept of small molecule OSCs consists of relatively large π-conjugated core with optional substituents enhancing its solubility. Building blocks for core structures usually include but not limited to mono-or polycyclic aromatic hydrocarbons (benzene, naphthalene, other oligoacenes and their structural isomers), heterocycles (thiophene, pyrrole, imidazole) and their fused variations as well as their combinations with unsaturated aliphatic groups (olefinic and acetylenic). Similar to conventional ISCs, their organic incarnations can favor either p-or n-type transport: P-type small molecule OSC are mostly represented by polycyclic aromatic hydrocarbons (for example, tetracene and pentacene (up to 5 cm 2 /(V s) [START_REF] Kelley | High Performance Organic Thin Film Transistors[END_REF]) (Figure 1.9, a)), their isomers (pyrene and perylene) and derivatives (rubrene (13 cm 2 /(V s)

(a) (b) O O CH 3 (c)
[62], Figure 1.9, b) and (triisopropylsilyl)ethynyl (TIPS) functionalized pentacene, 2.4 cm 2 /(V s) [START_REF] Hamilton | High-Performance Polymer-Small Molecule Blend Organic Transistors[END_REF]) and are able to compete with amorphous Si in terms of mobility (tenths of cm 2 /(V s)). Heterocyclic molecules have also proven their place in this class of materials: significant amount of studies was conducted on oligothiophenes [START_REF] Garnier | Molecular engineering of organic semiconductors: design of self-assembly properties in conjugated thiophene oligomers[END_REF], derivatives of tetrathiafulvalene [65] and other thienoacenes [66]. Molecules which contain nitrogen atoms are also not rare and include phtalocyanine [START_REF] Hu | The application of Langmuir-Blodgett films of a new asymmetrically substituted phthalocyanine, amino-tri-tert-butyl-phthalocyanine, in diodes and in all organic field-effect-transistors[END_REF] and porphyrin [START_REF] Che | A High-Performance Organic Field-Effect Transistor Based on Platinum(II) Porphyrin: Peripheral Substituents on Porphyrin Ligand Significantly Affect Film Structure and Charge Mobility[END_REF] derivatives with their organometallic complexes [START_REF] Wang | Semiconducting π-conjugated systems in field-effect transistors: A material odyssey of organic electronics[END_REF].

N-type small molecule OSC are less numerous than P-type ones, similarly to conjugated polymers. Compounds based on fullerene C 60 , which have high electron affinity (LUMO about 4 eV) have attracted the most attention and have been thoroughly studied in application to OPVs [START_REF] Li | Functional fullerenes for organic photovoltaics[END_REF] and OFETs [START_REF] Thomas | High performance n-channel organic field-effect transistors and ring oscillators based on C60 fullerene films[END_REF]: for example, phenyl-C61-butyric acid methyl ester (PCBM, Figure 1.9, c) has demonstrated good solubility and very high field effect electron mobility up to 6 cm 2 /(V s). Other more exotic subtypes of N-type small molecule OSC include fluorinated derivatives of acenes or compounds incorporating cyano-and carbonyl functional groups.

Liquid crystalline OSCs

The majority of liquid crystalline OSCs may be regarded as small molecule OSC which exhibit mesogenic behavior (though some examples of lyotropic behavior in conjugated polymers have been reported [START_REF] Gi | A molecular design principle of lyotropic liquid-crystalline conjugated polymers with directed alignment capability for plastic electronics[END_REF]). This concept is particularly attractive since liquidcrystalline properties offer exceptional control over intermolecular order. However, indepth review of LCs is not an objective of this thesis, and detailed physical and thermodynamical properties are not going to be covered here [START_REF] Ellmer | Liquid Crystalline Semiconductors[END_REF], but only the properties which are relevant to charge transport.

Calamitic organic semiconductors

This type of LCs consists of rod-like molecules with π-conjugated core with one or more often two aliphatic side chains which are responsible for their solubility and mesogenic properties. They form relatively fluid mesophases which still retain different degrees of order for their molecular long-axes (directors).

The most common and primitive case of mesophase is nematic (N ) phase [START_REF] Ellmer | Liquid Crystalline Semiconductors[END_REF]. In this phase the molecules have only one degree of order: the directors of molecules stay more or less parallel over a significant (for a microscopic scale) distance. Molecules are free to move along their molecular axes and therefore their mass centers are randomly distributed (Figure 1.10). Generally, nematic phase has low viscosity and is easily aligned by moderate electric or magnetic field and spontaneously align on pre-treated surfaces.

Another subtype of nematic phase is a so-called twisted nematic phase which is formed by chiral molecules and produces helix-like structures, however, this phase is not covered in this section.

Next step in mesophase ordering is the family of smectic phases. Smectic mesophases have two-dimensional order: in addition to director collinearity, the molecules are organised in layer structures. For smectic A (Sm A ) phase, the molecules are aligned mostly parallel to the layer plane normal, when for the more-ordered smectic B (Sm B ) phase the molecules tend be more strictly ordered and to form hexagonal pattern within the layer plane These smectic phases exhibit long range orientational order and short range positional order. The phases with both long-range orientational and positional order are often called smectic crystals and are outside of the subject of this study. Generally speaking, improved order of a mesophase is induced by higher interaction between molecules, which in turn renders this mesophase less susceptible to alignment control. McCulloch and T. Kreouzis [START_REF] Mcculloch | Electrical properties of reactive liquid crystal semiconductors[END_REF] [82], yielding hole mobility values in their most ordered mesophase up to 0.1 cm 2 /(V s) [START_REF] Funahashi | High carrier mobility up to 0.1 cm2 V-1 s -1 at ambient temperatures in thiophene-based smectic liquid crystals[END_REF] for quaterthiophene, 0.02 cm 2 /(V s) for phenyl trithiophene [START_REF] Funahashi | High ambipolar mobility in a highly ordered smectic phase of a dialkylphenylterthiophene derivative that can be applied to solution-processed organic field-effect transistors[END_REF] and 0.044 cm 2 /(V s) [START_REF] Mcculloch | Electrical properties of reactive liquid crystal semiconductors[END_REF] bisphenyl bithiophene-based molecules. LC OSCs tend to exhibit smectic phases with an exception for thienylethynyl terthiophene (Fig-

R S R S R S S R S S (a) Bisphenyl bithiophene (b) Quaterthiophene R S R S R R (c) Bisthienyl naphthalene (d) Phenyl naphthalene R S S R S S S R S R S (e) Bisthienyl bisthienothiophene (f) Benzothieno-benzothiophene R S S R S S S
(g) Thienylethynyl terthiophene Figure 1.12 -Examples of calamitic LC OSC cores, where R is the lateral alkyl chain. ure 1.12, (g))-based molecule, posessing also a nematic phase and showing relatively high mobility of 0.02 cm 2 /(V s) [START_REF] Albert | Large area liquid crystal monodomain fieldeffect transistors[END_REF] for the hexyl-substituted derivative. Fused core structures tend to possess higher mobility values (which can be explained by improved conjugation), however that is not always true. For instance, bisubstituted derivatives of bisthienylbisthienothiophene (Figure 1.12, (e)) attain mobility up to 0.02 cm 2 /(V s) [START_REF] Melucci | Liquid-crystalline rigid-core semiconductor oligothiophenes: influence of molecular structure on phase behaviour and thin-film properties[END_REF], and for octyl-substituted 2,5-bisthiophene thieno [3,2b]thiophene values up to 0.07 cm 2 /(V s) [85] were obtained. Bisthienyl-benzene with 0.1 cm 2 /(V s) [START_REF] Oikawa | A novel calamitic mesophase semiconductor with the fastest mobility of charged carriers: 1,4-di(5'-octyl-2'-thienyl)benzene[END_REF], -naphthalene (Figure 1.12, (c)) with 0.14 cm 2 /(V s) [START_REF] Shimizu | Mesophase semiconductors: The alignment control and selfassembling nature for transistor applications[END_REF], -anthracene with 0.48 cm 2 /(V s) [START_REF] Yamada | FET performance and substitution effect on 2,6-dithienylanthracene devices prepared by photoirradiation of their diketone precursors[END_REF] show, however, a remarkable succession in dependency of mobility on the core conjugation length.

One of the most interesting groups to date are the derivatives of [1]benzothieno[3,2-b]- [1]benzothiophene (Figure 1.12, (f)) which have outstanding charge carrier mobility. In particular, 2,7-dioctyl [1]benzothieno [3,2-b][1]benzothiophene [START_REF] Ebata | Highly soluble [1]benzothieno[3,2-b]benzothiophene (BTBT) derivatives for high-performance, solution-processed organic field-effect transistors[END_REF] prepared by Ebata et al. in 2007 has record mobility up to 43 cm 2 /(V s) [START_REF] Yuan | Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method[END_REF] measured in a highly-aligned film.

Discotic organic semiconductors

Discotic LCs represent another concept of mesogenic compound, notably a disc-shaped molecule. In relation to OSC, they usually have a large aromatic core with radial symmetry and several alkyl chains attached to perimeter of the core. Thereby, their structure is more adapted to be used as a high-performance OSC: large conjugated core allows to attain higher levels of π-interaction and is possibly more effective as a charge transporting moiety.

Discotic Nematic

Columnar Nematic Columnar Hexagonal which is in some way analogous to calamitic nematic: the planes of molecules are ordered in a more or less parallel disposition. Another subtype of nematic family is the columnar nematic (N Col , Figure 1.13) phase. Here, the elementary unit of system is not a single molecule (encountered in N D case) but a relatively short column of stacked molecules [91]. These units form a nematic "array", in a similar way with calamitic nematics. Both of these cases are rare among the discotics, proving the nematic ordering to be not largely inherent for this family of LCs.

Columnar mesophases occur more frequently. In columnar phases stacks of molecules are arranged in parallel structures, but they also have a certain degree of pattern-like ordering, which in turn is labeled by letters (in fact, very similar to the classification of smectic mesophases). As an example, the columnar hexagonal phase Col H (Figure 1.13) strongly resembles Sm B mesophase by means of its hexagonal pattern. The degree of order inside of a column also varies from substance to substance, and for same phases the molecules can possess significant tilt.

Most notable examples of π-conjugated structures of discotic LC OSC are perylene (often in a form of diimide), triphenylene, coronene (or other fused arenes), porphyrins, phtalocyanines and sometimes organometallic complexes. Perylene diimide (Figure 1.14, (a)) is a well-known n-type OSC, exhibiting π-stacking columnar properties [92], its derivatives are capable of high electron mobilities (0.6 cm 2 /(V s) [93]). Another similar structure, pyrene is well-known for its high fluorescent properties, discotics based on this core have interesting perspectives in light-emitting applications [START_REF] Hayer | Highly Fluorescent Crystalline and Liquid Crystalline Columnar Phases of Pyrene-Based Structures[END_REF]. Pyrene is also often used as a substitutent for discotic dendritic structures [START_REF] Teresa | Pyrene-Based Materials for Organic Electronics[END_REF]. However, one of the most interesting groups of discotic OSC are triphenylenes (Figure 1.14, (b)). A whole family of alkyl-substituted triphenylene derivatives exhibit hole mobility in order of 1 × 10 -3 cm 2 /(V s) for pentyl (R=C 5 H 11 ) lateral chain [START_REF] Hayer | Highly Fluorescent Crystalline and Liquid Crystalline Columnar Phases of Pyrene-Based Structures[END_REF] and up to 0.018 cm 2 /(V s) in case of shorter butyl (R=C 4 H 9 ) substituents [START_REF] Simmerer | Transient photoconductivity in a discotic hexagonal plastic crystal[END_REF]. Structures which possess bigger π-conjugated core such as hexabenzocoronene (Figure 1.14, (d)) exhibit much higher mobility, however it is very difficult to control the sample alignment. For decyl (R=C 10 H 21 ) substituted hexabenzocoronene, an extremely high mobility of 0.5 cm 2 /(V s) [START_REF] Atwood | Record Charge Carrier Mobility in a Room-Temperature Discotic Liquid-Crystalline Derivative of Hexabenzocoronene[END_REF] was observed on single-column scale, attaining however only 1 × 10 -3 cm 2 /(V s) [START_REF] Kastler | Room-temperature nondispersive hole transport in a discotic liquid crystal[END_REF] in the bulk. Phthalocyanines (Figure 1.14, (c)) and porphyrines were also extensively studied [START_REF] Fujikake | Time-of-flight analysis of charge mobility in a Cu-phthalocyaninebased discotic liquid crystal semiconductor[END_REF] [100] [START_REF] Schouten | Charge migration in supramolecular stacks of peripherally substituted porphyrins[END_REF], yielding hole mobility of about 0.2 cm 2 /(V s) in the bulk. These examples allow us to estimate the orders of carrier mobility value, typically obtainable for this class of materials.

To conclude, liquid crystalline semiconductors show best mobility values at their most ordered mesophase. But, for seemingly the most ordered crystalline phase, grain boundaries are significantly limiting their performance. Numerous studies have been conducted on this subject, and it is safe to say that there is a lot to be done in order to understand and take into account all the factors which influence semiconducting properties of organic molecules.

O N R O O N R O R R R R R R (a) Perylene diimide (b) Triphenylene N N N N H N N N NH R R R R R R R R R R R R R R (c) Phthalocyanine (d) Hexabenzocoronene

Photopolymers

Photopolymer is a composite of monomer and corresponding photoinitiator, which undergoes structural changes (construction of polymeric network, so-called curing) induced by light irradiation. The photopolymerisation process is followed by significant change of physical properties (hardening, formation of resins) which has a wide variety of applications:

varnishes, enamels, glues and photoresists, etc.

In application to OSC one should not confuse polymeric semiconductors with photopolymers, as for polymeric OSC the monomer units are π-conjugated with each other along the whole backbone of polymeric chain, but for now no method exist to create a π-π conjugated bond via conventional photopolymerizable groups. Photopolymerizable OSC could be a small semiconductor molecule, often of liquid crystal type and carry their functional groups on the end of an aliphatic chain. Excess of photoinitiator left after curing can also become a source of defects in liquid crystalline media and result in charge trapping [START_REF] Broer | Cross-Linked Liquid Crystaline Systems[END_REF]. For applications where light absorption or emission is required, excessive use of photoinitiator may cause unwanted shifts in spectrum. Together, these issues make a preparation of a polymer formulation with favorable properties a non-trivial task.

In fact, the idea of applying photopolymers to OSC, notably liquid crystal OSC, is not new by itself, as some attempts have already been made to functionalise calamitic and discotic semiconductors. A very comprehensive work was conducted by the group of Theo Kreozis to functionalise phenylnaphthalene (Figure 1.12, (d))-and quaterthiophene (Figure 1.12, (b))-based liquid crystals with different types of photo-crosslinkable groups of free-radical and cationic process [START_REF] Baldwin | A comprehensive study of the effect of reactive end groups on the charge carrier transport within polymerized and nonpolymerized liquid crystals[END_REF]. However, it is still difficult to estimate the Most of liquid crystalline OSC are limited by their π-interacting nature and usually exhibit highly-ordered smectic or plastic crystalline phases. These materials have excellent short-range order but stay poorly oriented on higher scale, exhibiting polydomain structure.

Reactive mesogens in turn are mostly nematic liquid crystals and their orientation is easily controlled. Combining both types of compounds in theory allows to increase the size of domains and render them more ordered, thus improving interdomain charge transport.

Most promising works in this direction were conducted on discotic semiconductors by preparation of physical (hydrogen-bonded) gels [START_REF] Hirai | Enhanced hole-transporting behavior of discotic liquid-crystalline physical gels[END_REF] [110] [START_REF] Santhosh Babu | Functional π -Gelators and Their Applications[END_REF], improving mechanical properties and alignment of mesophases. Considerable amount of such works do not benefit from possible liquid-crystalline properties of a gelator [112].

Figure 1.17 -Possible mechanism for cationic polymerization on the example of epoxide functional group.

Functional groups

Two methods of photopolymerization exist: cationic (anionic photoinitiators are considerably less investigated and are almost not present in commercial use) and free-radical.

Epoxide, vinylether and oxetane groups can be cross-linked with a cationic photoinitiator. One particular advantage of cationic process is its indifference to oxygen level, so it may be conducted under ambient conditions. Most popular choices of cationic curing agent include sulfonium, iodonium and pyridinium salts. The process itself is being initiated by a strong photo-acid (Brønsted or Lewis-type) which is formed after the photoinitiator absorbs light. The cationic photoinitiators got their name for the light-absorbing part of the molecule: the cation is being light-sensitive, and the anion becomes a photo-acid. Though the exact mechanism of cationic photopolymerization is yet to be understood, generalized schematic could be observed on the Figure 1.17 by the example of epoxide group. After the protonation by photo-acid, the charge transfers from one chain member to the next.

Free-radical process is usually applied to acrylate and methacrylate functional groups, and could be significantly hampered by presence of oxygen and requires inert gas environment, usually nitrogen. Also, functional groups of this type are shown to be much more sensible to high temperature and are prone to spontaneous polymerization, thus requiring some thermal stabilizers (for example, butylated hydroxytoluene or BHT). However, one of the most significant advantages compared to cationic process is the sensibility of photoinitiator. Contrary to cationic curing, it is the radical which is being transferred along the polymer chain (Figure 1.18). Salt which are used for cationic process mostly have their absorption maxima in deep UV (<250 nm), when free-radical photoinitiators possess more user-friendly absorption region (often above 300 nm). In fact, sometimes some free-radical sensitizer is used to assist a cationic curing process in order to use more convenient wavelength of irradiation [START_REF] Yuksel Durmaz | Visible Light Initiated Free Radical Promoted Cationic Polymerization Using Acylgermane Based Photoinitiator in the Presence of Onium Salts[END_REF].

1.5 Organic Field Effect Transistor

Introduction

Transistor is an electronic device based on a semiconductor material and is used to switch or amplify electrical signal. Today, transistor is the main building block of computer Basically, the geometry of an OFET strongly resembles one of a capacitor (Figure 1. 19) with one conventional electrode (gate) separated with a dielectric layer from the second electrode, which consists of two conducting parts (source and drain) and semiconductor (channel ).

In fact, the undoped OSC by itself behaves similarly to an insulator: the number of free charge carriers is significantly lower (orders of magnitude) than in its inorganic counterparts [START_REF] Zaumseil | Electron and Ambipolar Transport in Organic Field-Effect Transistors[END_REF]. (a) [115]. In "linear " regime I DS may be described by the following equation:

|V DS | < |V GS -V th | (b) |V DS | = |V GS -V th | (c) |V DS | > |V GS -V th |
I DS = W L µ F C i V GS -V th - V DS 2 V DS , (1.6)
where W is the channel width and L is the channel length, C i is the capacitance per unit area of the gate dielectric, µ F is the field-effect mobility and V th is the threshold voltage. As we discussed previously, in saturation regime the current I DS no longer depends on drain voltage when

|V DS | > |V GS -V th | (Figure 1.21, c
) and in this case is described by means of the following equation:

I DS = W 2L µ F C i (V GS -V th ) 2 . (1.7)
Another important parameter of OFET is its ON/OFF ratio, which is the ratio of I DS recorded for values of V GS below threshold and I DS in saturation regime at a given V DS (Figure 1.22, a). ON/OFF ratio is directly connected to the intrinsic conductivity of OSC and is heavily influenced by doping (several orders of magnitude [36]), which means that great care should be taken upon choosing appropriate doping concentration.

Architecture

Four types of OFET architecture are presented in the Figure 1.23. Each of these types has its own requirements for manufacturing process. For example, those types which presume deposition on OSC layer impose additional requirements to surface quality, thus favoring thermal evaporation methods. Bottom gate -bottom contact configuration (Figure 1.23, (a)) is the most forgiving in terms of deposition techniques and is one of the most widely employed. In fact, it is even possible to find some commercially distributed substrates which require only OSC deposition and are very easy to use.

Apart from layered sequence, different types of contacts exist, notably simple linear (Figure 1.24, (b)) and interdigitated (Figure 1.24, (a)). The latter one possesses significant advantage over former, since the drain current I D is directly proportional to width/length of channel ratio W L (Figure 1.24, in red). This approach allows to drastically increase channel width without substantial difference in geometrical size of device. However, it is generally more expensive to produce and the channel length is limited conventional photolithography (electron beam lithography is hardly applicable, if one opts for lowcost production). The linear configuration does not allow to obtain high current, but is significantly cheaper and less demanding for OSC layer uniformity. Doped silicon substrates are usually used as a laboratory standard. The gate is formed by heavily doped region of silicon, whether the dielectric layer consists of silicon dioxide. Flexible substrates (plastic with ITO or PEDOT:PSS electrodes) occur quite frequently in publications, which opens a path to study mechanical stability of OSC.

Device performance

This subsection is intended to give some information about numerous factors which may influence the performance of a complete device. The technological process of OFET production includes numerous steps, for example the most abundant device based on silicon substrate with bottom gate -bottom contact layout has to undergo the dielectric layer by thermal grow of silicon oxide or deposition of dedicated insulating material, thermal evaporation or sputtering of electrodes, sometimes followed by surfactant treatment and at last, the semiconducting layer. All these processes are sensible to preparation conditions (temperature, presence of oxygen, pressure, moist, dust) as well as to choice of materials. For example, polymeric dielectrics (PMMA, poly(methyl methacrylate)) and organic electrodes (conducting polymers as PEDOT:PSS) have naturally higher affinity to organic nature of OSC, thus improving the performance. This postulates the need of careful choice of materials and severe control over production conditions if repeatability is required.

Packing of semiconductor molecules

Apart from intrinsic OSC properties as mobility and carrier density, device performance is also influenced by intermolecular order of semiconducting layer. Several kinds of intermolecular packing are known for OSC molecules (Figure 1.25). The brick-wall (Figure 1.25, (d); example: TIPS-pentacene [116]) type stacking being believed to be In addition to these packing types, the alignment of OSC relatively to substrate can also be different. Such applications as transistor can benefit from the so-called "edge-on" conformation (Figure 1.26, (b)), which is optimal for charge transport in the direction parallel to substrate due to the π -π stacking of molecules (since drain and source electrodes of the transistor are situated in the same plane). "Face-on" alignment is useful for OLEDs and OPVs, since in these applications the charge carriers usually move perpendicularly to the substrate plane.

Since

Device production

As mentioned before in subsection 1.2.5, an organic transistor can be prepared by either solution, either vacuum deposition methods. Thermal evaporation allows to obtain a uniform film of very high quality with intermolecular assembly "on-the-go", as well as tune the thickness by order of monolayers. However, solution processes are gaining popularity, as they are expected to enable low production costs of organic electronics.

Significant efforts are being made just for the sake of optimizing the deposition process by improving already existing techniques or using new combinations of solvents or deposition parameters.

Gate -dielectric -semiconductor interface

Expand its role in performance, and, in particular, its interaction with LC OSC. Since intermolecular interactions in organic semiconductors are much weaker than in conventional inorganic solids, substrate properties such as roughness may significantly affect OSC molecular alignment and hence, performance of the device. For example, pronounced surface roughness is able to impede the formation of large-size crystalline domain and increase the number of charge traps, or even to induce undesirable interface energy states in organic semiconductor. Capacitance of insulator layer is directly responsible for threshold and operating voltage, thus reduction of dielectric layer thickness results in a drop of the latter [START_REF] Veres | Gate Insulators in Organic Field-Effect Transistors[END_REF]. Polarity of dielectric also plays an important role, as high-k dielectrics can broaden the distribution of site energies close to dielectric interface, which results in different behavior compared to bulk of OSC. Use of dielectrics with low polarity yields mobility values closer to intrinsic properties of OSC(measured by Time-of-Flight, for example [START_REF] Veres | Low-k Insulators as the Choice of Dielectrics in Organic Field-Effect Transistors[END_REF]).

One of the first dielectric materials applied in this scope was silicon dioxide SiO 2 . A very convenient way is to use a heavily doped Si wafer as gate electrode with thermally grown SiO 2 on it as an insulator, which comes from inorganic semiconductor industry and is well-optimized. Similarly, other oxides with higher dielectric constant , for example,

Al 2 O 3 ( = 8.4
) and TiO 2 ( = 41) could be used in order to decrease operating voltage [START_REF] Wang | Poly(3-hexylthiophene) field-effect transistors with high dielectric constant gate insulator[END_REF]. However, the main quest of OFET suggest flexibility of the device, and the number of published works on polymeric gate insulators is substantial. Organic dielectrics generally produce more smooth surface and can be solution-processed, which combines well with general OFET agenda. This class of substances also enables top gate architecture, since any surface treatment of conventional inorganic oxides could be troublesome. Use of such substances as PMMA (poly(methyl methacrylate)), PET (polyethylene terephthalate) and polyimide results in higher mobilities than that with SiO 2 [121] [122].

Electrode -semiconductor interface

As mentioned in subsection 1.2.2, the intrinsic density of charge carriers of OSC is low, and in order to conduct current, these carriers are injected from metal electrodes.

Together with previous subsection this makes the performance of such a device as OFET pentacene (E HOM O ≈ 4.9 eV) and gold (Φ Au = 5.2 eV) the barrier height due to energy level misalignment can reach significant values up to 0.85 eV [START_REF] Schroeder | Improving organic transistor performance with Schottky contacts[END_REF]. To overcome this difficulty, great care should be take to surface properties of contact material to reduce roughness or to gain control over surface dipole [START_REF] Devynck | Cumulative effects of electrode and dielectric surface modifications on pentacene-based transistors[END_REF]. Traditional solution for MOSFET is to employ contact doping (ion beam implantation, for example). These methods do not work in case of OFET due to molecular nature of OSC, however some attempts to dope contacts by co-evaporation of molecular dopants were published [START_REF] Lussem | Doped organic transistors operating in the inversion and depletion regime[END_REF] [25].

Conclusion

To summarize, Chapter 1 is composed with a purpose to give the reader an idea of the complexity of subject in scope: basic principles of organic semiconductors are introduced from the perspective of chemical structure and basic electronic properties, as well as from bulk charge transport standpoint. A connection to liquid crystal subject is made, and most interesting examples are presented. We have also presented different ways of improving the transporting properties of OSCs. These sections are highly relevant for the subject of present thesis: molecular doping serves as a one way to improve weak intrinsic conductivity of the OSCs by increasing charge carrier density, while dispersion and in situ formation of polymer network is aimed to improve structure and order of semiconductor, which is another key element on the way to better performance. Finally, we have introduced the field-effect transistor as one of potential applications: we have exposed numerous factors which are closely related to the choice of materials, molecular structure of semiconductor as well as the preparation procedure.

Introduction

This chapter describes the array of experimental methods used in this work. An attempt is made to briefly introduce the most significant techniques, with an emphasis on the type of obtained data and potential impact on the present study.

These techniques can be approximately divided in several groups: more generalized characterization of materials which includes morphological and structural study, spectroscopic techniques, and more application-specific methods: study of bulk charge transporting properties and preparation -characterization of the OFET, as well as electrochemical and optical bandgap estimation.

This work also includes several techniques more appropriate for a chemist, and their brief introduction is nonetheless included in order to lower the interdisciplinary barrier.

Protocols of some procedures are included, which details may help those who decide to pursuit their studies in this field.

Detailed explanation of software used in this work and computer programs elaborated during the course of this thesis is omitted in order to restrain the volume of this section.

Structural and morphological characterization techniques

Scanning Electron Microscopy (SEM)

In SEM, the imaging is performed by scanning the sample area with a narrow beam of the electrons in X-Y plane. The wavelength of electrons is much smaller (in the order of pm) than that of photons and depend on the accelerating voltage. This allows to obtain high resolution images on a scale of several nanometers, which is unattainable by conventional light microscopy due to optical diffraction phenomena. This imaging technique imposes some additional requirements to the sample which should be vacuum-stable: outgassing could compromise the experimental conditions, contaminate the chamber and/or damage the pump. The sample is also required to be able to evacuate the electrons, which are rapidly delivered to its surface by the beam.

Failing to do so often results in severe contrast degradation and beam instability due to the repulsion forces, especially on higher accelerating voltages. Samples are covered with a thin layer of conductive material (gold, carbon, etc.) during the preparation by sputtering prior to SEM analysis.

Energy-Dispersive X-ray analysis

An X-ray detector is often bundled with a SEM instrument, which allows to perform Energy-Dispersive Analysis (EDX). This method is based on the detection of the characteristic X-ray emission, which may occur upon absorbing an incoming electron by atoms of the sample. The energy of these X-ray photons is an intrinsic property of the chemical element, which allows to obtain an elemental composition for a desired spot under the electron beam, or perform a full elemental mapping of the scanned area. SEM images in this work were obtained using JEOL JSM-7100F Scanning Electron Microscope with 15 kV (otherwise explicitly cited) accelerating voltage. EDX analysis was performed by using Bruker QUANTAX (3x XFLASH 6130) detector system.

Atomic Force Microscopy (AFM)

This technique is a part of a wider family of scanning probe microscopy. Despite its name, it does not use lenses or sample irradiation to obtain an image, thus excluding optical limitations of wave optics and aberrations which may be introduced by lens. Instead, it is based on direct interaction of the probe tip (5 nm to 10 nm diameter) with the surface of sample, which allows to overcome diffraction limit imposed by conventional optical microscopy and attain resolution on a nanometric scale. A typical AFM instrument (Figure 2.5) consists of a cantilever with a sharp tip, which serves to probe the surface of the sample. The backside of this cantilever is made reflective in order to allow a laser beam to be reflected on a segmented photo-diode, which in turn enables readout of the cantilever deflection. The in-plane scanning, as well as Z-axis adjustment is performed with a piezoelectric element under the sample substrate.

Depending on the measurement mode, the probe tip may be kept in contact with the sample surface, or oscillate with a predefined frequency.

During the contact scan, the cantilever deflection creates an input for a feedback loop, which is employed to keep the deflection -distance between the tip apex and the sample constant (defined by a setpoint) by adjusting the Z-axis travel. The adjustment of Z-axis (sample topography) is coupled to the X-Y travel of the probe in order to form a pseudo-colored 2D image, where color represents the height.

Conductive AFM (C-AFM)

This technique was derived from the parental AFM in the nineties as a tool for semiconducting material science. It is based on measuring the current flowing through the sample at a given applied voltage and enables the user to obtain local conductivity data.

This data can be represented as a current-voltage characteristic curve of a certain contact area of a sample, or as a conductivity "mapping", usually performed at a constant bias and coupled to the topographic information of the sample surface (Figure 2.6) [START_REF] Musumeci | Electronic characterization of supramolecular materials at the nanoscale by Conductive Atomic Force and Kelvin Probe Force microscopies[END_REF]. The basic facilities for the topography imaging are the same as for the aforementioned AFM.

However, this technique imposes some additional requirements to the probe tip and the sample. C-AFM usually utilizes specially fabricated conductive probe tips. The sample itself has to be reasonably thin and conductive in order to obtain a measurable signal amplitude. AFM images in this work were obtained with the help of Veeco (Bruker) Multimode III with an optional C-AFM module. Samples used for conductive measurements were deposited on gold and ITO coated substrates. Small near-edge area of the conductive surface of these substrates was masked with a tape prior to coating. A small silver wire was attached by means of silver paste to make electrical contact of the conductive substrate layer and AFM sample holder in order to enable electrical field application between the aforementioned layer and conductive tip apex of the AFM instrument. Slow scanning speed (1.5 µm/s) was employed, as well as minimal contact force (deflection setpoint was adjusted prior to each image capture) was utilized in order to reduce tip wear and enhance reliability of conductivity data collected during measurements. An average of two conductive tips (Bruker TM SCM-PIC-V2) was utilized for each sample.

Differential scanning calorimetry (DSC)

DSC is the most widely used technique among different types of analysis methods to study thermal properties of materials. It helps to establish a connection between the temperature and particular thermodynamic properties which are characteristic for substances such as polymers, metallic glasses, liquid crystals, pharmaceutical substances and etc.

The DSC is an instrument of thermal analysis which is able to measure the heat flow between twin pans (reference and sample, Example of typical thermograms obtained by DSC is illustrated in Figure 2.8). If on heating a melting (endothermic process) phase transition occurs, in order to maintain the same temperature of the both pans, more heat flux must be transferred to the sample pan than to reference pan: a peak is then observed in the thermogram. The energy consumed as heat for this process (enthalpy change) is related to the area of the observed peak and can be easily calculated from the integral. In contrary, the sample is going to emit heat upon cooling during the crystallization. In this case more heat flux is delivered to the reference pan than the sample pan. This transition is going to appear in thermogram as an exothermic peak in the opposite direction.

The calorimetric studies in this work were made by a TA Instruments Q1000 Differential Scanning Calorimeter (-180 • C to 725 • C).

X-Ray Scattering

X-ray scattering techniques have proven themselves to become an indispensable tool in the fields of solid and condensed matter, macromolecular chemistry and nanoscience.

This technique is based on the elastic interaction of X-rays with the matter, and allows the user to obtain information on structural properties of the material under study, i.e. intermolecular distance, crystalline lattice type, etc.

In application to liquid crystals, Wide-Angle/Small-Angle X-ray Scattering has been historically utilized in order to identify and characterize the mesophase structure. The experimental data is often represented as a 2D detector image, which is further reduced to the 1D scattering profile. The d-spacing corresponds to the scattering vector

q: q = 4π sin θ λ , (2.1)
where θ is the scattering angle and λ is the wavelength of X-rays. In order to extract the d-spacing l out of the scattering vector, a following ratio is used: Duncan and Noel A. Clark are greatly acknowledged.

l = 2π q . ( 2

Spectroscopic techniques

Optical Spectroscopy

UV-Vis-NIR Absorption Spectroscopy

This technique relies on measuring the absorption of light by the sample as a function of it wavelength. A typical result of an experiment is an absorption spectrum. For the case of visible radiation (UV-Vis-NIR spectroscopy), the absorption of incident light photons is correlated to the change in the electronic state of the molecule, which makes it ubiquitous in the field of material science.

The absorbance (A) follows the Beer-Lambert law (for one absorbing moiety):

A = cl, (2.3)
where is the molar extinction coefficient, c is the molar concentration of the absorbing moiety and l is the optical path. This law implies that the absorption is directly proportional to the concentration of the molecule of interest.

A = -log 10 I I 0 , (2.4)
where I is the transmitted light intensity and I 0 is the intensity before entering the sample.

As mentioned before, UV/Vis light absorption process is connected to the electronic transitions in the atoms and molecules. In application to the OSC, this technique allows to probe the transition of an electron from HOMO to LUMO, thus allowing to obtain the band gap energy. The energy of a photon corresponding to the onset of the optical absorption enables the calculation of the optical bandgap (E Gopt ) using the following relation:

E Gopt = hc λ on , (2.5)
where h is the Planck's constant, c is the speed of light and λ on is the onset of the optical absorption.

The instruments used in this work were Varian (Agilent) Cary 100 and Perkin Elmer Lambda 2 spectrometers with a standard 1 cm fused quartz cuvette.

Fluorescence Spectroscopy is presented in Figure 2.12) and fitted with an exponential decay function, which allows to extract fluorescence lifetime. This may give additional information on molecular structure of organic semiconductors, as well as potential relevance for light emitting applications.

Fluorescence Lifetime spectra were recorded on Horiba DeltaFlex TCSPC system with 370 nm laser diode as an excitation source, colloidal silica was used for prompting.

Raman Spectroscopy

Raman spectroscopy is an analytical technique used to observe low frequency modes of vibration, rotation and others in a molecular system. Raman spectroscopy is commonly used in chemistry to identify various substances or to study chemical transformations.

Charge transport characterization techniques

Time Of Flight (TOF) mobility measurment

Generalities on charge carrier mobility measurement Electronic charge carrier (essentially, any charged particle: electron, hole or ion) mobility µ (m 2 V -1 s -1 , for simplicity very often expressed in cm 2 V -1 s -1 ) is defined as:

µ = v E , (2.6)
where v is charge carrier velocity (m s -1 ) and E is applied electric field (V m -1 ). There are many techniques which allow to estimate charge carrier mobility [START_REF] Wallace | Carrier Mobility in Organic Charge Transport Materials: Methods of Measurement, Analysis, and Modulation[END_REF]128]. The most widely employed are: time-of-flight [START_REF] Blakesley | Towards reliable charge-mobility benchmark measurements for organic semiconductors[END_REF] (TOF), space-charge-limited current [130] (SCLC), carrier extraction by linearly increasing voltage [START_REF] Albrecht | On the Field Dependence of Free Charge Carrier Generation and Recombination of PCPDTBT/PC70BM: Influence of Solvent Additives[END_REF] (CELIV), OFET I-V characterization [START_REF] Huang | Controllable thin-film morphology and structure for 2,7dioctyl[END_REF] and time-resolved microwave conductivity [START_REF] Dicker | Electrodeless time-resolved microwave conductivity study of charge-carrier photogeneration in regioregular poly(3-hexylthiophene) thin films[END_REF] (TRMC) as recently emerged but quickly gaining popularity. Each of these techniques has its own advantages and limitations. However their detailed overview is out of scope of this thesis. Hereby we are going to describe TOF method which was widely utilized during present research work. In order to obtain mobility value from extracted time of transit, expanded version of equation 2.6 is usually used: The latter is preferred for some applications, since it offers superior noise performance, stability, and simplicity of circuit layout. In our case, the noise do not pose a significant problem due to low repetition rate of Q-Switch (10 Hz), which enables us to use extensive signal averaging. Relatively large gains are required for acceptable signal-to-noise ratio (about 10 4 -10 5 ), which means feedback components of a typical current amplifier (Keithley 428) significantly limit the signal bandwidth (bandwidth is inversely proportional to the gain). Since the ability to measure short (in order of µs) transient times is essential for TOF experiment, a better solution must be employed which does not reduce the signal bandwidth (and the rise time t r ) as strongly as in the case of a single stage transimpedance amplifier.

µ = d 2 τ tr V , (2.7)
It was proposed to utilize a relatively small value shunt resistor to convert the current to voltage, which will be further amplified by a second stage non-inverting voltage amplifier. gain value and confront it with the initial 1 µA amplitude and 2.5 µs duration current pulse (Figure 2.21, gray line). We have calculated the rise time t r which is defined as a period of time required for output signal to go from 10% to 90% of its final value in response to ideal square wave input. In our case, the resulting t r for cascaded conversion scheme was almost four times lower than for single stage conversion (0.56 µs versus 2.12 µs).

Our experimental observations have confirmed the results of this simulation: use of cascaded scheme resulted in typical signal rise time t r of less than 1 µs, when for one stage current conversion via Keithley 428 we have obtained t r of about 5 µs (which is in good agreement with its specifications) for similar 10 5 gain, which has determined our choice of cascaded conversion scheme for better resolution in time domain.

This consideration also imposes its limits on µ values which can be measured by TOF setup: higher mobilities will necessitate use of low voltages to extend transit time. This will require further signal amplification, and, as a consequence, bandwidth reduction. It is also necessary to choose appropriate excitation wavelength and intensity, in order to have most of the light absorbed very close to electrode surface. The irradiation intensity should be high enough to provide reasonable amplitude of signal, but in the same time low enough to avoid charge carrier generation in the bulk of sample. Absorption in the bulk of material leads to curve distortion due to space-charge and inconsistency of transit time measurement.

Transistor I-V characterization

In this work we have used commercial OFET substrates (

Complimentary techniques

Molecular structure characterization techniques

The present manuscript contains a section dedicated to an entirely new material, synthesized during the course of this PhD thesis. This section assembles the techniques used to determine the molecular structure of the substance in scope. Due to the novelty of the material, basic analytical characterization is required in order to ensure the conformity to the expected molecular structure and validate the proposed synthetic route. However, these techniques are only briefly introduced, since the detailed chemical investigation is out of scope of this study.

Nuclear magnetic resonance spectroscopy

This technique is used primarily to assess the quality control of a substance, i.e. purity and conformity to a proposed molecular structure. There are many subtypes of this technique which are related to the nuclei type of interest. In this work, two primary sub-techniques were employed: 1 H and 13 C.

The sample is dissolved in a suitable solvent (deuterated solvents are used in order to avoid the contribution of its protons in 1 H) in a thin walled glass tube. The tube is then placed in a strong magnetic field, where the phenomena of the nuclear magnetic resonance is observed. The data is usually obtained in a form of the chemical shift spectrum. The chemical shift is a value between the measured resonance frequency and the reference frequency; being extremely small, it is often expressed in parts per million (ppm).

Chemical shift is directly related to the local electron environment of a given nucleus:

for example, electron-withdrawing environment will actively contribute to the nucleus de-shielding, which will result in a stronger chemical shift. This technique makes possible to quantify unique nucleus environments of 1 H and 13 C in the molecule and compare them to the expected numbers, as well as to directly correlate these environments with each other (2D-techniques).

The instrument used in this work was Bruker Avance III 400 MHz spectrometer. The spectra were obtained in chloroform-d, which was passed over dry alumina powder in order to remove acid and water traces prior to sample preparation.

High-resolution mass spectroscopy

This technique allows the detection of molecules with a respect to their mass with a 0.001 atomic mass unit precision. An actual mass spectrometer measures a mass-to-charge ratio, m/z, since only charged particles can be used as a subject of mass spectrometry.

A typical mass spectrometer includes three main components: ion source, mass analyzer and detector. The type of an ion source depends largely on the substance under study. In this work, an instrument with an electrospray-type source was used, since it is considered to be a "soft" ionization method, usually resulting in little fragmentation and often employed for macromolecules and polymers.

The mass analyzer is utilized to separate the ions or molecular pseudoions by their m/z ratio. One of the most popular mass analyzer types for organic substances include time-of-flight (TOF) and quadrupole analyzers. The TOF analyzer accelerates the ions by applying an electric field: for the ions of the same charge, their velocity will solely depend on their mass. Thus, it is possible to extract the ion's mass from its arrival time.

The quadrupole analyzer is capable to separate the ions by a RF quadrupole field created between four parallel rods. This type of analyzer is often used in fragmentation studies.

In application to organic molecules, ions are often created by addition of a proton or another cation, and the expected mass must be calculated as OCT1RFVpp (750 V).

[M+H + ].

Cyclic voltammetry

Cyclic voltammetry is an experimental technique which is purposed to study the process of electrochemical oxidation or reduction of molecules. Since theses processes are directly related to addition or withdrawal of an electron from a molecule, this technique is widely applied to the field of organic semiconductor characterization.It allows to extract the information about the frontier molecular orbitals, which is crucial for understanding of the charge transporting properties [START_REF] Ruzié | Design, synthesis, chemical stability, packing, cyclic voltammetry, ionisation potential, and charge transport of[END_REF][START_REF] Park | Voltammetric investigation for electrontransfer characteristics of organic semiconductors[END_REF].

The experimental setup (Figure 2 In a typical experiment, the voltage is scanned up to a certain limit which is determined by the so-called electrochemical window (this potential range is imposed by the solvent and supporting electrolyte in order to avoid their reduction/oxidation). Upon reaching the limiting potential (also called vertex potential), the scanning direction is reversed.

For example, in the case of oxidation event (Figure 2.25), the molecules situated near the WE loose one electron; when the scan direction is reversed, these molecules obtain an electron and revert to their initial neutral state (become de-oxidized). An increase in the scanning rate results in rise of the current. The ferrocene from this example exhibits where it is required to apply further potential to revert the molecule in its original state.

This behavior is directly connected to the rate of the electron exchange, which depends on the solvent used in the experiment, as well as the stability of oxidized/reduced molecule.

By tracing a tangent of the current corresponding to the oxidizing event (Figure 2.25, in red), we may extract the onset of oxidation for a given molecule, i.e. the energy which is required to extract an electron from its HOMO. By carefully referencing this energy to a known potential (reference electrode or internal reference substance with known oxidation/reduction potential, such as ferrocene), we may obtain an absolute energy value of its HOMO. In the case of reduction process, the same applies to calculation of the LUMO energy.

It is possible to carry out the experiment for the substance of interest in dissolved and solid state (deposited on the working electrode). The latter method enables the study of deposition and processing methods contribution to the potential landscape of the material, as well as study of insoluble substances. In this work however, the experiments were performed in a solution of dichloromethane, given the soluble nature of the studied substances. It is worth noting that the presence of oxygen interferes with the experiment (the molecule of oxygen, while being redox-active, undergoes a reduction, which manifests itself in the negative range of voltage sweep). The assembled cell was bubbled with the argon gas (saturated with dichloromethane vapors) prior to the experiment. The coating process involves creation of covalent bonds with Si atoms of OTS-18 trichlorosilane end group and Si-O groups of the substrate. First, the substrate is meticulously cleaned and treated with an oxidizing agent in order to break Si-O-Si bonds on the surface and attach a hydroxyl groups in their place. The substrate then is reacted with the SAM solution through the following scheme:

CH 3 (CH 2 ) 17 SiCl 3 + 3 H 2 O --→ CH 3 (CH 2 ) 17 Si(OH) 3 + 3 HCl CH 3 (CH 2 ) 17 Si(OH) 3 + 3 SiOH (solid) --→ CH 3 (CH 2 ) 17 Si(OSi) 3(solid) + 3 H 2 O
The presence of trace amounts of water is necessary for the first step of the reaction, however excessive moisture will result in uncontrollable SAM polymerization in solution.

The solution should be used only if it is transparent, and any cloudy deposits should be skimmed from the surface prior to substrate introduction.

The resulting SAM monolayer (Figure 2.27) is covalently bonded to the substrate and can withstand following ambient air handling and cleaning.

Coating with trichlorosilane-based SAMs is a delicate and moisture-sensitive process, which includes handling of highly corrosive, potentially explosive and carcinogenic sub- • SAM solution preparation: 20 ml of hexadecane is mixed with 5 µL of chloroform and 50 µL of OTS-18 (Alfa Aesar, 98%), stored in the cold while the substrate is in the oven.

• SAM coating: the substrate is immersed in the SAM solution and left undisturbed for 30 minutes

• Final cleaning -sonicating for 15 minutes in the following solutions:

-Chloroform

Spin-coating

Spin coating principle is based on utilizing of the centrifugal forces created by spinning the substrate with controlled speed. When the work solution is placed in the center of the substrate, the centrifugal forces make the liquid spread evenly on the surface of the substrate.

Often a so-called dynamic disperse method is employed, when the solution is placed on a relatively slowly spinning (about several hundred RPM) substrate, which ensures its complete and homogeneous coverage. The substrate is then ramped to the next high-speed step to obtain the desired coating thickness (Figure 2.29). There are many parameters which influence the thickness of resulting film including but not limited to: spin velocity, duration and acceleration, viscosity of the solvent, its evaporation rate, solution concentration, substrate wettability and the interfacial interactions between the substrates were immediately processed after the final step to avoid possible contamination.

Thin films in this work were prepared with Laurell WS-650 spin processor in a vertical laminar air flow workbench equipped with a high efficiency particulate filter (HEPA), removing 99.97% of all particles 0.3 µm or larger.

Drop-casting

This deposition method is very fast, simple and easily accessible, however it should not be overlooked. It is based on direct deposition of the solution on the substrate surface and requires no special equipment, however, it is difficult to control the resulting film quality.

In this work, this method was combined with solvent vapor annealing, which means it was performed in a controlled environment of saturated solvent vapor on a heatplate (about 50 • C). The substrate was placed in a small Petri dish cover, which in turn was placed in a bigger Petri dish to trap the solvent vapors. This dish itself was placed on a heatplate with a small inclination of about 2-3 • ) (Figure 2.30). This inclination is created in a direction perpendicular to the transistor channel width, and helps to orient the crystal growth in the direction favorable for the charge transport. Substrate preparation steps were identical to the aforementioned spin-coating procedure.

Conclusion

This chapter briefly describes the techniques and methods used during the course of present PhD thesis. It also demonstrates, in conjunction with Chapter 1, the vast complexity of the studied subject. These techniques are utilized in order to characterize the elaborated materials in the course of Chapters 3&4. In addition to description of some standard techniques, we have also included a short description of our work conducted on optimization of the TOF setup. We hope that the reader will find this chapter useful, and it will allow to better understand the experimental results which we are going to present further.

Chapter 3

Composite materials based on 2dodecyloxy-6-(4-octylphenyl)-naphthalene

General introduction

This chapter describes physical properties of various composites based on 2-dodecyloxy-6-(4-octylphenyl)-naphthalene (8-PNP-O12). This molecule is a well-known liquid crystalline semiconductor which exhibits two smectic mesophases on moderate temperatures [74].

First, the composite materials which contain a dispersed polymer network are studied.

We have thoroughly investigated the influence of reactive mesogen on thermophysical as well as alignment properties of the 8-PNP-O12, followed by characterization of hole transport in the mesophase. This section also serves as an introduction to the 8-PNP-O12 material and prepares the reader for the upcoming sections.

The next stage was the study of effects of chemical p-doping on the structural properties and charge transport performance. Extensive preliminary work was performed by employing ab initio (Gaussian 16, Rev. A03 software [137]) calculations in order to anticipate the charge transfer between the host and the dopant and to further correlate it with experimental data obtained by spectroscopic techniques. We have prepared a comprehensive range of dispersion concentrations, and studied their charge transporting properties in the mesophase by TOF technique. The next step of this section was to find favorable conditions to prepare a thin film of doped 8-PNP-O12, which has allowed us to prepare a field effect transistor.

The final stage is the investigation of a polymer stabilized p-doped system: we have investigated the feasibility of composites, where a reactive mesogen undergoes a lightactivated polymerization in a presence of doping impurity. This section concludes the research effort carried out on 8-PNP-O12 material.

Cross-linked "gel"-type composites

Composites of smectic OSC and nematic reactive mesogen undergo UV-activated polymerization in Sm B phase which yields the formation of chemical gels. The structure of resulting composites is characterized by means of optical and electronic microscopies.

The influence of polymer network on hole transport in smectic OSC is investigated by the time of flight charge carrier mobility measurements. This subsection summarizes the studies conducted on the composite materials.

Materials

One of the most widely known LC OSC is the 2-dodecyloxy-6-(4-octylphenyl)-naphthalene (8-PNP-O12, Figure 3.1) which exhibits two mesophases: Smectic B (Sm B , 66

PNP-O12

). The phase sequence on cooling: Cr

66 • C ← ---Sm B 101 • C ← ----Sm A 121 • C ← ----Iso.
A diacrylate reactive mesogen (RM82, [139] contains acrylate functional groups and has a nematic (N) mesophase in a range from 86 • C to 116 • C (Cr

86 • C ← ---N 116 • C ← ----Iso).
In order to make RM82 UV-cross-linkable, free-radical photoinitiator 2-benzyl-2-

Sample preparation

Optical microscopy and hole mobility measurements were carried out in a prefabricated commercial liquid crystal cells (predeposited ITO electrodes, homogeneous alignment layer and 5 µm spacer by AWAT, Poland).

The sample preparation procedure was as follows: first, the precursor mixtures (LC OSC and reactive mesogene) were prepared by dissolving the components for a desired weight concentrations in the chloroform and further magnetic stirring of the mixture in dark glass vial for one hour at 50 • C. After the stirring has ensured the sample homogeneity, the vials were kept open in dark area for 24 hours to attain complete solvent evaporation. Sample in dry powder form was placed on the edge of the cell. It was then was placed in a light tight hotstage, and was further heated to the isotropic temperature of the composite (about 130 • C), where it was maintained for about 20 min. This procedure has made possible to avoid curing at undesirable temperature due to significant sensibility of the RM82 precursor mixture at elevated temperatures. After the cell has been filled by capillary force, the hotstage was slowly (0.5 • C/min) cooled to 90 • C which would correspond to the Sm B phase of the composite. After the temperature was stabilized, the curing LED was introduced. Curing was performed at 1 mW/cm 2 power density of 365 nm wavelength radiation for 15 min.

Thermophysical, mesogenic and structural properties

Four composites have been prepared with the following mass fraction of RM82 in the mixture: 1% (g1), 3% (g2), 5% (g3) and 10% (g4). The DSC plots for cooling of the samples of prepared composites exhibit (Figure 3.4) some shift in the phase transitions to a lower temperature, as compared with that of pure 8-PNP-O12. This shift is merely explained by the mixture effect, and demonstrates that RM82 and 8-PNP-O12 molecules are miscible for the here studied concentrations range.

Adding RM82 to 8-PNP-O12 does not alter the texture of the host material: the mesogenic behavior of 8-PNP-O12 is preserved. The observed shift in phase transitions temperatures however is only valid for the precursor mixtures. Once the mixtures undergo polymerization, the obtained composites demonstrate the phase stability equivalent to the pure 8-PNP-O12, which is going to be addressed further. Texture of the mesophases was observed under polarizing optical microscope. Figure 3.5 assembles the texture of the cross-linked composites in Sm B , Sm A and I phases. One can notice that the similar focal conic pattern, which is characteristic for the smectic phase, is present in the corresponding smectic states of the prepared composites. It is also of interest to underline that the transition temperatures for cross-linked samples are almost similar to those of the pure 8-PNP-O12. It is proposed that the polymer network becomes micro-phase-separated from the host 8-PNP-O12, and it no longer interferes with the stability of the mesophases of host material.

The texture of the composites demonstrates a noticeable change in alignment: the smectic domains seem to be oriented in a preferred direction, as compared to the pure

8-PNP-O12

, where these domains are distributed randomly. This can be explained by the nematogenic nature of RM82: the directors in the nematic phase are aligned unidirectionally on surfaces with homogeneous anchoring. The good compatibility between RM82 and 8-PNP-O12 allows the nematic mesogene to "lead" otherwise difficult-toalign smectic host material, creating the observed alignment effect. A similar system based on the same host material and a conventional (non-mesogenic) cross-linker was studied by Hanna et al. [START_REF] Yoshimoto | A Novel Charge Transport Material Fabricated Using a Liquid Crystalline Semiconductor and Crosslinked Polymer[END_REF][START_REF] Yoshimoto | Charge transport in liquid crystalline semiconductor and crosslinked polymer composite[END_REF] and it does not exhibit any alignment effect, thus allowing to conclude that it solely the properties of RM82 which are responsible for the alignment and not the degradation of the smectic order. Moreover, the studies carried out with the same reactive mesogene, but a completely different host material with a mesophase of discotic nature [START_REF] Kondratenko | Hole transporting properties of discotic liquid-crystalline semiconductor confined in calamitic UV-crosslinked gel[END_REF] demonstrates similar strong effects on long-scale order.

These facts reveal that despite the relatively low order of the nematogenic RM82, it is however capable to significantly influence the long range order of much higher ordered mesophases.

The transition from the Sm A mesophase to the isotropic liquid phase allows to observe the polymeric network, which is formed in situ during the cross-linking process (right column of the Figure 3.5). Its thermal stability signifies that the curing under applied conditions was successful. The density of polymer network increases with the weight fraction of RM82. It is also possible to notice that the network looks like a very close imprint of the texture observed in the mesophase, which could be regarded as a sign of intimate contact between this network and host material. The reverse transition to the Sm A mesophase from the liquid phase, the resulting texture looks very similar to that before the melting to the liquid phase. The network acts as a "skeleton" for the mesophase, it stabilizes the liquid crystal order and preserves the alignment of the samples upon heating-cooling cycles.

The optically observed polymer network demands a closer structure inspection. Another set of "sacrificial" cells was prepared for SEM analysis in order to replicate the anchoring conditions which lead to the previously observed alignment effects. The prepared cells were then opened, and the host 8-PNP-O12 was washed out several times with acetone. The substrates (one "face" of the cell) were dried in the oven to ensure complete evaporation of the solvent (to prevent the swelling of polymer) prior to loading in the SEM.

SEM captures are assembled in Figure 3.6. Higher magnification reveals that this network is in fact composed of fiber-like shapes, which seem to follow the imprint of a smectic domain. The density of the observed network is in good agreement with the content of RM82. However, it should be underlined that due to the destructive method of sample preparation (the sample is cross-linked in situ while confined between two cell faces, which are further forced apart, thus tearing the sample in process), the density of an intact sample is going to be higher.

The sample of g2 exhibits some round shaped inclusions, which could be assigned as some signs of aggregation during the polymerization process, since the sample of g3 demonstrates an even higher content of these aggregates. This leads to a conclusion that despite seemingly good aligning properties of the RM82 and absence of any visible separation under POM, the host 8-PNP-O12 is unable to accommodate high quantity of the monomer. structures comprised of the same RM82 tended to form sponge-like porous network.

Charge transporting properties in the mesophase

Hole transporting properties were studied with the time-of-flight technique previously described in the Chapter 2. Measurements were performed for the range of applied electric field ,E, from 20 kV/cm to 44 kV/cm with a step of 4 kV/cm at Sm B mesophase and from 32 kV/cm to 60 kV/cm with the same step. Higher values of electric field are necessary to be applied in the Sm A mesophase in order to obtain a reasonable signal-to-noise ratio due to the lower carrier mobility intrinsic to this mesophase. Homogeneous alignment sample cells with the spacer thickness d=5 µm were used for these measurements.

(v = d τtr )
. The distribution of the calculated values is found to be linear, which means that the hole mobility is field-independent for the range of utilized electric fields. This behavior is in good agreement with the results previously reported for the 8-PNP-O12 and is not uncommon for this class of materials. The composites exhibit the same characteristic as the pure 8-PNP-O12: linear distribution of the hole velocity, but with smaller slope allows to suggest lower hole mobility for these samples. The data for hole mobility of g4 (sharp spike at 50 µs is related to measurement setup artifact). It could be noticed that the profile of photocurrent decay for composites remains unchanged when compared to that of the pure 8-PNP-O12.

This demonstrates that the polymer network does not contribute to significant trapping of the photo-generated holes or impede their withdrawal at electrode interface [75]: the transport mechanism of the pure 8-PNP-O12 is preserved for the composites.

The influence of the polymer network on the hole mobility is illustrated by the Figure 3.10. It is clear that the polymeric network has somewhat negative effect on the hole mobility and the magnitude of this effect is depends on the order of mesophase. As the Figure 3.10 (a) suggests, the hole mobility drops for about 15% upon adding 1% of the RM82, and then stays almost invariable for higher concentrations of RM82. However, We may propose that polymer network is able to introduce some trap states for charge carriers in the bulk of the composite. It is of interest ot compare these composites with the system of the same RM82 and the discotic OSC (HAT-5) [START_REF] Kondratenko | Hole transporting properties of discotic liquid-crystalline semiconductor confined in calamitic UV-crosslinked gel[END_REF], mentioned previously. Discotic LCs are notorious for having poor compatibility with calamitic materials, which results on a phase separation on a higher scale (i.e. the host mesophase is less permeable to the polymeric network, thus making it less possible to interfere with its short range positional order). That system exhibited enhanced mobility with a similar-looking alignment effect, showing a clear positive contribution of RM82 network [START_REF] Kondratenko | Hole transporting properties of discotic liquid-crystalline semiconductor confined in calamitic UV-crosslinked gel[END_REF]. In the case of 8-PNP-O12 as a host material, the compatibility between the two components of the precursor mixture is higher, which results in a significant penetration of the smectic domains of the host material by the polymer, which negates the positive effects of the long-range alignment.

The dependency of hole mobility on the temperature is plotted in the Figure 3.11.

This plot demonstrates that the charge carrier mobility does not depend on temperature in a range of single mesophase. One can also notice that the transition temperatures of the composites closely resemble those of the pure 8-PNP-O12. It is found that the pure 8-PNP-O12 and composites have comparable behavior: hole mobility is almost invariable within the limits of the mesophase, which is commonly seen in the LC OSCs.

The polymeric network does not disturb the thermodynamic nature of the charge transport.

A similar cross-linked system studied earlier [112] shows signs of thermal activation of carrier mobility, which is related to the increased disorder (structural and energetic) in the system. Temperature increase helps to overcome potential barriers related to system disorder, which further results in mobility increase [46]. For our system, it is possible to explain the stability of hole mobility by the improvement and stabilization of long range order of 8-PNP-O12 mesophase. All of the studied samples exhibit photocurrent in the isotropic phase, where the positive charge carrier mobility is found to be of the order of 1 × 10 -5 cm 2 /(V s), which is typically attributed to the ionic transport [START_REF] Iino | Electronic and ionic carrier transport in discotic liquid crystalline photoconductor[END_REF][START_REF] Macary | Electronic and ionic ambipolar transports in the isotropic, SmA, SmB and crystalline phases of a liquid crystal[END_REF].

Conclusion

The preparation of LC OSC -photopolymer composites has resulted in the apparition of a novel material which combines the beneficial properties of its both components. The in situ cured polymer network shows a fibrillar structure, which allowed to evaluate its influence on the charge transporting properties of the studied composites. The experimental data was confronted with results obtained on discotic LC -photopolymer composite, which has made possible to estimate the role of compatibility of the components in their contribution to the electrical properties of resulting composites.

The studies conducted on pure 8-PNP-O12 and composites have revealed a remarkable alignment effect of RM82 material in the conditions of homogeneous orientation of the composites. The hole mobility was found to be slightly lower for the composites than for pure 8-PNP-O12, this result may be correlated with minor trapping induced by the guest polymeric structures. By taking into account the temperature stability of prepared composites in the presence of in situ formed polymeric network, we may conclude that it does not provide significant contribution to the structural and energetic disorder of the resulting composites.

P-doped LC OSC

We have decided to apply another approach in an attempt to enhance hole transport in the LC OSC, since the preparation of LC OSC/polymer composites did not result in an increase of hole mobility. We explore the molecular nature of doping in OSC by employing the aforementioned phenyl naphthalene-based liquid crystalline organic semiconductor as a model substance.

The main idea of this research effort revolves around the so-called charge transfer complex (CTC), which is initially presumed to be formed between the host organic semiconductor (8-PNP-O12) and the introduced doping impurity, an electron acceptor (F 4 TCNQ). Firstly, the density functional theory (DFT) calculations were performed in order to predict the electronic properties of the CTC, which allowed us to anticipate its characteristic behavior, further uncovered by spectroscopy. The basic thermodynamic

properties and the mesophase nature of resulting system has been investigated by means of differential scanning calorimetry, polarized optical microscopy and x-ray scattering.

Optical and vibrational spectroscopies allowed us to estimate the amount of charge transfer for CTC and expose some properties which appear only in complexed stateand validate the geometry predicted by DFT.

These preliminary studies have enabled us to further investigate the influence of dopant introduction on the bulk hole mobility of 8-PNP-O12 and understand how the presence of CTC interacts with the hole transport in Smectic B and less ordered Smectic A mesophases. The results are confronted with local conductivity mapping via conductive atomic force microscopy.

Materials

For doping 8-PNP-O12, a fluorinated derivative of tetracyanoquinodimethane (TCNQ), the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ, Figure 3.12) was used as the electron acceptor. This dopant was provided by Apollo Scientific (Bredbury, United Kingdom), 97% purity. The molecules from tetracyanoquinodimethane family are well-known for their relevance in the field of OSCs and have been previously used in discotic semiconducting liquid crystals [130] in an attempt to improve hole transport. Their effects on optoelectronic properties were also studied in nematic [START_REF] Baise | Effect of charge-transfer acceptors on dynamic scattering in a nematic liquid crystal[END_REF] and ferroelectric LCs [START_REF] Moritake | Spontaneous polarization influence of V-shaped switching ferroelectric liquid crystals on pulse driving using active switch[END_REF].

Spectroscopic tools for studying of charge transfer

As we have discussed in Chapter 1 (Section 1.2.4), both cases of charge transfer between the host and dopant (integral and partial charge transfer (ICT and PCT)) result in noticeable change of electronic configuration of the host OSC moiety. It is possible to observe such changes by conventional spectroscopic methods. Furthermore, by the means of ab initio calculations it is possible to predict the character of this charge transfer [45].

In our study, the experimental results obtained from spectroscopy are confronted to the values deduced from ab initio calculations. This comparison provides us the basis to validate the predicted geometry for the CTC and utilize it to explain its effect on the charge transporting properties.

Computational study

Geometries of pristine 8-PNP-O12, F 4 TCNQ and the CTC were firstly optimized by semi-empirical methods with further DFT optimisation on B3LYP/6-31++G(d,p) level with additional empirical dispersion corrections (GD2) for the CTC. This have proven itself to significantly enhance [START_REF] Zhu | Charge transfer in molecular complexes with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ): A density functional theory study[END_REF] the accuracy of systems which involve intermolecular charge transfer. Optimized geometry had zero imaginary frequencies.

The molecular orbital diagram for CTC and its isolated components (Figure 3. 13) shows successful hybridisation of the HOMO of the 8-PNP-O12 and the LUMO of the F 4 TCNQ into a novel intramolecular bonding orbital at -6.29 eV.

As a result of complex "bonding", the geometry of its components undergoes some noticeable change. F 4 TCNQ molecule now exhibits some deviation of planarity (minor longitudinal torsion). For the 8-PNP-O12, a quite significant point is the change of the dihedral angle between the phenyl and naphthalene moieties of 8-PNP-O12 molecule, which changes from 141.57 to 155.83 • . This angle is of particular importance, since the planarity of molecule enhances the delocalization degree of electron density over the naphthalene: difference between bonds 10 to 13 increases, the same is true for bonds 2 to 4 in the phenyl moiety. The bond 5 between the two aromatic structures becomes shorter and approaches a C=C double bond (Figure 3.15, a). It is of interest to compare this data with bond lengths obtained for CTC: it is clear that the chemical structure of its components lies in between the neutral and charged states, which suggests some relocation of electron density from the 8-PNP-O12 molecule to F 4 TCNQ.

The geometry of neutral 8-PNP-O12 is not planar, since it has some noticeable torsion angle between phenyl and naphthalene parts. Due to this fact, the F 4 TCNQ molecule is not able to align itself completely parallel to the aromatic system of 8-PNP-O12, which results in the intermolecular distance between 8-PNP-O12 and F 4 TCNQ to be found in a range between 3.273 and 3.053 Å (Figure 3.14, side view). This fact presumably contributes to the significant asymmetry (Figure 3.13) of the shape of the HOMO of CTC as compared to that of more planar donor substances [39].

The CTC is calculated to have a bandgap of about 1.55 eV (Figure 3.13). Thus, it is then expected for the CTC to exhibit optical absorption in near infrared (NIR) part of the spectrum. UV-Visible-NIR spectroscopy is a suitable tool to investigate the character of newly formed CTC, given the absence of any electron transitions for initial products [START_REF] Hyun | Effectiveness of p -dopants in an organic hole transporting material[END_REF]39] in the NIR range. here studied concentration range.

Phase behavior, structural and optical properties

One can notice the color change, which becomes more apparent on higher concentrations (Figure 3.18, c -f). The color of the cells filled with the mixtures itself exhibits notable shift towards blue, though neither of initial products has this color. This coloration can be regarded as a result of interaction between 8-PNP-O12 and F 4 TCNQ.

To better explore this, we have performed UV-Vis-NIR optical spectroscopy, which is going to be covered in the corresponding section.

Structural studies by wide angle X-ray scattering (WAXS)

1D data extracted from wide-angle x-ray scattering images (inset of layer which results in a so-called modulated smectic mesophase [START_REF] Coleman | Polarization-modulated smectic liquid crystal phases[END_REF]. Such structures are encountered in lyotropic [START_REF] Sengupta | Structure of the ripple phase of phospholipid multibilayers[END_REF] as well as thermotropic [START_REF] Liu | Thermotropic liquid crystals from biomacromolecules[END_REF][START_REF] Vaupotič | Modulated general tilt structures in bent-core liquid crystals[END_REF] LCs and are not uncommon for assymetric molecules exhibiting longitudinal torsion [START_REF] Zhang | Two distinct modulated layer structures of an asymmetric bent-shape smectic liquid crystal[END_REF]. The leftmost intense peaks (a 4 and a * 4 , inset of Figure 3.20, (a)) are attributed to the direction orthogonal to the layer plane and can be utilized to extract the smectic interlayer distance, when the satellite peaks (a 1 -a 3 and a * 1 -a * 3 , inset of Figure 3.20) correspond to the layer modulation. Figure 3.20 allows us to compare the smectic layer separation distance for pure 8-PNP-O12 with the studied mixtures. The value of interlayer distance is found to be slightly greater than the 8-PNP-O12 molecule length in Sm B , whereas it is found to be slightly smaller in Sm A , indicating some interpenetration of layers. The layer spacing is found to be proportional to the content of F 4 TCNQ (as for the intermolecular distance distribution, Figure 3.19), the layers become less condensed as the doping ratio increases, which promotes greater layer flexibility and subsequently decreases the overall order of the mixtures [START_REF] Singh | CdSe quantum dots in chiral smectic C matrix: experimental evidence of smectic layer distortion by small and wide angle X-ray scattering and subsequent effect on electro-optical parameters[END_REF]. The observed behavior is encountered in all samples containing F 4 TCNQ (Figure 3.21).

Optical spectroscopy

Since the CTC forms new supramolecular orbitals with their own energy levels and bandgap, there should exist dedicated electron transition energy levels. In theory, it should be possible to observe the signature of CTC in the UV/Vis spectrum. Previous works, conducted on P3HT [37,39] have shown that it is indeed possible to observe the sharp peaks of fully ionized F 4 TCNQ -in case of ICT. P-doping of discotic LC HAT-6 with F 4 TCNQ has also demonstrated distinctive absorption features in both crystalline phase and columnar mesophase [130].

Pure 8-PNP-O12 exhibits optical absorption in UV region, which is explained by its wide optical bandgap [75]. F 4 TCNQ in turn, has it absorption onset at 2.6 eV, which leaves the remaining part of optical spectrum isolated and allows comfortable observation of their interaction product (CTC). The absorption spectra of initial products are assembled in Figure 3.22.

We have performed time-dependent DFT (TD-DFT) calculations in order to gain insight on the observed results. Observations of calculated optical absorption spectra (Figure 3.23) allows us to assign observed peaks in near infrared (peak at 1.45 eV with a sharp shoulder around 1.75 eV) and visible (two peaks at 2.27 eV and 2.57 eV) parts of spectrum (Figure 3.24) to the presence of CTC in the solution.

We are able to observe neutral F 4 TCNQ signatures on higher concentrations, however

Vibrational spectroscopy

Preliminary investigations

In order to investigate the possible contribution of structural order to the electron transfer in 8-PNP-O12/F 4 TCNQ system, we have carried out Raman spectroscopy in all phases displayed by this system. The first spectrum was recorded in crystalline phase Cr, then the sample was heated to isotropic phase Iso, where it was maintained for 30 minutes.

After annealing, each sample was slowly cooled back to crystalline phase through Smectic A (SmA) and Smectic B (SmB) mesophases. Raman spectra were recorded in every phase state, as well as two times in crystalline phase (before and after annealing). This was done in order to confirm the charge transfer and to investigate the thermal stability and possible influence of mesophase on the CTC. Pure 8-PNP-O12 does not exhibit any Raman vibrational frequency variation as a function of its phase state. This is not surprising, as the molecule itself is stable and does not undergo any structural change. We should also point out certain limits of this preliminary approach of integral charge transfer: it is not aimed to quantitatively evaluate the Raman band shift (instead, experimental data on F 4 TCNQ -1 is used [39,38]) in comparison between experimental and calculated spectra for all of the observable vibrational modes, but to give an idea of the changes in vibrational spectra of a mixture which exhibit evidence of successful charge transfer (i.e. the presence of binded complex between host and dopant) at this stage.

H 17 C 8 C 1 C 6 H C 5 H C 4 C 7 C 16 H C 15 C 14 H C 13 H C 12 O 17 C 12 H 25 C 11 H C 10 C 10 C 9 H C 8 H C 3 H C 2 H (a) N 1 C 2 C 3 C N C 4 C F C F C C C N C N C 6 F C 5 F (b)
We begin with the comparison of different doping concentrations in the form of as-prepared powder, as shown in the Figure 3.29. The spectra are normalized in respect of intensity of the peak at about 1300 1/cm. Pronounced peak at 1450 1/cm comes from the conjugated lateral double bond C=C stretching of the dopant and it is clear that its intensity is proportional to the dopant concentration. That is also true for the peak at The situation changes significantly upon heating to isotropic phase (Figure 3.30, Iso).

The nitrile group peak is shifted to the left 2212/cm and broadened. The quinoid ring Figure 3.29 -Raman spectra for mixtures in as-prepared powder: black for pure 8-PNP-O12, red for 0.1%, blue for 0.5%, green for 1%, magenta for 3% and brown for 5% F 4 TCNQ content. Black arrow highlights new band apparition. Intensity is normalized for peak at around 1300/cm. stretching band disappears, and stretching of lateral double bond shifts to the left (1437 1/cm). Peaks, related to the aromatic system (phenyl and naphthalene rings stretching) of 8-PNP-O12 also undergo noticeable shift to the left (1606 and 1624 1/cm, respectively).

Interestingly, the spectra recorded in SmA and SmB phases (Figure 3.30) seem to bear no difference with each other.

Finally, after crystallizing from Sm B mesophase, the annealed crystalline sample shows (Figure 3.30, Cr * ) correlation to the spectra, previously observed during annealing as we can notice in more pronounced peaks of aromatic system of 8-PNP-O12 (1600 and 1619 1/cm), as well as the reappeared shifted band connected to the quinoid ring (1650 1/cm). Samples of the other concentrations, as can be noticed in Figure 3.31, exhibit similar behavior: the other concentrations with less pronounced effects due to strong contribution of the initial components. The general trend observed for all of these mixtures allows us to attest successful formation of charge transfer complex between the host and the dopant.

In previous section, we have noticed that CTC has optical absorption in near-infrared seems to be in resonance with one of the electron transition energies of CTC. This can explain little to no signature of initial products in the spectra recorded during annealing, as well as for the annealed samples (Figure 3.30 for c5), despite the huge excess of pristine 8-PNP-O12 which is expected to contribute to the Raman spectrum.

These experimental Raman spectra suggest that the formed CTC is thermodynamically stable within applied temperature range of 50 • C to 130 • C, i.e. for all phase states displayed by the 8-PNP-O12/F 4 TCNQ mixtures. This result is in good agreement with the observations of Khan et al [130]: their system was based on HAT-5 and F 4 TCNQ.

The optical absorption spectra recorded in isotropic phase of their mixtures looked similarly to other phase states exhibited by their samples. Moreover, for our system, there seems to be no observable influence of the structural order on the quantity of the charge transfer between 8-PNP-O12 and F 4 TCNQ. In fact, the vibrational bands positions do not change between Sm B and Sm A mesophases. It is of interest to compare this observation to the case of PTCBI derivative [92,[START_REF] Gregg | Reversible switching between molecular and charge transfer phases in a liquid crystalline organic semiconductor[END_REF], where a phase transition introduces [39,38]. Frequency corresponding to partial charge transfer is extracted from Raman spectrum. The point in red represents the expected amount of charge transfer from the DFT calculations.

to frequencies of those b 1u modes which are only IR-active in F 4 TCNQ and F 4 TCNQ - (since it no longer posses a center of symmetry). It is possible to perform a quantitative estimation of the charge transfer degree δ between 8-PNP-O12 and F 4 TCNQ using the following equation [START_REF] Kampar | Degree of Charge Transfer in Donor-Acceptor Systems of the π-π Type[END_REF]:

δ = 2∆ν ν 0 1 - ν 2 1 ν 2 0 -1 , (3.1)
where the observed shift in vibrational frequency (∆ν) is compared to the position of the same band in pure F 4 TCNQ (ν 0 ) and the anion F 4 TCNQ -(ν 1 ).

The value for frequency of b 1u mode for F 4 TCNQ -is taken to be 2194/cm [39,38]. The observable red shift of this band (2227→22171/cm, Figure 3.33) yields a charge transfer degree δ of 0.31 (equation 3.1), which is in excellent agreement with DFT calculations (value of 0.31 obtained as an average of electrostatic potential (ESP, 0.33), natural bond orbital (NBO, 0.32) and Hirshfeld population (CM5, 0.29) analysis schemes). Figure 3.35 allows us to compare the calculated value (red) with the one extracted from the Raman spectrum (black).

To summarize, the spectroscopic studies have allowed us to confirm the predicted by ab initio calculations charge transfer between the 8-PNP-O12 and F 4 TCNQ and determine its character (partial charge transfer, PCT). The Raman spectroscopy has allowed us to quantify the amount of transferred charge, as well as to confirm the stability of the CTC in the phase states exhibited by the prepared mixtures. These results are going to help us in the investigations of their charge transporting properties, which are covered in the next section.

Charge transporting properties in the mesophase

It is usually assumed that introduction of dopant results in increase of notoriously weak intrinsic "free" charge carrier density in organic substances, though its effects on bulk mobility need to be addressed more profusely. In this section we have made an attempt to disclose the influence of charge transfer complex on bulk hole mobility of 8-PNP-O12

by utilizing the TOF technique [START_REF] Bushby | Photoconducting liquid crystals[END_REF][START_REF] Hudson | Radiation-induced trapping and charge transport in a smectic liquid crystal[END_REF]91,[START_REF] Kian | Guanine-oligothiophene conjugates: Liquid-crystalline properties, photoconductivities and ion-responsive emission of their nanoscale assemblies[END_REF][START_REF] Kondratenko | Hole transporting properties of discotic liquid-crystalline semiconductor confined in calamitic UV-crosslinked gel[END_REF].

Due to the nature of this technique, the obtained results are decoupled from the absolute number of charge carriers which participate in the transport (quantity of carriers impacts the amplitude of signal and not the photocurrent decay profile). Mobility (µ) is calculated by using the equation 2.6 [128].

Hole mobility values are measured for the range of applied electric field E from 10 kV/cm to 50 kV/cm in Sm B mesophase and 25 kV/cm to 60 kV/cm in Sm A mesophase, [START_REF] Lüssem | Doping of organic semiconductors[END_REF], which results in easier charge injection and subsequent rise of the DC conductivity (Figure 3.39). One of the basic requirements for TOF measurements is injection-blocking contacts, since under high applied field (often used in TOF measurements) possible contribution of space charge-limited current may interfere with transient signal [128].

A possible explanation for the mobility increase may be the change of dihedral angle 5-4-7-8 (Figure 3.27, a), which result in its planarization of 8-PNP-O12 molecule, thus improving the electron density delocalization between phenyl and naphthalene aromatic systems. Similar phenomena has been previously reported by works dedicated to the role of regioregularity of P3HT in its charge transporting properties [59,[START_REF] Jiang | Doping-induced change of carrier mobilities in poly(3-hexylthiophene) films with different stacking structures[END_REF].

Hole mobility as a function of temperature is presented in the Figure 3.40. Results obtained for pure 8-PNP-O12 are in good agreement with those previously reported [START_REF] Shiyanovskaya | Electronic transport in smectic liquid crystals[END_REF]78]: hole mobility is of 1.5 × 10 -3 cm 2 /(V s) in Sm B and 3.6 × 10 -4 cm 2 /(V s) in Sm A and temperature independent in each smectic phase. As we have previously noticed, doping with F 4 TCNQ results in the increase of hole mobility in Sm B , whereas in Sm A it decreases with the concentration of dopant. However, unlike for pre 8-PNP-O12, the mobility seems to be increasing with the temperature (for the mixtures). This thermal activation may be explained by the fact that CTC is interfering with the smectic order, thus creating some trap states [START_REF] Zuo | Molecular Doping and Trap Filling in Organic Semiconductor Host-Guest Systems[END_REF]. These trap states can be overcome on higher temperature due to broadening of the density of states (DOS) distribution [46,[START_REF] Hanna | Charge carrier transport in liquid crystals[END_REF]. We have calculated the activation energy E a for mobility in both mesophases (Figure 3.41).

It is found that in Sm B , E a is increasing with dopant concentration, whereas in Sm A it is decreasing.

In order to address to prepare the discussion on the results TOF mobility measurements in smectic mesophases we have carried out EDX analysis on previously prepared samples of c3 and c5, which were used for TOF experiments. These cells were slowly cooled from the isotropic phase state in order to preserve "smectic" polydomain structure in its crystalline state. The purpose of this analysis is not to demonstrate the morphology of thin films, but to investigate the presence of dopant aggregates, which may allow us to obtain more information and shed some light on the charge transporting properties of the 8-PNP-O12/F 4 TCNQ system. The fluorine signatures appear to be grouped more densely in the areas populated by the mixture. A higher magnification on a "smectic" domain reveals largely uniform distribution of dopant sites across the field of view.

Images of c5 cell (Figure 3.43) demonstrate similar picture and show correlation and shows good compatibility between the components of the mixtures.

These results allow us to conclude that the observed hole-transporting behavior is not related to the "macroscopic" dopant distribution, since the thickness of the cell is of the order of one smectic domain.

The behavior of the hole mobility in the Sm A of 8-PNP-O12/F 4 TCNQ system can be compared to the case of binary mixture between 8-PNP-O12 and small amounts (in order of tenths to hundreds of ppm) of dihexyl terthiophene (DH3T). This system was studied by Funahashi et al [START_REF] Funahashi | Impurity effect on charge carrier transport in smectic liquid crystals[END_REF] in an attempt to understand the contribution of charge trapping to the hole transport by deliberately introducing traps to the 8-PNP-O12 material. the HOMO of DH3T lies higher than that of 8-PNP-O12 (5.1 eV vs 5.6 eV)

Thus the DH3T indeed acts as a hole trap for the host 8-PNP-O12. It was found that the hole transport was considerably affected by carrier trapping in the smectic phases:

the hole mobility was decreasing with the concentration of DH3T. These results have demonstrated that the behavior of their system is described by the following equation [172]:

µ = µ 0 (1 + τ detrap τ hop C 1 2 ) (3.2)
where µ 0 is the mobility in pristine 8-PNP-O12, C is the dopant molar fraction, τ hop and τ detrap is the hopping time and detrapping time (when the carrier is released) respectively. For the 8-PNP-O12 doped with DH3T the ratio of τ hop /τ detrap was found to be around 8.6 × 10 -4 The expression 3.2 was used to fit the experimental data of hole mobility found in our system (Figure 3.44), and τ hop /τ detrap ratio is found it is found to be of about 0.26.

By comparing the results reported by Funahashi et al [START_REF] Funahashi | Impurity effect on charge carrier transport in smectic liquid crystals[END_REF] to our data, it becomes clear that for holes trapped by D3HT it is significantly harder to leave their trapped state than for the our case of doping by F 4 TCNQ. Results reported in their work demonstrate negative effect of DH3T on hole transport, it can even induce complete loss of electronic transport in Smectic A mesophase on higher DH3T loading (50 ppm). In our case, the HOMO of CTC lies lower (6.3 eV estimated from DFT calculations, Figure 3.13) than that of 8-PNP-O12, thus acting as a potential barrier rather than a trap. It is well-known for the potential barriers to have less negative effects on the carrier transport, since they repel carriers (when traps attract them), thus the carriers have more chances to find their way around the barrier instead of being retained by a trap state [93].

The behavior of mobility in Sm B mesophase is unexpected. The main challenge in interpretation of results of TOF measurements comes from the observation of orthogonal mobility behavior: hole mobility is rising as a function of increased dopant content in the Sm B and decreasing in the Sm A . However, one should consider the two experimental facts:

• As demonstrated by X-ray scattering section, the mesophase order is decreasing as a function of dopant similarly for Sm B (broadening of intermolecular distance distribution within the smectic layers, Figure 3.19, (a)) and Sm A (slight increase of layer separation, Figure 3.20)

• The Raman spectra demonstrate the invariant character of charge transfer degree These observations do not show any orthogonality of aforementioned properties between two mesophases. Thus, the introduction of F 4 TCNQ induces some disorder to both mesophases. This rules out, for example, promotion of hole mobility due to order increase or difference in charge transfer degree in between the aforementioned molecules in CTC.

It has been widely demonstrated in the literature that some materials, notably, polythiophenes, demonstrate a nonlinear behavior of mobility as a function of dopant concentration. Arkhipov et al. [START_REF] Arkhipov | Weak-field carrier hopping in disordered organic semiconductors: The effects of deep traps and partly filled density-of-states distribution[END_REF] have developed a model which is meant to explain the influence of doping impurity on the charge carrier mobility behavior in bulk OSCs.

This model has been successfully applied to the doping of amorphous OSCs, for example P3HT, and may be applied to other materials which exhibit charge transport by carrier hopping (which is the case for 8-PNP-O12). It describes two doping regimes in regard to the concentration of the dopant content in the OSC: it is proposed that the dopant is able to induce potential traps on low concentrations, where the dopant sites are essentially isolated. For increased concentrations, the overlap of these dopant-induced trap states is able to smooth the potential landscape of the material (i), and alongside with trap filling (ii) by increased charge carrier density, this may be held accountable for the mobility increase.

It is worth noting that due to the homogeneous alignment (i.e. the long molecular axis is parallel to the electrode plane) of the studied samples, the charge transport takes place within the smectic layers. Though the WAXS data demonstrates that the introduction of F 4 TCNQ does not result in the changes of phase sequence of the pristine 8-PNP-O12, since the smectic phases are maintained in the prepared mixtures. Taking into account the pseudo-hexagonal close range order of the Sm B mesophase [START_REF] Huang | Non-Chiral Smectic Liquid Crystals: Physical Properties of Non-Chiral Smectic Liquid Crystals[END_REF], it is possible to propose that the CTC moieties are distributed more uniformly within the smectic layers (Figure 3.45, top), contrary to the case of the Sm A phase, where is no preferred intermolecular order inside of the smectic layer (Figure 3.45, bottom). Thus, the short range order of the Sm B mesophase is able to maintain the favorable order of CTC and adjacent 8-PNP-O12 sites by reducing average distance b and height ∆ (Figure 3.46), resulting in reduction of potential barrier between these sites as compared to the case of Sm A phase (Figure 3.46, bottom). This may result in improved charge carrier mobility inside of the smectic layer. However, in the less ordered Sm A phase,

CTC is allowed to be less regularly distributed or even aggregated inside the layer, which results in the less favorable overlapping between the potentials of neighboring CTC and 8-PNP-O12 sites, and, as a result, increase of potential barrier.

Chen et al [93] have demonstrated that carrier density in doped PTCBI derivative is thermally activated. This result may explain the mobility increase by trap filling in Sm B and the rise of E a (Figure 3.41), but the decrease of E a in Sm A with increasing F 4 TCNQ content suggest that traps are generated faster than new carriers. This contradicts to some extent with the phase behavior of the mixtures: Sm A → Iso transition temperature does not show significant shift with dopant concentration increase, which allows us to propose that the stability of Sm A mesophase is less affected by the dopant presence than that of Sm B .

If we take a closer look at possible trap filling hypothesis, it is difficult to explain the abrupt mobility decrease in the Sm A mesophase. The possibility of a drastic change in the carrier density (which from a theoretical standpoint may be also held accountable for the change of mobility by filling the trap states) upon Sm B to Sm A transition seems to be unlikely, especially taking into account thermal activation of carrier density in doped OSCs.

One possible explanation may be that the mobility in the Sm B mesophase is indeed increased due to the trap filling, and after Sm B to Sm A transition, a much higher quantity of traps is abruptly introduced, and the aforementioned trap filling effect is no longer able to compensate the increase in trap density, which results in mobility decrease.

Topology and current mapping of thin polycrystalline films

Thin film deposition

An array of attempts was performed on depositing a thin film of 8-PNP-O12 on oxidated surface (soda lime glass and quartz plates), however, the resulting films were highly nonuniform due to poor wetting properties of the non-polar solvents utilized to dissolve 8-PNP-O12 and its mixtures.

In order to improve the wettability the substrate, the latter has undergone the procedure of OTS-18 coating, as described in the Chapter 2. The solvent choice was based on chemical properties of 8-PNP-O12: unsurprisingly, best results were demonstrated with chlorinated and aromatic non-polar solvents. Xylene (mixture of isomers, Sigma Aldrich) was chosen due to its higher vapor pressure and viscosity as compared to toluene, in addition to lower toxicity compared to chlorinated solvents (chloroform and chlorobenzene). Films prepared with chloroform-based solutions exhibited numerous defects and overall inhomogenity with obvious signs of solvent evaporation during thinning part of coating process. After testing a vast range of speed, acceleration and duration settings, following parameters were retained, as they tended to give adequate results upon optical observations: speed of 3000 rpm, acceleration of 800 rpm/s and time range between 10 and 45 seconds.

Thickness of prepared films was estimated by means of SEM microscopy after dissection of a substrate with thin film, further mounted on the SEM sample holder. Thickness was measured by examining 4 field of views of 3 µm by 2.25 µm (Figure 3.47), 6 measurements were taken on each field of view. For films prepared at 10s thinning time average thickness was measured to be 633±44nm; for 20s thinning time -135±12nm. For greater thinning times, the films were found to be non-homogeneous with a significant number of defects.

Conductive atomic force microscopy

Thin films of pure 8-PNP-O12, c3 and c5 were deposited on gold-coated glass slides by previously described spin coating method for duration of 20 seconds. Optical observations have demonstrated good adhesion on clean gold surface. Further, these films were annealed for 2h at 110 • C (Sm A ), followed with slow cooling speed (1 • C/min) to ambient temperature. The annealing was performed in order to ensure more complete CTC formation, as demonstrated in previous subsection dedicated to vibrational spectroscopy.

In order to study the topology of the deposited films we have performed AFM analysis in contact mode, as well as current mapping in order to better understand the consequences of the molecular p-doping, which allows us to establish a connection between the topology of the films and charge transporting properties.

Example of topological and conductive properties of pure 8-PNP-O12 thin film is presented in the Figure 3.48. The overall topology shows presence of small grains of 8-PNP-O12 material. The current mapping (recorded at -6V constant bias) reveals strong correlation of conductive regions to the topology of the thin film: it seems that conductive regions follow the cracks and pits found on the surface of thin film. This image also demonstrates behavior expected from a pure material, as relatively flat areas exhibit similar conductivity. It is possible to propose two explanations for this observations:

firstly, the collected current is higher due to reduced thickness (and thus, lower resistance); secondly, effective surface of the probe tip is increased upon contact with sloped surface of the crack, increasing current amplitude, which may also explain why crack edges have asymmetric shape in the conductivity mapping image. Selected current-voltage data (scanned from 10 V to -10 V, 2 V/s) confirms the observations of current mapping, with similar voltage threshold values, as well as a symmetric shape, illustrating the ambipolar host [43]). The overall current mapping of this sample confirms the complex picture which we have observed for the c3. Selected current-voltage data further demonstrates the predisposition of doped 8-PNP-O12 to enhanced hole transport.

As for the c3 thin film, we have included a zoomed section of the C-AFM images of c5 (Figure 3.52). The current mapping reveals highly conductive grain boundaries which are in large contrast with grain summits: relatively low variation of height in order of 10nm

(less than 10%) results in more than threefold increase of collected current. Thus, it is possible to suggest that sample thickness is not accountable for the observed phenomena, and the aforementioned explanation of high grain boundary conductivity due to molecular packing appears more adequate.

Conductive AFM has allowed us to establish a link between structural and conductive properties of 8-PNP-O12 doped with F 4 TCNQ in a form of thin polycrystalline films. We have also validated deposition procedure (spin-coating) whic provides us homogeneous dispersion of dopant sites in the bulk of 8-PNP-O12. The local currenvoltage characteristics have revealed a noticeable change from ambipolar transport in We have extracted hole mobility with the help of equation 1.7 from the saturation current at gate bias V G = -100V, µ = (1.3 ± 0.6) × 10 -8 cm 2 /(V s) as compared to the mobility before annealing µ = (1.0 ± 0.8) × 10 -9 cm 2 /(V s). This drastic change is attributed to the enhancement of near-dielectric interface alignment of semiconductor molecules, which corresponds well to the observations of threshold voltage decrease.

ON/OFF ratio is also improved from 3 to 83. Despite this improvement, the value of ON/OFF ratio is still low, which is a fundamental consequence of chemically doped materials [36].

The performance of the present mixtures is less than spectacular when compared to the best examples of organic transistors [START_REF] Braga | High-Performance organic field-effect transistors[END_REF]. We also notice here that the extracted hole mobility is significantly lower than that obtained by TOF technique in Sm B mesophase.

This may seem counterintuitive, since the more ordered crystalline phase should be more It is necessary to point out that high-performance devices are usually prepared by utilizing vacuum deposition techniques, where highest molecular order of semiconducting material can be attained. However, application of doping to these deposition techniques is less straightforward, since the doping impurity is usually introduced by co-evaporation [25], which arises additional requirements for the molecular structure of dopant and experimental conditions. The use of solution process, as well as preliminary dispersion of the doping impurity in the host semiconductor significantly simplifies the procedure.

Conclusion

In this work we have explored different aspects of a LC OSC -electron acceptor complex.

We have confirmed the stability of LC mesophase up to significant concentrations of This observation has enabled us to use the TOF technique to find the bulk hole mobility in the mesophase. It is found that despite the stability of CTC remains high across both mesophases, the CTC itself has some effects on the mesophase structure. The hole mobility in each mesophase appears to increase with temperature, meaning that some charge trapping takes place. It is proposed that CTC is somehow able to disrupt the order within the layers of smectic mesophases, inducing some structural defects which may act as traps. The isolated CTC by itself appears to have better charge transporting properties than the pristine 8-PNP-O12 due to the aforementioned change in the geometry. Thus, for the more ordered Sm B mesophase, the role of structural defects is partially negated by the improvement of the hole mobility of the CTC and sufficient overlap of the dopant sites. This can explain why the hole mobility is higher in this mesophase for mixture samples than that of pristine 8-PNP-O12. However, in the Sm A mesophase, whose in-layer order is inherently weaker compared to the Sm B , the structural defects seem to provoke the charge carrier trapping. This explains why the hole mobility decreases with the F 4 TCNQ content. It is also possible to suggest that the short-scale order of the Sm A mesophase is more "vulnerable" to the disorder-inducing impurities than that of the Sm B mesophase, as the resulting from doping charge carrier density increase is not able to compete with introduced traps. Thus, for the more ordered Sm B mesophase, the role of structural defects may be partially negated by sufficient overlap of the dopant site potentials resulting from a more homogeneous CTC distribution in the layer, or trap filling by increased carrier density, which results in improvement of the hole mobility of the mixtures compared with that of pristine 8-PNP-O12. In order to gain more insight on the observed mobility behavior it is necessary to reliably estimate the carrier density in both mesophases, which may be proven a non-trivial task due to strong ionic current contribution occurring on DC measurements in Sm A mesophase.

Generally, it is proposed that for adequate doping results the LUMO of the acceptor should be as close as possible to the HOMO of the donor to ensure efficient electron transfer. In this work we have showed that despite very close energy levels (less than 0.2 eV offset for 8-PNP-O12 and F 4 TCNQ as opposed to 0.06 eV offset between 4T

and F 4 TCNQ [39]) the charge transfer amount is not strictly proportional to the value of the LUMO of the electron acceptor. We may also connect this behaviour to steric effects induced by the geometry of the pure 8-PNP-O12. Relatively large dihedral angle (141.57 • ) between phenyl and naphthalene rings obstructs the access for one wing of the F 4 TCNQ, which prevents it from being parallel to the molecule of 8-PNP-O12. The result of this is the incomplete participation of electron-deficient nitrile groups on one wing of the F 4 TCNQ, which is reflected in the asymmetric hybridization observed on the Figure 3.13. This asymmetry may also explain the results we have observed in the Raman spectra.

The main goal of our work is to postulate the necessity of scrupulous research of factors which reflect the influence of geometry on the efficiency of molecular doping. This underlines the importance of the term "molecular" in this process: (i) choice of the electron donor-acceptor system should be based not only on frontier molecular orbital energy levels, but also on the individual geometry; (ii) the result of donor-acceptor interaction in case of partial charge transfer (CTC formation) should be regarded as a unique supramolecular entity in order to predict its properties; (iii) careful attention should be addressed to the application of vibrational spectroscopy, as it is largely influenced by the molecular symmetry; (iv) the nature of change in the charge transport should be properly inspected:

the carrier mobility measurement should be accompanied by carrier density estimation in order to obtain the comprehensive picture.

We have made further investigations of mixtures in form of thin polycrystalline films.

A protocol for thin film deposition was elaborated, which enabled us to study the resulting thin films with C-AFM technique. We have confirmed preferable p-type nature of the charge transport in the prepared mixtures, as well as established a correlation between the topology of films and C-AFM current. Finally, we have prepared a field effect transistor with the highest-performing concentration, c5, which demonstrated encouraging results,

Polymer-stabilized p-doped composites

This section is purposed to study the effects of in situ formed polymer network on the p-doped 8-PNP-O12. As it was demonstrated earlier, the polymer network formed from a nematic reactive mesogen is able to promote the large-scale order and results in a minor decrease of the charge carrier mobility. The p-doped samples however show that it is possible to overcome some negative consequences of positional order decrease by improving charge carrier density and overall homogenity of the smectic layer structure. It is then expected that a polymer network will be able to improve the stability of mesophase in p-doped materials presented previously, increasing their performance.

Materials and sample preparation

The materials in this study were based on the "gel" type composites containing 3% of the reactive mesogen RM82 (g3). This RM82 concentration was chosen as it gives a reasonable polymeric network density without severe negative impact on the charge transport in the semiconducting matrix. After the gel precursor was prepared, three composite concentrations were elaborated with variable p-dopant content: of 0.1 (gc1), 0.5 (gc2), 1 (gc3) %wt of F 4 TCNQ. Higher concentrations of F 4 TCNQ have shown to significantly inhibit the process of photo-activated polymerization.

The prepared composites were placed inside of a LC sandwiched cell by capillary forces on the isotropic temperature, then slowly cooled to the temperature of Sm B mesophase (90 • C), where the cells were irradiated with 365 nm for 15 minutes, which ensured cross-linking of the RM82 molecules.

Thermophysical, mesogenic and structural properties

As in the case with all liquid-crystalline materials, prepared precursor mixtures served as an object of DSC study. Transition temperatures and peak amplitudes seem to be largely unaffected by the introduction of F 4 TCNQ. It is of interest to compare the resulting thermograms (Figure 3.57, a) of precursor with those of doped mixtures prepared in previous section. In the case of absence of RM82, the phase transition temperature shift is quite noticeable; the most affected phase transition was that of between Sm B and Sm A mesophases. However, in case of RM82 introduction, the phase transition temperatures seem to be stabilized, in contrast to previously described doped system (Figure 3.57, b).

It is possible to suggest that stabilization of long-range order by RM82 is able to prevent the degradation of Sm B mesophase positional order, previously observed in the case of p-doped 8-PNP-O12. However, the most interesting change is in the in situ formed polymeric network: one can notice that the density of network seems to be inversely proportional to the content of F 4 TCNQ (Figure 3.58, isotropic phase). This can be explained by the inhibition of free-radical mechanism by the doping impurity. Indeed, electron-poor systems including quinoids, chloranil, trinitrotoluene and benzylidene malononitrile are capable of inhibiting the polymerization process [START_REF] Kice | Inhibition of Polymerization. I. Methyl Methacrylate[END_REF][START_REF] Yeon | Synthesis and free radical polymerization of vinyl ethers containing two electron acceptors[END_REF]. The F 4 TCNQ molecule is able to concur with the functional groups [START_REF] Iwatsuki | Spontaneous Alternating Copolymerization of Tetracyanoquinodimethane with Styrene[END_REF] and even form copolymers [START_REF] Iwatsuki | Alternating Copolymerization of 7,7,8,8-Tetracyanoquinodimethane with Methyl Methacrylate in Acetonitrile[END_REF][START_REF] Washino | Cross-linking and postfunctionalization of polymer films by utilizing the orthogonal reactivity of 7,7,8,8-tetracyanoquinodimethane[END_REF]. Presumably, the F 4 TCNQ molecules interfere with chain propagation, and even sometimes cap the functional groups of RM82, thus disrupting the formation of polymer network (Figure 3.59).

Charge transporting properties in smectic mesophases

The charge transport in cross-linked composites was studied by TOF technique in both Sm B and Sm A mesophases. Measurements were performed for the range of applied electric field E from 20 kV/cm to 44 kV/cm with a step of 4 kV/cm at Sm B mesophase and from 32 kV/cm to 60 kV/cm with the same step and d=10 µm sample cell thickness.

Transient photocurrent plots for holes are presented in the insets of the Figures 3.60 By comparing this behavior with that previously encountered for polymer stabilized The overall behavior of hole mobility for a given mesophase is presented in the Figure 3.62 (a and b). These plots allow us to suggest that composites gc1 and gc2 behave similarly despite significant difference in the content of F 4 TCNQ. However, the gc3 composite shows a decrease of hole mobility in the Sm B mesophase and a slight increase in Sm A phase. It is also of interest to underline the evolution of hole mobility in the Sm A phase.

N N F F N N F F OR O N N F F N N OR O F F           n       m
Immediately after the Sm B to Sm A transition, the hole mobility of gc1 and gc2 is comparable with that of the undoped g2. However, the evolution of hole mobility in the Sm A mesophase demonstrates thermal activation, which was also previously encountered in the doped 8-PNP-O12 (Figure 3.40). This observation may be explained by previously discussed broadening of DOS with increased temperature. Thus, thermally activated hole mobility of polymer-stabilized p-doped 8-PNP-O12 is able to overcome the thermally stable mobility of g2 in the Sm A mesophase.

The overall contribution of RM82 polymer network may be concluded by partial stabilization of the p-doped 8-PNP-O12 phases. The Sm A phase of polymer stabilized p-doped 8-PNP-O12 composites is more prone to the destabilizing effects of F 4 TCNQ, and despite the minor mobility increase, its behavior is still quite similar to the p-doped 8-PNP-O12 mixtures. We may conclude that in situ polymerized reactive mesogen network allows the hole mobility to become more or less decoupled from F 4 TCNQ concentration, as well as from temperature in Sm B phase.

In order to address the previously discussed results, as well as to obtain some grounds

Conclusion

This brief section illustrates the work aimed to study a potentially cooperative effects of cooperation between the two approaches utilized in previous sections: polymer dispersion One should not forget that in this case we are dealing with a strongly polycomponent system (host semiconductor, doping impurity, reactive mesogen, photoinitiator and thermal inhibitor). It has been widely demonstrated in the field of semiconducting materials that any impurity indeed can and will actively contribute to the charge transporting properties of the host material. We have performed exhaustive studies of the particular cases of pdoped 8-PNP-O12 and polymer-stabilized p-doped 8-PNP-O12. We have also proposed that some undesirable interaction (e.g. copolymerization) between F 4 TCNQ and RM82 may take place. This proposal may indeed explain well the limited compatibility of these two approaches.

In application to the charge transport in the two smectic phases, we have noticed some improvements over the samples without polymer network stabilization: hole mobility was slightly improved, as well as the transition temperate was not as seriously affected by the F 4 TCNQ presence as before. However, larger concentrations of F 4 TCNQ seem to act against the positive contribution of the RM82 network, which effectively destabilizes the system and results in slight decrease of hole mobility in the Sm B phase as the dopant loading increases.

General conclusion

This chapter exposes the work conducted on the newly developed materials based on 8-PNP-O12 semiconducting liquid crystal. We have performed a thorough characterization of a wide range of host-guest systems based on a well-known liquid crystalline OSC. This allowed us to obtain original and interesting results, which we hope will be useful for the scientific community and help to discover new and better ways to attain higher performance in the field of organic electronics.

It has allowed the author to explore such subfields of condensed matter physics as mesogenic materials and organic electronics, as well as acquire, improve and cement the knowledge of a larger variety of experimental techniques required for physico-chemical characterization of materials. This includes various types of microscopy, spectroscopic techniques, ab initio calculations (which were strongly correlated with experimental data), X-ray data treatment and etc. The Time-Of-Flight measurement setup has required the author to acquire knowledge and develop skills of EMI shielding, signal treatment and amplification, software coding and instrument communication. Furthermore, the attempt on device prototyping has allowed the development of skills and knowledge related to thin film deposition and device characterization.

Introduction

We present synthesis and characterization of a novel liquid crystalline semiconductor in the current chapter. This molecule will become the first step in the direction which final goal is to prepare a photo-reactive organic LC semiconductor in order to prepare aligned cross-linked films for potential applications. We have conceived an easy-to-synthesize semiconductor which is based on inexpensive and readily available precursors. First, we present a brief overview of the semiconductors based on the building block of choice -anthracene. Further, we describe synthetic procedure, as well as characterization of the chemical structure in order to prove that proposed synthetic route indeed affords the expected molecule. The next step is to present some fundamental properties of the molecule: band gap energy, frontier molecular energy levels, thermo-physical properties.

Charge transport is investigated by preparation of a field-effect transistor. Finally, we investigate remarkable photoconducting properties of the molecule in scope, which may be relevant for potential light-sensitive applications.

Elaboration of (E)-N-(anthracen-2-yl)-1-(4-(decyloxy)

-phenyl)methanimine

Overview of anthracene-based semiconductors

As we have already mentioned in the introduction to Chapter 1, one of the first evidence of charge transport in organic molecules was discovered in anthracene crystals. Since then, enormous progress has been made in this field, and many other molecules have been identified as promising organic semiconductors. Nevertheless, anthracene still attracts significant attention as a building block for the OSC materials. anthracene, [START_REF] Ito | Oligo(2,6-anthrylene)s: Acene-oligomer approach for organic field-effect transistors[END_REF] µ hole * = 0.13 cm 2 /(V s); thiophene , µ hole * = 0.037 cm 2 /(V s); etc.).

These efforts have proven the great potential of anthracene, and in addition to improving charge transport, significant efforts were dedicated to improve the processability. * These mobilities were measured for a single crystal field effect transistor.

Elaboration of (E)-N-(anthracen-2-yl)-1-(4-(decyloxy) -phenyl)methanimine 179

Use of (triisopropylsilyl)ethynyl (TIPS) functional groups is a remarkable example of improvement of solubility, with a (9,10)-TIPS-functionalized molecule substituted with naphthalene in 2,6-positions which attained µ hole = 3.7 cm 2 /(V s) [START_REF] Sung | High mobility organic single crystal transistors based on soluble triisopropylsilylethynyl anthracene derivatives[END_REF].

In fact, favorable conditions for charge transport, such as strong π -π overlap which may lead to enhancement of transfer integral and improve charge hopping from site to site may seem counterintuitive for the liquid crystalline properties, since they demand a certain degree of "flexibility" for intermolecular order, notably, inside of the smectic layer, where π -π interactions are strong. This makes a creation of a mesogenic molecule with a reasonable mesophase temperature range a non-trivial task. It is now of interest to address some anthracene-based liquid crystals which possess semiconducting properties. Meŕy et al have made a significant contribution to the investigation of 2,6-substituted anthracene derivatives [START_REF] Méry | Liquid crystals containing a 2,6-disubstituted anthracene core-mesomorphism, charge transport and photochemical properties[END_REF]: it was found that direct attachment of flexible alkyl chains to the end positions of anthracene is not sufficient to obtain mesomorphic properties. Insertion of additional phenyl moieties between the anthracene and alkyl spacer has allowed not only to increase the overall flexibility of the molecule (which increases probability to attain mesomorphic properties), but to additionally increase the size of π-conjugated backbone. However, it is worth mentioning that hole mobility measured in Sm C and Sm A mesophases were of about 2 × 10 -3 cm 2 /(V s) at maximum, which is of the same order of value as for 8-PNP-O12 molecule in Sm B mesophase. In addition to relatively small gain in hole mobility, the mesomorphic behaviour was exhibited on very high temperature (> 200 • C).

Another interesting (and insoluble) molecule was prepared recently by Chen et al [START_REF] Chen | Thermal and Optical Modulation of the Carrier Mobility in OTFTs Based on an Azo-anthracene Liquid Crystal Organic Semiconductor[END_REF]: it is a 2-substituted derivative of anthracene, which in addition to semiconducting properties exhibits photosensitivity due to incorporation of an azo-group. This substance has a relatively lower mesophase transition temperature (about 140 • C), which makes it more convenient to investigate its charge transport. The cooperative effects of UVirradiation (molecular structure rearrangement) and thermal annealing (crystalline order improvement) have resulted in mobility increase from 0.019 cm 2 /(V s) to 0.746 cm 2 /(V s). It begins with the alkylation of para-hydroxy-benzaldehyde (Figure 4.3). The mechanism of this reaction is a simple alkylation (Williamson ether synthesis). We placed 0.3 g of para-hydroxy-benzaldehyde (Sigma Aldrich) in a round bottom flask with 0.65 g of 1-bromo-decane (molar ratio 1.2 to 1 with para-hydroxy-benzaldehyde). 0.41 g of K 2 CO 3

Synthetic route

(the same molar ratio with 1-bromo-decane) was added to the reagents to serve as a base for the alkylation reaction. Finally, 15 ml of acetone were introduced, and the mixture was set to reflux with stirring for two days (about 48 h). After the reaction was finished, we have filtered the resulting suspension on a sintered glass buchner funnel to remove K 2 CO 3 . The acetone was evaporated and the crude para-decyloxy-benzaldehyde (PDBA) was purified by column chromathography: the reaction product was suspended on the silica gel and then eluted with a mixture of dichloromethane/hexane ∼ 1:1 ratio.

This procedure allowed us to recover 0.53 g of PDBA which corresponds to 82% yield.

Imine coupling

The next reaction allows us to obtain the desired compound 10-OPIA (Figure 4.4). To perform this reaction, we have placed in a round bottom flask 0.53 g of the purified PDBA which we have obtained previously with 0.39 g of 2-amino-anthracene (2-AA).

The initial products were dissolved in 25 ml of toluene, and a few drops of glacial acetic acid were introduced as a catalyst. The reaction mixture is allowed to reflux with stirring for two days (about 48 h). The final product precipitated from the reaction mixture upon cooling, and was filtered and washed with cold toluene to remove unreacted compounds and further with acetone to remove toluene. The crude 10-OPIA was dried in the oven and then recrystallized in small amount of chloroform to improve the purity of final product. Finally, we have obtained 0.55 g of 10-OPIA which corresponds to 57% yield.

Additionally, a Dean-Stark trap may be employed to remove water from the reaction and improve yield.

Characterization of the chemical structure

Synthesis of a novel material requires an extensive and unambiguous investigation of its molecular structure in order to confirm the proposed interpretations of the encountered physical and/or chemical properties. Figure 4.5 allows us to map the atoms of interest (notably, hydrogen and carbon) in order to discuss the further results obtained from the NMR spectroscopy.

C 9 H 6 C 8 H 5 C 7 C 6 H 4 C 5 C 4 H 3 C 3 H 2 C 2 N C

NMR spectroscopy

The 1 H NMR spectrum of 10-OPIA is presented in Figure 4.6.

Optical spectroscopy

Absorption spectroscopy

The absorption spectrum has been recorded with a 10 -5 M solution of 10-OPIA in dichloromethane in a standard 1 cm quartz cuvette on a range from 230 nm to 500 nm.

By drawing a tangent at the first absorption band in Figure 4.11, it is possible to extract the onset of optical absorption of the studied material, which usually corresponds to the energy of the first electronic transition (S 0 → S 1 , HOMO → LUMO).

We have also recorded the absorption spectrum for a thin film of 10-OPIA deposited on a quartz slide by drop-casting 0.5% wt. chlorobenzene solution. It was found that 10-OPIA exhibits a redshift of 22 nm when in thin film, which may be explained by pronounced π -π interactions of neighboring molecules while being in close contact in the crystal as opposed to that of in solution. This results in a band gap energy reduction for the bulk 10-OPIA.

By utilizing the Planck-Einstein relation:

E = hc λ , (4.1)
where E is the energy, h is the Planck's constant and λ is the wavelength; the optical bandgap is calculated to be E g = 2.87 eV for the onset of absorption of 431 nm in dichloromethane solution, and E g = 2.74 eV for the onset of absorption of 453 nm in the form of thin film, respectively.

Fluorescence spectroscopy

The 10-OPIA molecule is expected to have semiconducting properties, thus an estimation of its light emitting properties has been carried out by fluorescence spectroscopy analysis in order to study its possible applications.

For this study, a dilute (10 -5 M) solution of 10-OPIA in acetone was prepared. The solvent was degassed by freeze-pump-thaw method on a Schlenck line prior to solution preparation in order to avoid quenching.

The excitation and emission spectra are presented in Figure 4.12. Excitation spectrum exhibits a slight red shift when compared to the absorption spectrum (Figure 4.11) recorded in dichloromethane, which may be explained by a small degree of solvatochromism. The peak of emission is situated at about 480 nm (small Stokes shift is commonly encountered in polyacenes).

Fluorescence lifetime of 10-OPIA was measured in acetone solution. We may speculate that for 10-OPIA molecule, the non-planar (dihedral angle between the planes of anthracene and phenyl moieties is about 140˚) geometry results in partial inclusion of lone electron pair on nitrogen atom to the delocalized electron cloud system of aromatic anthracene, thus hindering the rotation around imine linkage. Similar effects were observed by Yoshino et al [START_REF] Yoshino | Fluorescent azobenzenes and aromatic aldimines featuring an N-B interaction[END_REF], when molecules with azo-and aldimine linkages were constrained by modifying their structure, which resulted in noticeable enhancement of fluorescence.

Cyclic voltammetry

We have used cyclic voltammetry to experimentally obtain frontier MO energy levels.

This method is less precise than initially adopted in semiconductor research X-ray and UV photo-electron spectroscopy, but is by far less expensive and time consuming, while Cyclic voltammogramms were collected in the range of -2.2 to 1.8 V against silver wire pseudoreference with scanning speeds ranging from 50 to 500 mV/s. Electrolyte was agitated for about 1 minute with a magnetic stirrer between the scans to ensure the presence of pristine 10-OPIA molecules in the near-electrode environment. and almost negligible cathodic current, which is however more visible on higher scanning speeds) indicates irreversibility of these two processes, which may be related to the instability of oxidated/reduced 10-OPIA species in solution.

After collection of 10-OPIA voltammogramms, 2 ml of 1 mmol solution of ferrocene (98%, Sigma Aldrich) in supporting electrolyte was injected in the cell and degassed for 20 minutes prior to measurements. Additional data was recorded to include the potential range of Fc/Fc+ oxidation process (Figure 4.15, in blue). The data of ferrocene oxidation was further used to extract its half-wave potential (mean value between the anodic and cathodic peaks, since Fc/Fc+ process is quasi-reversible) and use it as a reference point [START_REF] Gagne | Ferrocene as an Internal Standard for Electrochemical Measurements[END_REF] for the previously obtained data for 10-OPIA.

Charge transport

Organic Field Effect Transistor

We have made several attempts to perform TOF carrier mobility measurements on 10-OPIA. However, the resulted photocurrent profile was highly dispersive in crystalline and smectic phases (it was impossible to extract carrier transit time) irrelevant to sample thickness. Probable reasons for this behavior may include inadequate alignment of 10-OPIA in the cell, negative contribution of grain boundaries and/or insufficient charge carrier generation by the available laser wavelength (λ = 355 nm).

Linear configuration

In order to characterize the charge transporting of this new material, current-voltage measurements were carried out on 10-OPIA OFET at room temperature. We have succeeded in preparation of OFET in bottom gate -bottom contact configuration by simple drop casting of 0.5% wt. solution of 10-OPIA in chlorobenzene (Sigma Aldrich) on clean substrate covered with OTS-18. The protocol (previously described in Chapter

2) has allowed us to grow relatively big crystals (greater than channel length) in the direction favorable for charge transport (π -π stacking normal to electrodes). µ = (4.1 ± 2.7) × 10 -5 cm 2 /(V s). This value is comparable to that obtained by Chung et al [START_REF] Sung | High mobility organic single crystal transistors based on soluble triisopropylsilylethynyl anthracene derivatives[END_REF] for 9,10-TIPS functionalized antracene derivatives substituted in 2,6-positions with phenylene vinylene (µ = (5.21 ± 3.55) × 10 -5 cm 2 /(V s)) and hexyl thiophene (µ = (2.05 ± 1.17) × 10 -6 cm 2 /(V s)) in thin film form. It should be noted that these reported results were obtained for devices in bottom gate/top contact configuration, which, according to the literature [START_REF] Liu | Contact engineering in organic field-effect transistors[END_REF][START_REF] Zhang | Device Engineering of Ofet[END_REF], usually exhibits better results than that of bottom gate/bottom contact geometry (used in this work).

The transfer characteristic and threshold voltage of 10-OPIA OFET in linear con- the ON/OFF ratio (10 4 ) of the device, which is considerably higher than that of doped 8-PNP-O12 due to low intrinsic conductivity of 10-OPIA. The threshold voltage for this device is found to be -27.6 V, which is lower than previously presented value for doped 8-PNP-O12 OFET (order of µ = 10 -7 cm 2 /(V s)). We would like to underline a relatively low threshold voltage obtained with 10-OPIA material, especially for this fabrication method (drop casting), which amplitude in some cases may be greater than -50 V [85].

Inter-digitated configuration

In order to investigate the possibility of 10-OPIA application to a larger-scale device, we have prepared another OFET with different electrode configuration (the same we have previously used to prepare field-effect transistors with doped 8-PNP-O12). Previously disclosed protocol has allowed us to obtain 17 working devices out of 20 per substrate on average. The device was characterized in ambient air conditions similar to previously described. The output characteristic of 10-OPIA OFET in inter-digitated configuration is presented in the Figure 4.24. This device, as in the case of linear configuration, demonstrates characteristic features of a field-effect transistor: drain-source current is proportional to the negative gate voltage, and the curve has two distinct regions: quasilinear on lower voltage and saturation on higher voltage. Field effect hole mobility is calculated from the saturation part of the drain source current: µ = (2.6 ± 1.2) × 10 -5 cm 2 /(V s). We may speculate that higher channel length (L = 50 µm) results in higher amount of structural micro-defects in 10-OPIA layer, which may be responsible for lower average mobility. found to be of the same order as for linear OFET: V th =-20.5 V.

We find that performance of 10-OPIA in these devices is fairly typical for a small molecule OSC. However, one should note simplicity and accessibility of the 10-OPIA synthesis process, which is an important factor promoting the widespread of organic electronics [START_REF] Po | all that glisters is not gold": An analysis of the synthetic complexity of efficient polymer donors for polymer solar cells[END_REF]. It is necessary to underline the working conditions as well: this molecule is remarkably stable in solid state (we have compared the NMR spectra after 8 months of storage in ambient conditions: no traces of degradation were detected). Easy and undemanding process (which may be easily upscaled with inkjet printing) of OFET preparation yields devices with reasonable performance. Interestingly, thermal annealing of devices did not result in improvement of carrier mobility, but in contrary it dramatically degraded their performance: by examining the substrates under optical microscope, we have found that annealing on temperatures above 80 • C results in apparition of significant amount of cracks in the channel area, which act as structural defects and impede charge transport. There are several ways to improve the performance which can be proposed:

• Purity of the 10-OPIA material may be improved. Although we demonstrate that initial purity obtained after successive recrystallizations from toluene and chloroform is good enough to demonstrate stable transistor behavior, there still may be some traces of impurity which act as traps for charge carriers. We may propose vacuum sublimation as the most suitable purification way for this kind of material.

• Geometry of the device may be changed for bottom gate/top contact. Devices in this geometry tend to demonstrate better performance for both mobility and threshold voltage. However, this type of geometry would impose additional requirements to the OSC film thickness and roughness and may be regarded as less attractive for low-cost applications.

• Additional attention may be focused on both contact and dielectric material and their interfaces with OSC. Despite rigorous cleaning, it is still possible for the gold contacts have a surface dipole which may interfere with hole transport. It has been demonstrated that some perfluorinated self-assembled monolayers (SAMs) may be efficiently deployed on the contacts via thiol functional group (covalently bound to gold) to remedy this issue [START_REF] Li | Improving performance of OFET by tuning occurrence of charge transport based on pentacene interaction with SAM functionalized contacts[END_REF]. Device preparation in a dry environment may additionally result in performance improvement, since the samples which were exposed to humidity for prolonged periods of time have demonstrated noticeable degradation of performance.

• Modifications to chemical structure may be introduced without significantly complicating the synthetic procedure. The most straightforward way is to modify the length of alkyl spacer unit -it not only gives us the control over mesogenic properties, but also allows us to modify the structure of crystal, which will in turn have direct consequence on transfer integral, and thus charge carrier mobility.

We may also introduce supplementary functionality to this molecule by attaching different groups to the alkyl spacer -for instance, our group continues to work on a photopolymerisable molecule based on 10-OPIA. In addition to conventional parameters (field effect mobility, threshold voltage, ON/OFF ratio), phototransistors are characterized by two additional parameters: photosensitivity P (ratio of photocurrent to dark current) and responsivity R (factor of proportionality of photocurrent to incident light power). We present these parameters in the Figure 4.27, (a). It is of interest to notice how responsivity gradually increases with negative gate voltage and essentially mimics a transfer characteristic of an OFET. This parameter exceeds 1 A/W at higher gate voltages, which may be considered reasonably high when compared to typical organic materials [START_REF] Hamilton | Thin-Film Organic Polymer Phototransistors[END_REF][START_REF] Narayan | Light responsive polymer field-effect transistor[END_REF] and even some inorganic semiconductors [START_REF] Di | Electrical transport and persistent photoconductivity in monolayer MoS2 phototransistors[END_REF].

The photosensitivity of this device is found to be maximal at intermediate gate voltage, which is directly related to a significant decrease of threshold voltage under illumination due to photogating effect (from V th = -20.2V to +20.3V), and decreases on higher gate field values (where I SD increases in dark conditions), stabilizing around 100. This may be explained by taking into account that the process of photoexcitation is somewhat similar to the field effect in regard to the increase in local density of charge carriers. The more than two order increase observed for current amplitude may indicate filling of significant amount of trap states by photogenerated carriers. In cooperation with field effect it may bring the device closer to the regime where the source-drain current becomes limited by imperfect contacts [START_REF] Campbell | Bulk photoconductive gain in poly(phenylene vinylene) based diodes[END_REF]. Visible saturation of I SD as a function of illumination power (inset of Figure 4.26) confirms this observation.

It is of interest to compare this behavior with the work published by Pal et al [START_REF] Tanusri Pal | High performance organic phototransistor based on regioregular poly(3-hexylthiophene)[END_REF],

where a field effect transistor prepared with P3HT in bottom gate/bottom contact configuration showed high performance under illumination. The ON/OFF ratio degraded significantly upon illumination (several orders of magnitude) which is in distinct contrast with our device. For 10-OPIA OFET, the ON/OFF ratio upon illumination is found to be virtually unchanged from dark conditions (around 10 4 ) even on relatively high illumination power (greater than 1 mW/cm 2 , see the inset of Figure 4.27, a) which may indicate a relatively high recombination barrier for photogenerated electrons. A larger band gap of 10-OPIA (as compared to that of P3HT) allows this device with gold electrodes to retain its P-channel character even upon significant illumination ("photodoping"). Another possible explanation includes a substantial difference between electron and hole mobilities in the bulk of 10-OPIA, which limits the number of photogenerated electrons participating in the channel current. This may explain large photoinduced gain of OFET's transfer curve on the majority of gate voltage range.

Before we continue our investigations of photoconductive properties of 10-OPIA, it is necessary to explain the nature of this large photoconductive gain. As we have mentioned, subjection of 10-OPIA to incident light results in the increase of channel conductivity (photoconductive effect) and threshold voltage reduction (photogating effect):

• Incident light creates electron-hole pairs in the semiconductor, which become separated under applied electric field V DS and participate in the increase of sourcedrain current(Figure 4.28, b). This effect is responsible for conductivity boost and positive photocurrent (equation 1.2, p. 10).

• Photogating effect is responsible for the aforementioned change of the threshold voltage upon illumination ( The resulting photoconductive gain G is expressed by following equation [START_REF] Campbell | Bulk photoconductive gain in poly(phenylene vinylene) based diodes[END_REF]:

G = cτ pc τ tr = cτ pc µV L 2 , (4.4)
where τ pc is the lifetime of photogenerated charge carriers, τ tr is the transit time, c is the concentration of successfully dissociated into electrons and holes light-induced excitons, µ is the mobility of majority carriers, V is the applied voltage and L is the distance between two electrodes. In the case of photogating, the τ pc is limited by the recombination of carriers residing in the trap states.

Field effect dependency of persistent photoconductivity

We have performed transient photoconductivity measurements in order to better understand the recombination behavior of these "gating" states, which has led us to another interesting feature of 10-OPIA -a distinct and non-negligible hysteresis of dark conductivity. Figure 4.29 serves to illustrate this phenomena: the sample is illuminated for 180s with 1.16 mW/cm 2 of 455nm light, which yields higher dark current after the light source is turned off than before it was turned on. We demonstrate that gate voltage has pronounced effect on the transient photoconductivity. First of all, we notice that OFET reacts significantly faster while the gate is being negatively biased than in unbiased state. We have succeeded in fitting the photocurrent growth with a single exponential growth function:

I = I 0 1 + exp t -t 0 τ , (4.5)
where I 0 is the initial current (vertical offset), t 0 is the horizontal offset, and τ is the response time. Application of relatively moderate voltage (V GS = -50V) results in almost one order decrease of saturation time for the photocurrent (7.5s vs 53.8s). We may attribute this result to faster filling of trap states with photogenerated carriers, as well as improved exciton dissociation on the semiconductor/dielectric interface.

The decay part of persistent "photo" current also bears differences in respect to gate bias. By judging the decay profile on a semilogarithmic plot, we have deduced it to have two exponential components, which has enabled us to fit it with double exponential decay function:

I = I 0 + I 1 exp - t -t 0 τ 1 + I 2 exp - t -t 0 τ 2 , (4.6)
where I 1,2 are the pre-exponential factors and τ 1,2 are the relaxation times. The persistent photocurrent has two components: a fast and a slow relaxation time. This behavior is encountered in polycomponent [START_REF] Yan | Highly photosensitive thin film transistors based on a composite of poly(3-hexylthiophene) and titania nanoparticles[END_REF][START_REF] Sun | Highly sensitive organic near-infrared phototransistors based on poly(3-hexylthiophene) and PbS quantum dots[END_REF][START_REF] Xie | Ultrasensitive broadband phototransistors based on perovskite/organicsemiconductor vertical heterojunctions[END_REF] as well as pure materials, which is our case for 10-OPIA. Some researches attribute the existence of two decay time components to two different energy levels of trap states: deeper traps are considered to recombine slower [START_REF] Di | Electrical transport and persistent photoconductivity in monolayer MoS2 phototransistors[END_REF][START_REF] Mubashshir | Structural, optical and photoconductivity study of ZnO nanoparticles synthesized by annealing of ZnS nanoparticles[END_REF], contrary to more shallow trap states. Similarly to photocurrent growth, the decay of persistent photoconductivity is affected by gate bias.

By comparing the extracted relaxation times (Figure 4.29) we find that the sample under continued negative V GS shows slower current decay. In order to interpret these experimental results, we should consider the following:

• As demonstrated for photocurrent growth, current saturates significantly faster under negative gate voltage, which for same duration of illumination may result in higher occupancy ratio of persistent photogating states. Prolonged periods of illumination have also shown noticeable increase in both time components of current decay, which supports this suggestion.

• The gate bias may also improve charge separation on the semiconductor/dielectric interface or even allow the photogenerated charge carriers to access states which are normally (at V GS =0V) unavailable due to band bending. This delays the recombination of populated trap states, thus prolonging their lifetime.

Temperature dependency of persistent photoconductivity

In order to demonstrate the difference between trap state energy levels, we have performed another series of experiments where we have measured the persistent photoconductivity decay time components as a function of temperature. We have performed measurements in a temperature range from 20 • C to 60 • C, since higher temperatures have resulted in substantial instability of gate dielectric layer. The extracted decay time components are assembled in the Table 4.1. We notice that temperature increase results in noticeable decrease of both time components. However, it seems that faster decay component τ 1 is affected stronger than the slower τ 2 . Tokumoto et al [116] have performed investigations on TIPS-functionalized pentacene, which exhibited similar persistent photoconductivity properties. The decay time was found to be inversely proportional to the temperature. However, this dependency was found to be more pronounced in the case of TIPS-pentacene than in the case of 10-OPIA. This may be explained by relatively low energy trap states in a semiconductor with a smaller bad gap photoresponsivity R ∼ 1 A/W and pronounced photogating effect), photodetectors and optoisolators (response time may be reduced with smaller channel length and higher gate bias) and memory devices (long persistent photoconductivity decay times, positive gate voltage "erases" persistent state). which results in immediate breakdown of photocurrent to values below initial (before illumination). We restore zero gate bias after 20 seconds, which allows current amplitude to recover in the same manner as in the beginning of illumination. After 85 seconds under light, we apply negative gate bias (V GS =-50V), which results in immediate saturation of photocurrent. 30 seconds later, short pulse (1s) of positive voltage is applied, which resets the photocurrent in a similar manner. After the recovery of photocurrent, the gate is once again turned to -50V, however this time we let the photocurrent saturate for 30 seconds and then turn off the LED in order to observe decay of persistent photoconductivity. After about one minute we remove gate bias (V GS =0V), which to our surprise acts similarly to application of positive field -the persistent photoconductivity effect is removed. Further part of the plot demonstrates persistent effect in absence of gate voltage modulation.

This demonstration serves as a confirmation of the great potential of control over the conductivity of an easy to fabricate and stable material by electric field and light illumination.

Conclusion

In this chapter we have described a novel semiconducting material, from synthesis to application. Our first goal was to create a novel family of liquid crystalline OSCs, which will have improved charge transporting properties over classic materials as 8-PNP-O12.

The incorporation of heteroatom (nitrogen) in π-conjugated system of this new molecule makes it easier to synthesize, since no carbon-carbon bonds need to be created. The resulting substance turned out to be air-stable, which is an important advantage over classic OSCs based on polyacenes. We have also provided all the necessary information to confirm the chemical structure of our product.

We have employed classic tools (optical spectroscopy and cyclic voltammetry) in order to characterize frontier molecular orbital energy levels as well as band gap energy. We have found that HOMO of 10-OPIA aligns well with gold, which allowed us to use this classic material for electric contacts in order to create a P-type transistor. These studies have led us to the expected field of application for this molecule -organic field effect transistors. We have developed a simple protocol of deposition for 10-OPIA, which has allowed us to prepare OFETs in bottom gate/bottom contact configuration, as well as to control preferable direction of crystal growth. We have found that 10-OPIA exhibits reasonable field effect hole mobility (relevant properties are assembled in Table 4.2) in solution-processed thin film. formed electrically inactive polymeric network on the alignment, structure and, most importantly, charge transporting properties. We have used a classic approach which involves employment of prefabricated sandwich-type liquid crystal cells with ITO electrodes and polymer alignment layer. Prepared composites have demonstrated good alignment properties under this conditions, which may be interesting in large surface area applications.

Charge transport was found to be largely unaffected by the polymeric network -the liquid crystalline matrix was able to tolerate high (up to 5% wt.) concentrations of dispersed polymer and the hole mobility only slightly decreased in the polymerized composites.

Further studies have brought us to the second part of the Chapter 3: we have studied the link between mesophase order and efficiency of the molecular doping, as well as their effects on the hole mobility of the resulting mixture. To do so, we have prepared a series of samples where we have dispersed an electron acceptor impurity (a derivative of tetracyanoquinodimethane,F 4 TCNQ) in the 8-PNP-O12. We have applied a wellorganized and structured investigation procedure: first, we have performed ab initio calculations on the 8-PNP-O12/F 4 TCNQ complex in order to predict its electronic properties, as well as the degree of charge transfer between the two components of the mixture. This has allowed us to successfully confirm the formation of complex in the host 8-PNP-O12 and prove partial charge transfer experimentally. We have also carried out the investigations of mesophase structure by X-ray scattering in order to better understand the interactions of host and dopant. Next, we have characterized the hole transport in both smectic mesophases of 8-PNP-O12, which in addition with all the previous results has allowed us to construct a comprehensive picture of doping process and its influence on the physical properties of 8-PNP-O12. Finally, we have made an attempt to corroborate our observations with conductive atomic force microscopy which has enabled us to directly investigate charge transport of composite films on the nanometric scale:

the doped mixtures have exhibited enhanced hole transporting properties. Finally, we have prepared a field effect transistor (OFET) with one of these mixtures in order to demonstrate its superior to the pure 8-PNP-O12 charge carrier transport and obtain a more appropriate estimation of its performance.

We wrap up this chapter by showcasing the system which combines the two aforementioned approaches: we have tried to mitigate the inevitable mesophase order degradation due to doping impurity by incorporating the polymer network. Our investigations have shown that despite seemingly incompatible chemical nature of reactive mesogen (possessing electron-rich terminal functional groups) and doping impurity (strong electrophilic properties), they are able to coexist up to significant concentrations (1% wt. of F 4 TCNQ with 3% wt. RM82). Moreover, we have seen some positive results of this approach:

hole transport was improved while retaining mesophase transition temperatures close to the pure 8-PNP-O12.

The second part of this work (Chapter 4) is dedicated to a novel material based on anthracene. Our first intention was to prepare two materials -a normal semiconducting liquid crystal and its photopolymerizable version -with an additional functional group attached. In fact, the presented molecule (10-OPIA) is not our first attempt on this subject. Initially, we have succeeded in preparation of another molecule, which closely resembles 10-OPIA, with the only difference of having an azo-linkage between anthracenyl and phenyl moieties, instead of imine. A photo-polymerizable version was prepared as well, by attaching an oxirane cycle to the end of alkyl chain. However, we did not succeed in obtaining any quantifiable semiconducting performance in the case of this material, which has given us some motivation to prepare its "sibling" -the 10-OPIA. We decided not to include the synthesis and characterization of these molecules in the current manuscript in order to restrain its volume.

Nevertheless, the new molecule was thoroughly investigated for its possible applications in the organic electronics. After finalizing synthetic procedure and confirming its structure and purity, we have proceeded to estimate its frontier molecular orbital energy levels.

First, we have performed some estimations by ab initio calculations, which were followed by optical band gap measurements with absorption spectroscopy in both solution and thin film form. This data was then confirmed by our measurement of HOMO and LUMO levels in solution by cyclic voltammetry. The HOMO energy of 10-OPIA has allowed us to utilize gold as material for contacts in the OFET, which we have fabricated by simple drop-casting. Hole transporting performance of 10-OPIA was found to be significantly higher than that of 8-PNP-O12-based materials, which encourages us to continue our investigation on this material. The final part of this chapter is dedicated to remarkable photoconducting properties of 10-OPIA, which were discovered during our initial studies.

It was found that these properties in combination with remarkable control over dark-and photo-conductivity makes 10-OPIA a promising candidate for light-sensitive applications.

Figure 1 -

 1 Figure 1 -Système π-conjugué de base, poly-acétylène.

Figure 2 -

 2 Figure 2 -(a) Structure des liaisons σ et π dans la molécule d'éthylène. (b) Diagramme des niveaux d'énergies des orbitaux moléculaires pour une simple molécule (à gauche) et dans le SCO massif (à droite).

Figure 3 -

 3 Figure 3 -Exemples des SCOs les plus connus: (a) pentacène et (b) poly-thiophène .

Figure 4 -

 4 Figure 4 -Représentation schématique des mésophases calamitiques: nématique (N ), smectique A (Sm A ), smectique C (Sm C ) et smectique B (Sm B ).

Figure 5 -

 5 Figure 5 -Polymérisation in situ d'un mésogène réactif dans le massif d'un SCO.

Figure 6 -

 6 Figure 6 -Relation entre les niveaux d'énergie des orbitales moléculaires d'une molécule de SCO et d'une molécule de dopant.

Figure 7 -

 7 Figure 7 -Montage expérimental de TOF dans le cas des études de transport des trous.

Figure 8 -

 8 Figure 8 -Exemple d'un profil de photo-courant en fonction de temps permettant l'extraction d'un temps de transit τ tr .

xRÉSUMÉFigure 9 -

 9 Figure 9 -Structure schématique d'un OFET de type p : (a) le dispositif se trouve en état OFF; (b) le canal conducteur est formé grâce a la tension de grille négative V GS . Les caractéristiques d'un OFET de type p : (c) de transfert en échelle logarithmique (en bleu) et de tension de seuil V th (en rouge), l'encart représente la géométrie du canal d'un OFET (vue de dessus) ; (d) caractéristique courant-tension pour les valeurs de V GS divers (|V G3 | > |V G2 | > |V G1 |).

Figure 10 -

 10 Figure 10 -Structure moléculaire du 2-dodecyloxy-6-(4-octylphényle)-naphthalène (8-PNP-O12).

Figure 11 -Figure 12 -

 1112 Figure 11 -Texture des échantillons : (a) 8-PNP-O12 et (b) g2 par la microscopie optique en lumière polarisée.

Figure 13 -Figure 14 -

 1314 Figure 13 -(a) Mobilité des trous en fonction de la température pour 8-PNP-O12 pur (symbols noires), g1 (rouges), g2 (bleus), g3 (verts) et g4 (roses). (b) Mobilité des trous en fonction de la concentration de RM82 en phases Sm B (85 • C, noir) et Sm A (115 • C, rouge).

Figure 15 -

 15 Figure 15 -(a) Diagramme des niveaux d'énergie des orbitales moléculaires des composés initiaux (8-PNP-O12 et F 4 TCNQ) et du complexe avec transfert de charge (CTC) entre ces deux composés. (b) La géométrie de CTC. Ces données sont obtenues au niveau théorique B3LYP/6-31++G(d,p).

Figure 16 -

 16 Figure 16 -Spectres d'absorption optique pour les matériaux étudiés : 8-PNP-O12 pur (noir), F 4 TCNQ (noir pointillé) c1 (rouge), c2 (bleu), c3 (vert), c4 (rose) et c5 (violet).

Figure 17 -

 17 Figure 17 -(a) Spectres Raman de 8-PNP-O12 pur (bleu), F 4 TCNQ (vert) et composite c5 (rouge). (b) Spectres vibrationels calculés pour CTC (noire, Raman) et F 4 TCNQ (Raman en gris et FTIR en rouge). Encart : Fréquence de mode b 1u de F 4 TCNQ en fonction du degré de transfert de charge.

Figure 18 -

 18 Figure 18 -(a) Mobilité des trous en fonction de la température pour 8-PNP-O12 pur (symboles noirs), c1 (rouges), c2 (bleus), c3 (verts), c4 (roses) and c5 (violets). (b) Mobilité des trous en fonction de la concentration de F 4 TCNQ en mésophases Sm B (85 • C, noir) et Sm A (115 • C, rouge).

Figure 19 -Figure 20 -Figure 21 -

 192021 Figure 19 -(a) Caractéristique de transfert et V th d'un OFET (L = 50um, W=18mm) réalisé avec c5. (b) Caractéristique de sortie du même dispositif. Encart : photo de canal.

Figure 22 -Figure 23 -Figure 24 -

 222324 Figure 22 -(a) HOMO et LUMO de 10-OPIA calculées en niveau théorique de B3LYP/6-31G++(d,p). (b) Voltammogramme cyclique d' une solution 2mmol de 10-OPIA avec 0.1M TBAPF6 en dichlorométhane versus Fc/Fc+. Encart : absorption optique en solution 10 -5 M de 10-OPIA en dichlorométhane (noir) et en couche mince (rouge).

Figure 25 -

 25 Figure 25 -Diagramme des bandes d'énergie pour le canal d'un OFET (a) dans l'obscurité (b) sous illumination.

Figure 26 -Figure 27 -

 2627 Figure 26 -(a) Caractéristique de transfert d'un OFET (L = 50um, W=18mm) réalisé avec 10-OPIA pour diverses puissances lumineuses. Encart : I P h de la partie de saturation en fonction de la puissance lumineuse. (b) Réponse normalisée (noir) et sensibilité (rouge) en fonction de V GS pour 1.44 mW/cm 2 . Encart : caractéristiques V th sans (noir) et avec (rouge) illumination.

  present work was conducted in the Unité de Dynamique et Structure des Matériaux Moléculaires (UDSMM) under the supervision of Professors Christian Legrand, Abdelylah Daoudi and Dr. Yahia Boussoualem. It deals with structural and electrical charac-terization of organic materials exhibiting mesogenic and semiconducting properties, as well as synthesis of new materials. This is the second thesis conducted on the organic semiconductors, and the first one dealing with organic field effect transistors in this laboratory.

Figure 1 . 1 -

 11 Figure 1.1 -(a) Schematic representation of orbitals and σ -and π -bonds in ethylene. (b) Energy diagram of molecular orbitals in ethylene molecule demonstrates separation between σ -and π -energetic framework.

Figure 1 . 3 -

 13 Figure 1.3 -Diagrams for (a) ground state and (b) excited state of single OSC molecule; (c) representation of energy levels in bulk material.

Figure 1 . 4 -

 14 Figure 1.4 -Simplified representation of π-π stacking for two pentacene molecules.

Figure 1 . 5 -

 15 Figure 1.5 -Relation between molecular orbitals of host organic semiconductor and doping agents.

Figure 1 . 6 -

 16 Figure 1.6 -(a) Energy splitting between MOs in a hydrogen molecule, (b) Splitting between bonding and antibonding MOs in a helium dimer.

Figure 1 . 7 -

 17 Figure 1.7 -(a) Splitting between bonding and antibonding MOs in OSC dimer, (b) Energy splitting between supramolecular MOs in a complex between the OSC and P-dopant.

Figure 1 . 8 -

 18 Figure 1.8 -Molecular structure of some polymeric OSCs: (a) polyparaphenylene, (b) polyfluorene, (c) polythiophene. R denotes optional alkyl sidechain.

Fluorene-based polymers

  share the same light-emitting applications with phenylenebased materials. Studies of different substituents for fluorene moiety (Figure 1.8, b)has enabled first fabrication of blue organic light-emmiting diodes[START_REF] Marsitzky | Self-Encapsulation of Poly-2,7-fluorenes in a Dendrimer Matrix[END_REF] [START_REF] Scherf | Semiconducting Polyfluorenes-Towards Reliable Structure-Property Relationships[END_REF]. Different copolymers of fluorene are capable of showing bulk hole mobility in the range from 10 -5 cm 2 /(V s) to 10 -3 cm 2 /(V s)[START_REF] Rawcliffe | Comparison between bulk and field effect mobility in polyfluorene copolymer field effect transistors[END_REF].Thiophene-based polymers (Figure1.8, c) are the classic and most studied polymeric materials for charge transporting applications. Profound understanding of their electronic properties as well as good control over film morphology makes them the pinnacle of high-performing charge conducting polymers. This group makes a great example to illustrate the significance of structural planarity in π-conjugated systems. The remarkable comparison of poly(3-hexylthiophene) (P3HT) fractions with different degree of regioregularity *[59] shows carrier mobility spanning for almost four orders of magnitude from 2 × 10 -5 cm 2 /(V s) for 70% regioregular blend to extraordinary 0.1 cm 2 /(V s) for sample with 96% of regioregularity.N-type polymers are still relatively rare and present a growing field of interest. One of the most remarkable examples is the poly(benzobisimidazobenzophenanthroline)

Figure 1 . 9 -

 19 Figure 1.9 -Molecular structure of some small molecule OSCs: (a) pentacene, (b) rubrene, (c) phenyl-C61-butyric acid methyl ester (PCBM).

Figure 1 . 10 -

 110 Figure 1.10 -Schematic representation of most common calamitic mesophases: nematic (N ), smectic A (Sm A ), smectic C (Sm C ) and smectic B (Sm B ).

Figure 1 .

 1 Figure 1.11 illustrates by means of a generic calamitic smectic mesogen the relation between charge carrier mobility and structural order of system. Further in this section we are going to describe two types of LCs: calamitic and discotic OSCs with relevant examples.Typical core structures for semiconducting calamitic LCs include oligothiophenes (bisphenyl bithiophene, quarterthiophene), carbazole, benzothiadiazole, fused acenes (naphthalene and anthracene) or thiophenes (thienothiophene and dithienothiophene), fluorene, or less common pyridine and their various combinations (benzothienobenzothiophene, thienonaphthalene, etc.). Some notable examples are assembled in the Figure1.12.

Figure 1 . 11 -

 111 Figure 1.11 -Relation between phase state (from left to right: crystalline, liquidcrystalline, isotropic liquid) and charge carrier mobility.

Figure 1 . 13 -

 113 Figure 1.13 -Schematic representation of most common discotic mesophases: discotic nematic N D , columnar nematic N Col and columnar hexagonal Col H

Figure 1 . 14 -

 114 Figure 1.14 -Examples of discotic LC OSC cores, where R is the lateral alkyl chain.

Figure 1 .

 1 Figure 1.15 -Cross-linking of a smectic reactive mesogens.

  contribution of polymeric groups to charge carrier transport. Discotic semiconductors can also undergo chemical functionalization[START_REF] Bleyl | Photopolymerization and Transport Properties of Liquid Crystalline Triphenylenes[END_REF] [107][START_REF] Inoue | Effect of Photopolymerization on Photoconductive Behavior in Triphenylene Discotic Liquid Crystals[END_REF] in order to form crosslinked structures, effectively stabilizing charge-transporting properties of their mesophases at room temperature. Design and synthesis of molecules which combine these properties (semiconducting, mesogenic and photopolymeric) is a non-trivial task and often leads to fairly unpredictable results.Such problems give rise to a more simple solution: creating a bi-component system, consisting of a conventional liquid crystalline semiconductor and a so-called reactive mesogen: a bi-or monofunctionalised liquid crystal, usually of calamitic nature (Figure1.16).

Figure 1 . 16 -

 116 Figure 1.16 -In situ creation of polymeric backbone within OSC host by introduction of a reactive mesogen.

  logic and is an indispensable component of many circuit concepts, employed either as a standalone component, or as a part of integrated circuit. There are two most popular basic types of transistors: bipolar junction transistor (BJT) and field effect transistor (FET).In this thesis the attention would be focused on the FET family of devices. The FET group in turn divides in two sub-types, insulated-gate FETs (notable examples are metaloxide-semiconductor FET (MOSFET) and organic FET (OFET)) and junction-gate FETs. MOSFET devices usually operate in depletion or enhancement mode, when most of OFETs works in accumulation mode except for some rare examples[START_REF] Lussem | Doped organic transistors operating in the inversion and depletion regime[END_REF]). Inorganic semiconductor devices are not covered in this study.

Figure 1 . 19 -

 119 Figure 1.19 -Schematic structure of OFET: (a) device is switched off; (b) current is driven by voltage V DS while OFET is swiched on by negative gate voltage V GS ; (c) geometry of OFET channel: L is the channel length, W is the channel width.

Figure 1 . 20 -

 120 Figure 1.20 -Energy band diagram of gate -insulator -p-type OSC interface: (a) zero gate voltage, (b) negative gate voltage and (c) positive gate voltage. E V is the vacuum level, Φ M is the work function of gate material, Φ OSC is the work function of OSC, EA is the electron affinity, IP is the ionization potential and V GS is gate voltage.

Figure 1 . 21 -

 121 Figure 1.21 -Charge carrier density in OFET channel: (a) in linear regime; (b) pinch-off; (c) in saturation regime.

Figure 1 .

 1 Figure 1.22 represents characteristics of a p-type OFET in respect of gate voltage V GS . Transfer characteristic represents the dependency of channel current I DS for a fixed V DS as a function of V GS (Figure 1.22, a): it is often plotted in logarithmic scale and demonstrates distinct difference between the ON and OFF states of OFET. By tracing the square root of I DS as a function of V GS , we may extract the threshold voltage V th of OFET: the intersection of linear fit of √ I DS with x-axis (I DS = 0) affords us this parameter. Output characteristic is obtained by plotting I DS for different values of V GS as a function of V DS (Figure 1.22, b). The current I DS is proportional to charge carrier density accumulated in the conducting channel and hence to the value of V GS . Typical output characteristic of an OFET can be divided in two regimes: so-called "linear " regime with a non-linear region close to higher V DS values and saturation regime. The "linear " regime is observed when |V DS | < |V GS -V th | (Figure 1.21, a): the current I DS increases proportionally to V DS in almost linear manner.Channel current is described by the model of Sze[115]. In "linear " regime I DS may be described by the following

Figure 1 . 22 -

 122 Figure 1.22 -Examples of characteristic plots for a generic p-type OFET: (a) transfer characteristic in logarithmic scale (in blue) and threshold voltage V th (in red) characteristic (b) output characteristic for various gate voltage values (|V G3 | > |V G2 | > |V G1 |).

  (a) Bottom gatebottom contact (b) Bottom gatetop contact (c) Top gatetop contact (d) Top gatebottom contact Figure 1.23 -Four types of OFET architecture.

Figure 1 .Figure 1 . 25 -

 1125 Figure 1.24 -Top view on two geometry types of OFET contacts with common gate electrode: (a) interdigitated; (b) linear.

  LCs possess extraordinary alignment and self-organizing properties, this class of substances is of particular interest in OFET application. Hydrophobic coating, such as OTS-18 (octadecyltrichlorosilane), can induce the homeotropic orientation of molecules, which corresponds to edge-on alignment. Recent studies of semiconducting LC deposition show the possibility to induce in-plane (in other words, face-on) alignment by coating of an additional layer of poly(vinyl alcohol) (PVA, one of the classic alignment layer material in LC field) over semiconductor layer [117].

Figure 1 . 26 -

 126 Figure 1.26 -Possible orientations of OSC molecules (dioctyl-substituted benzothienobenzothiophene as an example) on a substrate: (a) face-on; (b) edge-on.

Figure 1 . 27 -

 127 Figure 1.27 -Energy band diagram of metal -p-type OSC interface: (a) before and (b) after contact. E V is the vacuum level, Φ M is the work function of metal, Φ OSC is the work function of OSC, Φ Bn is the barrier height for electrons, Φ Bp is the barrier height for holes, EA is the electron affinity and IP is the ionization potential

2. 2 . 1

 21 Polarized optical microscopyOptical microscopy is a relatively simple, accessible yet effective method of material study. Polarized optical microscopy (POM) has become one of the most widely employed techniques for optically anisotropic (birefringent) materials, notably liquid crystals. In a typical experimental setup, the LC sample is placed in a special cell of so-called "sandwich-type", where it is held between two glass surfaces. The LC orientation could be controlled by the means of surface interactions, thickness of the cell gap and magnetic or electric field. The LC could be spontaneously oriented in the case of an untreated cell surface (discotic mesogenes are especially prone to it) or aligned at different angles, depending on the nature and treatment (rubbing force and direction) of alignment layer, which is often pre-deposited during the process of cell fabrication.

Figure 2 . 1 -

 21 Figure 2.1 -Schematic representation of light path throug a polarizing optical microscope with an angle Θ = 90˚between the analyzer/polarizer.

  (a) Smectic A (b) Nematic (c) Isotropic

Figure 2 . 2 -

 22 Figure 2.2 -Examples of sample texture in various phases: (a) smectic A, (b) nematic and (c) isotropic. Scale bar represents 50 µm.

Figure 2 . 3 -

 23 Figure 2.3 -Schematic representation of a SEM instrument.

Figure 2 . 4 -

 24 Figure 2.4 -Example of elemental mapping: (a) SEM image of an organic compound on a glass substrate and (b) corresponding carbon (red) and oxygen (blue) element map. Scale bar represents 10 µm.

Figure 2 . 5 -

 25 Figure 2.5 -Schematic representation of an AFM instrument. The conductive module is highlighted in yellow.

Figure 2 . 6 -

 26 Figure 2.6 -Example of C-AFM mapping, sample demonstrates some non-conductive agglomerate in the organic semiconductor thin film deposited on gold. Left to right: sample topography, current mapping (-5V sample bias), local current-voltage characteristics. Scale bar represents 100 nm.

Figure 2 . 7 )

 27 as well as the sample temperature. During a physical or chemical transformation (chemical reaction or phase transition) of the sample, requiring energy exchange, the DSC measures a heat quantity emitted (exothermic process) or absorbed (endothermic process) by the sample.

Figure 2 . 7 -

 27 Figure 2.7 -Schematic representation of heat flux DSC setup.

Figure 2 . 8 -

 28 Figure 2.8 -Example of a DSC output curve (thermogram) -bottom for an endothermic phase transition (melting) and top for an exothermic (crystallization).

Figure 2 . 9 -

 29 Figure 2.9 -Schematic representation of the X-ray scattering experiment.

  .2) X-ray scattering data in this work was obtained with Forvis Technologies instrument. The x-ray source was 30 W Genix 3D (Cu anode, λ = 1.5405 Å and E = 8.050 92 keV), the detector was Dectris Eiger R 1M. The beam size was 0.8 × 0.8mm 2 , the x-ray flux was 4 × 10 7 photons/s and sample-to-detector distance was 190 mm. Wide-Angle X-ray scattering experiments were performed during the course of collaboration with University of Colorado, Boulder, USA. The contributions of Rayshan Visvanathan, Alexandra E.

Figure 2 . 10 -

 210 Figure 2.10 -Schematic representation of UV-Visible absorption experimental setup.

Figure 2 .Figure 2 . 12 -

 2212 Figure 2.11 -(a) Schematic representation of Fluorescence spectrometer. (b) Simplified Jablonsky diagram: S 0 and S 1 are ground and first excited singlet states, T 1 is the first triplet state.

  The method is based on inelastic scattering, or Raman scattering by the sample of monochromatic light from a laser source in the near-ultraviolet, near-infrared, or visible range. Laser light interacts with molecular vibrations or other excitations in the system, which results in a positive or negative change of scattered photons. This shift gives information on the vibrational modes of the molecular system.

Figure 2 . 13 -

 213 Figure 2.13 -Schematic energy diagram of elastic and inelastic scattering.

Figure 2 . 14 -

 214 Figure 2.14 -Schematic representation of Raman spectrometer.

Figure 2 . 15 -

 215 Figure 2.15 -Example of Raman spectrum of F 4 TCNQ molecule (recorded with 785 nm excitation wavelength), the corresponding vibrational modes are referred to the parts of molecular structure.

Figure 2 . 16 -

 216 Figure 2.16 -Schematic representation of TOF experiment in case of hole transport.

Figure 2 . 17 -

 217 Figure 2.17 -Generalized transient photocurrent curve for non-dispersive charge transport in linear scale.

  Figure 2.19 -3D model of sample cell: two ITO electrodes, top (green, T-shaped) and bottom (blue, O-shaped) are deposited on glass slides and glued together with a precision spacer (yellow). The active area of 5x5 mm is situated in the center of the cell where top electrode crosses the bottom one.

Figure 2 . 20 -

 220 Figure 2.20 -Basic electric circuit of TOF which includes the DC voltage source Vb, sample cell which is represented by a photocurrent source in parallel with a resistor and a capacitor (typical values are presented), coaxial cable and current conversion stage: shunt resistor followed by voltage amplifier (a) or transimpedance amplifier (b). The resulting voltage V out is fed into oscilloscope.

Figure 2 . 21 -

 221 Figure 2.21 -Transient signals obtained from circuit simulation: current pulse ( 1 µA amplitude, in gray), cascade configuration (1 kΩ shunt × 100 non-inverting gain, in red) and one stage configuration (inverted, 100 kΩ+10 pF feedback, in green) and their corresponding rise times.

Figure 2 .

 2 Figure 2.21 demonstrates transient voltage output for the circuits presented in the Figure 2.20 (simulated in NI Multisim 14.0). We have included typical cell parameters (capacitance C=133 pF and resistance R=1 GΩ), as well as cable capacitance (20 cm of RG58 coaxial cable account for about 20 pF) in the simulation. An idealized operational amplifier (unity gain bandwidth 100 MHz, open loop gain 2 × 10 5 , input resistance 10 MΩ) was used in both cases. The value of feedback capacitor (10 pF) in Figure 2.20, (b) corresponds to that in Keithley 428 schematic for 10 5 gain. The results of this simulation allow us to compare time response of two current conversion approaches with the same

Figure 2 .

 2 22) in bottom gate/bottom contact configuration provided by Ossila (Sheffield, UK), following information on manufacturing process and electrical characteristics are provided by the supplier. Each substrate had 20 source/drain electrode pairs with a common gate. The size is 20×15 mm, thickness is (725 ± 25) µm.

Figure 2 . 22 -

 222 Figure 2.22 -Schematic representation of OFET substrate.

Figure 2 . 23 -

 223 Figure 2.23 -Schematic representation of transistor test setup.

  High resolution mass spectrum (HRMS) in this work was recorded in positive mode using an Agilent 6540 Accurate-Mass Quadrupole Time-of-Flight (Q-TOF) LC/MS system equipped with an Agilent Jet Stream dual electrospray source. Analysis parameters were as follows: Gas temperature (150 • C, 5 L/min), Nebulizer (35 psig), Sheath gas (375 • C, 12 L/min) Vcap (3500 V), nozzle voltage (2000 V), fragmentor (100 V), skimmer(65 V)

. 24 )

 24 consists of an electrochemical cell which is connected to the potentiostat (Metrohm AUTOLAB PGSTAT302). The electrochemical cell is composed of the electrolyte containment vessel with three electrodes: a working electrode (WE), a counter electrode (CE, sometimes called an auxiliary electrode) and a reference electrode (RE). The RE, as its name suggest, provides a potential reference, which remains constant during the experiment, while the current flows between the WE and CE. The WE used in this work is represented by a platinum disc of 3 mm diameter (Ø), platinum wire was used as the CE. Two REs were used in this work -a silver wire pseudoreference and an Ag/Ag + RE, which was filled with an electrolyte for non-aqueous measurements ([NEt 4 Br]=0.4 M in ethylene glycol). Both the WE and Ag/Ag + RE were provided by Metrohm, while the CE was purchased from Radiometer Analytical, the pseudoreference was custom-made.

Figure 2 . 24 -

 224 Figure 2.24 -Schematic representation of cyclic voltammetry setup.

Figure 2 . 25 -

 225 Figure 2.25 -Cyclic voltammetry of 2 mmol/L ferrocene in dichloromethane ([NBu 4 PF 6 ]=0.1 M) on Pt electrode (Ø=3 mm). Color corresponds to the scanning rate.

2. 5 . 3

 53 Self-Assembled Mono-layer (SAM) coating Numerous failed attempts to spin-coat OSC on bare (and thoroughly cleaned) glass/ITO substrates have showed poor compatibility with utilized solvents (toluene, anisole, THF and chloroform). Both types of substrates used in this work, glass and Si wafers are covered with a layer of SiO 2 . In order to overcome excessive hydrophilicity, it was proposed to use octadecyltrichlorosilane (OTS-18, Figure 2.26) as a well-known self-assembled mono-layer adhesion promoter[136].

Figure 2 . 26 -

 226 Figure 2.26 -Molecular structure of OTS-18.

Figure 2 . 27 -••-

 227 Figure 2.27 -Schematic representation of OTS-18 mono-layer grafted on the substrate.

•

  Substrate drying -2h at 50 • C in a closed Petri dish in the oven This protocol was successfully used to coat soda lime glass slides, as well as prefabricated Si OFET substrates. No sign of gold electrode degradation was found.

Figure 2 .

 2 Figure 2.28 -Droplet (V = 5 µL) profile of toluene (non-polar solvent) on OTS-18 coated glass substrate. Average contact angle calculated to be about 21.3 • after 4 measurements.

Figure 2 .

 2 Figure 2.28 demonstrates superior wetting properties of a soda lime glass slide coated with a OTS-18 monolayer: the contact angle (measured wit KRUSS DSA100 Drop Shape Analyzer) of toluene drop is close to 20 • which indicates hydrophobic properties of the functionalized surface.

Figure 2 . 29 -

 229 Figure 2.29 -Diagram of spin-coating procedure. Size of the solute molecules is greatly exaggerated.

Figure 2 . 30 -

 230 Figure 2.30 -Schematic representation of drop-casting setup. α is the substrate inclination angle.

Figure 3 . 1 -

 31 Figure 3.1 -Molecular structure of 2-dodecyloxy-6-(4-octylphenyl)-naphthalene (8-

Figure 3 .Figure 3 . 2 -

 332 Figure 3.2 -Molecular structure of the components of reactive mesogene precursor mixture: monomer (a) 2-methyl-1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate) (RM82), photoinitiator (b) 2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone (Irgacure 369) and thermal stabilizer (c) 2,6-di-tert-butyl-4-methylphenol (BHT).

(

  dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone (Irgacure 369, Figure 3.2, b) (Ciba) and thermal stabilizer 2,6-di-tert-butyl-4-methylphenol (BHT, Figure 3.2, c) were added to pure RM82 by 0.5% and 1% of total RM mixture mass, respectively. The photoinitiator has a pronounced absorption in UV part of optical spectra (Figure 3.3), which has enabled the use of a UV LED with emission wavelength of 365 nm for UV-initiated polymerization process.

Figure 3 . 3 -

 33 Figure 3.3 -Optical absorption spectrum of the Irgacure 369 in acetonitrile. Vertical black dashed line corresponds to the wavelength of emission peak of the LED used for curing.

Figure 3 . 4 -

 34 Figure 3.4 -(a) DSC plots for pure 8-PNP-O12 (black), g1 (red), g2 (blue), g3 (green) and g4 (pink) composites obtained with 10 • C/min cooling program. (b) Phase transition temperatures of precursor composites as a function of RM82 concentration: Cr to Sm B (black squares), Sm B to Sm A (red squares) and Sm A to I (blue squares).

Figure 3 . 5 -

 35 Figure 3.5 -Sample texture under crossed analyzer-polarizer: for pure 8-PNP-O12 and cross-linked mixtures g1, g2, g3 and g4 (in Sm B at 85 • C, Sm A mesophase at 115 • C and isotropic phase at 130 • C). Scale bar represents 100 µm.

Figure 3 .Figure 3 . 6 -

 336 Figure 3.7 contains a higher magnification SEM image of g2, which allows us to gain insight of the fibrillar nature of the polymer network. As previously demonstrated, the network consists of fibers of about one micron diameter (insets of Figure 3.6, b and c), which are in turn composed of smaller fibers, as demonstrated by the inset of the Figure 3.7 on nanometric scale. Taking into account the strength of the optically observed aligningeffects, it is possible to conclude that the host 8-PNP-O12 is densely inter-penetrated by the RM82 fibers. This observation is in contrast with the system based on discotic host OSC (2,3,6,7,10,11 -hexapentyloxytriphenylene, HAT-5)[START_REF] Kondratenko | Hole transporting properties of discotic liquid-crystalline semiconductor confined in calamitic UV-crosslinked gel[END_REF], where the polymer

Figure 3 . 7 -

 37 Figure 3.7 -High magnification SEM image of the polymer backbone of g2.

Figures 3 .

 3 Figures 3.8 and 3.9 demonstrate the dependency of the calculated hole velocity

Figure 3 . 8 -

 38 Figure 3.8 -Plot of the hole velocity as a function of applied electric field for pure 8-PNP-O12 (black squares), g1 (red circles), g2 (blue triangles) and g3 (green triangles) in Sm B mesophase at 85 • C. Inset: Transient photocurrent plots for pure 8-PNP-O12 (black), g1 (red), g2 (blue) and g3 (green) in Sm B mesophase at 28 kV/cm and 85 • C.

Figure 3 . 9 -

 39 Figure 3.9 -Plot of the hole velocity as a function of applied electric field for pure 8-PNP-O12 (black squares), g1 (red circles), g2 (blue triangles), g3 (green triangles) and g4 (magenta squares) in Sm A mesophase at 115 • C. Inset: Transient photocurrent plots for pure 8-PNP-O12 (black), g1 (red), g2 (blue), g3 (green) and g4 (magenta) in Sm A mesophase at 40 kV/cm (80 kV/cm for the g4, marked with asterisk) and 115 • C.

Figure 3 . 10 -

 310 Figure 3.10 -Plots of the hole mobility as a function of RM82 content in Sm B mesophase at 85 • C (a) and Sm A mesophase at 115 • C (b).

Figure 3 . 11 -

 311 Figure 3.11 -Plot of the hole mobility as a function of temperature for pure 8-PNP-O12 (black squares), g1 (red circles), g2 (blue triangles), g3 (green triangles) and g4 (magenta squares). Vertical dashed lines indicate the transition temperatures of pure 8-PNP-O12.

Figure 3 . 12 -

 312 Figure 3.12 -Molecular structure of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ).

Figure 3 . 13 -Figure 3 . 14 -

 313314 Figure 3.13 -Energy diagram of frontier molecular orbital levels for CTC and isolated molecules of 8-PNP-O12 and F 4 TCNQ. MO energy levels of CTC and pristine substances are calculated on B3LYP/6-31++G(d,p) level.

Figure 3 . 15 -

 315 Figure 3.15 -Bond lengths calculated on B3LYP/6-31++G(d,p) level of theory for (a) the isolated neutral 8-PNP-O12, the cation 8-PNP-O12 +1 and the charge-transfer complex with F 4 TCNQ; for (b) the isolated neutral F 4 TCNQ, the anion F 4 TCNQ -1 and the charge-transfer complex with 8-PNP-O12 (CTC).

Five

  mixtures were prepared by mixing selected weight fraction of dopant of 0.1 (c1), 0.5 (c2), 1 (c3), 3 (c4), 5 (c5) %wt. with host material in chloroform solvent in glass vial at 50 • C for 1 hour. Vials then were left open for 24 hours under vent hood to ensure complete solvent evaporation. As a first step, we have performed DSC over the whole sample sequence in order to ensure mesophase stability over its temperature range. The thermograms of DSC analysis for all studied concentrations are presented in Figure 3.16. Interestingly, the mixtures exhibit relatively minor changes in melting and clarification (Sm A → Iso) temperatures (Figure 3.17,b).

Figure 3 .

 3 Figure 3.16 -DSC plots for pure 8-PNP-O12 (black), c1 (red), c2 (blue), c3 (green), c4 (pink) mixtures obtained with 10 • C/min cooling program and c5 (purple) samples obtained with 5 • C/min cooling program. Dashed rectangles denote the scaled part of the thermogram.

Figure 3 .

 3 Figure 3.17 -(a) Plot for inverse temperature (1000/T) of the Sm A to Sm B phase transition as a function of F 4 TCNQ concentration. (b) Phase transition temperatures of 8-PNP-O12/F 4 TCNQ mixtures as a function of F 4 TCNQ concentration: Cr to Sm B (black squares), Sm B to Sm A (red circles) and Sm A to I (blue triangles).

Figure 3 . 18 -

 318 Figure 3.18 -Sample texture under crossed analyzer-polarizer: for pure 8-PNP-O12 (in Sm B (a) and Sm A (b) mesophase), c3 (in Sm B (c) and Sm A (d) mesophase), and c5 (in Sm B (e) and Sm A (f) mesophase). Scale bar represents 100 µm.

Figure 3 .Figure 3 .Figure 3 .Figure 3 . 21 -

 333321 Figure 3.19 -(a)Plot for FWHM of Sm B in-layer order (peak b) as a function of F 4 TCNQ content. Inset: 1D WAXS data for pure 8-PNP-O12 in Sm B at 85 • C (black line), in Sm A mesophase at 115 • C (blue line) and for c5 in Sm B at 85 • C (red line), in Sm A at 115 • C (green line). (b) Pseudo-hexagonal order spacing in Sm B as a function of temperature: black squares correspond to pure 8-PNP-O12 (black squares), c1 (red circles), c2 (blue triangles), c3 (green triangles), c4 (magenta squares) and c5 (purple diamonds). Inset: intermolecular Sm B spacing as a function of F 4 TCNQ content at 85 • C.

Figure 3 .

 3 Figure 3.22 -UV-Vis spectra for starting products: pure 8-PNP-O12 (black) and F 4 TCNQ (red), 2 mg/ml chloroform solution.

Figure 3 .

 3 Figure 3.23 -UV-Vis spectrum of c5 sample (solid red line) and the result of TD-DFT calculation of CTC (dashed black line). Black vertical lines denote transition energies and corresponding oscillator strengths.

Figure 3 .

 3 Figure 3.24 -UV-Vis spectra for prepared samples taken in 2 mg/ml chloroform solution: c1 (red), c2 (blue), c3 (green), c4 (pink) and c5 (purple) samples. Gray vertical dotted lines correspond to the expected energies of the anion F 4 TCNQ -absorption peaks [39][155]. Peaks assigned to CTC are marked with red asterisks.

Figure 3 . 25 -

 325 Figure 3.25 -Raman spectra for (a) pure 8-PNP-O12 and (b) pure F 4 TCNQ: black for calculated (scaling factor of 0.9626[START_REF] Kudoh | Infrared spectra of Dewar 4-picoline in low-temperature argon matrices and vibrational analysis by DFT calculation[END_REF][START_REF] Osaki | Quadratic scaling functions for obtaining Normal vibrational wavenumbers from the B3LYP calculation[END_REF]) and red for experimental spectra. Intensity is normalized for highest peak.

Figure 3 .

 3 Figure 3.26 -HOMO of neutral 8-PNP-O12 molecule.

Figure 3 . 27 -

 327 Figure 3.27 -Atom indexing of molecular structure of (a) 2-dodecyloxy-6-(4-octylphenyl)naphthalene (8-PNP-O12) and (b) 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ).

1667 1 /

 1 cm previously assigned to quinoid ring C=C stretching. The cyano (C≡N) bands are not observed until high concentrations, but then still follow the observed dependency on F 4 TCNQ content. One can notice the apparition of new peak, clearly visible at 1655 1/cm, which is connected to the dopant molecule after charge transfer (highlighted with arrow in Figure 3.29).

Figure 3 . 28 -

 328 Figure 3.28 -Calculated Raman spectra for (a) 8-PNP-O12 and (b) F 4 TCNQ: green for neutral and blue for charged molecule. Intensity is normalized for highest peak, scaling factor of 0.9626[157, 158] is applied

Figure 3 . 30 -

 330 Figure 3.30 -Example of Raman spectra recorded for c5 sample in different phase: black dashed for as-prepared powder, green for I, blue for Sm A , red for Sm B and purple for annealed Cr. Intensity is normalized for naphthalene ring stretch band (1635/cm before annealing, 1624/cm in liquid and liquid crystalline phase and 1618/cm after annealing).

Figure 3 . 31 -Figure 3 . 32 -

 331332 Figure 3.31 -Raman spectra for as-prepared (green) and annealed (red) samples (a) pure 8-PNP-O12, (b) for 0.1%, (c) for 0.5%, (d) for 1%, (e) for 3% and (f) for 5% F 4 TCNQ content. Green and red arrows indicate Raman bands shifts after annealing.

Figure 3 . 33 -

 333 Figure 3.33 -Raman spectra for Cr * of c5 sample (red solid line), pure 8-PNP-O12 (dark blue filled plot) and pure F 4 TCNQ (green filled plot) recorded at room temperature. The Raman bands of interest are labeled with the corresponding color.

Figure 3 . 34 -Figure 3 . 35 -

 334335 Figure 3.34 -Calculated vibrational spectra (scaling factor of 0.9626 [157, 158]) for pure F 4 TCNQ (Raman in dashed gray and IR in solid red) and CTC (Raman in solid black and vibrational frequencies in blue). The corresponding vibrations are assigned respectively.

Figure 3 . 36 -

 336 Figure 3.36 -Plots of the photocurrent as a function of F 4 TCNQ content in Sm B at 85 • C (a) and Sm A at 115 • C (b): : pure 8-PNP-O12 (black), c1 (red), c2 (blue), c3 (green), c4 (magenta) and c5 (purple).

Figure 3 . 37 -

 337 Figure 3.37 -Plot of the hole velocity as a function of applied electric field for pure 8-PNP-O12 (black squares), c1 (red circles), c2 (blue triangles), c3 (green triangles), c4 (magenta squares) and c5 (purple diamonds) in Sm B at 85 • C (a) and Sm A at 115 • C (b).

Figure 3 . 38 -

 338 Figure 3.38 -Plots of the hole mobility as a function of F 4 TCNQ content in Sm B mesophase at 85 • C (a) and Sm A mesophase at 115 • C (b).

Figure 3 . 39 -

 339 Figure 3.39 -Conductivity of samples extracted from current-voltage plot in Sm B at 85 • C as afunction of F 4 TCNQ content. Sample was placed in Au/ITO measurement cell with gap of 12 µm and 5×5 mm active area.

Figure 3 .

 3 Figure 3.36 assembles examples of photocurrent transient plots obtained by TOF measurements as a function of F 4 TCNQ content in the two smectic mesophases. The

Figure 3 . 40 -

 340 Figure 3.40 -Plot of the hole mobility as a function of temperature for pure 8-PNP-O12 (black squares), c1 (red circles), c2 (blue triangles), c3 (green triangles), c4 (magenta squares) and c5 (purple diamonds).

Figure 3 .

 3 Figure 3.42 demonstrates two SEM images of the c3 cell after it has been opened, which explains large amount of film defects on a relatively thick sample (about 10 µm).

Figure 3 . 41 -

 341 Figure 3.41 -Arrhenius plot of the hole mobility for pure 8-PNP-O12 (black squares), c1 (red circles), c2 (blue triangles), c3 (green triangles), c4 (magenta squares) and c5 (purple diamonds).

Figure 3 .

 3 Figure 3.42 -SEM and EDX images corresponding to c3. Top: SEM morphology images. Bottom: fluorine elemental mapping of the sample area shown above.

Figure 3 .

 3 Figure 3.43 -SEM and EDX images corresponding to c5. Top: SEM morphology images. Bottom: fluorine elemental mapping of the sample area shown above.

Figure 3 . 44 -

 344 Figure 3.44 -Hole mobility measured in the Sm A at 115 • C as a function of F 4 TCNQ concentration. The red line corresponds to the fit with equation 3.2.

Figure 3 . 45 -

 345 Figure 3.45 -Schematic representation of possible CTC distribution in a smectic layer of corresponding mesophase of 8-PNP-O12.

Figure 3 . 46 -

 346 Figure 3.46 -Possible effect of the intermolecular positional order in the smectic layer on the potential distribution in the 8-PNP-O12/F 4 TCNQmixture: U is the potential, E is the external field, b is the width of barriers, ∆ is the barrier height and x is the space coordinate.

Figure 3 . 47 -

 347 Figure 3.47 -Example of SEM profile of pure 8-PNP-O12 thin film deposited on OTS-18 functionalized soda lime glass at 3000 rpm, 20 seconds, 1% wt 8-PNP-O12 in xylene.

Figure 3 .Figure 3 . 49 -

 3349 Figure 3.48 -Clockwise: AFM topology image, 3D reconstruction, selected I-V curves and conductivity mapping (constant -6V) of a pure 8-PNP-O12 thin film. Image dimensions are 3 by 3 µm.

Figure 3 .Figure 3 . 50 -

 3350 Figure 3.50 represents a zoomed version of AFM data in Figure 3.49. Despite somewhatsoft topological image (which is the consequence of low contact force, necessary for the current mapping), it is possible by correlation with the 3D image to observe the grain boundaries (highlighted with white circles). By comparing these areas with the conductivity mapping, one can conclude that these grain boundaries are more conductive than the surrounding material. In order to explain this observation, we should imagine the pathways which electrical current would take across the sample when electrical field is applied. Hunter et al.[START_REF] Hunter | Observation of unusual, highly conductive grain boundaries in high-mobility phase separated organic semiconducting blend films probed by lateral-transport conductive-AFM[END_REF] have observed similar behavior of small-molecule(2,8-

Figure 3 .

 3 Figure 3.51 represents the results of C-AFM study conducted on the thin film of c5. The surface topology is noticeably different to that of pristine 8-PNP-O12: the roughness is significantly increased (we may relate this observation to the CTC acting as an impurity, which interferes with the growth of crystalline thin films of 8-PNP-O12

Figure 3 .

 3 Figure 3.51 -Clockwise: AFM topology image, 3D reconstruction, selected I-V curves and conductivity mapping (constant -6V) of a c5 thin film. Image dimensions are 3 by 3 µm.

Figure 3 . 52 -

 352 Figure 3.52 -Left to right: AFM topology image, conductivity mapping and a 3D reconstruction of a c5 thin film. Contrast is artificially enhanced.

3. 3 . 8 Figure 3 . 53 -

 38353 Figure 3.53 -Microscope captures with 5x lens (a) and 50x lens (b) of an OFET (L = 50 µm, W=18mm) prepared with c5 mixture, as-spun at 3000 rpm, 10 seconds.

Figure 3 . 54 -Figure 3 . 55 -

 354355 Figure 3.54 -Output (a) and transfer (b) characteristic of an OFET (L = 50 µm, W=18mm) prepared with c5 mixture, as-spun at 3000 rpm, 10 seconds.

Figure 3 .

 3 Figure 3.55 demonstrates characteristics of the same device after thermal annealing.It should be noted that the overall conductivity, as well as the response to gate bias (the field effect) are significantly improved in comparison to the as-prepared OFET. A more prominent effect of thermal annealing is the improvement of threshold voltage: it is reduced from -54.5V to -35.6V (Figure3.56).

Figure 3 . 56 -

 356 Figure 3.56 -Threshold voltage characteristic of an OFET (L = 50 µm, W=18mm) prepared with c5 mixture: before (black squares) and after (blue squares) annealing at 115 • C for 30 minutes.

introduced F 4

 4 TCNQ. DFT calculations have allowed us to expect the partial charge transfer between the host 8-PNP-O12 and the acceptor F 4 TCNQ, as well as some noticeable molecular geometry change. Furthermore, we have predicted the behavior of CTC in optical and vibrational spectroscopy. Experimental data has confirmed that it is indeed possible to distinguish partial and integral charge transfer by UV-Vis-NIR spectroscopy alone.The Raman spectroscopy has allowed us to probe relatively small content of CTC in the host 8-PNP-O12 and obtain a reliable confirmation of the results which we have previously observed in the optical spectra. More importantly, it has provided us a solid proof of the predicted geometry by observation of the previously inactive vibrational bands. This allows us to propose the Raman spectroscopy as a valid tool alongside with IR spectroscopy to quantitatively estimate the amount of charge transfer. It is proposed that the molecular symmetry change upon the formation of CTC deserves a more in-depth inspection, since it may result in a significant change of the vibrational signature expressed by the apparition of previously inactive vibrational modes or by a significant change in their intensity and/or frequency. The results of vibrational spectroscopy have also confirmed that the CTC is not dependent on the mesophase nature and is thoroughly stable up to isotropic phase.

Figure 3 .

 3 Figure 3.57 -(a) DSC plots for g2 (black), gc1 (red), gc2 (blue) and gc3 (green) composites obtained with 10 • C/min cooling program prior to photo-polymerization. (b) Phase transition temperatures of precursor mixtures as a function of F 4 TCNQ concentration: Cr to Sm B (black squares), Sm B to Sm A (red circles) and Sm A to I (blue triangles). Empty symbols represent transition for doped mixtures without RM82 from previous section.

Figure 3 . 58 -

 358 Figure 3.58 -Sample texture under crossed analyzer-polarizer: for polymerized gc1, gc2 and gc3 (in Sm B at 85 • C, Sm A mesophase at 115 • C and isotropic phase at 130 • C). Scale bar represents 100 µm.

and 3 . 61 .

 361 The profile of a photocurrent decay shows a kink point when plotted in a double logarithmic scale, which corresponds to the charge carrier arrival time, similarly to the previously described case of p-doped mixtures (Figure3.36).The charge carrier velocity, v, as a function of applied electric field is presented in the Figures 3.60 and 3.61. v is linearly increasing with respect to the applied field.

Figure 3 . 59 -

 359 Figure 3.59 -Possible copolymerization mechanism between the RM82 functional group and F 4 TCNQ.

Figure 3 .

 3 Figure 3.63 represents the hole mobility as a function of temperature for the prepared composites in comparison to the g3. It is clear that in Sm B phase, the hole mobility remains independent of the temperature. This is in contrast with our observations of doped 8-PNP-O12 (Figure 3.40), which means that the polymeric network consisting of RM82 stabilizes the structure of Sm B phase.

Figure 3 . 60 -

 360 Figure 3.60 -Plot of the hole velocity as a function of applied electric field for polymerized gc1 (black squares), gc2 (red circles) and gc3 (blue triangles) in Sm B at 85 • C. Inset: Transient photocurrent plots for gc1 (black), gc2 (red) and gc3 (blue) in Sm B at 20 kV/cm and 85 • C.

Figure 3 . 61 -

 361 Figure 3.61 -Plot of the hole velocity as a function of applied electric field for polymerized gc1 (black squares), gc2 (red circles) and gc3 (blue triangles) in Sm A at 115 • C. Inset: Transient photocurrent plots for gc1 (black), gc2 (red) and gc3 (blue) in Sm A at 34 kV/cm and 85 • C.

Figure 3 . 64 shows

 364 SEM captures of an opened cell of gc3 composite. Top images correspond to the SEM morphology and bottom images represent elemental mapping of fluorine. Images show no correlation of fluorine signatures with topology. This demonstrates that polymer-stabilized composites behaves similarly to p-doped mixtures in regard to the dopant distribution. The absence of dopant aggregation inside of the polymer backbone indicates that the possible copolymerisation side reaction between the RM82 and F 4 TCNQ does not take place on high scale nor impedes the formation of the CTC, as indicated by enhanced hole mobility.

Figure 3 . 62 -

 362 Figure 3.62 -Hole mobility of polymer stabilized p-doped 8-PNP-O12 composites as a function of F 4 TCNQ content in Sm B at 85 • C (a) and Sm A at 115 • C (b).

Figure 3 . 63 -Figure 3 . 64 -

 363364 Figure 3.63 -Hole mobility as a function of temperature of samples of g2 (black squares), gc1 (red circles), gc2 (blue triangles) and gc3 (green triangles).

Figure 4 . 1 -

 41 Figure 4.1 -Substituent indexing of anthracene molecule.

OFigure 4 . 2 -Figure 4 . 3 -

 4243 Figure 4.2 -(E)-N-(anthracen-2-yl)-1-(4-(decyloxy)-phenyl)methanimine (10-OPIA)

Figure 4 . 4 -

 44 Figure 4.4 -Condensation of para-decanoxy-benzaldehide with 2-amino-anthracene (2-AA)

  ) ppm. The signals situated in the aromatic part of the spectrum show presence of seven unique proton environments, which integrate to the total number of 14 protons, which assignments are listed previously. The single proton of the imine linkage can be identified at δ = 8.58 ppm. This part of the spectrum also contains the signal of trace ammouns of CHCl 3 present at CDCl 3 as a singlet at δ = 7.26 ppm. Aliphatic part of the spectrum allows us to observe 4 proton environments, which integrate to the total of 21 atoms, as expected for the alkyl chain. The signal at δ = 1.56 ppm is assigned to the trace water content in the sample. Observations of 1 H COSY spectrum (Figure 4.7) allows us to confirm the assignments

Figure 4 . 7 -Figure 4 . 10 -

 47410 Figure 4.7 -COSY spectrum of 10-OPIA.

Figure 4 . 11 -

 411 Figure 4.11 -Optical absorption spectrum of 10-OPIA recorded for 10 -5 M solution in dichloromethane. Inset: absorption for drop-casted thin film of 10-OPIA (0.5% wt. in chlorobenzene) on quartz substrate.

Figure 4 . 13 presents 19 Figure 4 . 12 -

 41319412 Figure 4.12 -Excitation (black line, emission wavelength -490nm) and emission (blue line, excitation wavelength -435nm) spectrum of 10-OPIA.

Figure 4 . 13 -

 413 Figure 4.13 -Fluorescence decay of dilute (10 -5 M) 10-OPIA solution in acetone. Empty circles: instrument response function (recorded with 1% wt LUDOX TM HS-40 colloidal silica in deionized water). Red circles: experimental data for 10-OPIA solution. Blue line: data fit.

Figure 4 .

 4 Figure 4.14 represents the raw data of 10-OPIA solution cyclic voltammogramms. It is possible to observe distinct features corresponding to the electrochemical oxidation and reduction of 10-OPIA molecules. The peak positions are independent to the scanning speed, while the current is proportional to it. The asymmetric profile of oxidation (large cathodic current and almost negligible anodic current) and reduction (large anodic current

Figure 4 . 14 -

 414 Figure 4.14 -Cyclic voltammetry data of 2mmol 10-OPIA in 0.1M TBAPF6 in DCM, degassed with Ar 20 minutes versus silver wire pseudoreference. Asterisk indicates the start of data collection.

Figure 4 .

 4 Figure 4.15 depicts a cyclic voltammogram of 10-OPIA solution recorded at 50 mV/s referenced versus Fc/Fc+ redox couple (in blue). Two tangents were drawn (in red) at the reduction and oxidation parts of the data in order to determine the onsets E REDand E OX on their intersection with the background current tangent. These extracted potentials were employed to evaluate the HOMO and LUMO energy levels of 10-OPIA using the following relations[START_REF] Brédas | Chain-Length Dependence of Electronic and Electrochemical Properties of Conjugated Systems: Polyacetylene, Polyphenylene, Polythiophene, and Polypyrrole[END_REF]:

Figure 4 . 15 -

 415 Figure 4.15 -Normalised cyclic voltammetry data of 2mmol 10-OPIA in 0.1TBAPF6 in DCM, degassed with Ar (20 minutes) at 50mV/s versus Fc/Fc+. Fc/Fc+ oxidation process is represented in blue, vertical scale is not respected for clarity.

Figure 4 .

 4 Figure 4.16 -DSC thermograms of 10-OPIA sample obtained on heating and cooling with a rate of 10 • C/min.

Figure 4 . 17 -

 417 Figure 4.17 -Sample texture under crossed analyzer-polarizer for pure 10-OPIA (a) in crystalline phase (Cr ) at 120 • C, (b) Sm 1 mesophase at 140 • C, Sm 2 mesophase at 148 • C and nematic (N ) mesophase at 159 • C.

Figure 4 . 18 -

 418 Figure 4.18 -SEM images of 10-OPIA film prepared by drop-casting of 0.5% wt. solution in chlorobenzene on OTS-18 functionalized glass slide at 3 degree substrate inclination: (a) topography (red arrow indicates direction of crystallization) (b) profile of the same thin film.

Figure 4 .

 4 Figure 4.18, (a) illustrates the alignment of thin film drop-casted on a soda lime glass substrate functionalized with OTS-18. Note how the grain boundaries aligned in the

Figure 4 .

 4 Figure 4.19 -AFM topology (left) and a cross-section (right) of drop-casted 10-OPIA film on OTS-18 functionalized glass slide at 3 degree substrate inclination.

Figure 4 . 20 -

 420 Figure 4.20 -Microscope captures (in polarized light) with 5x lens (a) and 50x lens (b) of a linear electrode OFET (L = 30 µm, W=1mm) prepared with 10-OPIA, drop-casted, 3 degree substrate inclination.

Figure 4 .

 4 Figure 4.20 shows magnification of transistor channel area with a deposited 10-OPIA thin film, which reveals large crystalline domains crossing both (source and drain) electrodes. The yield of 18 working devices out of 20 per substrate on average was achieved.

Figure 4 . 21 -

 421 Figure 4.21 -Output characteristic of an OFET (L = 30 µm, W=1mm) prepared with 10-OPIA, drop-casted, 3 degree substrate inclination.

Figure 4 .

 4 Figure 4.22 -Transfer (a) and threshold voltage (b) characteristic of an OFET (L = 30 µm, W=1mm) prepared with 10-OPIA, drop-casted, 3 degree substrate inclination.

Figure 4 .

 4 [START_REF] Pingel | Effect of molecular p-doping on hole density and mobility in poly ( 3-hexylthiophene ) Effect of molecular p-doping on hole density and mobility[END_REF] demonstrates microscope capture of an OFET prepared with 10-OPIA in inter-digitated (IE) geometry. It is of interest to notice the size of crystalline domains and how efficiently their alignment could be controlled with substrate inclination during drop-casting.

Figure 4 . 23 -

 423 Figure 4.23 -Microscope captures (in polarized light) with 5x lens (a) and 50x lens (b) of an inter-digitated electrode OFET (L = 50 µm, W=18mm) prepared with 10-OPIA, drop-casted, 3 degree substrate inclination. Dark spots on the left image correspond to crystalline agglomerates on the surface of thin film.

Figure 4 . 24 -

 424 Figure 4.24 -Output characteristic of an IE OFET (L = 50 µm, W=18mm) prepared with 10-OPIA, drop-casted, 3 degree substrate inclination.

Figure 4 .

 4 Figure 4.25 -Transfer (a) and threshold voltage (b) characteristics of an IE OFET (L = 50 µm, W=18mm) prepared with 10-OPIA, drop-casted, 3 degree substrate inclination.

4. 5 . 2

 52 PhototransistorDuring the initial testing of devices fabricated with 10-OPIA, we have noticed a pronounced photosensitivity of 10-OPIA material to the 3300K light source used to micromanipulator positioning. To further investigate this phenomena, we have employed a 455 nm (bandwidth FWHM = 18 nm) light source (variable power LED) which corresponds to the edge of absorption for 10-OPIA in a solid film form (inset of Figure4.11).

Figure 4 .

 4 Figure 4.26 represent the transfer characteristic of previously described OFET device in interdigitated configuration. We would like to underline how the current I SD is amplified by incident light, while retaining distinct ON and OFF states -a characteristic feature of the field-effect transistor.

Figure 4 . 26 -

 426 Figure 4.26 -Plot of transfer characteristic of an IE OFET (L = 50 µm, W=18mm) prepared with 10-OPIA, drop-casted, 3 degree substrate inclination. Illumination wavelength is λ=455 nm, V SD = -100V. Inset: Plot of saturation drain-source current as a function of light intensity.

Figure 4 .

 4 27, b). This effect is explained by the presence of trap states such as structural defects on 10-OPIA/SiO 2 interface, for example (on the edge of LUMO "band" in the Figure4.28, a), which become populated upon irradiation (Figure4.28, b). The contribution of these charged states acts as an additional local "gate" electric field and is able to noticeably shift the threshold voltage of a device[START_REF] Buscema | Photocurrent generation with two-dimensional van der Waals semiconductors[END_REF], especially if we take into account weak field screening in the bulk of material (we expected low dielectric permittivity of 10-OPIA due to its organic nature).

Figure 4 . 28 -

 428 Figure 4.28 -Energy band (approximation used here for simplicity) diagram for OFET channel (a) in dark conditions and (b) under illumination. Trap states (horizontal bars near the edge of conduction band) and trap filling by minority carriers is shown.

Figure 4 . 29 -

 429 Figure 4.29 -Plot for current as a function of time. V DS = -50V, V GS = 0V (black symbols), V GS = -50V (red symbols). T L -start of illumination.

Figure 4 .

 4 30 assembles transient current plots for different temperatures. Every measurement was preceded by 5s pulse of positive gate voltage (V GS =50V) in order to "erase" persisting photoconductive states. Current was measured during 30s of illumination, then additional 140s of current decay were recorded. The data was then normalized in respect to the peak photocurrent and the decay part was fitted with the equation 4.5.

Figure 4 . 30 -

 430 Figure 4.30 -Plot for normalized photocurrent as a function of time and temperature. Vertical offset is introduced for clarity. V SD = -100V, V GS = 0V. Purple lines represent double exponential fit. T L -start of illumination.

Figure 4 . 32 -

 432 Figure 4.32 -Plots for OFET channel current (bottom) and gate voltage (top) as a function of time. Irradiation power was 1.16 mW/cm 2 at 455nm. T L -start of illumination.

Figure 4 .

 4 Figure 4.32 demonstrates the channel current plot as a function of time, illumination and gate voltage. The device is initially illuminated at zero gate bias, and the current begins to rise relatively slowly. After 50 seconds, positive voltage (V GS =50V) is applied,

E 4 Table 4 . 2 -

 442 g (eV) E HOM O (eV) E LU M O (eV) µ hole (cm 2 /(V s))V th (V) I ON/OF F Relevant properties of charge transport for 10-OPIA material.Finally, we have characterized photoconductive properties of this new material. We have found large photoconductive gain (∼ 100) and photoresponsivity (R ∼ 1 A/W)for OFETs based on 10-OPIA despite relatively large channel length (50 µm). These devices have also demonstrated non-negligible photogating effect which in connection to remarkable control of over photoconductivity via gate bias allows us to endorse further investigations of this new class of materials in relation to photosensitive applications.

  -clés : semiconducteurs organiques, cristaux liquides, photopolyméres, dopage, temp de vol, transistors organiques à effet de champ Differential scanning calorimetry (DSC) . . . . . . . . . . . . . . . . 2.2.5 X-Ray Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Spectroscopic techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . Optical Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Charge transport characterization techniques . . . . . . . . . . . . . . . . 2.4.1 Time Of Flight (TOF) mobility measurment . . . . . . . . . . . . . 2.4.2 Transistor I-V characterization . . . . . . . . . . . . . . . . . . . . . 2.5 Complimentary techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Molecular structure characterization techniques . . . . . . . . . . . 2.5.2 Cyclic voltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.3 Self-Assembled Mono-layer (SAM) coating . . . . . . . . . . . . . . 2.5.4 Spin-coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.5 Drop-casting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

	xxvi	Contents
	Contents General Introduction 2.3.1 3 Composite materials based on 8-PNP-O12
	xxv

1 Organic semiconductors: concepts and theoretical basics 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Organic semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Structure of organic semiconductors . . . . . . . . . . . . . . . . . . 1.2.3 Charge transport in organic semiconductors . . . . . . . . . . . . . 1.2.4 Doping in organic semiconductors . . . . . . . . . . . . . . . . . . . 1.2.5 Applications and processing . . . . . . . . . . . . . . . . . . . . . . 1.3 Classification of organic semiconductors . . . . . . . . . . . . . . . . . . . 1.3.1 Polymeric OSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Small molecule OSCs . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Liquid crystalline OSCs . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Photopolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1 Functional groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Organic Field Effect Transistor . . . . . . . . . . . . . . . . . . . . . . . . 1.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.3 Device performance . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Experimental methods and techniques 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Structural and morphological characterization techniques . . . . . . . . . 2.2.1 Polarized optical microscopy . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Scanning Electron Microscopy (SEM) . . . . . . . . . . . . . . . . . 2.2.3 Atomic Force Microscopy (AFM) . . . . . . . . . . . . . . . . . . . 2.2.4 3.1 General introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Cross-linked "gel"-type composites . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Thermophysical, mesogenic and structural properties . . . . . . . . 3.2.4 Charge transporting properties in the mesophase . . . . . . . . . . . 3.2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 P-doped LC OSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Spectroscopic tools for studying of charge transfer . . . . . . . . . . 3.3.3 Phase behavior, structural and optical properties . . . . . . . . . . 3.3.4 Structural studies by wide angle X-ray scattering (WAXS) . . . . . 3.3.5 Vibrational spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 3.3.6 Charge transporting properties in the mesophase . . . . . . . . . . . 3.3.7 Topology and current mapping of thin polycrystalline films . . . . . 3.3.8 Preparation and characterization of a field-effect transistor . . . . . 3.3.9 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Polymer-stabilized p-doped composites . . . . . . . . . . . . . . . . . . . 3.4.1 Materials and sample preparation . . . . . . . . . . . . . . . . . . . 3.4.2 Thermophysical, mesogenic and structural properties . . . . . . . . 3.4.3 Charge transporting properties in smectic mesophases . . . . . . . . 3.4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 4 Novel liquid crystalline semiconductor based on anthracene 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Elaboration of (E)-N-(anthracen-2-yl)-1-(4-(decyloxy) -phenyl)methanimine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Overview of anthracene-based semiconductors . . . . . . . . . . . . 4.2.2 Synthetic route . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

  To do so, we have looked into another mature and well-established field of material science -the liquid crystals. The first reports on this type of materials date to the end of the XIX century, when F. Reinitzer et al have discovered an unusual state of matter (which today we call mesophase) in a derivative of cholesterol. Since then, extraordinary progress has been made to this research field, which has resulted in numerous applications including but not limited to displays and light modulation. In this work, however, we are not going to disclose any light modulation-related properties. Instead, we are going to get interested in structural

  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 3.5 General conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

	3.4.4
	3.4 Polymer-stabilized p-doped composites . . . . . . . . . . . . . . . . . . . .
	3.4.1 Materials and sample preparation . . . . . . . . . . . . . . . . . . .
	3.4.2 Thermophysical, mesogenic and structural properties . . . . . . . .
	3.4.3 Charge transporting properties in smectic mesophases . . . . . . .
	95

3.1 General introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Cross-linked "gel"-type composites . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Thermophysical, mesogenic and structural properties . . . . . . . . 3.2.4 Charge transporting properties in the mesophase . . . . . . . . . . 3.2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 P-doped LC OSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Spectroscopic tools for studying of charge transfer . . . . . . . . . 3.3.3 Phase behavior, structural and optical properties . . . . . . . . . . 3.3.4 Structural studies by wide angle X-ray scattering (WAXS) . . . . . 3.3.5 Vibrational spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 3.3.6 Charge transporting properties in the mesophase . . . . . . . . . . 3.3.7 Topology and current mapping of thin polycrystalline films . . . . 3.3.8 Preparation and characterization of a field-effect transistor . . . . . 3.3.9 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* DOS describes the distribution of available energy states, which charge carriers may occupy ("populate") per unit volume. In relation to traditional ISC, this distribution is usually described via parabolic function, whereas for OSCs it is expressed by Gaussian distribution due to higher energetic disorder of the latter.

Acknowledgment

Aromatic part of the spectrum

Aliphatic part of the spectrum 13 C spectrum obtained from the same sample as the previous spectra. 13 C spectrum found: 13 C-NMR (100 MHz, CDCl 3 ) δ = 162. 14, 159.76, 149.56, 132.48, 132.24, 131.47, 130.76, 130.48, 129.40, 129.24, 128.39, 128.08, 126.32, 126.09, 125.68, 125.64, 125.24, 122.16, 117.04, 114.91

General Conclusion

This document regroups the research efforts conducted on organic liquid crystalline semiconductors during the period from 2016 to 2019. The goal of this thesis project was to develop a novel liquid crystalline organic semiconductor as well as composites for organic electronics.

Extensive literature research has been performed in order to gain insight on the subject in scope. The theoretical part of this document (Chapters 1&2) includes the general information on organic semiconductors with a brief overview on the essential topics: chemical structure, processing techniques and charge transport; particular attention is addressed towards doping, since many aspects of this concept take an important part in the Chapter 3. We have also made an attempt to provide a short review of the recent progress in organic semiconductors field from the material point of view, with an additional focus of liquid crystals. We have also presented a short glimpse into the vast diversity of mesophases and their structure. With this, we have also introduced the concept of reactive mesogens -another important notion which we have addressed in more detail during the course of Chapter 3. At this point, we have passed to the presentation of basic principles of a field-effect transistor, since it is going to be the object of a significant effort in this thesis. The author have made attempts to introduce the theory of its functioning as briefly as possible. However, numerous details and factors affecting its functioning and performance have been exposed.

The second Chapter contained the experimental techniques used by the author and his collaborators to the benefit of this work. We have divided these techniques in relevant subgroups (thermophysical and structural characterization; spectroscopy; charge transport measuring techniques; etc) for which the author has provided appropriate descriptions, as well as relevant experimental details and protocols.

The first part of experimental work was conducted on the 8-PNP-O12 and its composites. We have started with preparation of the semiconductor(8-PNP-O12)/reactive mesogen (RM82) composites. This has allowed the author to study the effects of in situ

217

Structural and charge transporting properties of pure liquid crystalline organic semiconductors and composites for applications in organic electronics Abstract

This thesis is dedicated to various aspects of liquid crystalline (LC) organic semiconductors (OSCs) in regard to their applications in the field of organic electronics.

The first part of this work deals with a well-known LC OSC based on phenyl-naphthalene. Two major ways of performance improvement are proposed and investigated: stabilization of LC structure by in situ photo-polymerization and introduction of electron acceptor doping impurity.

In the first case, the influence of polymer network on mesophase order and charge transport is investigated by conventional experimental techniques and Time-of-Flight (TOF) mobility measurements. For the doped materials, ab initio calculations are employed to predict their spectroscopic properties which is exhaustively compared with the experimental data obtained by optical and vibrational spectroscopy. The charge transport is studied by TOF method in the mesophase, while crystalline phase is investigated via conductive atomic force microscopy. A prototype of organic field effect transistor (OFET) is prepared to obtain an estimate of performance for a relevant real-world application.

The second part of this work includes design and synthesis of a novel LC semiconductor based on anthracene, additional attention is made to obtain an easy-to-make and low production cost material. Novel molecule is fully characterized: molecular structure is confirmed by relevant techniques; frontier molecular energy levels are studied by optical spectroscopy and cyclic voltammetry and confronted to values obtained via ab initio calculations; mesophase properties are investigated by optical microscopy and scanning calorimetry. Charge transporting properties are characterized by means of an OFET device: it is found that new anthracene-molecule exhibits significant improvement of field-effect hole mobility over previously studied phenyl naphthalene derivative. Finally, photoconductive properties of the novel material are addressed in order to investigate its potential applications to organic phototransistors.