Keywords: Semi-conducteurs de polymères, Accepteurs de polymères, Cellules solaires entièrement polymères Polymer semiconductors, Polymer acceptors, All-polymer solar cells Reaction conditions: toluene/ Pd(PPh 3

Titre : Nouveaux polymères conjugués π de type N pour les cellules solaires entièrement polymères

Résumé :

Les cellules solaires organiques (OSC) apparaissent comme une technologie prometteuse pour les énergies renouvelables en raison de leur poids léger, leur grande flexibilité et leur processus de fabrication peu coûteux. Jusqu'à présent, la plupart des OPV ont utilisé des dérivés de Fullerene, tels que PCBM ou PC71BM, en tant qu'accepteur d'électrons dans la couche active, qui s'est avéré être un goulet d'étranglement pour cette technologie. Par conséquent, le développement d'accepteurs non-fullerene est devenu la nouvelle force motrice de ce domaine. Les cellules solaires tout-polymères (tous-PSC) qui ont les avantages de la robustesse, de la stabilité et de l'accessibilité ont déjà atteint PCE jusqu'à 9%. Ainsi, le développement de nouveaux matériaux accepteurs est impératif pour améliorer les performances de tous les PSC.

Résumé en français

Les cellules solaires organiques (OSC), également appelées photovoltaïques organiques (OPV), apparaissent comme une technologie prometteuse pour les énergies renouvelables en raison de leur poids léger, leur grande flexibilité et leur processus de fabrication peu coûteux. Jusqu'à présent, la plupart des OPV ont utilisé des dérivés de Fullerene, tels que PCBM ou PC71BM, en tant qu'accepteur d'électrons dans la couche active, qui s'est avéré être un goulet d'étranglement pour cette technologie. Par conséquent, le développement d'accepteurs non-fullerene est devenu la nouvelle force motrice de ce domaine. Les cellules solaires tout-polymères (tous-PSC) qui ont les avantages de la robustesse, de la stabilité et de l'accessibilité ont déjà atteint PCE jusqu'à 9%. Cependant, il existe encore un écart important entre les PSC et les OSC basés sur Fullerene (PCE s'approchant de 12%) malgré les efforts considérables qui ont été mis sur l'optimisation du matériel et de l'appareil. Ainsi, le développement de nouveaux matériaux accepteurs est impératif pour améliorer les performances de tous les PSC. Dans cette thèse, trois classes de polymères π-conjugués ont été conçus et synthétisés pour l'application de tous les PSC. La première classe de polymères est basée sur un nouveau fragment déficient en électrons, (3E, 7E) -3,7-bis (2-oxoindolin-3-ylidène) -5,7dihydropyrrolo [2,3-f] indole-2,6 (1H, 3H) -dione (IBDP). Les polymères à base d'IBDP (P1 et P2) ont montré une propriété de transport ambipolaire équilibrée (mobilité des électrons jusqu'à 0,10 cm2 V-1 s-1 et mobilité des trous jusqu'à 0,19 cm2 V-1 s-1) dans les OTFT. En plus des bonnes propriétés de transport de charge, les polymères IBDP présentent un profil d'adsorption fort et large dans la région visible et NIR jusqu'à 1100 nm ainsi que des niveaux élevés de LUMO à -3,70 eV. Avec ces caractéristiques avantageuses, ces polymères IBDP ont été utilisés comme accepteur avec du poly (3hexylthiophène-2,5-diyle) (P3HT) comme donneur dans tous les PSC. Après l'optimisation du ratio donneur / accepteur, les dispositifs tous-PSC résultants ont montré une PCE élevée de 3,38%, ce qui est le PCE le plus élevé qui a été obtenu à partir de tous les PSC basés sur P3HT jusqu'à présent. La deuxième classe consiste en trois (3E, 7E) -3,7-bis (2oxoindolin-3-ylidène) benzo [1,2-b: 4,5-b '] difurane-2,6 (3H, 7H ) -dione (IBDF) à base de polymères qui présentent un nouveau type de chaînes latérales qui contiennent un groupe ester. Les polymères IBDF résultants présentaient d'excellentes propriétés de transport d'électrons avec une mobilité électronique jusqu'à 0,35 cm2 de V-1 s-1 dans des OTFT. Lorsqu'il a été utilisé comme accepteur dans tous les PSC avec PTB7-Th en tant que donneur, des PCE faibles (<0,4%) ont été obtenus, ce qui a été causé par la faible miscibilité du donneur et de l'accepteur, ainsi que par les propriétés de transport de charge de masse inférieure des polymères IBDF. Enfin, un nouveau bloc de construction, le dihydroxylnaphtalène diimide (NDIO), a été introduit pour la première fois dans des polymères conjugués π. En raison des groupes alcoxy, l'affinité électronique du polymère NDIO est significativement plus élevée que les analogues NDI, ce qui a conduit à une propriété améliorée de transport d'électrons et à une performance plus stable dans les OTFT lors de l'exposition à l'air. Lorsqu'il a été utilisé comme accepteur dans tous les PSC avec PTB7-Th en tant que donateur, un PCE décent de 3,25% a été réalisé. En particulier, le FF (0,61) des appareils de cellules solaires est beaucoup plus élevé que celui des polymères de NDI basés sur tous les PSC, ce qui a été attribué au transport de charge équilibré pour le trou et l'électron dans la couche active, ainsi que le supprimé recombinaison bimoleculaire.

La thèse est organisée comme suit:

1) Au chapitre 2, on décrira la conception et la synthèse d'un nouveau bloc de construction déficient en électrons (IBDP) et de deux polymères IBDP (P1 et P2). Les polymères résultants seront ensuite caractérisés en utilisant plusieurs techniques telles que le microscope à force atomique (AFM), la diffraction des rayons X (XRD), la spectroscopie d'absorption UV-Vis et la voltamétrie cyclique (CV) seront utilisés pour étudier les propriétés optiques et électroniques. Ensuite, le polymère sera utilisé pour les OTFT de contact inférieur (BGBC) de base pour étudier leurs propriétés de transport de charge. En outre, les polymères seront utilisés comme accepteurs dans tous les PSC avec P3HT comme donneur. Seuls les résultats préliminaires seront présentés en raison de la quantité limitée de matériaux.

2) Dans le chapitre 3, une série de chaînes latérales d'ester sera synthétisée et utilisée pour les copolymères d'IBDF et de bithiophène (BT) pour produire des polymères à base de IBDF solubles dans la solution de type n. Tout d'abord, les propriétés optiques et électrochimiques de ces polymères seront systématiquement étudiées en utilisant UV et CV. Ensuite, les propriétés de transport de ces polymères seront évaluées dans les OTFT de contact inférieur (BGBC) du bas de la porte. Les polymères seront ensuite mélangés avec PTB7-Th dans une structure de dispositif standard: ITO / PEDOT: PSS / Polymer blend / LiF / Al pour étudier les propriétés photovoltaïques. Enfin, SCLC, l'efficacité quantique externe (EQE), AFM et XRD seront utilisés pour étudier les relations structure-propriété.

3) Dans le chapitre 4, un nouveau bloc de construction, le dihydroxylnaphtalène diimide (NDIO), sera utilisé pour former deux polymères à base de n-unipolaire à base de NDIO.

Les polymères ont démontré des niveaux de HOMO et LUMO plus profonds. Les polymères seront ensuite mélangés avec PTB7-Th dans une structure de dispositif standard: ITO / PEDOT: PSS / Polymer blend / LiF / Al. Les PSC seront caractérisés et comparés avec le matériel PA de repère, P (NDI2OD-T2). Les OTFT, SCLC, l'efficacité quantique externe (EQE), AFM et XRD seront utilisés pour étudier les relations structurepropriété. of side chains that contain an ester group. The resultant IBDF polymers exhibited excellent electron transport properties with electron mobility up to 0.40 cm 2 V -1 s -1 in OTFTs. When used as acceptor in all-PSCs with PTB7-Th as donor, low PCEs (<0.4%) were obtained, which was found to be caused by the poor miscibility of the donor and acceptor, as well as the inferior bulk charge transport properties.

Finally, a new building block, dihydroxylnaphthalene diimide (NDIO), was introduced for the first time into π-conjugated polymers. Due to the hydroxylamine groups, the electron affinity of the NDIO polymer is significantly higher than the NDI analogues, which led to an enhanced electron transport property and more stable performance in OTFTs upon air-exposure. When used as acceptor in all-PSCs with PTB7-Th as the donor, a decent PCE of 3.25 % was realized, which represents an improvement from its NDI analogues (PCE of 2.87% under the same conditions). In particular, the FF (0.61) of the solar cell devices is much higher than those of the NDI polymers based OSCs, which is attributed to the balanced charge transport for both hole and electron in the active layer, as well as the suppressed bimolecular recombination. vii The blend film (1:1 wt ratio) was spin-coated at 1300 rpm using a solution (25 mg/mL) in o-DCB. . Table 2-

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Chapter 1. Introduction 1.1 Research Context

As the demand for energy has been skyrocketing in last 30 years, it has been more and more imminent for us to seek new energy sources other than traditional ones such as petroleum and coal. [START_REF]Peaking of World Oil Production Impacts[END_REF] On the other hand, there are growing environmental concerns over the climate change and air pollution associated with using these traditional energy sources. [START_REF] Stoppato | Life cycle assessment of photovoltaic electricity generation[END_REF] Therefore, numerous efforts have been spent

finding new energy sources in the last two decades.[3,4] Solar energy is an ideal energy source, since it is completely renewable and environmentally friendly. Moreover, it is free of charge. A solar cell (Figure 1-1) is an electrical device that can convert solar energy into electricity, which can be of various uses such as residential supply and automotive transportation, without generating any air pollution or causing any negative impacts to the environment. As far as the technology of solar cell (also known as photovoltaics) has developed, it can be generally categorized into three generations. [START_REF] Badawy | A review on solar cells from Si-single crystals to porous materials and quantum dots[END_REF][START_REF] Brown | Third generation photovoltaics[END_REF][START_REF] Green | Third Generation Photovoltaics: Advanced Solar Energy Conversion[END_REF] The first generation of solar cells is based on single crystal silicon. With considerable efficiency and long lifespan, single crystal silicon solar cell panels are massively used for residential and plants nowadays, however, the initial production cost and heavy weight have refrained it from application in other areas, for instance, portable electronic devices. [START_REF][END_REF] The second generation of solar cells is called thin film solar cells, because they are made of thin layers of inorganic semiconductors such as cadmium telluride (CdTe) and copper indium gallium diselenium (CIGS). The combination of use of less material and lower production cost allows the manufacturers to produce and sell solar cells at a much lower price.

However, it has turned out to be difficult to produce thin film solar cells in massive quantity with comparable efficiency level to that of single crystal silicon solar cells. [START_REF] Green | Thin-film solar cells: review of materials, technologies and commercial status[END_REF] The third generation of solar cells usually refers to a variety of technologies that are still under research and development now, which xxi include organic solar cells, dye sensitized solar cells and quantum dot solar cells. Organic solar cells, also known as organic photovoltaics (OPVs), are mainly based on organic semiconductors. Thus, they have several distinguished features. First, they can be solution processed, which fundamentally allows the possibility of manufacturing a solar cell panel using printing technologies at high throughput.

Compared with the fabrication of inorganic solar cells, using printing technologies will substantially lower the manufacturing cost and increase manufacturing efficiency. [START_REF] Dennler | Polymer-fullerene bulk-heterojunction solar cells[END_REF][START_REF] Liu | Spin-Coated Small Molecules for High Performance Solar Cells[END_REF] Second, organic materials generally have light weight (due to the low density) and good flexibility. This feature is highly desired by the portable electronic devices. [START_REF] Kaltenbrunner | Ultrathin and lightweight organic solar cells with high flexibility[END_REF][START_REF] Kim | Flexible, highly efficient all-polymer solar cells[END_REF] Third, the diversity of organic materials provides means to tune and optimize the material properties to meet the criteria set by the applications. [START_REF] He | New building blocks for π-conjugated polymer semiconductors for organic thin film transistors and photovoltaics[END_REF] Owing to these features, organic solar cells have been considered one of the promising photovoltaic technologies.

Figure 1-1

The illustration of the structure of a typical solar cell device. [15] xxi i

Organic Solar Cells

Solar cell is an electrical device that can steadily harvest the sunlight and generate electricity. In 1979,

Tang introduced the first organic solar cell and achieved power-conversion efficiencies (PCEs) of around 1%. [START_REF] Tang | Two-layer organic photovoltaic cell[END_REF] The cell was based on the two-layer planar heterojunction of two organic materials: copper phthalocyanine (CuPc) as donor and 3,4,9,10-perylenetetracarboxyl-bis-benzimidazole (PV) as acceptor. These two organic layers were sandwiched between ITO-coated glass (front contact) and Ag (back contact) to form a solar cell (Figure 1-2). When the light photon reaches the front contact, it transmits through the front contact and to the CuPc layer, where the photon will be absorbed by CuPc and generates an exciton. As the exciton travels to the interface between CuPc and the PV, the exciton gets split into an electron and a hole, and then the electron will transfer to the PV layer. Finally, the hole and electron will be collected by respective electrodes, forming the photovoltaic current. The driving force for the charge separation is provided by the energy difference between the lowest unoccupied molecular orbital (LUMO) of the donor (CuPc) and highest occupied molecular orbital (HOMO) of the acceptor (PV). One improvement to the OSC device was to use C60 fullerene and its derivatives to replace the n-type PV, since fullerene has stronger electronegativity and higher electron mobility. In 1993, researchers introduced the use of fullerene derivative, [START_REF] Brown | Third generation photovoltaics[END_REF][START_REF] Brown | Third generation photovoltaics[END_REF]-phenyl-C61-butyric acid methyl ester (PCBM, Figure 1-3) as the acceptor in planar heterojunction OSCs. [START_REF] Sariciftci | Semiconducting polymers (as donors) and buckminsterfullerene (as acceptor): photoinduced electron transfer and heterojunction devices[END_REF] However, the cell only demonstrated a low PCE of 0.04%. It was because the lifetime of the exciton in organic semiconductors (~10 nm) [START_REF] Shaw | Exciton Diffusion Measurements in Poly(3hexylthiophene)[END_REF][START_REF] Stapper | Ab initio[END_REF] is much shorter than that in single crystal silicon, which means only small portion of the generated excitons turn into charge carriers. [START_REF] Li | Polymer solar cells[END_REF] This problem was later addressed by introducing a bulk heterojunction (BHJ) system (Figure 1-4), which involves blending donor and acceptor materials in a single layer device. It was first introduced by Hiramoto et al. through the coevaporating donor and acceptor molecules simultaneously under high-vacuum condition. [START_REF] Hiramoto | p-i-n like behavior in three-layered organic solar cells having a co-deposited interlayer of pigments[END_REF] In 1995,

xxi ii

Heeger's group reported the first polymer-fullerene (MEH-PPV and PCBM) bulk heterojunction solar cell with a high PCE. [START_REF] Yu | Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions[END_REF] This opened a new rea of polymer-fullerene system in the field of highefficiency OSCs. Up to now the polymer-fullerene solar cell's PCE has reached 11.7% [START_REF] Zhao | Efficient organic solar cells processed from hydrocarbon solvents[END_REF], indicating remarkable progress towards a promising future.

Figure 1-2

The device structure of the first OSC reported by Tang et al. [START_REF] Tang | Two-layer organic photovoltaic cell[END_REF] xxi v There are four parameters to characterize the performance of a solar cell: Open circuit voltage (V OC ), short circuit current density (J SC ), fill factor (FF) and power conversion efficiency (PCE), which all can be obtained from the V-J (voltage-current density) curve of a cell. As depicted in Figure 1-5, V OC is the xx v output voltage when J is at zero, which represents the maximum voltage that can be obtained from the cell. There are many factors that can influence V OC of a solar cell (Figure 123456). For organic solar cells, Cowan et al. proposed an equation derived from both theoretical study and experimental data [START_REF] Leong | Role of trace impurities in the photovoltaic performance of solution processed small-molecule bulk heterojunction solar cells[END_REF]:

𝑉 𝑂𝐶 = 1 𝑒 (𝐸 𝐿𝑈𝑀𝑂 𝐴 -𝐸 𝐻𝑂𝑀𝑂 𝐷 -∆) - 𝑘𝑇 𝑒 ln ( 𝑁 𝑒 𝑁 ℎ 𝑁 𝐶 )
Where N e is termed the electron density in the acceptor domains and N h is termed the hole density in the donor domains under open circuit conditions, and N C is the density of states (DOS) at the conduction band edge of the donor and acceptor. The energy shift, Δ, arises from disorder within the phase separated donor and acceptor domains. J SC is the measured current density when the external resistance is zero, which represents the maximum current density that can be obtained from the cell. This equation was also verified by Scharber's work, [START_REF] Scharber | Design Rules for Donors in Bulk-Heterojunction Solar Cells-Towards 10 % Energy-Conversion Efficiency[END_REF] which gave a simplified equation:

𝑉 𝑂𝐶 = 1 𝑒 (𝐸 𝐿𝑈𝑀𝑂 𝐴 -𝐸 𝐻𝑂𝑀𝑂 𝐷 ) -0.3𝑉
A simple way to increase the J SC of a solar cell is to use a low bandgap and broaden the adsorption profile of semiconductor to promote the light harvesting and charge generation. In a device, however, J SC could also be determined by several non-trivial factors such as exciton dissociation, charge transport (bulk charge transport mobility) and germinate recombination. Achieving a proper morphology of the active layer is a key to getting the intrinsically maximum Jsc for given a combination of donor/acceptor.

The preferred morphology of the active layer is a bi-continuous interpenetration network (Figure 1234567) [START_REF] Yang | Toward High-Performance Polymer Solar Cells: The Importance of Morphology Control[END_REF]. Donor and acceptor domains should be twice the size of the exciton diffusion length (around 10 nm), which allows excitons to diffuse to the donor-acceptor interface and thus achieves efficient exciton dissociation and charge generation. After charge separation at the donor-acceptor interface, holes and electrons must travel to the positive and negative electrodes through the continuous donor and acceptor networks, respectively. Although V OC and J SC can indeed be improved by employing these xx vi strategies, it remains a challenge to improve both values simultaneously. Narrowing the bandgap can improve J SC , but V OC may correspondingly be decreased because of the higher HOMO level that results.

Researchers recently demonstrated that structural fine-tuning is a powerful approach for improving both V OC and J SC simultaneously. [START_REF] Chen | Polymer Solar Cells with Enhanced Open-Circuit Voltage and Efficiency[END_REF][START_REF] Zhou | Development of Fluorinated Benzothiadiazole as a Structural Unit for a Polymer Solar Cell of 7 % Efficiency[END_REF][START_REF] Liang | Highly Efficient Solar Cell Polymers Developed via Fine-Tuning of Structural and Electronic Properties[END_REF][START_REF] Price | Fluorine Substituted Conjugated Polymer of Medium Band Gap Yields 7% Efficiency in Polymer-Fullerene Solar Cells[END_REF] For example, introducing a fluorine atom into the TT unit reduces both HOMO and LUMO levels simultaneously, while also improving V OC and retaining the bandgap. [START_REF] Zhou | Development of Fluorinated Benzothiadiazole as a Structural Unit for a Polymer Solar Cell of 7 % Efficiency[END_REF][START_REF] Price | Fluorine Substituted Conjugated Polymer cx xxi x of Medium Band Gap Yields 7% Efficiency in Polymer-Fullerene Solar Cells[END_REF] Fine-tuning the side-chain structure can also result in a similar effect. One example is to simultaneously lower both the HOMO and LUMO levels by replacing the electron-rich alkoxy side chain with the less electron-rich alkyl chain. [START_REF] Liang | Highly Efficient Solar Cell Polymers Developed via Fine-Tuning of Structural and Electronic Properties[END_REF] FF is the ratio of the maximum power output to the product of V OC and J SC . It is affected by many factors, including the balance of charge carrier mobility, interface recombination (bimolecular recombination), series and shunt resistances, film morphology and miscibility between the donor and acceptor. However, obtaining a clear understanding and the ability to modulate the FF remains a hurdle in the development of PSCs. Apart from the parameters above, the series resistance (R S ) and shunt resistance (R SH ) can also estimate from the J-V curve by take the slopes at the open-circuit point and short-circuit point. With present of R S and R SH , the J-V curve is given by the following equation:

𝐽 = 𝐽 𝐿 -𝐽 0 exp ( 𝑞(𝑉 + 𝐽𝑅 𝑆 ) 𝑛𝐾𝑇 ) - 𝑉 + 𝐽𝑅 𝑆 𝑅 𝑆𝐻
Where J L is the photocurrent, J o is the dark saturation current and n is the ideality of the device.

According to this equation, the higher R S or the low R SH is, the lower V OC and FF will become. The PCE is ratio of maximum output to the power input (100 mW/cm 2 for the standard AM1.5G spectrum multiplied by the area of the cell). Therefore, we have the expression for PCE:

𝑃𝐶𝐸 = 𝐽 𝑆𝐶 ×𝑉 𝑆𝐶 ×𝐹𝐹 100 𝑚𝑊/𝑐𝑚 2 xx vii
Since PCE represents the overall performance of a solar cell, the focus of organic photovoltaics has been achieving high PCE to compete with silicon solar cells. With all these parameters affecting the PCE of a solar cell, improving the PCEs of OSCs still presents a great challenge to the researchers.

Although optimizing the fabrication process can probe the intrinsic maximum performance of the materials, more efforts should be made to develop novel materials with excellent properties that can simultaneously boost all those parameters in order for OSCs' performance to reach that of Silicon solar cells.

Figure 1-5

The illustration of the J-V characteristics of a solar cell. [33] xx viii

Organic Semiconductors

Semiconductors are materials that show low conductivity at room temperature but significantly higher conductivity at elevated temperatures. This phenomenon can be explained by the band gap between valence band and conduction band in an inorganic solid. For a solid material, if the band gap is less than 3.0 eV [START_REF] Ashish | Basic Electronics Engineering & Devices[END_REF], an appreciable number of electrons in the valence band will jump the gap and reach the conduction band (Figure 12345678). This behavior can be described by the following equation [START_REF] Misiakos | Accurate measurements of the silicon intrinsic carrier density from 78 to 340 K[END_REF]:

𝑛 𝑖 = 𝐴 1/2 𝑇 3/2 exp (- 𝐸 𝑔 2𝑘 𝐵 𝑇 )
where A is a constant depending on the characteristics of the valence and conduction bands of the material, T is the absolute temperature, and k B is the Boltzmann constant. Thus, there will be free electrons populating in conduction band and the material becomes conductive when the temperature was increased. The modern theory of semiconductor physics is based on quantum mechanics to explain the movement of charge carriers in a crystal lattice. Depending on which type of charge carriers (electrons and holes) the material is conducting, the semiconductor can be categorized into 3 types: n-type (electron conduction), p-type (hole conduction) and ambipolar type (both electron and hole conduction). The semiconductor materials can be doped under precise conditions to control the charge carrier concentration and regions of p-and n-type dopants to realize hole conduction and electron conduction (Figure 123456789).

xx x The illustration of impurity doping to realize n-type or p-type semiconductor. [START_REF]The doping of semiconductor[END_REF] In the 1954, researchers discovered that polycyclic aromatic compounds can form semiconducting charge-transfer complex salts with halogens. Electrical conductivity of 0.12 S/cm was observed in perylene-iodine complex. [START_REF] Naarmann | Polymers, Electrically Conducting[END_REF] This finding showed that materials could carry charge. In theory, carbon is a Group IV atom with four valence electrons, which can form four bonds through orbital hybridization in three forms, sp, sP2, and sp 3 (Figure 1-10). The sP2 orbital is formed by hybridization of s, px and py orbitals along a plane separated from each other by 120 degrees while the remaining pz orbitals (out of plane) form a delocalized electron cloud. This is called π-

xx xi
conjugation. When a π-bond is formed, there are two orbitals formed, one bonding orbital (π orbital) and one anti-bonding (π*). The bonding orbital will be occupied by two electrons with spin-up and -down, while the anti-bonding orbital is unoccupied. When many π-bonds are conjugated together, for example, π-conjugated polymers, these orbitals will split and form two continuous energy level bands as shown in Figure 1-11 and the π electrons will fill the energy levels from lowest to highest.

The frontier orbital of the π-orbitals is called HOMO (the highest occupied molecular orbital) and the frontier orbital of the π* orbitals is called LUMO (the lowest occupied molecular orbital). This concept is similar to the top of the valence band and the bottom of the conduction band in inorganic semiconductors. As π-conjugation grows larger, the HOMO-LUMO band gap becomes smaller. If the band gap is small enough (< 3.0 eV), the organic material starts to have semiconducting properties, just like the inorganic semiconductors. For organic semiconductors, the HOMO and LUMO levels determine the type of charge carriers they can conduct. An organic semiconductor with a high HOMO level tends to transport positive holes and it is called a p-type semiconductor or an electron donor material, a term widely used for OSCs. In contrast, an organic semiconductor with a low LUMO level favors the transport of negative electrons and behaves as an n-type semiconductor or an electron acceptor material. Previous studies have already shown that a LUMO level that is close or below -4.0 eV is needed to stabilize the electron transport in the organic semiconductor. [START_REF] Li | High-Mobility n-Type Conjugated Polymers Based on Electron-Deficient Tetraazabenzodifluoranthene Diimide for Organic Electronics[END_REF][START_REF] Jones | Tuning Orbital Energetics in Arylene Diimide Semiconductors. Materials Design for Ambient Stability of n-Type Charge Transport[END_REF] Similarly, a HOMO level close or below -5.0 eV is also required for hole transport in organic semiconductor. [START_REF] Li | Solution processable poly(2,5-dialkyl-2,5-dihydro-3,6-dicxl 2-thienyl-pyrrolo[3,4-c]pyrrole-1,4-dione) for ambipolar organic thin film transistors[END_REF][START_REF] Sun | Record High Electron Mobility of 6.3 cm 2 V -1 s -1 Achieved for Polymer Semiconductors Using a New Building Block[END_REF] The HOMO and LUMO levels of an organic semiconductor are governed by the chemical functional groups in the π-conjugation. An electron deficient or electron-accepting moiety would decrease the energy levels such as carboxyl groups. On the other hand, an electron rich or electron-donating moiety would increase the energy levels such as alkoxyl groups. Due to the variety of functional groups available for organic compounds, the development of organic xx xii semiconductors becomes promising and diversified. In particular polymeric semiconductors have the merits of high mechanical robustness and fully compatibility with printing technology and therefore receive a lot of research attention. [START_REF] Guo | Recent Progress in High Mobility Polymer Semiconductors for Organic Thin Film Transistors[END_REF] For charge transport within π-conjugated polymer semiconductors, two aspects can be looked at: i) intrachain transport and ii) interchain transport (Figure 1-12). The intrachain charge transport mainly depends on the effective π-conjugation length along the polymer backbone. Generally high coplanarity along the backbone will result in effective π-conjugation and hence efficient intrachain charge transport. The interchain charge transport is determined by the interchain distance and the charge transport between polymer chains is highly anisotropic. The most favored direction is along the π-π stacks, which usually has the shortest interchain distance among all directions and it is the most beneficial for charge hopping between chains. In general, since the intrachain transport is much faster than the interchain transport, interchain charge hopping is usually the rate limiting step. [START_REF] Kim | Flexible, highly efficient all-polymer solar cells[END_REF] as donor material in PSCs. [START_REF] Wan | 8% Efficiency Polymer Solar Cells Based on PTB7-Th and PC71BM via Binary Solvent Additives Treatment[END_REF][START_REF] He | Singlejunction polymer solar cells with high efficiency and photovoltage[END_REF] Nowadays this low-bandgap polymer has been a research focus and served as a benchmark material in this field. [START_REF] Lu | Recent Advances in Bulk Heterojunction Polymer Solar Cells[END_REF] Numerous structural parameters such as side chain length and molecular weight, and fabrication conditions such as solvent, additives and post-treatments were engineered to achieve higher PCE and establish more accurate structure-property relationships. [START_REF] Wu | n-Type Water/Alcohol-Soluble Naphthalene Diimide-Based Conjugated Polymers for High-Performance Polymer Solar Cells[END_REF][START_REF] Etxebarria | Inverted vs standard PTB7:PC70BM organic photovoltaic devices. The benefit of highly selective and extracting contacts in device performance[END_REF][START_REF] Chen | Single-Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency[END_REF][START_REF] He | Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells[END_REF] Due to these efforts and guidance, researchers were able to discover more donor materials and now the PCE of PSC has reached 11.7%. [START_REF] Zhao | Efficient organic solar cells processed from hydrocarbon solvents[END_REF] xx xvi the visible and infrared region. [START_REF] Falke | Raman spectroscopic identification of fullerene inclusions in polymer/fullerene blends[END_REF] Besides, several studies have shown that fullerene will undergo dimerization under photo stress, which also hurts the device's long-term stability. [START_REF] He | Fullerene derivative acceptors for high performance polymer solar cells[END_REF][START_REF] Heumueller | Morphological and electrical control of fullerene dimerization determines organic photovoltaic cxl iii stability[END_REF] In this regard, alternative systems with non-fullerene acceptors attracted intensified research attention in recent years. [START_REF] Kang | From Fullerene-Polymer to All-Polymer Solar Cells: The Importance of Molecular Packing, Orientation, and Morphology Control[END_REF][START_REF] Fan | High-Performance Nonfullerene Polymer Solar Cells based on Imide-Functionalized Wide-Bandgap Polymers[END_REF][START_REF] Zhao | Fullerene-Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability[END_REF][START_REF] Liu | Fast charge separation in a non-fullerene organic solar cell with a small driving force[END_REF]Among them, all-polymer solar cells (all-PSCs), consisting of polymer donors (PD) and polymer acceptors (PA), have recently been studied extensively as replacement for conventional fullerene based PSCs. [START_REF] Kim | Flexible, highly efficient all-polymer solar cells[END_REF][START_REF] Hwang | Side chain engineering of n-type conjugated polymer enhances photocurrent and efficiency of all-polymer solar cells[END_REF][START_REF] Li | Polymer solar cells with diketopyrrolopyrrole conjugated polymers as the electron donor and electron acceptor[END_REF][START_REF] Mori | Highly efficient charge-carrier generation and collection in polymer/polymer blend solar cells with a power conversion efficiency of 5.7%[END_REF][START_REF] Earmme | All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor[END_REF][START_REF] Hwang | 7.7% Efficient All-Polymer Solar Cells[END_REF][START_REF] Jung | Fluoro-Substituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%[END_REF][START_REF] Lee | High-Performance All-Polymer Solar Cells Via Side-Chain Engineering of the Polymer Acceptor: The Importance of the Polymer Packing Structure and the Nanoscale Blend Morphology[END_REF][START_REF] Choi | Importance of Electron Transport Ability in Naphthalene Diimide-Based Polymer Acceptors for High-Performance, Additive-Free, All-Polymer Solar Cells[END_REF] For example, Figure 1-15a represents the dramatically increasing trend in the number of SCI publications regarding all-PSCs. In accordance with these trends, the PCE values of all-PSCs have risen from 2% to 8% over the last three years (Figure 1-15b). Currently most of the acceptor polymers are based on the electron-deficient building block naphthalene diimide (NDI)

xx xiii

and their all-PSCs have already achieved high PCEs over 9% with the efforts of material and device optimization. [START_REF] Jo | Effect of Molecular Orientation of Donor Polymers on Charge Generation and Photovoltaic Properties in Bulk Heterojunction All-Polymer Solar Cells[END_REF][START_REF] Long | Polymer Acceptor Based on Double B←N Bridged Bipyridine (BNBP) Unit for High-Efficiency All-Polymer Solar Cells[END_REF][START_REF] Jung | Controlling Molecular Orientation of Naphthalenediimide-Based Polymer Acceptors for High Performance All-Polymer Solar Cells[END_REF][START_REF] Li | High Performance All-Polymer Solar Cells by Synergistic Effects of Finecxl v Tuned Crystallinity and Solvent Annealing[END_REF][START_REF] Lee | Side Chain Optimization of Naphthalenediimide-Bithiophene-Based Polymers to Enhance the Electron Mobility and the Performance in All-Polymer Solar Cells[END_REF][START_REF] Gao | All-Polymer Solar Cells Based on Absorption-Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%[END_REF] Despite all these efforts, there is still a non-negligible gap in PCE between fullerene-based PSCs and all-PSCs. Specifically, the J SC for PSCs can be up to 20 mA/cm 2 and only around 12 mA/cm 2 for all-PSCs. [START_REF] Kang | From Fullerene-Polymer to All-Polymer Solar Cells: The Importance of Molecular Packing, Orientation, and Morphology Control[END_REF][START_REF] Yan | Third-generation solar cells: a review and comparison of polymer:fullerene, hybrid polymer and perovskite solar cells[END_REF] Similarly, the FF for PSCs usually falls in the range of 60-80% while for all-PSCs the FF is rarely over 55%. [START_REF] Kang | From Fullerene-Polymer to All-Polymer Solar Cells: The Importance of Molecular Packing, Orientation, and Morphology Control[END_REF][START_REF] Yan | Third-generation solar cells: a review and comparison of polymer:fullerene, hybrid polymer and perovskite solar cells[END_REF] Different from the fullerene/polymer system, the charge transport is fully anisotropic in polymer/polymer blends, and therefore enhancing the electron mobility of acceptor and matching the molecular orientation of donor and acceptor as face-to-face at the interface become keys for improving charge carrier generation, charge carrier transport, and the resulting PCE of all-PSCs. [START_REF] Gregg | Entropy of Charge Separation in Organic Photovoltaic Cells: The Benefit of Higher Dimensionality[END_REF] In addition, precise control of the kinetically trapped BHJ morphology should be achieved in a rational manner. Different from fullerene/polymer systems, optimizing the BHJ morphology of polymer/polymer blends is particularly challenging due to the entropic penalty caused by the presence of long PA chain, so it is of great importance to control the molecular weight of both PD and PA to realize highly crystalline domains of PD and PA as well as well-intermixed D-A phases.

One of the reasons accountable for the low FF in all-PSCs is the inferior electron transport property in

xx xvi ii
those NDI-based polymers. For example, the space charge limited current (SCLC) measured electron mobility in PC 71 BM is usually around 5×10 -3 cm 2 /V s [START_REF] You | A polymer tandem solar cell with 10.6% power conversion efficiency[END_REF][START_REF] Zhang | Efficient Polymer Solar Cells Based on Benzothiadiazole and Alkylphenyl Substituted Benzodithiophene with a Power Conversion Efficiency over 8%[END_REF] while that of the NDI-based polymers is usually on the order of 10 -4 cm 2 /V s. [START_REF] Earmme | All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor[END_REF][START_REF] Hwang | 7.7% Efficient All-Polymer Solar Cells[END_REF][START_REF] Gao | All-Polymer Solar Cells Based on Absorption-Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%[END_REF] On the other hand, NDI polymers have been found to

Π-conjugated Polymers for Organic Thin Film Transistors

Charge mobility is one of the most important parameters to evaluate the charge transport property of a material. Organic thin film transistor (OTFT) is a useful tool to test the charge carrier mobility extrinsically. In the same time, OTFTs can be used for various applications such as logic circuits, biosensors, photodetectors and flexible displays. By controlling the gate voltage, the current from source to drain can be adjusted for the transistor to function as a swtich. In the last few years, several new polymer semiconductors with mobilities above 1 cm 2 V -1 s -1 have been reported [START_REF] Bronstein | Thieno[3,2-b]thiophene-Diketopyrrolopyrrole-Containing Polymers for High-Performance Organic Field-Effect Transistors and Organic Photovoltaic Devices[END_REF][START_REF] Chen | Highly π-Extended Copolymers with Diketopyrrolopyrrole Moieties for High-Performance Field-Effect Transistors[END_REF][START_REF] Ha | 2,5-Bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)dione-Based Donor-Acceptor Alternating Copolymer Bearing 5,5′-Di(thiophen-2-yl)-2,2′biselenophene Exhibiting 1.5 cm2•V-1•s-1 Hole Mobility in Thin-Film Transistors[END_REF][START_REF] Sonar | High mobility organic thin film transistor and efficient photovoltaic devices using versatile donoracceptor polymer semiconductor by molecular design[END_REF], outperforming amorphous silicon (0.1~1 cm 2 V -1 s -1 ). [START_REF] Snell | Application of amorphous silicon field effect transistors in integrated circuits[END_REF] xl Most of these high mibilities were achieved with polymers having alternating electron donating (D) and accepting (A) building blocks in the backbone. [START_REF] He | New building blocks for π-conjugated polymer semiconductors for organic thin film transistors and photovoltaics[END_REF][START_REF] Guo | Designing π-conjugated polymers for organic electronics[END_REF] These D-A polymers also have larger conjugated units. The strong intermolecular static attraction between backbones due to the D-A interaction and large overlapping area gives a much closer π-π stacking distance, which dramatically facilitates the inter-chain charge hopping. Thus, the D-A strategy has been widely adopted for developing high mobility π-conjugated polymers.

For complementary logic applications, p-type and n-type transistors with balanced hole and electron mobilities are required. While many high-performance p-type polymer semiconductors have been reported, high performance n-type unipolar polymer semiconductors are rare. Currently there are 4 n-type polymer semiconductors with electron mobility higher than 1 cm 2 V -1 s -1 listed in Figure 1234567891011121314151617.

For example, the widely investigated P(NDI2OD-T2) developed by Yan et al. achieved electron mobility as high as 0.85 cm 2 V -1 s -1 in top gate bottom contact (TGBC) OTFTs. [START_REF] Yan | A high-mobility electron-transporting polymer for printed transistors[END_REF] In addition, the electron mobility can be improved to up to 3 cm 2 V -1 s -1 by controlling the defect amount in the backbone. [START_REF] Matsidik | Defect-free Naphthalene Diimide Bithiophene Copolymers with Controlled Molar Mass and High Performance via Direct Arylation Polycondensation[END_REF] Lei et.al. reported a PPV-like acceptor based polymer, BDOPV-2T, which displayed unipolar electron mobility up to 1.75 cm 2 V -1 s -1 . Recently, our group reported two electron deficient building blocks: pyridine-flanked diketopyrrolopyrrole (DPPy) and thiophene-S,S-dioxidized indophenine (IDTO). The DPPy-BT copolymer, PDBPyBT, showed record high electron mobility up to 6.3 cm 2 /V s. These 4 polymers represent best n-type polymers available for OTFTs, while ptype polymer semiconductors can already achieve hole mobility up 52.7 cm 2 /V s. [START_REF] Luo | General Strategy for Self-Assembly of Highly Oriented Nanocrystalline cxl vii Semiconducting Polymers with High Mobility[END_REF] xli

3,7-bis(2-oxoindolin-3-ylidene)-5,7-dihydropyrrolo[2,3-f]indole-2,6(1H,3H)-dione (IBDP) is
designed and synthesized. The resultant IBDP-based polymers will be characterized to study the optical and electronic properties. A suitable donor material and the standard device configuration will be used to fabricate the all-PSCs to investigate the photovoltaic properties of the polymers.

2) A new series of ester side chains will be designed and synthesized for IBDF-based polymers with enhanced solubility. The side chain for IBDF-based polymers will be engineered to optimize the xlii i electron transport property and then the resultant polymers will be used with PTB7-Th in active layer of all-PSCs. The standard device configuration will be used in this work. The structureproperty relationship will be studied using several tools, which include EQE, XRD, AFM and SCLC measurement.

3) A novel electron-deficient building block, dihydroxylnaphthalene diimide (NDIO), will be used for the first time to form a D-A polymer towards air-stable unipolar n-type charge transport.

With addition of alkoxy side chain, the LUMO and HOMO levels of the NDIO polymers are substantially lowered, yielding larger HOMO/LUMO offsets in all-PSCs, which is believed to help facilitate the charge generation and supress bimolecular recombination. The polymers will first be tested in OTFTs to evaluate the electron transport property in nitrogen and air, respectively, to test the performance robustness and stability in air. The all-PSCs will be fabricated using the NDIO polymers as PA and PTB7-Th as PD in the standard device configuration. The structure-property relationship will be studied using several techniques, which includes external quantum efficiency (EQE), AFM and XRD and SCLC measurement.

The thesis is organized as follows:

1) In chapter 2, the design and synthesis of a novel electron deficient building block (IBDP) will be described and two IBDP polymers (P1 and P2). The resultant polymers will then be characterized using several techniques such as atomic force microscope (AFM), X-ray diffraction (XRD), UV-Vis absorption spectroscopy and cyclic voltammetry (CV) will be used to study the optical and electronic properties. Then the polymer will be used for bottom-gate bottom-contact (BGBC)

OTFTs to investigate their charge transport properties. Furthermore, the polymers will be used

xli v
as acceptor in all-PSCs with P3HT as the donor and as donor in OSCs with PC 71 BM as acceptor.

Only preliminary results will be presented due to the limited amount of materials.

2) In chapter 3, a series of ester side chains will be synthesized and used for the copolymers of IBDF and bithiophene (BT) to yield n-type dominant solution-processable IBDF-based polymers.

First, the optical and electrochemical properties of these polymers will be systematically studied using UV and CV. Then the charge transport properties of these polymers will be evaluated in bottom-gate bottom-contact (BGBC) OTFTs. The polymers then will be mixed with PTB7-Th in a standard device structure: ITO/PEDOT:PSS/Polymer blend/LiF/Al to inversitgate the photovoltaic properties. Finally, SCLC, external quantum efficiency (EQE), AFM and XRD will be used to study the structure-property relationships.

3) In chapter 4, a new building block, dihydroxylnaphthalene diimide (NDIO), will be used to form two NDIO-based unipolar n-type polymers. The polymers demonstrated deeper HOMO and LUMO levels. The polymers then will be mixed with PTB7-Th in a standard device structure:

ITO/PEDOT:PSS/Polymer blend/LiF/Al. The all-PSCs will be characterized and compared with the bench mark PA material, P(NDI2OD-T2). OTFTs, SCLC, external quantum efficiency (EQE), AFM and XRD will be used to study the structure-property relationships.

4) In chapter 5, the thesis work will be concluded and then some future work will be proposed for further study to this work.

xlv

Chapter 2. Synthesis and Characterization of IBDP-based Polymers

Part of this chapter was published in Yinghui He, Chang Guo, Bin Sun, Jesse Quinn and Yuning Li, Chem. Commun., 2015, 51, 8093-8096.

Introduction

D-A polymers are useful materials for low-cost organic electronics such as organic thin film transistors (OTFTs) and organic photovoltaics (OPVs) due mainly to their excellent charge transport properties and fine-tuned HOMO/LUMO levels. [START_REF] Li | Polymer solar cells[END_REF][START_REF] Guo | Recent Progress in High Mobility Polymer Semiconductors for Organic Thin Film Transistors[END_REF][START_REF] Li | High mobility diketopyrrolopyrrole (DPP)-based organic semiconductor materials for organic thin film transistors and photovoltaics[END_REF][START_REF] Boudreault | Processable low-bandgap polymers for photovoltaic applications[END_REF] In recent years, intensified research effort has been devoted to the development of novel electron-rich and -deficient building blocks in order to enhance the charge carrier mobility for a wider range of application of polymer semiconductors. While there are numerous electron-rich building blocks reported in literature, there are only a handful electrondeficient building blocks available. [START_REF] He | New building blocks for π-conjugated polymer semiconductors for organic thin film transistors and photovoltaics[END_REF][START_REF] Baeg | Toward Printed Integrated Circuits based on Unipolar or Ambipolar Polymer Semiconductors[END_REF] Recently our group reported an novel electron deficient ). The two polymers will be first characterized by UV-Vis-IR absorption spectroscopy, cyclic voltammetry and used for BGBC OTFTs. Besides, the structure-property relationship will be investigated using AFM and XRD. Finally the two polymers will be used as acceptor in all-PSCs with P3HT as donor material and preliminary results will be presented.

Results and Discussion

We started our study by conducting computational simulations on two model compounds: IBDF-Me and IBDP-Me (Figure 2-2), whose substituents are methyl groups for simplifying the calculation. The simulation results showed that IBDP-Me has a very small dihedral angle (~10°) between an indolin-2one unit and the pyrroloindoledione core, which is comparable to that of IBDF-Me (~8°). This indicates that IBDP has very similar coplanarity with IBDF. We suspected one of the intermediates (Figure 2-3) of this reaction was very unstable, which might have failed the reaction. Therefore, we turned to Route 2, whose details are given in Scheme 2-1. First, p-phenylenediamine was N,N'-dialkylated with dodecyl bromide in ethanol to form 1, which was used to prepare 4 following a similar synthetic route reported previously. [START_REF] Rumer | BPTs: thiophene-flanked benzodipyrrolidone conjugated polymers for ambipolar organic transistors[END_REF] The synthesis of compound 6 was usually done by heating the isatin in hydrazine. In our case, however, such treatment only yielded the intermediate hydrazine. This was believed to be caused by to the steric effect induced by the bulky side chains since a similar isatin with a larger spacer group between isatin and alkyl branches was converted into oxindole in hydrazine. [START_REF] Cao | N-Fused BDOPV: a tetralactam derivative as a building block for polymer field-effect transistors[END_REF] Inspired by a two-step reduction reported in literature [START_REF] Lötter | Synthesis of the dibenzopyrrocoline alkaloid skeleton: indolo[2,1-a]isoquinolines and related analogues[END_REF],

IBDF IBDP

we first chlorinated 6-bromo-1-(2-decyltetradecyl)indoline-2,3-dione with PCl 5 to give the showed a blue-shift in absorption from solution (711 nm) to the film (698 nm), which could be caused by the pre-aggregation in the solution. [START_REF] Lei | Influence of alkyl chain branching positions on the hole mobilities of polymer thin-film transistors[END_REF] The adsorption coefficients of the peak for film absorption were determined to be 1.3×10 6 cm -1 and 1.1×10 6 cm -1 for P1 and P2 respectively, which are an order higher than the common donor materials that are being used for OSCs. For example, the adsorption coefficient for PTB7-Th is only on the order of 10 5 cm -1 . [START_REF] Wan | 8% Efficiency Polymer Solar Cells Based on PTB7-Th and PC71BM via Binary Solvent Additives Treatment[END_REF][START_REF] He | Singlejunction polymer solar cells with high efficiency and photovoltage[END_REF] The optical band gaps of the polymer films are calculated to be 1.23 eV for P1 and 1.22 eV for P2 using the absorption onsets. As shown in Figure 2-6, both polymer shown extended absorption into the NIR region up to 1100 nm. This is probably due to the nitrogen donating effect that raised the HOMO levels of the polymers. Cyclic voltammetry (CV) was used to reveal electrochemical property of the polymers (Figure 234567). By using the oxidative onset potentials, HOMO levels were calculated to be -5.60 eV and -5.71 eV for P1 and P2, respectively. LUMO levels were calculated from the reduction onset potentials to be -3.71 eV and -3.70 eV for P1 and P2, respectively. Both the HOMO and LUMO levels are raised compared to the IBDF analogues by 0.1-0.2 eV. [START_REF] Zhang | Bis(2-oxoindolin-3-ylidene)benzodifuran-dione-based D-A polymers for high-performance n-channel transistors[END_REF][START_REF] Zhou | Balanced Ambipolar Organic Thin-Film Transistors Operated under Ambient Conditions: Role of the cxl ix Donor Moiety in BDOPV-Based Conjugated Copolymers[END_REF] Based on these HOMO/LUMO levels, P1 and P2 are suitable materials for ambipolar charge transport in OTFTs and potentially can be both the donor and acceptor materials in OSCs provided the compatible counterpart.

lii 2-1). For devices based on P1, the best performance with hole mobility of 0.19 cm 2 V -1 s -1 and electron mobility of 0.088 cm 2 V -1 s -1 was achieved for the 150 °C-annealed films (Figure 2345678). Annealing at a higher temperature of 200 °C led to a slight increase in the electron mobility (up to 0.089 cm 2 V -1 s -1 ), but a significant drop in the hole mobility (0.057 cm 2 V -1 s -1 ). For devices based on P2, the best overall performance with hole/electron mobilities of 0.10 cm 2 V -1 s -1 / 0.075 cm 2 V -1 s -1 was obtained for the 200 °C-annealed polymer films (Figure 2345678). Annealing at a higher temperature of 250 °C did not further improve the device performance. The decent charge transport properties of these two polymers and their considerable absorption in the NIR region may enable the NIR detection application for P1 and P2, which is limited by the lack of suitable materials that can response to NIR signals. [START_REF] Zhu | Enhanced near-infrared photoresponse of organic phototransistors based on single-component donor-acceptor conjugated polymer nanowires[END_REF][START_REF] Li | High-Performance Near-Infrared Phototransistor Based on n-Type Small-Molecular Organic Semiconductor[END_REF][START_REF] Qi | High-Performance Thermally Stable Organic Phototransistors Based on PSeTPTI/PC61BM for Visible and Ultraviolet Photodetection[END_REF][START_REF] Hendriks | Small-Bandgap Semiconducting Polymers with High Near-Infrared Photoresponse[END_REF][START_REF] Peng | High performance near infrared photosensitive organic field-effect transistors realized by an organic hybrid planarbulk heterojunction[END_REF] liv We noticed that P1 and P2 showed lower charge transport performance than some of the IBDF based polymers. [START_REF] Lei | A BDOPV-Based Donor-Acceptor Polymer for High-Performance n-Type and Oxygen-Doped Ambipolar Field-Effect Transistors[END_REF][START_REF] Zhang | A bis(2oxoindolin-3-ylidene)-benzodifuran-dione containing copolymer for high-mobility ambipolar transistors[END_REF] One possible reason for their lower mobility is that an optimal side chain combination is not reached in these polymers. P1 and P2 showed greatly improved solubility due to the presence of four large side chains on IBDP. However, the excessively strong side chain interaction in the solid state might hinder the - stacking of the polymer main chains. Transmission X-ray diffraction (XRD) measurement was used to investigate the packing of polymer flakes of both polymers. P1 and P2 showed intense primary diffraction peaks at 2θ = 3.26° and 3.51°, which correspond to the interlayer lamellar d-spacing of 2.71 nm and 2.52 nm, respectively. The weak diffraction peak at 2θ = 25.04° for P1 is originated from the π-π stacking distance of polymer chains, which was calculated to be 0.355 nm. No noticeable π-π stacking peak was observed for P2. The presence of four long side chains on IBDP might hinder the π-π stacking and lead to the lower carrier mobility of these IBDP polymers compared to the IBDF polymers as discussed previously. The very weak diffraction peaks corresponding to the - stacking distances for both polymers might substantiate this side chain interference. Using shorter side chains may help strengthen the - interaction. [START_REF] Lei | A BDOPV-Based Donor-Acceptor Polymer for High-Performance n-Type and Oxygen-Doped Ambipolar Field-Effect Transistors[END_REF][START_REF] Lei | Influence of alkyl chain branching positions on the hole mobilities of polymer thin-film transistors[END_REF] Furthermore, most of the high mobility IBDF polymers reported so far bear the 4-octadecyldocosyl side chain, which has a farther distance from the polymer backbone compared to the 2-decyltetradecyl side chain in P1 and P2. The branching point of branched side chains was reported to have a great impact on the ordering of the polymer chains and the carrier mobility. [START_REF] Dou | Systematic Investigation of Side-Chain Branching Position Effect on Electron Carrier Mobility in Conjugated Polymers[END_REF][START_REF] Lei | Influence of alkyl chain branching positions on the hole mobilities of polymer thin-film transistors[END_REF][START_REF] Chen | Influence of side chain length and bifurcation point on the crystalline structure and charge transport of diketopyrrolopyrrolequaterthiophene copolymers (PDQTs)[END_REF] The use of branched side chains with a lvi larger spacer between the branching point and the backbone such as 4-decylhexadecyl or four straight alkyl side chains on IBDP may lead to better charge transport performance. Following this work, Cao et al. used 4-decyltetradecyl as the side chain on IBDP and improved the mobility significantly. [START_REF] Cao | N-Fused BDOPV: a tetralactam derivative as a building block for polymer field-effect transistors[END_REF] The surface morphology of P1 and P2 thin films spin-coated on dodecyltrichlorosilane (DDTS) modified SiO 2 /Si wafer substrates was examined by atomic force microscopy (Figure 18). P1 thin films showed fibre-like domains, which grew slightly as the annealing temperature increased from 100 °C to 150 °C and remained similar at an annealing temperature of 200 °C. In general, P2 thin films showed poorly defined domains compared to P1 films, indicative of the less ordered chain packing of P2. device based on P2 achieved the best PCE at 2.14%. As preliminary optimization, we tuned the D/A ratio for P1:P3HT to improve the device performance. As shown in Figure 2-13 and Table 2-3, the optimal D/A ratio was determined to be 1:1.5, where the best PCE of 3.38% was realized. This is the highest PCE that has been obtained from P3HT-based all-PSCs by far. [START_REF] Zhou | All-Polymer Solar Cells from Perylene Diimide Based Copolymers: Material Design and Phase Separation Control[END_REF][START_REF] Mori | Polymer/Polymer Blend Solar Cells cl Improved by Using High-Molecular-Weight Fluorene-Based Copolymer as Electron Acceptor[END_REF][START_REF] Schubert | Influence of Aggregation on the Performance of All-Polymer Solar Cells Containing Low-Bandgap Naphthalenediimide Copolymers[END_REF] EQE spectrum for the same cell was shown in Figure 2-13. Owing to the complementary absorption of these two polymers to each other, the spectrum showed full coverage over the visible region as well as part of the NIR region (up to 1000 nm). However, the EQE is much lower those of the P3HT:PCBM cells (up to 60%) [START_REF] Li | High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends[END_REF] and NDI polymer based all-PSCs (usually 45% to 55%) [START_REF] Kang | From Fullerene-Polymer to All-Polymer Solar Cells: The Importance of Molecular Packing, Orientation, and Morphology Control[END_REF][START_REF] Benten | Recent research progress of polymer donor/polymer acceptor blend solar cells[END_REF], indicating blend morphology is probably suboptimal. The calculated J SC from the EQE spectrum is 8.2 mA/cm 2 , which is within 5% error of the measured value. As a preliminary demonstration, we also tested P1 as donor in OSCs with PC 71 BM (whose HOMO/LUMO levels are -5.9 eV and -4.1 eV respectively) as acceptor. A decently high PCE of 5.3% with a high V OC of 0.9 V was obtained from the best device (Figure 2 -14), which

V -20 V -40 V -60 V -80 V -100 V V G 0 V 20 V 40 V 60 V 80 V 100 V V G V DS (V) I DS (
shows the great potential of this polymer as donor for OSCs. The blend film (1:1 wt ratio) was spin-coated at 1300 rpm using a solution (25 mg/mL) in o-DCB. All devices used P3HT as the donor and the same device configuration: ITO/PEDOT:PSS/Polymer blend/LiF/Al. The active layers were spin-coated under the same conditions (1300 rpm and 25 mg/mL). The numbers in the parenthesis are average values for 4 devices on the same substrate.

lxi

Summary and Future Direction

In conclusion, a novel electron deficient building block, IBDP, is designed, synthesized, and incorporated into D-A polymers. With lactam groups replacing the lactone groups, IBDP gets two extra side chains to yield good solution processability, which is requisite for the fabrication of OSCs. On the other hand, the donating effect of lactam groups has raised the LUMO levels of P1 and P2 to ~-3.7 eV from -4.0 eV of those IBDF-based polymers. With a raised LUMO level, the electron injection barrier now is higher and electron transport will become less favourable. [START_REF] Jones | Tuning Orbital Energetics in Arylene Diimide Semiconductors. Materials Design for Ambient Stability of n-Type Charge Transport[END_REF][START_REF] De Leeuw | Stability of n-type doped conducting polymers and consequences for polymeric microelectronic devices[END_REF] Therefore IBDP-based lxii polymers exhibited balanced ambipolar semiconductor performance in OTFTs with high electron (up to 0.10 cm 2 V -1 s -1 ) and hole (up to 0.19 cm 2 V -1 s -1 ) mobility. Both polymers showed enhanced absorption with absorption coefficient as high as 1.3×10 6 cm -1 and a broad absorption up to 1100 nm according to the UV-Vis-NIR absorption spectra. The high absorption coefficient and broad absorption range in Visible and NIR region are extremely desired for OSC application. On the other hand, the relatively deep HOMO levels and high LUMO levels would also help to increase the V OC of the resultant solar cells. Given these features, these two IBDP-based polymers were used as acceptor for all-PSCs in combination with regioregular P3HT as donor, which has suitable HOMO/LUMO levels.

Initially all-PSC device based on P1 exhibited better PCEs up to 2.56% than 2.14% from the cell based on P2. We furthered our study by optimizing the D/A ratio of P1:P3HT. A PCE of 3.38% was obtained from the cell with a D/A ratio of 1:1.5, which is the highest value that has been obtained from P3HTbased all-PSCs. The EQE spectrum of the same cell showed a full coverage over the Vis-NIR region (up to 1000 nm) due to the complementary absorption. Furthermore, we also tested P1 as donor in OSC using PC 71 BM as acceptor. A high PCE of 5.3% was obtained from the non-optimized device, indicating the great potential of P1 as donor in OSCs. Further study is limited by the available amount of materials. For future direction, the synthesis of the IBDP monomers need to be optimized to achieve higher yield and cleaner chemistry (thiophenol was used in the synthesis). Second of all, the extended absorption into NIR may enable the photo-detection applications such as phototransistor in NIR region, which has been rarely explored and believed to be a useful technology in various fields. [START_REF] Wang | Highly Polarized Photoluminescence and Photodetection from Single Indium Phosphide Nanowires[END_REF][START_REF] Gong | High-Detectivity Polymer Photodetectors with Spectral Response from 300 nm to 1450 nm[END_REF][START_REF] Konstantatos | Ultrasensitive solution-cast quantum dot photodetectors[END_REF] What is more, these IBDP polymers can be also used in ternary organic solar cells, which is also a new direction for the development of OSCs. In a nutshell, the preliminary results demonstrated that IBDP is a promising electron acceptor building block for constructing ambipolar type of polymer semiconductors, which can be used as both acceptor and donor in OSC applications. lxii i

Experimental

Materials and Characterization

All chemicals were purchased from commercial sources and used without further purification unless specified. 6-Bromo-1-(2-decyltetradecyl)indoline-2,3-dione [START_REF] Yan | Novel stable (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5b[prime or minute]]difuran-2,6(3H,7H)-dione based donor-acceptor polymer semiconductors for n-type organic thin film transistors[END_REF] and (E)-1,2-bis(5-(trimethylstannyl)thiophen-2-yl)ethene [START_REF] Kim | High-Mobility Air-Stable Naphthalene Diimide-Based Copolymer Containing Extended π-Conjugation for n-Channel Organic Field Effect Transistors[END_REF] were synthesized according to the literature.

Computational simulations were performed using density function theory (DFT) calculation with the 6-311G+(d,p) basis set and all the orbital pictures were obtained using GaussView 5.0 software. GPC measurements were performed on a Waters SEC system using 1,2,4-

trichlorobezene

Fabrication and Characterization of OTFT devices

The bottom-contact, bottom-gate configuration was used for all OTFT devices. The preparation procedure of the substrate and device is as follows. A heavily n-doped SiO 2 /Si wafer with ~300 nm-thick SiO 2 was patterned with gold source and drain pairs by conventional photolithography and thermal deposition. Then the substrate was treated with air plasma, followed by cleaning with acetone and isopropanol in an ultra-sonic bath. Subsequently, the substrate was placed in a solution of dodecyltrichlorosilane (DDTS) in toluene (3 % in toluene) at room temperature for 20 min. the substrate was washed with toluene and dried under a nitrogen flow. Then a polymer solution in chloroform (5 mg mL -1 ) was spin-coated onto the substrate at 3000 rpm for 60s to give a polymer film (~40 nm), which was further subject to thermal annealing at different temperatures for 20 min in a glove box. All the OTFT devices have a channel length (L) of 30 µm and a channel width (W) of 1000 µm, and were characterized in the same glove box using an Agilent B2912A Semiconductor Analyser.

Fabrication and Characterization of all-PSC devices

All all-PSC devices were fabricated using the conventional configuration ITO/PEDOT:PSS/PTB7-Th:PA/LiF/Al. ITO glass substrates were sonicated in water, acetone and IPA. Then the substrates were treated with plasma cleaning. A thin layer of PEDOT:PSS (Al 4083) was deposited through spincoating at 4000 rpm and dried subsequently at 150 °C for 20 min in air. Then the substrates were transferred to a nitrogen glove box, where the polymer blend layer (~140 nm) was spin-coated onto the PEDOT:PSS layer. The active layer was formed using a o-dichlorobenzene solution of P3HT:PA (1:1).

Finally, a thin layer of LiF (1 nm) and a layer of Al (100 nm) electrode were deposited in vacuum onto the substrate at P ≈5.0 × 10 -6 Pa. The active area is 0.07 cm 2 . The current density-voltage (J -V) lxv characteristics of the all-PSCs were measured on an Agilent B2912A Semiconductor Analyser with a ScienceTech SLB300-A Solar Simulator. A 450 W xenon lamp and an air mass (AM) 1.5 filter were used as the light source.

Synthetic Procedures

Synthesis of N,N ' -didodecylbenzene-1,4-diamine (1)

To a solution of p-phenylenediamine (4.33 g, 40.0 mmol) in ethanol (120 mL), 1-bromododecane (19.94 g, 80.0 mmol) was added. The reaction mixture was stirred under reflux for 24 h. Upon cooling to room temperature, the reaction mixture was filtered. The filter cake was washed with ethanol and then dried in vacuo to give 1. Yield: 4.45 g (25%). Compound 1 was used immediately for the next step without further purification due to its very poor stability in air. 1 H-NMR (300 MHz, DMSO-d 6 ) δ 6.90

(s, 4H), 3.07 (t, J = 7.0 Hz, 4H), 1.60 -1.48 (m, 4H), 1.24 (br, 36H), 0.85 (t, J = 6.4 Hz, 6H).

Synthesis of N,N'-(1,4-phenylene)bis(N-dodecyl-2-hydroxyacetamide) (2)

lxv i

To a solution of 1 (4.45 g, 10.0 mmol) and triethylamine (5.

Synthesis of (3E,7E)-3,7-bis(6-bromo-1-(2-decyltetradecyl)-2-oxoindolin-3-ylidene)-1,5didodecyl-5,7-dihydropyrrolo[2,3-f]indole-2,6(1H,3H)-dione (7) lxx

To a solution of 4 (0.25 g, 0.45 mmol) and 5 (0.50 g, 0.50 mmol) in acetic acid (6 mL), was added concentrated HCl aqueous solution (0.05 mL). The reaction mixture was heated to reflux and stirred for 24 h. Upon cooling to room temperature, the precipitate was filtered off from the reaction mixture and was further purified by silica gel column chromatography with chloroform : hexane (1:1) to give 6. Yield: 0.232 g (32%).

Introduction

So far for development of novel n-type materials, most efforts have been directed to the development of novel electron deficient building blocks, [START_REF] He | New building blocks for π-conjugated polymer semiconductors for organic thin film transistors and photovoltaics[END_REF][START_REF] Guo | Designing π-conjugated polymers for organic electronics[END_REF][START_REF] Guo | Imide-and Amide-Functionalized Polymer Semiconductors[END_REF] particularly fused ring acceptor building blocks [START_REF] Yuen | Strong acceptors in donor-acceptor polymers for high performance thin film transistors[END_REF] side chain and used it to solubilise an isoindigo-based polymer, achieving much improved hole mobility of up to 2.48 cm 2 V -1 s -1 compared with the alkyl-substituted polymer. [START_REF] Mei | Siloxane-Terminated Solubilizing Side Chains: Bringing Conjugated Polymer Backbones Closer and Boosting Hole Mobilities in Thin-Film Transistors[END_REF] We previously found that branched acetal groups (Scheme 3-1) could be used as solubilising side chains for D-A polymers. [START_REF] Hong | Synthesis and thin-film transistor performance of benzodipyrrolinone and bithiophene donor-acceptor copolymers[END_REF][START_REF] Hong | Dipyrrolo[2,3-b:2′,3′e]pyrazine-2,6(1H,5H)-dione based conjugated polymers for ambipolar organic thin-film transistors[END_REF] lxx iii Several other studies have shown that engineering of alkyl side chains has significant impacts on chain packing, film morphology and hence charge transport properties. [START_REF] Dou | Systematic Investigation of Side-Chain Branching Position Effect on Electron Carrier Mobility in Conjugated Polymers[END_REF][START_REF] Lei | Influence of alkyl chain branching positions on the hole mobilities of polymer thin-film transistors[END_REF][START_REF] Back | Investigation of Structure-Property Relationships in Diketopyrrolopyrrole-Based Polymer Semiconductors via Side-Chain Engineering[END_REF] For example, Lei et al.

conducted a study on the branching position of the branched alkyl chains on the copolymers of isoindigo (IID) and bithiothene (BT). [START_REF] Lei | Influence of alkyl chain branching positions on the hole mobilities of polymer thin-film transistors[END_REF] They found that π-π stacking was hindered by the side chains as the spacer group between the IID unit and the branching point of the side chain was CH 2 or C 2 H 4 (simply, C1 or C2) (m = 1 or 2 in Scheme 3-1). When the branching position was moved away further, the backbone became more exposed so that the π-π stacking distance was reduced, which led to improved charge transport property. The copolymer (IIDDT-C3, Figure 3-1) with a spacer group of C3 (m = 3) lxx iv achieved the highest OTFT performance. We also found that the side chain length and the branching position greatly influenced the crystallinity, morphology, as well as the - stacking distance of a DPPbased polymer, PDQT (Figure 3-1). [START_REF] Chen | Influence of side chain length and bifurcation point on the crystalline structure and charge transport of diketopyrrolopyrrolequaterthiophene copolymers (PDQTs)[END_REF] By increasing the size of the side chain from 2-octyldodecyl (C20) to 2-decyltetradecyl (C24), the crystallinity of the polymer improved, resulting an increase in mobility from 2.10 cm 2 V -1 s -1 to 3.37 cm 2 V -1 s -1 . When the distance of the branching point from the backbone was increased from C1 (for 2-octyldodecyl) to C3 (for 4-decylhexadecyl), the - distance decreased from 0.386 nm to 0.368 nm, leading to further improved mobility up to 6.90 cm 2 V -1 s -1 . processable. [START_REF] Lei | A BDOPV-Based Donor-Acceptor Polymer for High-Performance n-Type and Oxygen-Doped Ambipolar Field-Effect Transistors[END_REF] High electron mobility up to 1.74 cm 2 V -1 s -1 was achieved for this polymer in topgate bottom-contact (TGBC) devices. (The electron mobility is 0.13 cm 2 V -1 s -1 in the BGBC devices, somehow lower than in the TGBC devices.) The same group also studied the branching point position of a series of side chains, C38-C43, and found that the side chain (C40) with a C3 spacer gave the best lxx v charge transport performance. [START_REF] Dou | Systematic Investigation of Side-Chain Branching Position Effect on Electron Carrier Mobility in Conjugated Polymers[END_REF] However, these giant alkyl side chain precursors, alkyl iodide or bromide compounds, are not commercially readily available[136] and the synthesis of these compounds is very tedious, [START_REF] Jian Pei | Compound with branching alkyl chains, method for preparing the same, and use thereof in photoelectric device[END_REF] which would limit their use in large-scale application. We also synthesized PIBDFBT-40 and found that even with such large side chains this polymer still showed poor solubility in common solvents (see below). In this regard, we propose to use halogenated branched alkyl ester compounds as precursors to substitute IBDF to form soluble IBDF-BT copolymers. These halogenated branched alkyl esters can be conveniently prepared using an exemplary two-step route shown in Scheme 3-2. The branching point position and the branch length can be varied by using different commercially readily available starting materials. A C3 spacer was chosen for this series of ester side chains since it has been found optimal lxx vi for achieving high charge transport performance. We synthesized three PIBDFBT polymers with these new branched alkyl ester side chains with different branch length (-C 14 H 29 , -C 16 H 33 and -C 18 H 37 ), which are named P-33, P-37 and P-41 respectively. These polymers will be first fully characterized with several techniques. Then the charge transport properties will be evaluated in OTFTs and finally these polymers will be used as acceptor in all-PSCs with PTB7-Th as donor and the structure-property relationship will be investigated using several techniques such as AFM, XRD and SCLC.

Results and Discussion

The syntheses of all four polymers having branched alkyl ester side chains, P-33, P-37 and P-41 are outlined in Scheme 3-2. Three branched alkyl ester bromides (8a-c) were conveniently synthesized by reacting respective secondary alcohols with 4-bromobutyryl chloride, which was prepared in situ by using 4-bromobutyric acid and thionyl chloride. The two-step yields were around 65%. 8a-c were then reacted with 6-bromoisatin to form the branched alkyl ester-substituted 6-bromoisatins 9a-c. Compared to the C40 branched alkyl side chain, the C37 branched alkyl side chain with the similar size appears to have stronger solubilizing ability. High temperature-gel permeation chromatography (HT-GPC) was used to evaluate the molecular weights of these polymers. The column temperature was set at 140 °C and 1,2,4-trichlorobenzene was used as the eluent. P-33 has a rather low number average molecular weight (M n ) of 16 kDa, which is probably due to its poor solubility in the polymerization medium (CB as the solvent). The molecular weights of P-37 and P-41 are much higher with M n of 40 kDa and 39 kDa, respectively, owing to their better solubility enabled by their larger side chains.

Condensation

PIBDFBT-40 has a similar M n of 40 kDa, but the polydispersity (PDI) is extremely large (8.4), which is thought to be caused by the strong polymer chain aggregation or poorer solubility even at such as high temperature (Figure 3-3). The thermal stability of the polymers was characterized by thermogravimetric analysis (Figure 34). The 5% weight loss temperatures (T -5% ) are 290 °C, 300 °C and 300 °C for P-33, P-37 and P-41, respectively, indicating their decent thermal stability. However, these polymers started to decompose at lower temperatures compared to that of PIBDFBT-40 (T -5% = ~380 C) probably due to the less thermally stable ester groups.

lxx viii

Scheme 3-2 Synthetic route to branched ester-substituted IBDF polymers P-33, P-37 and P-41.

Reaction conditions: i) CH 2 Cl 2 /SOCl 2 /0 °C to r.t., THF/reflux; ii) DMF/K 2 CO 3 /50 °C; iii) AcOH/ptoluenesulfonic acid/115 °C; iv) chlorobenzene/P(o-tolyl) 3 /Pd 2 (dba) 3 /130 °C. 3-1). All three polymers showed a significant redshift in the maximum absorption (λ max ) compared to that of PIBDFBT-40 in solutions and films [START_REF] Lei | A BDOPV-Based Donor-Acceptor Polymer for High-Performance n-Type and Oxygen-Doped Ambipolar Field-Effect Transistors[END_REF].

These results suggest that the ester side chains have an impact on the backbone conjugation. P-41

showed a λ max at ~840 nm in solution and the solid state, which is blue-shifted compared to those of P-33 and P-37. This implies that the backbone of P-41 is less coplanar than those of P-33 and P-37, likely caused by the stronger interactions of its larger side chains with solvent molecules in solution and lxx x between themselves in the film to make the backbone slightly more twisted. [START_REF] Chen | Influence of side chain length and bifurcation point on the crystalline structure and charge transport of diketopyrrolopyrrolequaterthiophene copolymers (PDQTs)[END_REF][START_REF] Hong | 3Z,3'Z)-3,3'-(Hydrazine-1,2-diylidene)bis(indolin-2-one) as clii a new electron-acceptor building block for donor-acceptor [small pi]-conjugated polymers for organic thin film transistors[END_REF] All polymers showed vibronic splitting adsorption peaks at ~750 nm in both dilute solutions and films. In solution, the vibronic splitting adsorption for P-33 is much weaker compared to the other two polymers, which might be due to its low molecular weight. In films, the vibronic splitting adsorption peaks for all polymers became more resolved as compared to those in the solution spectra. This could be attributed to the exitonic-vibronic coupling induced by interchain aggregation in the solid state [START_REF] Spano | The Spectral Signatures of Frenkel Polarons in H-and J-Aggregates[END_REF], indicating a more ordered packing of chains. The optical bandgaps were estimated using the onset absorption wavelengths to be ~1.31 eV for all three polymers, which is the same as that of PIBDFBT-40 [START_REF] Lei | A BDOPV-Based Donor-Acceptor Polymer for High-Performance n-Type and Oxygen-Doped Ambipolar Field-Effect Transistors[END_REF]. It is worth noting that all IBDF-based polymers showed considerable absorption from 350 nm to 600 nm, which can be idea complementary absorption to PTB7-Th in the same region. The absorption coefficient for these polymers (P-33, P-37 and P-41) were determined to be 7.4×10 5 cm -1 , 7.1×10 5 cm - 1 and 6.6×10 5 cm -1 , respectively. Cyclic voltammetry (CV) was used to study the electrochemical properties of the polymers (Figure 3-6 and Table 3-1). All polymers showed both the reductive and oxidative peaks. The highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO and LUMO) levels were estimated using the onset oxidation and reduction potentials. All three polymers have similar HOMO levels (ranging from -5.68 eV to -5.73 eV) and LUMO levels (ranging from -3.85 eV to -3.87 eV). The similar HOMO and LUMO levels of these polymers indicate that side chains have minimal impacts on the electrochemical properties of the polymers. The frontier orbital levels of these polymers fall in the ranges where n-type dominant ambipolar charge transport performance might be observed. [START_REF] Zaumseil | Electron and Ambipolar Transport in Organic Field-Effect Transistors[END_REF][START_REF] Zhao | 25th Anniversary Article: Recent Advances in n-Type and Ambipolar Organic Field-Effect Transistors[END_REF] Moreover, we measured electrochemical characteristic of PTB7-Th with CV as well. With the HOMO/LUMO being -5.36 eV and -3.55 eV respectively, [START_REF] Zhang | Efficient Nonfullerene Polymer Solar Cells Enabled by a Novel Wide Bandgap Small Molecular Acceptor[END_REF] the HOMO and LUMO offsets between the IBDF-based polymers (P-33, P-37 and P-41) and PTB7-Th are larger than 0.3 eV, which made them suitable active layer materials in all-PSCs. [START_REF] Scharber | Design Rules for Donors in Bulk-Heterojunction Solar Cells-Towards 10 % Energy-Conversion Efficiency[END_REF] lxx xi To probe how the length of the alkyl branch (R) will impact the charge transport, these polymers were used as channel semiconductor materials in BGBC OTFT devices using SiO 2 /Si wafer as the substrate.

All three polymers showed n-type dominant ambipolar charge transporting characteristics (Table 3-2).

When the annealing temperature was increased from 100 C to 200 C, the carrier mobility improved gradually for all devices. However, it was found that further increasing the annealing temperature to 250 °C led to performance degradation. Among three polymers, P-37 with C 16 H 33 as the branch showed the best device performance with the average electron mobility of 0.32 cm 2 V -1 s -1 and hole mobility of 0.15 cm 2 V -1 s -1 when the films were annealed at 200 °C (Figure 34567). P-33 showed the average electron and hole mobilities of 0.10 cm 2 V -1 s -1 and 0.051 cm 2 V -1 s -1 , respectively, for the films annealed at 200 °C. The lower performance of P-33 is considered due to its lower molecular weight as well as its poorer thin film morphology (see below). The best performance for P-41 was obtained for the films annealed at 200 °C, with average electron mobility of 0.072 cm 2 V -1 s -1 and hole mobility of 0.027 cm 2 V -1 s -1 .

The poorest charge transport performance of P-41 among three polymers is probably the result of its more twisted back bone as previously discussed (UV-Vis-NIR absorption spectra) and its poor film morphology (see below). To compare the ester chains with alkyl side chains, we also tested PIBDFBT-40 as channel semiconductor in BGBC devices under the same conditions. Ambipolar charge transport was also observed for the devices based on PIBDFBT-40 (Table 3-2). For the devices annealed at 200 °C, the average electron mobility was 0.39 cm 2 V -1 s -1 and the average hole mobility was 0.22 cm 2 V -1 s -1 , which are slightly higher than those of P-37. Our results indicate that ester side chains can be used as solubilizing chains for D-A polymers and the resulting polymers can achieve comparable charge transporting properties to those with alkyl side chains in BGBC OTFTs. The annealing temperature was chosen for the best performance according to the references. To further understand why P-37 showed better charge transport property than P-33 and P-41, we characterized the microstructures of the polymer thin films with atomic force microscopy (AFM, Figure 26) and X-ray diffraction (XRD, Figure 345678). The thin film samples were prepared under the same conditions as those used for the OTFT devices. In the AFM images of the P-33 films, very smooth surfaces with low root mean squared roughness (R q ) of 1.3 nm were observed. However only small grains with poor interconnections can be seen, which might be due to the low molecular weight of P-

lxx xiv

33.

For the P-37 films, well-interconnected large fibre-like domains are observed, which is regarded to be beneficial for the charge transport. [START_REF] He | 3E,7E)-3,7-Bis(2-oxoindolin-3-ylidene)-5,7dihydropyrrolo[2,3-f]indole-2,6(1H,3H)-dione based polymers for ambipolar organic thin film transistors[END_REF]. The as-spun P-41 films showed very large grains and grain boundaries with R q of 23 nm. With increasing annealing temperature, the film morphology slightly improved, but the films are still very rough. The very poor film morphology might be another reason for the poor charge transport performance of P-41. Reflective mode XRD was employed to investigate the chain packing of these polymers in spin-coated thin films (Figure 3456789). The as-spun P-33, P-37 and P-41 films showed the primary diffraction peaks at 2θ = 3.37°, 2.97° and 2.90°, which correspond to their inter-lamellar d-spacing distances of 2.62 nm, 2.97 nm and 3.04 nm, respectively, agreeing with the increasing size of their side chains. Since there are no diffractions corresponding to the co-facial ππ-distance (normally at 2θ = ~20-25), the polymer chains presumably adopted an edge-on orientation motif with respect to the substrate. [START_REF] Winokur | Structural and absorption studies of the thermochromic transition in poly(3-hexylthiophene)[END_REF][START_REF] Sirringhaus | Twodimensional charge transport in self-organized, high-mobility conjugated polymers[END_REF] As the annealing temperature was increased, the intensity of the primary diffraction peaks increased and the secondary diffraction peaks became more visible, indicating that longer range ordering was achieved. That explains why the carrier mobility increased with increasing annealing temperature. The 200 C-annealed P-37 film showed the strongest diffraction peaks and thus the most ordered chain packing, which is also accounted for its best charge transport performance among three polymers.

-20 V -40 V -60 V -80 V -100 V V G 0 V 20 V 40 V 60 V 80 V 100 V V G V DS (
V -20 V -40 V -60 V -80 V -100 V V G 0 V 20 V 40 V 60 V 80 V 100 V V G V DS (
V -20 V -40 V -60 V -80 V -100 V V G 0 V 20 V 40 V 60 V 80 V 100 V V G V DS (

lxx xvi

P-33

R q = 1.3 nm R q = 1.3 nm R q = 1.3 nm 100 C

P-37

R q = 3.6 nm R q = 2.5 nm R q = 2.7 nm

P-41

R q = 23 nm R q = 20 nm R q = 17 nm we were unable to process P-33 into thin film. Figure 3-11 shows the J-V characteristics of the devices measured under AM 1.5G illumination (100 mW cm -2 ). The devices based on P-37 exhibited on average the V OC of 0.72 V, J SC of 1.2 mA cm -2 , and FF of 0.39, corresponding to a PCE of 0.34%. The devices based on P-41 exhibited on average the V oc of 0.57 V, J sc of 0.54 mA cm -2 , and FF of 0.37, corresponding to a PCE of 0.11%. We can clearly tell the low PCEs mainly resulted from the extremely low Jsc and relatively low FF, which is most likely caused by the poor morphology. It has been reported

that post treatments such as solvent annealing and film aging can significantly improve the morphology and device performance. In our case, however, the PCEs of the devices did not get improved by solventannealing nor film-aging (Table 3- [START_REF] Samuel | Increasing the Energy Yield of Generation from New and Renewable Energy Resources[END_REF]. illumination. The blend film was spin-coated at 1500 rpm using a solution in chlorobenzene with a total concentration of 10 mg/mL. Solvent annealing was performed in a Petri dish at room temperature and film aging was performed by storing the device in a glovebox.

lxx xvi ii
To understand why these devices have such poor performance, we investigated the morphology of the PTB7-Th:PA blend by AFM (Figure 3-12). The samples were prepared under the same conditions to emulate the solar cell devices. Both samples showed pretty rough film morphology with root mean xc squared roughness of 4.3 nm for P-37 blend and 5.5 nm for P-41 blend respectively. In addition, largesize and poorly defined fabril structures can be observed in the AFM images with size over 1µm.

Similar observation has been previous reported in literature and usually correlated to large phase separation. [START_REF] Hwang | Side chain engineering of n-type conjugated polymer enhances photocurrent and efficiency of all-polymer solar cells[END_REF][START_REF] Li | High Performance All-Polymer Solar Cells by Synergistic Effects of Finecxl v Tuned Crystallinity and Solvent Annealing[END_REF][START_REF] Wang | Measurement of Exciton Diffusion in a Well-Defined Donor/Acceptor Heterojunction based on a Conjugated Polymer and Cross-Linked Fullerene Derivative[END_REF] [START_REF] He | New building blocks for π-conjugated polymer semiconductors for organic thin film transistors and photovoltaics[END_REF]. We calculated the charge carrier mobility using the following equation:

𝐽 = 9 8 µ𝜀𝜀 0 𝑉 2 𝐿 3
Where ε is 3 for organic semiconductors and L is the thickness of the semiconductor films. The electron mobilities for P-37 and P-41 were determined to be 7.24×10 -6 cm 2 /V s and 4.39×10 -6 cm 2 /V s, which are orders lower than those of PCBM (~10 -3 cm 2 /V s) and P(NDI2OD-T2) (~10 -4 cm 2 /V s) in literature. [START_REF] Jo | Effect of Molecular Orientation of Donor Polymers on Charge Generation and Photovoltaic Properties in Bulk Heterojunction All-Polymer Solar Cells[END_REF][START_REF] Benten | Recent research progress of polymer donor/polymer acceptor blend solar cells[END_REF] On the other hand, the calculated hole mobilities for PTB7-Th were 2.54×10 -4 cm 2 /V s and 1.75×10 -4 cm 2 /V s in PTB7-Th:P-37 and PTB7-Th:P-41 respectively, which are in good agreement with previous reports. [START_REF] Mori | Highly efficient charge-carrier generation and collection in polymer/polymer blend solar cells with a power conversion efficiency of 5.7%[END_REF][START_REF] Jung | Fluoro-Substituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%[END_REF][START_REF] Lee | High-Performance All-Polymer Solar Cells Via Side-Chain Engineering of the Polymer Acceptor: The Importance of the Polymer Packing Structure and the Nanoscale Blend Morphology[END_REF] The poor electron transport properties has led to unbalanced xci i charge transport in the polymer blend, which is known to have negative impact on FF and J SC according previous studies. [START_REF] He | Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells[END_REF][START_REF] Neher | Impact of charge transport on current?voltage characteristics and power-conversion efficiency of organic solar cells[END_REF][START_REF] Kotlarski | Role of balanced charge carrier transport in low band gap polymer:Fullerene bulk heterojunction solar cells[END_REF] This may partially explain why the FFs of these cells have such low values (<0.4). We also tested the electron-only devices based on the P-37 and P-41 neat films (Figure 3-15). Similar electron mobilities were found for P-37 (8.36 ×10 -6 cm 2 /V s) and P-41 (3.91×10 -6 cm 2 /V s). These results show that the low electron mobilities for bulk charge transport result from the polymers themselves. To investigate the polymer molecular packing in solid state, we performed thin film XRD on the polymer neat films for P-37 and P-41 (Figure 345678910111213141516). The samples were prepared by spin-coating the polymer films on ZnO/ITO substrates under the same conditions to emulate the electron-only device. XRD patterns of both the P-37 and P-41 neat films showed almost completely amorphous morphology. A weak (100) at diffraction peak 2θ = ~3.10 ° was observed for P-37, corresponding to a d-spacing of 2.84 nm, indicating the presence of edge-on oriented chains on the substrate. The amorphous morphology of these two polymers is likely because the bulky side chains have imposed a large hindrance for the polymer chain to form order packing, this has also been observed by previous studies. [START_REF] Fan | Side-chain engineering for efficient non-fullerene polymer solar cells based on a wide-bandgap polymer donor[END_REF][START_REF] Tu | Side-chain engineering of diindenocarbazole-based large bandgap copolymers toward high performance polymer solar cells[END_REF][START_REF] Wang | Side-Chain Architectures of 2,7-Carbazole and Quinoxaline-Based Polymers for Efficient Polymer Solar Cells[END_REF][START_REF] Cabanetos | Linear Side Chains in Benzo[1,2-b:4,5-b′]dithiophene-Thieno[3,4c]pyrrole-4,6-dione Polymers Direct Self-Assembly and Solar Cell Performance[END_REF] The intrinsic disordered packing property of these polymers may explain why low SCLC electron mobilities were observed in the electron-only devices on both the blend films and neat films.

xci ii

Summary and Future Direction

In conclusion, a series of ester side chains have been developed and used as solubilizing side chains for the copolymers of IBDF and BT. Three IBDF-BT polymers were synthesized and demonstrated excellent solubility, suitable HOMO/LUMO levels and excellent charge transport properties in OTFTs with electron mobility up to 0.40 cm 2 /V s. When the polymers were incorporated in all-PSCs in conjunction with PTB7-Th, all devices showed very poor performance (PCEs <0.4% with J SC < 1 xc vi mA/cm 2 and FF<0.4). AFM images suggested there were large polymer fabrils (>1000 nm) present in the polymer blends, indicating a large phase separation for the blends. This has limited the charge generation that happens at the D-A interface, and led to a very low J SC . In addition, SCLC measurement of the blends exhibited quite low electron mobilities (~10 -6 cm 2 /V s) and led to unbalanced charge transport in the polymer blend, which could be one of the reasons why the devices showing low FFs.

Through XRD and SCLC studies, we found the spin-coated polymer thin films were intrinsically amorphous and the bulky side chains seemed to disrupt the chain packing, which might have caused the poor bulk electron charge transport in the solar cell devices. To further improve the performance of IBDF-based polymers as acceptor in organic solar cells, large phase separation needs to be avoided.

This will take a comprehensive consideration of various measures such as reducing the side chain's length [START_REF] Lee | Influence of Alkyl Side Chain on the Crystallinity and Trap Density of States in Thiophene and Thiazole Semiconducting Copolymer Based Inkjet-Printed Field-Effect Transistors[END_REF][START_REF] Wang | Impact of the Nature of the Side-Chains on the Polymer-Fullerene Packing in the Mixed Regions of Bulk Heterojunction Solar Cells[END_REF], introducing a twisted backbone [START_REF] Le Borgne | Synthesis and properties of a novel narrow band gap oligomeric diketopyrrolopyrrole-based organic semiconductor[END_REF][START_REF] Le Borgne | New 3,3′-(ethane-1,2diylidene)bis(indolin-2-one) (EBI)-based small molecule semiconductors for organic solar cells[END_REF] and tuning the backbone structures [START_REF] Hwang | 7.7% Efficient All-Polymer Solar Cells[END_REF][START_REF] Choi | Importance of Electron Transport Ability in Naphthalene Diimide-Based Polymer Acceptors for High-Performance, Additive-Free, All-Polymer Solar Cells[END_REF][START_REF] Shi | Improved All-Polymer Solar Cell Performance by Using Matched Polymer Acceptor[END_REF] to improve the miscibility of the acceptor polymer and donor polymer. A good balance between crystallinity (charge transport) and solubility/miscibility (blend morphology) must be realized to achieve considerable solar cell performance.

Experimental

Materials and Characterization

All chemicals were obtained from commercial sources and used as received. Nonacosan-15ol, [START_REF] Roosjen | Synthesis and Characteristics of Biodegradable Pyridinium Amphiphiles Used for in vitro DNA Delivery[END_REF] tritriacontan-17-ol, [START_REF] Roosjen | Synthesis and Characteristics of Biodegradable Pyridinium Amphiphiles Used for in vitro DNA Delivery[END_REF] heptatriacontan-19-ol, [START_REF] Li | Monomeric, oligomeric and polymeric semiconductors containing fused rings and their devices[END_REF] [START_REF] Lei | Electron-Deficient Poly(p-phenylene vinylene) Provides Electron Mobility over 1 cm2 V-1 s-1 under Ambient Conditions[END_REF] and PIBDFBT-40 [START_REF] Lei | A BDOPV-Based Donor-Acceptor Polymer for High-Performance n-Type and Oxygen-Doped Ambipolar Field-Effect Transistors[END_REF] were prepared according to the literature methods. High-temperature gel permeation chromatography (HT-GPC) measurements were performed on a Malvern 350 HT-GPC system using 1,2,4-trichlorobenzene as eluent and polystyrene as standards at a column temperature of 140 °C. Thermogravimetric analysis (TGA)

3,7-dihydrobenzo[1,2-b:4,5- b']difuran-2,6-dione
xc vii was carried out on a TA Instruments SDT 2960 at a scan rate of 10 °C min -1 under nitrogen.

The UV-Vis absorption spectra of polymers were recorded on a Thermo Scientific model GENESYS™ 10S VIS spectrophotometer. Cyclic voltammetry (CV) data were obtained on a CHI600E electrochemical analyser using an Ag/AgCl reference electrode and two Pt disk electrodes as the working and counter electrodes in a 0.1 M tetrabutylammonium hexafluorophosphate solution in acetonitrile at a scan rate of 50 mV s -1 . Ferrocene was used as the reference, which has a HOMO energy value of -4.8 eV. [START_REF] Pommerehne | Efficient two layer leds on a polymer blend basis[END_REF] NMR spectra were recorded with a Bruker DPX 300 MHz spectrometer with chemical shifts relative to tetramethylsilane (TMS, 0 ppm). Reflective XRD measurements were carried out on a Bruker D8 Advance diffractometer with Cu Ka radiation (λ = 0.15406 nm) using polymer films spin coated on SiO 2 /Si substrates. Atomic force microscopy (AFM) images were taken with a Dimension 3100 scanning probe microscope.

Fabrication and Characterization of OTFT devices

The bottom-contact bottom-gate (BGBC) configuration was used for all OTFT devices. The preparation procedure of the substrate and device is as follows. A heavily n-doped Si wafer with ~300 nm-thick SiO 2 layer was patterned with gold source and drain pairs by conventional photolithography and thermal deposition. Then the substrate was treated with air plasma, followed by cleaning with acetone and isopropanol in an ultrasonic bath. Subsequently, the substrate was placed in a 3% dodecyltrichlorosilane (DDTS) solution in toluene at room temperature for 20 min. The substrate was washed with toluene and dried under a nitrogen flow. Then a polymer solution in chloroform (5 mg mL -1 ) or 1,1,2,2-tetrachloroethane (TCE) (10 mg mL -1 ) was spin-coated onto the substrate at 3000 rpm for 60 s to give a polymer film (~40 nm), which was further subjected to thermal annealing at different temperatures for 20 min in a glove box. All OTFT devices were characterized in the same glove box xc viii using an Agilent B2912A Semiconductor Analyser. The hole and electron mobilities are calculated in the saturation regime according to the following equation:

𝐼 𝐷𝑆 = 𝜇𝐶 𝑖 𝑊 2𝐿 (𝑉 𝐺 -𝑉 𝑇 ) 2
where I DS is the drain-source current, μ is charge carrier mobility, C i is the gate dielectric layer capacitance per unit area (~ 11.6 nF cm -2 ), V G is the gate voltage, V T is the threshold voltage, L is the channel length (30 µm), and W is the channel width (1000 µm).

Fabrication and Characterization of all-PSC devices

All all-PSC devices were fabricated using the conventional configuration ITO/PEDOT:PSS/PTB7-Th:PA/LiF/Al. ITO glass substrates were sonicated in water, acetone and IPA. Then the substrates were treated with plasma cleaning. A thin layer of PEDOT:PSS (Al 4083) was deposited through spincoating at 4000 rpm and dried subsequently at 150 °C for 20 min in air. Then the substrates were transferred to a nitrogen glove box, where the polymer blend layer (~90 nm) was spin-coated onto the PEDOT:PSS layer. The active layer was formed using a chlorobenzene solution of PTB7-Th:PA (1:1).

Finally, a thin layer of LiF (1 nm) and a layer of Al (100 nm) electrode were deposited in vacuum onto the substrate at P ≈5.0 × 10 -6 Pa. The active area is 0.07 cm 2 . The current density-voltage (J -V) characteristics of the all-PSCs were measured on an Agilent B2912A Semiconductor Analyser with a ScienceTech SLB300-A Solar Simulator. A 450 W xenon lamp and an air mass (AM) 1.5 filter were used as the light source.

Synthetic Procedures

General procedure for the synthesis of brominated branched alkyl esters 8a~8c xci x

To a solution of 4-bromobutyric acid (8.4 g, 50 mmol) in dichloromethane (50 mL) under ice/water bath, thionyl chloride (8.3 g, 70 mmol) was added. The solution was stirred at room temperature for 4 h. Then solvent and the excess thionyl chloride were removed under reduced pressure and then the residual was dissolved by tetrahydrofuran (THF) (50 mL). The solution was added drop wise to a suspension of a secondary alcohol (40 mmol) (nonacosan-15-ol for 8a, tritriacontan-17-ol for 8b, and heptatriacontan-19-ol for 8c) in THF (200 mL) at 0 °C. After addition, the reaction mixture was gradually heated to reflux. After 12 h, the reaction mixture was cooled down in an ice/water bath and water was added to quench the reaction. Diethyl ether was added to extract the product. The separated organic phase was washed with saturated NaHCO 3 aqueous solution and brine. After removal of solvent under reduced pressure, the crude product was purified by column chromatography using hexanes/ethyl acetate as the eluent, affording 8 as a white solid. (m, 4H), 1.25 (br, 64H), 0.88 (t, J = 6.8 Hz, 6H).

Nonacosan-15-yl 4-bromobutanoate (8a

General procedure for the synthesis of 9a~9c c

To a two-neck round bottom flask, 6-bromoisatin (1.13 g, 5 mmol), potassium carbonate (1.38 g, 10 mmol), 1 (5 mmol) and N,N'-dimethylformamide (40 mL) were added. The reaction mixture was then stirred at 50 °C for 18 h. The solvent was removed and the residual was dissolved in dichloromethane and washed with water. After removal of solvent, the crude product was purified by column chromatography using a mixture of hexanes and ethyl acetate as the eluent, affording 9 as an orange solid.

Nonacosan-15-yl 4-(6-bromo-2,3-dioxoindolin-1-yl)butanoate (9a). Yield: 2.55 g, 71%. General procedure for the synthesis of P-33, P-37, and P-41.

To a Schlenk flask, 10 (0.0453 mmol), 5,5'-bis(trimethylstannyl)-2,2'-bithiophene (22.6 mg, 0.0453 mmol), tri(o-tolyl)phosphine (1.1 mg, 3.6 µmol), tris(dibenzylideneacetone)dipalladium (0.8 mg, 0.9 µmol) and anhydrous chlorobenzene (4 mL) were added under argon atmosphere. The reaction mixture was then stirred at 130 °C for 48 h. Upon cooling to room temperature, the reaction mixture was poured into methanol and the precipitate was collected by filtration, followed by Soxhlet extraction.

P-33:

This polymer was purified by Soxhlet extraction using acetone, hexanes, chloroform and TCE.

Yield: 71 mg, 98% (from the TCE fraction).

P-37:

This polymer was purified by Soxhlet extraction using acetone, hexanes and chloroform. Yield: OTFTs, showing electron mobility between 0.45 and 0.85 cm 2 /V s [START_REF] Yan | A high-mobility electron-transporting polymer for printed transistors[END_REF], while at that time, the other ntype organic semiconductors were only showing electron mobility around 0.01 cm 2 /V s. [START_REF] Hüttner | n-type organic field effect transistors from perylene bisimide block copolymers and homopolymers[END_REF][START_REF] Letizia | n-Channel Polymers by Design: Optimizing the Interplay of Solubilizing Substituents, Crystal Packing, and Field-Effect Transistor Characteristics in Polymeric Bithiophene-Imide Semiconductors, clv i[END_REF][START_REF] Zhan | A High-Mobility Electron-Transport Polymer with Broad Absorption and Its Use in Field-Effect Transistors and All-Polymer Solar Cells[END_REF] With such a huge breakthrough, this polymer was commercialized by Polyera and became the benchmark ntype polymer semiconductor for OTFTs. [START_REF] Matsidik | Defect-free Naphthalene Diimide Bithiophene Copolymers with Controlled Molar Mass and High Performance via Direct Arylation Polycondensation[END_REF][START_REF] Kim | High-Mobility Air-Stable Naphthalene Diimide-Based Copolymer Containing Extended π-Conjugation for n-Channel Organic Field Effect Transistors[END_REF][START_REF] Sommer | Conjugated polymers based on naphthalene diimide for organic electronics[END_REF] Meanwhile researchers also attempted to use P(NDI2OD-T2) to replace PCBM as acceptor material in OSCs given the promising electron transport properties. Due the immiscibility and energy level mismatch between P(NDI2OD-T2) and the donor materials such as P3HT, the PCEs of these OSCs were all below 2%. [START_REF] Schubert | Influence of Aggregation on the Performance of All-Polymer Solar Cells Containing Low-Bandgap Naphthalenediimide Copolymers[END_REF][START_REF] Fabiano | Role of photoactive layer morphology in high fill factor all-polymer bulk heterojunction solar cells[END_REF][START_REF] Moore | Polymer Blend Solar Cells Based on a High-Mobility Naphthalenediimide-Based Polymer Acceptor: Device Physics, Photophysics and Morphology[END_REF] It was not until 2014 when Mori et al. introduced the donor polymer, PTB7-Th, in all-PSCs and achieved high PCE up to 5.7% [START_REF] Mori | Highly efficient charge-carrier generation and collection in polymer/polymer blend solar cells with a power conversion efficiency of 5.7%[END_REF] that P(NDI2OD-T2) gained considerable attention in the field of fullerene-free solar cell. Since then, numerous efforts were made to further improve the performance of P(NDI2OD-T2)based all-PSCs, including using different donor building blocks for the polymer backbone [START_REF] Earmme | All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor[END_REF][START_REF] Hwang | 7.7% Efficient All-Polymer Solar Cells[END_REF][START_REF] Choi | Importance of Electron Transport Ability in Naphthalene Diimide-Based Polymer Acceptors for High-Performance, Additive-Free, All-Polymer Solar Cells[END_REF],

side chain engineering [START_REF] Lee | High-Performance All-Polymer Solar Cells Via Side-Chain Engineering of the Polymer Acceptor: The Importance of the Polymer Packing Structure and the Nanoscale Blend Morphology[END_REF][START_REF] Lee | Side Chain Optimization of Naphthalenediimide-Bithiophene-Based Polymers to Enhance the Electron Mobility and the Performance in All-Polymer Solar Cells[END_REF], using different donor materials [START_REF] Kim | Flexible, highly efficient all-polymer solar cells[END_REF][START_REF] Gao | All-Polymer Solar Cells Based on Absorption-Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%[END_REF][START_REF] Li | High-Performance and Stable All-Polymer Solar Cells Using Donor and Acceptor Polymers with Complementary Absorption[END_REF][START_REF] Deshmukh | Performance, morphology and photophysics of high open-circuit voltage, low band gap all-polymer solar cells[END_REF] for the solar cells and fabrication optimization [START_REF] Hwang | 7.7% Efficient All-Polymer Solar Cells[END_REF][START_REF] Li | High Performance All-Polymer Solar Cells by Synergistic Effects of Finecxl v Tuned Crystallinity and Solvent Annealing[END_REF][START_REF] Fan | Optimisation of processing solvent and molecular weight for the production of green-solvent-processed all-polymer solar cells with a power conversion efficiency over 9%[END_REF]. For instance, Fan et al. reported the optimization by choosing the processing solvent judiciously, controlling the molecular weight of P(NDI2DT-T2) and optimizing the active layer thickness. A record high PCE of 9.16% was realized [START_REF] Fan | Optimisation of processing solvent and molecular weight for the production of green-solvent-processed all-polymer solar cells with a power conversion efficiency over 9%[END_REF], which is already approaching the high PCEs of fullerene-based OSCs. [START_REF] Zhao | Efficient organic solar cells processed from hydrocarbon solvents[END_REF][START_REF] He | Singlejunction polymer solar cells with high efficiency and photovoltage[END_REF][START_REF] Wu | n-Type Water/Alcohol-Soluble Naphthalene Diimide-Based Conjugated Polymers for High-Performance Polymer Solar Cells[END_REF][START_REF] Chen | Single-Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency[END_REF] civ Despite all these advancements in tuning the performance of NDI-based polymers in all-PSCs, the FF (0.40-0.55) and J SC (<15 mA cm -2 ) [START_REF] Jo | Effect of Molecular Orientation of Donor Polymers on Charge Generation and Photovoltaic Properties in Bulk Heterojunction All-Polymer Solar Cells[END_REF][START_REF] Schubert | Influence of Aggregation on the Performance of All-Polymer Solar Cells Containing Low-Bandgap Naphthalenediimide Copolymers[END_REF][START_REF] Li | High-Performance and Stable All-Polymer Solar Cells Using Donor and Acceptor Polymers with Complementary Absorption[END_REF][START_REF] Dai | Perylene and naphthalene diimide copolymers for all-polymer solar cells: Effect of perylene/naphthalene ratio[END_REF] of these solar cells are still significantly behind those (0.7-0.8 for FF and ~20 mA cm -2 for J SC ) of high-performance fullerene based OSCs. [START_REF] Zhao | Efficient organic solar cells processed from hydrocarbon solvents[END_REF][START_REF] Chen | Single-Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency[END_REF] It is generally believed the FF of all-PSCs is limited due to three main causes: relatively low bulk charge carrier mobilities, imbalanced charge transport and bimolecular recombination. Realizing a high FF requires simultaneously improvement on these three aspects. The HOMO level of P(NDI2OD-T2) was found to be ~5.7 eV, yielding an HOMO offset of 0.34 eV with PTB7-Th. Given that the binding energy of an exciton in organic semiconductors can be up 1.0 eV, a larger HOMO offset can facilitate that charge separation and help reduce the bimolecular recombination, improving the J SC and FF. Jung demonstrated a deeper HOMO level of -5.9 eV and a high PCE of 6.29% was obtained by the fluorosubstituted polymer compared to 5.21% of P(NDI2OD-T2) under the same conditions. [START_REF] Jung | Fluoro-Substituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%[END_REF] Their cv success was mainly attributed to the enhancement of charge generation and less bimolecular recombination, leading to the raised short-circuit current density (J SC ) and fill factor (FF), and PCE. On the other hand, a number of studies have shown that the NDI-based polymers generally adopted an mixture of edge-on and face-on molecular orientations in solid state while PTB7-Th is completely face-on orientated. [START_REF] Schubert | Influence of Aggregation on the Performance of All-Polymer Solar Cells Containing Low-Bandgap Naphthalenediimide Copolymers[END_REF][START_REF] Liao | Fullerene Derivative-Doped Zinc Oxide Nanofilm as the Cathode of Inverted Polymer Solar Cells with Low-Bandgap Polymer (PTB7clv ii Th) for High Performance[END_REF][START_REF] Li | Thermally stable high performance non-fullerene polymer solar cells with low energy loss by using ladder-type small molecule acceptors[END_REF][START_REF] Weng | A new polymer acceptor containing naphthalene diimide and 1,3,4-thiadiazole for all-polymer solar cells[END_REF] The suboptimal orientation match leads to less efficient charge dissociation at the D-A interface and higher germinate recombination than fullerene/polymer BHJ, which are believed to be one of the main causes for inferior J SC and FF in all-PSCs, respectively. Their studies have triggered the thought that chemically modifying the NDI moiety to enhance the bulk electron transport and improve the energy level matchup is of significance for achieving high performance all-PSCs. LUMO and HOMO levels. [START_REF] Kantchev | N′ -Dihydroxynaphthalenediimides (DHNDI): First Principles Designed Organic Building Blocks for Materials Science[END_REF] Herein, we managed to brominate NDIO and used it to form a D-A polymer, PNDIOT, whose backbone is comprised of NDIO and thiophene (T). The polymer will be first fully characterized with several techniques. Then the charge transport properties will be evaluated in OTFTs in both glovebox and air to assess the performance robustness in ambient conditions. Finally, the polymer will be used as acceptor in all-PSCs with PTB7-Th and the structure-property relationship will be investigated by using AFM, XRD and SCLC.

Results and Discussion

We started our synthesis with the monomer, NDIO20-Br (R = 2-octyldodecyl), which was provided by our collaborator. At first, bithiophene was chosen as the donor building block. The polymerization was conducted via Stille coupling using the toluene/Pd(PPh 3 ) 2 Cl 2 system. The resultant polymer, PNDIOBT (Scheme 4-1), showed very poor solubility in chlorobenzene. Soxhlet extraction with chlorobenzene only yielded 15% of the crude polymer, with the remaining as insoluble solid. The dissolved polymer fraction was subject to HT-GPC measurement to confirm the molecular weight. M n of 22.3 kDa was found with a PDI of 6.2. In previous studies, the NDI analogue, P(NDI2DT-2T), demonstrated good solubility and processability in chlorobenzene even with a high M n of 91.7 kDa. Such a dramatic difference implies that NDIO based polymers have a stronger π-π interaction among polymer chains than the NDI-based polymers, which is due to large spacer group between the alkyl branches and the NDI core. The stronger π-π interaction can bring the polymer chains closer to each other and favor the interchain charge hopping and the overall charge transport performance. [START_REF] Guo | Recent Progress in High Mobility Polymer Semiconductors for Organic Thin Film Transistors[END_REF][START_REF] Dou | Systematic Investigation of Side-Chain Branching Position Effect on Electron Carrier Mobility in Conjugated Polymers[END_REF][START_REF] Lei | Influence of alkyl chain branching positions on the hole mobilities of polymer thin-film transistors[END_REF][START_REF] Chen | Influence of side chain length and bifurcation point on the crystalline structure and charge transport of diketopyrrolopyrrolequaterthiophene copolymers (PDQTs)[END_REF] To improve the solubility of the polymer, thiophene was used as the donor building block instead to form PNDIOT (Scheme 4-1), which was synthesized by reacting NDIO20-Br with 2,5-bis(trimethylstannyl)thiophene via. The crude polymer was precipitated in methanol and then subject to Soxhlet extraction with acetone, hexanes and chloroform. The chloroform fraction was dried under vacuum to afford PNDIOT as a purple solid in a yield of 94%. The molecular weight was determined by HT-GPC using 1,2,4trichlorobenzene at 140 °C (Figure 4-4). The number average molecular weight (M n ) was found to be This red-shifting is attributed to the chain packing in the solid state that makes the backbone more coplanar and reduces the bandgap. It is interesting that the ICT peak has become stronger in thin film absorption, which implies potentially better visible light harvesting for the all-PSC devices. The absorption coefficient (α) of the as-spun thin film was determined to be ~2.7× 10 4 cm -1 , comparable to those of PNDIT (3~5×10 4 cm -1 ). [START_REF] Mori | Highly efficient charge-carrier generation and collection in polymer/polymer blend solar cells with a power conversion efficiency of 5.7%[END_REF] The optical bandgap of the thin film was estimated to be 1.89

eV by using the absorption onset. As shown in Figure 4 cxi ii The numbers can be found in the reference. [START_REF] Earmme | All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor[END_REF] The charge transport property of PNDIOT was investigated by fabricating the OTFTs. The BGBC OTFT device was fabricated on SiO 2 /Si wafer with Au electrode pairs as the source and drain contacts.

The polymer film was spin-coated using a chloroform solution (5 mg/mL) and then subject to different annealing temperatures (100 °C, 150 °C and 200 °C) in a nitrogen filled glovebox. The devices were tested in the same glovebox. All devices showed unipolar n-type charge transport characteristics (Table 3). The best performance was achieved at annealing temperature of 150 °C, where the devices showed electron mobility up to 5.4×10 -3 cm 2 V -1 s -1 with an average of 4.1×10 -3 cm 2 V -1 s -1 . With PNDIT's FET mobility only being on the order of 10 -4 to 10 -5 cm 2 V -1 s -1 , [START_REF] Earmme | All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor[END_REF][START_REF] Guo | Naphthalene Diimide-Based clv Polymer Semiconductors: Synthesis, Structure-Property Correlations, and n-Channel and Ambipolar Field-Effect Transistors[END_REF] our result represents a significant enhancement in electron charge transport performance, which can be attributed to the stronger π-π interaction and higher electron affinity. Under the same conditions, previously, several n-type polymer OTFTs were reported to show stable or decently stable operation under ambient conditions. For instance, the fluorinated PIBDFBT-40, F 4 -BDOPV-2T, showed excellent electron charge transport property in air, with electron mobility up to 14.9 cm 2 V -1 s -1 . [START_REF] Zheng | Strong Electron-Deficient Polymers Lead to High Electron Mobility in Air and Their Morphology-Dependent Transport Behaviors[END_REF] Zhang et al. showed long term stability of the IBDF polymer based OTFT in air. They found the OTFT device was still able to show 0.65 cm 2 V -1 s -1 electron mobility and 10 4 on/off current ratio after exposed in air for 70 days. [START_REF] Zhang | A bis(2oxoindolin-3-ylidene)-benzodifuran-dione containing copolymer for high-mobility ambipolar transistors[END_REF] It cxi v is worth noting that all these OTFT devices were in a top-gate bottom-contact device configuration, in which the conductive channel is protected by the dielectric and gate contact layers. Herein, we tested the air stability of the BGBC devices in an ambient environment with RH of ~50%. For the PNDIOT based devices, the electron mobility was able to remain 65% of what it was in the glovebox, with an electron mobility of 2.8×10 -3 cm 2 V -1 s -1 on average and 3.9×10 -3 cm 2 V -1 s -1 at maximum. On the other hand, we also fabricated the OTFTs using P(NDI2OD-T2) as the channel material under the same conditions. In the glovebox, the best performance of P(NDI2OD-T2)-based devices was achieved at 150 °C annealing temperature. The highest electron mobility observed was up to 0.013 cm 2 V -1 s -1 with an average of 1.1×10 -2 cm 2 V -1 s -1 . In the air, however, P(NDI2OD-T2)-based devices showed significant decrease in the electron mobility with only 2.2×10 -3 cm 2 V -1 s -1 on average and 3.4×10 -3 cm 2

P(NDI2OD-T2

) has a similar HOMO level with PIBDFBT-40 (both around -5.7 eV) [START_REF] Jung | Fluoro-Substituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%[END_REF][START_REF] Lei | A BDOPV-Based Donor-Acceptor Polymer for High-Performance n-Type and Oxygen-Doped Ambipolar Field-Effect Transistors[END_REF] and therefore P(NDI2OD-T2) showed a similar behaviour in the air. While PNDIOT has a HOMO level of ~6.0 eV, it seems that the HOMO is deep enough to resist the oxygen doping and the polymer can maintain unipolar n-type charge transport characteristic. Finally, we stored the device in the air over a period of 24 days and monitored the device performance. In the first two weeks, the electron mobility slowly dropped to around 1.6×10 -3 cm 2 V -1 s -1 and after that the electron mobility seemed to stabilize at cx v around 1.6 ×10 -3 cm 2 V -1 s -1 . Our results showed that PNDIOT with deep-lying LUMO and HOMO levels is robust n-type semiconductor and suitable for n-type OTFT operation in the air. We used PNDIOT as electron acceptor and PTB7-Th as electron donor to fabricate all-PSC devices.

The device configuration was ITO/PEDOT:PSS/PTB7-Th:PNDIOT/LiF/Al. Chlorobenzene was chosen as the processing solvent for the active layer since both polymers demonstrated excellent solubility in chlorobenzene (>10 mg/mL). Initially, the optimal donor/acceptor ratio and active layer thickness were determined to be 1:1 and 113±5 nm (Table 4-4 & 4-5). Fabricated under these optimal conditions, the best device exhibited a promising PCE of 2.92% (V OC = 0.75 V, J SC = 6.73 mA cm -2 , and FF = 0.59) with an average PCE of 2.68%. Previously solvent annealing was reported to have a positive effect on the device performance. [START_REF] Wei | Solvent-Annealed Crystalline Squaraine: PC70BM (1:6) Solar Cells[END_REF][START_REF] Zheng | Effects of different polar solvents for solvent vapor annealing treatment on the performance of polymer solar cells[END_REF][START_REF] Kumar | Solvent Annealing Control of Bulk Heterojunction Organic Solar Cells with 6.6% Efficiency Based on a clv iii Benzodithiophene Donor Core and Dicyano Acceptor Units[END_REF] We tried the solvent annealing treatment for 15 minutes on the polymer blend. However, it only led to a slight decrease in performance with the highest cx viii PCE of 2.73% and average PCE of 2.55% (Table 456). Furthermore, we tried aging the film before the anode deposition to improve the device performance. After the film aged in the glovebox for 24 hours, the PCE improved up to 3.25% (Figure 4-12 & Table 4-6) (V OC = 0.75 V, J SC = 7.11 mA cm -2 , and FF = 0.61) with an average PCE of 3.03%, which is superior to the NDI analogue in the same device configuration [START_REF] Choi | Importance of Electron Transport Ability in Naphthalene Diimide-Based Polymer Acceptors for High-Performance, Additive-Free, All-Polymer Solar Cells[END_REF] and slightly lower than the devices based on P(NDI2OD-T2) under the same device configuration [START_REF] Li | High Performance All-Polymer Solar Cells by Synergistic Effects of Finecxl v Tuned Crystallinity and Solvent Annealing[END_REF]. The J SC value was well matched (within 10% error) with the integrated J SC values obtained from the EQE spectrum (Figure 4-12). EQE spectrum showed two peaks with EQEs around 40% at 710 nm and 370 nm, respectively, which can be attributed to the absorption of PTB7-Th and PNDIOT. The relatively low EQE values for the PNDIOT-based device seem to originate from the poor light harvesting in the region of 400 nm to 600 nm. This can be further improved by modifying the backbone structure to boost the absorption [START_REF] Dai | Perylene and naphthalene diimide copolymers for all-polymer solar cells: Effect of perylene/naphthalene ratio[END_REF][START_REF] Zhang | Perfluoroalkyl-substituted conjugated polymers as electron acceptors for allpolymer solar cells: the effect of diiodoperfluoroalkane additives[END_REF] or using an donor that has complementary absorption. [START_REF] Li | High-Performance and Stable All-Polymer Solar Cells Using Donor and Acceptor Polymers with Complementary Absorption[END_REF] Another reason accountable for the low EQE was identified to be the mismatch of molecular orientation of the donor and acceptor, which will be discussed in the following section. A solution with a total concentration of 11 mg/mL and a spin speed of 1500 rpm were used for spincoating the active layer for these solar cells. a The average value was obtained from a batch of 4 devices. The active layers were formed by using a blend solution (11 mg/mL) at different spin speeds (1500 rpm, 1300 rpm and 1000 rpm). a The average value was obtained from a batch of 4 devices. The active layers were formed by using a blend solution (D/A ratio 1:1, 11 mg/mL) at 1300 rpm. a The average value was obtained from a batch of 4 devices. Solvent annealing was performed in a Petri dish at room temperature and film aging was performed by storing the device in a glovebox.

It was worth noting that the FF of 0.61 for the PNDIOT-based device is much higher than those of the P(NDI2OD-T2) or PNDIT devices, for which the FF is usually between 0.40 to 0.55. The improved FF can be related to the suppressed bimolecular recombination (see below) and balanced hole and cx xi electron mobilities (1.35×10 -4 cm 2 V -1 s -1 for hole and 6.93×10 -5 cm 2 V -1 s -1 for electron), which were obtained from the SCLC measurement of the hole-only device and electron-only device (Figure 4-13) mentioned in the previous chapter. On the other hand, FF is strongly related to the charge recombination, the light intensity dependence of the J-V characteristics was investigated to probe the bimolecular recombination that affects the device performance. It has been reported that the correlation between J SC and illumination intensity (P) can be addressed by J SC ∝ P α where α should be unity when the bimolecular recombination is negligible (maximum carrier swept out). We investigated the relationship between J SC and P and found the α for the solar cell based on PTB7-Th: PNDIOT. The higher α value of the device based on PTB7-Th: PNDIOT (0.945) than that of PTB7-Th: P(NDI2OD-T2) (0.886) [START_REF] Jung | Fluoro-Substituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%[END_REF] clearly indicates that bimolecular recombination is significantly suppressed when PNDIOT was used as the electron acceptor. All these results prove that NDIO polymers is a promising class of n-type semiconductors for high-performance all-PSCs, and with further material optimization [START_REF] Dai | Perylene and naphthalene diimide copolymers for all-polymer solar cells: Effect of perylene/naphthalene ratio[END_REF][START_REF] Deng | Naphthalene diimidedifluorobenzene-based polymer acceptors for all-polymer solar cells[END_REF] they can achieve an excellent photovoltaic performance. (P H ), medium molecular weight (P M ) and low molecular weight (P L ). [START_REF] Jung | Controlling Molecular Orientation of Naphthalenediimide-Based Polymer Acceptors for High Performance All-Polymer Solar Cells[END_REF] They found P H showed mix of face-on and edge-on molecular orientation while P L showed merely edge-on orientation. As the resultant all-PSC devices, the device based on P H exhibited much higher PCE, J SC and EQE than the device based on P L and P M , which is correlated to the beneficial molecular orientation.

cx xv

Summary and Future Direction

In conclusion, we reported the use of NDIO in D-A polymer for the first time and we synthesized a copolymer of NDIO and thiophene, PNDIOT. The HOMO/LUMO levels of PNDIOT were determined to be -5.98 eV/-4.08 eV, significantly lower than the those of NDI analogue. As the channel semiconductor in BGBC OTFTs, PNDIOT exhibited unipolar n-type charge transport characteristic with an electron mobility up to 5.9×10 -3 cm 2 /V s, an order higher than the NDI analogue. In addition, the BGBC OTFTs still maintained unipolar n-type charge transport characteristic with an electron mobility of 3.9×10 -3 cm 2 /V s under ambient conditions, indicating the good device performance robustness. Moreover, the electron mobility of the BGBC OTFT devices showed decent long-term cx xvi i stability when stored in air for up to 24 days. Finally, PNDIOT was used as acceptor in all-PSCs with PTB7-Th as donor. PCE as high as 3.25% was realized in the standard device configuration. In particular, the FF (0.61) of the PNDIOT-based devices is much higher than those of the NDI polymer based all-PSCs. This is due to the balanced charge transport in the polymer blend and the suppressed bimolecular recombination. Our preliminary results suggested that NDIO polymers represent a class promising n-type semiconductors for air-stable OTFTs and high-performance organic solar cells.

For future work on the NDIO-based materials, the focus should be placed on the backbone structure optimization. For high performance OTFTs, bithiophene (BT) or thiophene-vinylene-thiophene (TVT) is proven to be a better donor building block for D-A polymers. The reason is being that BT and TVT will bring higher symmetry into the polymer backbone, leading to a higher crystallinity for better charge transport. In addition, BT and TVT can provide larger spacer between two NDIO units to avoid the steric effect, which helps to improve the coplanarity of the backbone and intramolecular charge transport. It was also reported that the increase of coplanarity and crystallinity may help turn the molecular orientation from edge-on to face-on. [START_REF] Jo | Effect of Molecular Orientation of Donor Polymers on Charge Generation and Photovoltaic Properties in Bulk Heterojunction All-Polymer Solar Cells[END_REF] For OSCs, the current hurdles are the poor absorption in the visible region (1ow absorption coefficient and narrow coverage) and undesired molecular orientation. Using BT and TVT in the backbone can improve the ICT efficiency and lower the bandgap for better light harvesting. On the other hand, the increase of crystallinity will lower the solubility and affect the processability of the material. Therefore, a sweet balance between the crystallinity and solubility needs to be reached by tuning the side chain and molecular weight of the polymer to achieve the higher performance in OSCs than the NDI polymers and fullerene. Ferrocene was used as the reference, which has a HOMO energy value of -4.8 eV. [START_REF] Pommerehne | Efficient two layer leds on a polymer blend basis[END_REF] NMR spectra were recorded with a Bruker DPX 300 MHz spectrometer with chemical shifts relative to tetramethylsilane (TMS, 0 ppm). Reflective XRD measurements were carried out on a Bruker D8 Advance diffractometer with Cu Ka radiation (λ = 0.15406 nm) using polymer films spin coated on PEDOT:PSS/ITO substrates. Atomic force microscopy (AFM) images were taken with a Dimension 3100 scanning probe microscope on the samples prepared the same way.

Fabrication and Characterization of OTFT devices

The bottom-contact bottom-gate (BGBC) configuration was used for all OTFT devices. The preparation procedure of the substrate and device is as follows. A heavily n-doped Si wafer with ~300 nm-thick cx xix

SiO 2 layer was patterned with gold source and drain pairs by conventional photolithography and thermal deposition. Then the substrate was treated with air plasma, followed by cleaning with acetone and isopropanol in an ultrasonic bath. Subsequently, the substrate was placed in a 3% dodecyltrichlorosilane (DDTS) solution in toluene at room temperature for 20 min. The substrate was washed with toluene and dried under a nitrogen flow. Then a polymer solution in chloroform (5 mg mL -1 ) was spin-coated onto the substrate at 3000 rpm for 60 s to give a polymer film (~40 nm), which was further subjected to thermal annealing at different temperatures for 20 min in a glove box. All OTFT devices were characterized in the same glove box using an Agilent B2912A Semiconductor

Analyser. The hole and electron mobilities are calculated in the saturation regime according to the following equation:

𝐼 𝐷𝑆 = 𝜇𝐶 𝑖 𝑊 2𝐿 (𝑉 𝐺 -𝑉 𝑇 ) 2
where I DS is the drain-source current, μ is charge carrier mobility, C i is the gate dielectric layer capacitance per unit area (~ 11.6 nF cm -2 ), V G is the gate voltage, V T is the threshold voltage, L is the channel length (30 µm), and W is the channel width (1000 µm).

Fabrication and Characterization of all-PSC devices

All all-PSC devices were fabricated using the conventional configuration ITO/PEDOT:PSS/PTB7- In chapter 2, a novel electron-accepting building block, IBDP, is designed, synthesized, and incorporated into D-A polymers. With lactam groups replacing the lactone groups, IBDP gets two extra side chains to yield good solution processability, which is requisite for the fabrication of OSCs. On the other hand, the donating effect of lactam groups has raised the LUMO levels of P1 and P2 to ~-3.7 eV from -4.0 eV of those IBDF-based polymers. With a raised LUMO level, the electron injection barrier now is higher and electron transport will become more unstable. [START_REF] Jones | Tuning Orbital Energetics in Arylene Diimide Semiconductors. Materials Design for Ambient Stability of n-Type Charge Transport[END_REF][START_REF] De Leeuw | Stability of n-type doped conducting polymers and consequences for polymeric microelectronic devices[END_REF] Therefore the electron transport is supressed and the IBDP-based polymers exhibited balanced ambipolar semiconductor performance in OTFTs. Since both polymers exhibited high hole mobility up to 0.19 cm 2 V -1 s -1 and electron mobility up to 0.10 cm 2 V -1 s -1 , these two IBDP-based polymers was used as acceptor material in combination with P3HT as donor. As preliminary results, the devices based on P1 exhibited a high PCE of 3.38%. To our best knowledge, this is the highest PCE that has been obtained from P3HT-based all-PSCs. Furthermore, P1 was also used as donor material in OSCs with PC 71 BM as acceptor. A high PCE of 5.3% was obtained from the non-optimized devices. Besides, both IBDP-based polymers cx xxi i demonstrate a broad absorption up to 1100 nm according to the UV-Vis-NIR absorption spectra. This may enable the photo-detection applications such as phototransistor in NIR region, which has been rarely explored and believed to be a useful technology in various areas [START_REF] Mukherjee | Flexible organic N-channel phototransistor and integrated logic devices[END_REF][START_REF] Lee | Gadolinium-Enriched Polyaniline Particles (GPAPs) for Simultaneous Diagnostic Imaging and Localized Photothermal Therapy of Epithelial Cancer[END_REF][START_REF] Zhou | NIR photothermal therapy using polyaniline nanoparticles[END_REF][START_REF] Gong | Near-Infrared Absorbing Polymeric Nanoparticles as a Versatile Drug Carrier for Cancer Combination Therapy[END_REF][START_REF] Sliney | Visual sensitivity of the eye to infrared laser radiation[END_REF].

Th
In chapter 3, a series of ester side chains have been developed and used as solubilizing side chains for the copolymers of IBDF and BT. Three IBDF-BT polymers were synthesized and demonstrated excellent solubility, suitable HOMO/LUMO levels and excellent charge transport properties in OTFTs with electron mobility up to 0.40 cm 2 /V s. When the polymers were incorporated in all-PSCs in conjunction with PTB7-Th, all devices showed very poor performance (PCES < 0.4% with J SC < 1 mA/cm 2 and FF < 0.4). AFM images suggested there were large polymer fabrils (>1000 nm) present in the polymer blends, indicating a large phase separation for the blends. This has limited the charge generation that happens at the D-A interface, and led to a very low J SC . In addition, SCLC measurement of the blends exhibited quite low electron mobilities (~10 -6 cm 2 /V s) and led to unbalanced charge transport in the polymer blend, which could be one of the reasons why the devices showing low FF.

Through XRD and SCLC studies, we found the spin-coated polymer thin films were intrinsically amorphous and the bulky side chains seemed to disrupt the chain packing, which might have caused the poor bulk electron charge transport in the solar cell devices.

In chapter 4, we reported the use of NDIO in D-A polymer for the first time and we synthesized a copolymer of NDIO and thiophene, PNDIOT. The HOMO/LUMO levels of PNDIOT were determined to be -5.98 eV/-4.08 eV, significantly lower than the those of NDI analogue. As the channel semiconductor in BGBC OTFTs, PNDIOT exhibited unipolar n-type charge transport characteristic with an electron mobility up to 5.9×10 -3 cm 2 /V s, an order higher than the NDI analogue. In addition, the BGBC OTFTs still maintained unipolar n-type charge transport characteristic with an electron mobility of 3.9×10 -3 cm 2 /V s under ambient conditions, indicating the good device performance cx xxi ii robustness. Moreover, the electron mobility of the BGBC OTFT devices showed decent long-term stability when stored in air for up to 24 days. Finally, PNDIOT was used as acceptor in all-PSCs with PTB7-Th as donor. PCE as high as 3.25% was realized in the standard device configuration. In particular, the FF (0.61) of the PNDIOT-based devices is much higher than those of the NDI polymer based all-PSCs. This is due to the balanced charge transport in the polymer blend and the suppressed bimolecular recombination. Our preliminary results suggested that NDIO polymers represent a class promising n-type semiconductors for air-stable OTFTs and high-performance organic solar cells.

The major contributions of this thesis include:

• First to report the challenging synthesis of the electron deficient building block, IBDP, and two IBDP polymers, which showed high hole mobility of 0.19 cm 2 V -1 s -1 and electron mobility of 0.10 cm 2 V -1 s -1 in OTFTs and demonstrated broad absorption up to 1100 nm. In the all-PSCs that use P3HT as donor, P1 exhibited a decently high PCE of 3.38%, which is comparable to the devices based on PCBM:P3HT. P1 also demonstrated a high PCE of 5.3% as donor in OSCs. The results showed IBDP polymers are very promising materials for organic photovoltaics

• Designed and synthesized a series of ester side chain for solution processable IBDF-polymers, which needed expensive carbon 40 side chain as solubilizing group. The resultant polymers showed excellent electron transport property with electron mobility up to 0.4 cm 2 V -1 s -1 .

• This new type of side chains provides a tool to solubilize other n-type semiconductors such as, PDPPT-BTz copolymers. [START_REF] Quinn | Converting a Semiconducting Polymer from Ambipolar into n-Type Dominant by Amine End-Capping[END_REF][START_REF] Guo | Dramatically different charge transport properties of bisthienyl diketopyrrolopyrrole-bithiazole copolymers synthesized via two direct (hetero)arylation polymerization routes[END_REF] cx xxi v

• Frist to incorporate NDIO into D-A π-conjugated polymer. The resultant polymers exhibited high electron affinity, which led to enhanced electron transport property and more stable BGBC OTFT performance in the air.

• The NDIO polymer, PNDIOT, showed superior photovoltaic performance with PCE up to 3.25% in all-PSCs compared to the its NDI analogue. In particular, the FF (0.61) is much higher than those of the NDI polymer based all-PSCs, which has been a hindrance to achieving high PCE in all-PSCs.

Recommend Future Research

As discussed in Chapter 2, the synthesis of IBDP polymers needs to be improved in terms of yield and toxicity in order for further studies and optimization. The two IBDP polymers showed considerable absorption in the NIR region (800 nm to 1100 nm), which has been rarely reported for a single component organic semiconductor. Therefore, photodetection application such as phototransistor can be tried using these materials. Meanwhile, the simultaneously deep HOMO levels and excellent absorption profile of these materials make them favorable donor materials in OSCs for increasing the V OC without compromising the light harvesting. With the ambipolar charge transport characteristic, the polymer can be used in ternary OSCs as the third component, which may help to improve the absorption, smoothen the exciton dissociation and tune the blend morphology. [START_REF] Huang | Recent advances in organic ternary solar cells[END_REF][START_REF] Angmo | Fullerene alloy formation and the benefits for efficient printing of ternary blend organic solar cells[END_REF][START_REF] Xu | Interface Engineering for Ternary Blend Polymer Solar Cells with a Heterostructured Near-IR Dye[END_REF] Although IBDF polymers have shown excellent electron transport property in OTFTs, the required bulky side chains might have greatly limited the bulk heterojunction morphology and bulk electron transport. Introducing a twisted backbone in the IBDF polymers may be a solution to both issues. For one thing, it can lower the crystallinity of the polymer to achieve better miscibility with other donors.

cx xx v

For another, the twisted polymer may not require the bulky side chains and thus improve the bulk charge transport property.

With the introduction of alkoxy groups into NDIO, the electron affinity has been increased and the ππ interaction of the resultant polymers are much stronger. This has led to enhanced performance in OTFTs but difficulty in solubilizing the NDIO-BT copolymer, which can provide better adsorption in the visible and NIR region. Solution processable PNDIOBT can be realized by controlling the molecular weight or introducing defects into the backbone (random copolymers). On the other hand, the device performance can be substantially increased by using a donor with complementary absorption profile.

cx xx vi

4 )

 4 Au chapitre 5, les travaux de thèse seront conclus et des travaux futurs seront proposés pour une étude plus approfondie de ce travail. [1,2-b:4,5-b']difuran-2,6(3H,7H)-dione (IBDF)-based polymers that feature a new type

Figure 1 - 1

 11 Figure 1-1 The illustration of the structure of a typical solar cell device.[15] .....................................

Figure 1 - 2

 12 Figure 1-2 The device structure of the first OSC reported by Tang et al.[16] .....................................

Figure 1 - 3

 13 Figure 1-3 The chemical structure of PCBM. ......................................................................................

Figure 1 - 4

 14 Figure 1-4 The concept of bulk heterojunction (BHJ) that is used for OSCs. .....................................

Figure 1 - 5

 15 Figure 1-5 The illustration of the J-V characteristics of a solar cell.[33] ............................................

Figure 1 - 6

 16 Figure 1-6 Factors that can affect the V OC in organic solar cells. ........................................................

Figure 1 - 7

 17 Figure 1-7 The ideal morphology (bi-continuous and interpenetrating network) for a bulk heterojunction of donor-acceptor. ........................................................................................................

Figure 1 - 8

 18 Figure 1-8 The difference among insulator, semiconductor and conductor.[36] .................................

Figure 1 - 9

 19 Figure 1-9 The illustration of impurity doping to realize n-type or p-type semiconductor.[37] ..........

Figure 1 -

 1 Figure 1-10 The geometry of sp, sP2 and sp3.[44] ..............................................................................

Figure 1 -

 1 Figure 1-11 The formation of π-bonding orbital and anti-bonding orbital. .........................................

Figure 1 -

 1 Figure 1-12 Intramolecular charge transport and intermolecular charge transport mechanism. ..........

Figure 1 -

 1 Figure 1-13 The chemical structure of polymer PCPDTBT and PTB7-Th. ........................................

Figure 1 -

 1 Figure 1-14 The chemical structure of PC 71 BM. .................................................................................

Figure 1 -

 1 Figure 1-15 (a) Numbers of SCI papers and (b) efficiency enhancements regarding all-PSCs for the last two decades.[67] ............................................................................................................................

Figure 1 -

 1 Figure 1-16 Two types of OTFTs (BGBC and TGBC) that are commonly used. ...............................

Figure 1 -

 1 Figure 1-17 Some of the best n-type/n-type dominant polymer semiconductors available. ................

Figure 2 - 1

 21 Figure 2-1 The chemical structures of IBDF and IBDP. .....................................................................

Figure 2 - 2

 22 Figure 2-2 The chemical structures and optimized geometry of IBDF-Me and IBDP-Me. ................

Figure 2 - 3

 23 Figure 2-3 Two synthetic routes towards the brominated IBDP monomer. ........................................

Figure 2 - 4

 24 Figure 2-4 The molecular weight distribution for P1 (a) and P2 (b) obtained by HT-GPC. ...............

Figure 2 - 5

 25 Figure 2-5 The TGA diagrams for P1 (a) and P2 (b) obtained in nitrogen. .........................................

Figure 2 - 6

 26 Figure 2-6 The UV-Vis-IR absorption spectra in solution (chloroform) and in thin film. ..................

Figure 2 - 7 Figure 2 - 8

 2728 Figure 2-7 The cyclic voltammetry diagrams for P1 (a and b) and P2 (c and d) at a scan rate of 0.1 V/s. .......................................................................................................................................................

Figure 2 - 9

 29 Figure 2-9 The transmission XRD patterns of P1 flake (a) and P2 flake (b). ......................................

Figure 2 -

 2 Figure 2-10 AFM height images (2µm × 2µm each) of P1 (a) and P2 (b) thin films on SiO 2 /Si substrates annealed at different temperatures. ......................................................................................

Figure 2 -

 2 Figure 2-11 The comparison of the HOMO/LUMOs of P3HT, P1 and P2 as well as their absorption coefficients. ..........................................................................................................................................

Figure 2 -

 2 Figure 2-12 The J-V characteristics of the devices based on P1 and P2 under AM 1.5G illumination.

Figure 2 -

 2 Figure 2-13 The J-V characteristics of the devices based on P1:P3HT with different weight ratio under AM 1.5G illumination and the EQE spectrum of the cell with 1:1.5 D/A ratio that showed the best PCE. ..............................................................................................................................................

Figure 2 -

 2 Figure 2-14 The J-V characteristic of the OSC device based on the blend of P1:PC 71 BM. The blend film (1:1 wt ratio) was spin-coated at 1000 rpm using a solution (25 mg/mL) in o-DCB. ..................

Figure 3 - 1

 31 Figure 3-1 The chemical structures of IIDT-C3 and PDQT polymers. ................................................

Figure 3 - 2

 32 Figure 3-2 The comparison of three different types of IBDF-BT copolymers. ....................................

Figure 3 - 3

 33 Figure 3-3 The molecular weight distribution of P-33, P-37, P-41 and PIBDEFBT-40 obtained with HT-GPC................................................................................................................................................

Figure 3 - 4

 34 Figure 3-4 The TGA diagrams for P-33, P-37, P-41 and PIBDFBT-40 obtained in nitrogen. ............

Figure 3 - 5

 35 Figure 3-5 The UV-Vis-NIR adsorption spectra in solution (chloroform) and in film. .......................

Figure 3 -

 3 Figure 3-6 the CV diagrams for P-33, P-37 and P-41. .........................................................................

Figure 3 - 7

 37 Figure 3-7 Transfer (top) and output (bottom) curves of BGBC OTFT devices with 200 C-annealed P-33, P-37 and P-41 films. Device dimensions: channel length (L) = 30 m; channel width (W) = 1000 m. ...............................................................................................................................................

Figure 3 - 8

 38 Figure 3-8 AFM height images (4μm × 4μm) of polymer films annealed at different temperatures. ..

Figure 3 - 9

 39 Figure 3-9 Reflective XRD patterns of P-33, P-37 and P-41 films annealed at different temperatures. ..............................................................................................................................................................

Figure 3 -

 3 Figure 3-10 The energy level alignment and adsorption coefficient spectra of the IBDF-polymers and PTB7-Th. ..............................................................................................................................................

Figure 3 -

 3 Figure 3-12 AFM height images (4μm × 4μm) of polymer blend films spin-coated on PEDOT:PSS/ITO substrates. ...............................................................................................................

Figure 3 -

 3 Figure 3-13 AFM height images (4μm × 4μm) of P(NDI2OD-2T):P3HT blend films: processed with p-xylene:1-CN (a)[122] and o-DCB (b)[147]. .....................................................................................

Figure 3 -

 3 Figure 3-14 The J-V characteristics of the electron-only and hole-only devices for PTB7-Th:P-37 and PTB7-Th:P-41. .....................................................................................................................................

Figure 3 -

 3 Figure 3-15 The J-V characteristics of the electron-only and hole-only devices for P-37 neat film and P-41 neat film. ......................................................................................................................................

Figure 3 -

 3 Figure 3-16 The XRD patterns of P-37 and P-41 on ZnO/ITO fabricated under the same conditions as the electron-only devices. ....................................................................................................................

Figure 4 - 1

 41 Figure 4-1 The chemical structures of NDI and P(NDI2OD-2T). .....................................................

Figure 4 - 2 Figure 4 - 3

 4243 Figure 4-2 The importance of molecular orientation control at the D-A Interface of all-PSCs.[67] Figure 4-3 The chemical structure of NDIO moiety. .........................................................................

Figure 4 - 4

 44 Figure 4-4 The molecular weight distribution for PNDIOBT (a) and PNDIOT (b) obtained by HT-GPC. ...................................................................................................................................................

Figure 4 - 5

 45 Figure 4-5 The TGA diagrams for PNDIOT conducted under nitrogen. ...........................................

Figure 4 - 6

 46 Figure 4-6 The DSC diagram for PNDIOT obtained at 20 °C/min. ..................................................

Figure 4 - 7

 47 Figure 4-7 The absorption profile of in PNDIOT solution (chloroform) and thin film. ....................

Figure 4 - 8

 48 Figure 4-8 The cyclic voltammetry diagrams for PNDIOT: reduction process (a) and oxidation process (b). .........................................................................................................................................

Figure 4 - 9

 49 Figure 4-9 Transfer and output curves of BGBC OTFT devices annealed at 150 C for PNDIOT films tested in a glovebox. Device dimensions: channel length (L) = 30 m; channel width (W) = 1000 m. ...........................................................................................................................................................

Figure 4 -

 4 Figure 4-10 Transfer curves of BGBC OTFT devices annealed at 150 C and tested in nitrogen or air: PNDIOT, in a glovebox (a) and air (b); P(NDI2OD-T2), in nitrogen (c) and air (d). Device dimensions: channel length (L) = 30 m; channel width (W) = 1000 m. ........................................

Figure 4 -Figure 4 -

 44 Figure 4-11 The long-term stability of PNDIOT-based OTFT device in ambient conditions. ..........

Figure 4 -

 4 Figure 4-13 The J-V characteristics for the hole-only and electron-only devices for PTB7-Th:PNDIOT blend. .............................................................................................................................

Figure 4 -

 4 Figure 4-14 The J SC dependence on light intensity for the PNDIOT-based solar cell. .......................

Figure 4 -

 4 Figure 4-15 The XRD pattern and AFM image of the PTB7-Th:PNDIOT blend on PEDOT:PSS/ITO substrate. .............................................................................................................................................

Figure 4 -

 4 Figure 4-16 The XRD patterns of the PTB7-Th and PNDIOT neat films on PEDOT:PSS/ITO substrate. .............................................................................................................................................

Figure 4 -

 4 Figure 4-17 The correlation between molecular orientation and device performance for the blend of PTB7-Th and P(NDI2OD-T2).[81] ....................................................................................................

1

 1 The summary of BGBC OTFT performance of P1 and P2. ................................................ Table 2-2 The summary of the all-PSC device performance for P1 and P2. ....................................... Table 2-3 The D/A optimization of the P1:P3HT blend in all-PSC devices ........................................ Table 3-1 The summary of properties of polymers. .............................................................................

  PNDIOT and PNDIT. ........................................................................................................................ Table 4-2 A summary of the OTFT performance of PNDIOT and P(NDI2OD-T2). ........................ Table 4-3 A summary of all-PSCs based on different acceptors ....................................................... Table 4-4 The optimization of D/A ratio in the active layer .............................................................. Table 4-5 The optimization of active layer thickness ........................................................................ Table 4-6 Device performance optimization through post-treatment ................................................

e ( 1 .

 1 60218×10-19 C) elementary charge ε dielectric constant E g optical band gap EQE external quantum efficiency FF fill factor h (6.6207×10-34 m2 kg s-1) Planck's constant I DS drain-source current I ON /I OFF current ON/OFF ratio J SC short-circuit current λ max wavelength of incident light with maximum absorption L channel length M n numerical average molecular weight PDI polydispersity index PCE power conversion efficiency R S series resistance R SH shunt resistance V OC open-circuit voltage V TH threshold voltage V G gate voltage V DS drain-source voltage μ charge carrier mobility W width, gate width

Figure 1 - 3

 13 Figure 1-3 The chemical structure of PCBM.

Figure 1 - 4

 14 Figure 1-4 The concept of bulk heterojunction (BHJ) that is used for OSCs.

Figure 1 - 6 Figure 1 - 7

 1617 Figure 1-6 Factors that can affect the V OC in organic solar cells.

Figure 1 - 8 Figure 1 - 9

 1819 Figure 1-8The difference among insulator, semiconductor and conductor.[START_REF] Conductor | semiconductor and insulator[END_REF]

Figure 1 - 10

 110 Figure 1-10The geometry of sp, sP2 and sp3.[START_REF]Orbital hybridization[END_REF]

Figure 1 - 11 Figure 1 - 12

 111112 Figure 1-11 The formation of π-bonding orbital and anti-bonding orbital.

Figure 1 - 13

 113 Figure 1-13 The chemical structure of polymer PCPDTBT and PTB7-Th.

  show narrow adsorption (600-700 nm) and fairly low adsorption coefficient (~10 4 cm -1 ) in the visible region. Therefore, future work should be focused on the development of new PA materials with high electron mobility and broader absorption in the visible region to bring new possibilities of achieving high performance all-PSCs.

Figure 1 - 14

 114 Figure 1-14 The chemical structure of PC 71 BM.

Figure 1 -

 1 Figure 1-15 (a) Numbers of SCI papers and (b) efficiency enhancements regarding all-PSCs for the

Figure 1 -

 1 [START_REF] Tang | Two-layer organic photovoltaic cell[END_REF] shows 2 types of OTFT devices: bottom gate bottom contact (BGBC) and top gate bottom contact (TGBC) strucutres, wherein the gate controls the current between the source and drain electrodes, similar to a tap that controls the water flow. The source electrode (S) is where the majority carriers enter the channel, the drain (D) is where the majority carriers leave the channel, and the gate (G) terminal controls the channel conductivity.

Figure 1 - 16

 116 Figure 1-16 Two types of OTFTs (BGBC and TGBC) that are commonly used.

Figure 1 - 17 1 . 6 1 )

 117161 Figure 1-17 Some of the best n-type/n-type dominant polymer semiconductors available.

  Scheme 1). The two polymers will be first characterized by UV-Vis-IR absorption spectroscopy, cyclic

Figure 2 - 1

 21 Figure 2-1 The chemical structures of IBDF and IBDP.

Figure 2 - 2

 22 Figure 2-2 The chemical structures and optimized geometry of IBDF-Me and IBDP-Me.

5 ,Figure 2 - 3

 523 Figure 2-3 Two synthetic routes towards the brominated IBDP monomer.

Figure 2 - 4

 24 Figure 2-4 The molecular weight distribution for P1 (a) and P2 (b) obtained by HT-GPC.

Figure 2 - 5

 25 Figure 2-5 The TGA diagrams for P1 (a) and P2 (b) obtained in nitrogen.

  Vis-IR absorption spectra of the polymers in solution and in thin film are shown in Figure2-6. Both polymers exhibited typical dual band absorption, wherein the low energy band can be attributed to the internal charge transfer (ICT) transition and the high-energy band is the π-π* transition of IBDP moieties. In solution, the maximum absorption wavelengths (λ max ) are 699 nm for P1 and 711 nm for P2. The λ max of the P1 film remained at 699 nm, but a prominent shoulder at 822 nm appeared, indicating the more extended conjugation of the polymer chains in the solid state. Interestingly, P2

Figure 2 - 6 Figure 2 - 7

 2627 Figure 2-6 The UV-Vis-IR absorption spectra in solution (chloroform) and in thin film.

Figure 2 - 8

 28 Figure 2-8 The output curves for OTFTs based on P1 (a) and P2 (b), and transfer curves for OTFTs

   0.012 (0.093) 0.037  0.008 (0.045) 200 0.10  0.02 (0.10) 0.055  0.016 (0.075) 250 0.091  0.020 (0.11) 0.055  0.015 (0.067) a The average mobility  standard deviation (maximum mobility) calculated from the saturation regions of five devices.

Figure 2 - 9

 29 Figure 2-9 The transmission XRD patterns of P1 flake (a) and P2 flake (b).

Figure 2 -

 2 Figure 2-10 AFM height images (2µm × 2µm each) of P1 (a) and P2 (b) thin films on SiO 2 /Si

Figure 2 - 11 Figure 2 - 12

 211212 Figure 2-11 The comparison of the HOMO/LUMOs of P3HT, P1 and P2 as well as their absorption coefficients.

Figure 2 -

 2 Figure 2-13 The J-V characteristics of the devices based on P1:P3HT with different weight ratio under AM 1.5G illumination and the EQE spectrum of the cell with 1:1.5 D/A ratio that showed the best PCE.

Figure 2 -

 2 Figure 2-14 The J-V characteristic of the OSC device based on the blend of P1:PC 71 BM. The blend film (1:1 wt ratio) was spin-coated at 1000 rpm using a solution (25 mg/mL) in o-DCB.

  as eluent and polystyrene as standards at 140°C. TGA measurementss were carried out on a TA Instruments SDT 2960 at a scan rate of 10°C min -1 under nitrogen. The UV-Vis-IR absorption spectra of polymers were recorded on a Thermo Scientific model GENESYS™ 10S VIS spectrophotometer. Cyclic voltammetry (CV) data were obtained on a CHI600E electrochemical analyser using an Ag/AgCl reference electrode and two Pt disk electrodes as the working and counter electrodes in a 0.1 M tet-rabutylammonium hexafluorophosphate solution in acetonitrile at a scan rate of 100 mV s -1 . Ferrocene was used as the reference, which has a HOMO energy value of -4.8 eV.[START_REF] D'andrade | Relationship between the ionization and oxidation potentials of molecular organic semiconductors[END_REF] NMR data was recorded with a Bruker DPX 300 MHz spectrometer with chemical shifts relative to tetramethylsilane (TMS, 0 ppm). Transmission XRD measurements were carried out on a Bruker Smart 6000 CCD 3-circle D8 diffractometer with a Cu RA (Rigaku) X-ray source (λ = 0.15406 nm) and the polymer flakes stacked between two Mylar substrates. Atomic force microscopy (AFM) images were taken on polymer thin films spin-coated on the dodecyltrichlorosilane (DDTS)-modified SiO 2 /Si substrates with a Dimension 3100 scanning probe microscope. lxi v

Synthesis of 6 - 5 ) 6 )

 656 bromo-3,3-dichloro-1-(2-decyltetradecyl)indolin-2-one (To a solution of 6-bromo-1-(2-decyltetradecyl)indoline-2,3-dione (1.13 g, 2.0 mmol) in toluene (15 mL), was added anhydrous PCl 5 (0.92 g, 4.4 mmol). The reaction mixture was stirred at room temperature for 24 h. Then a saturation NaHCO 3 aqueous solution was added to quench the reaction and the organic layer was washed with brine three times and dried over anhydrous Na 2 SO 4 . Removing solventin vacuo gave the unstable 5, which was used for the next step without further purification.To a solution of the intermediate in acetic acid (10 mL), zinc powder (0.29 g, 4.4 mmol) was added portionwise. After the resultant mixture was stirred at room temperature for 20 minutes the excess zinc powder was filtered off and washed with dichloromethane. The combined organics was washed brine and saturation NaHCO 3 aqueous solution. The solution was then concentrated in vacuo and purified by silica gel column chromatography with hexane : ethyl acetate (10:1) to give 5. Yield: 0.86 g (78%). 1 H-NMR (300 MHz, CDCl 3 ) δ 7.16 -7.07 (q, 2H), 6.92 (s, 2H), 3.54 (d, J = 7.5 Hz, 2H), 3.46 (s, 2H), 1.83 (m, 1H), 1.38 -1.25 (br, 40H), 0.88 (t, J = 6.4 Hz, 6H).

  such as IBDF, (3E,8E)-3,8-bis(2-oxoindolin-3-ylidene)naphtho-[1,2-b:5,6-b']difuran-2,7(3H,8H)-dione (INDF) and thiophene-S,S-dioxidized indophenine (IDTO). In general, sufficiently long branched alkyl side chains (Scheme 3-1) are required to offset the strong aggregation tendency of polymer main chains to render these D-A polymers soluble. Mei et al. reported a siloxane-terminated

Scheme 3- 1

 1 Scheme 3-1 (a) Side chains used for solubilising D-A polymers. (b) An exemplary synthetic route to

Figure 3 - 1

 31 Figure 3-1 The chemical structures of IIDT-C3 and PDQT polymers.

Figure 3 - 2

 32 Figure 3-2 The comparison of three different types of IBDF-BT copolymers.

  of 3,7-dihydrobenzo[1,2-b:4,5-b']difuran-2,6-dione with two equivalents of 9a-c produced the IBDF monomers 10a-c. Finally three copolymers P-33, P-37, and P-41 were synthesized via the Stille coupling polymerization following a similar procedure reported previously.[99] The yields for P-33, P-37, and P-41 are 98%, 95% and 92% respectively, after purification by Soxhlet extraction. We tested the solubility of the polymers (P-33, P-37, P-41 and PIBDFBT-40) in various common organic solvents including 1,1,2,2-tetrachloroethane (TCE), chlorobenzene (CB), o-dichlorobenzene (DCB), chloroform (CF), dichloromethane (DCM), toluene (TL), and m-xylene (XL) at room temperature (

Figure 3 - 3

 33 Figure 3-3 The molecular weight distribution of P-33, P-37, P-41 and PIBDEFBT-40 obtained with

aFigure 3 - 4

 34 Figure 3-4 The TGA diagrams for P-33, P-37, P-41 and PIBDFBT-40 obtained in nitrogen.

Figure 3 - 5 Figure 3 - 6

 3536 Figure 3-5 The UV-Vis-NIR adsorption spectra in solution (chloroform) and in film.

Figure 3 - 7

 37 Figure 3-7 Transfer (top) and output (bottom) curves of BGBC OTFT devices with 200 C-annealed

Figure 3 - 8

 38 Figure 3-8 AFM height images (4μm × 4μm) of polymer films annealed at different temperatures.

Figure 3 - 9

 39 Figure 3-9 Reflective XRD patterns of P-33, P-37 and P-41 films annealed at different temperatures.

  Given that LUMO/HOMO offsets (E LUMO,D -E LUMO,A and E HOMO,D -E HOMO,A ) between these polymers and PTB7-Th are larger than 0.3 eV (Figure3-10), these three polymers were used as electron acceptor with PTB7-Th as electron donor to fabricate all-PSC devices. The device configuration was ITO/PEDOT:PSS/ PTB7-Th:PA/LiF/Al. The active layer was spin-coated from the blend of the two polymers (1:1 wt ratio) in chlorobenzene solution. Due to the poor solubility of P-33 in chlorobenzene,

Figure 3 - 10 Figure 3 - 11

 310311 Figure 3-10 The energy level alignment and adsorption coefficient spectra of the IBDF-polymers and PTB7-Th.

Figure 3 - 12 Figure 3 - 13

 312313 Figure 3-12 AFM height images (4μm × 4μm) of polymer blend films spin-coated on

Figure 3 - 14 Figure 3 - 15 Figure 3 - 16

 314315316 Figure 3-14 The J-V characteristics of the electron-only and hole-only devices for PTB7-Th:P-37 and

  1 H-NMR (300 MHz, CDCl 3 ) δ 7.46 (d, J = 7.8 Hz, 1H), 7.28 (dd, J = 8.1 Hz, 1.5 Hz, 1H), 7.23 (d, J = 1.5 Hz, 1H), 4.94 -4.86 (m, 1H), 3.77 (t, J = 7.5 Hz, 2H), 2.42 (t, J = 6.9 Hz, 2H), 2.05 -1.96 (m, 2H), 1.54 -1.52 (m, 4H), 1.25 (br, 48H), 0.88 (t, J = 6.6 Hz, 6H). Tritriacontan-17-yl 4-(6-bromo-2,3-dioxoindolin-1-yl)butanoate (9b): Yield: 2.56 g, 66%. 1 H-NMR (300 MHz, CDCl 3 ) δ 7.46 (d, J = 7.8 Hz, 1H), 7.28 (dd, J = 8.1 Hz, 1.4 Hz, 1H), 7.23 (d, J = 1.4 Hz, 1H), 4.94 -4.86 (m, 1H), 3.77 (t, J = 7.4 Hz, 2H), 2.42 (t, J = 6.9 Hz, 2H), 2.05 -1.95 (m, 2H), 1.55 -1.51 (m, 4H), 1.25 (br, 56H), 0.88 (t, J = 6.9 Hz, 6H). Heptatriacontan-19-yl 4-(6-bromo-2,3-dioxoindolin-1-yl)butanoate (9c). Yield: 2.45 g, 59%. 1 H-NMR (300 MHz, CDCl 3 ) δ 7.46 (d, J = 7.8 Hz, 1H), 7.28 (dd, J = 8.1 Hz, 1.5 Hz, 1H), 7.23 (d, J = 1.5Hz, 1H), 4.94 -4.86 (m, 1H), 3.77 (t, J = 7.4 Hz, 2H), 2.42 (t, J = 6.9 Hz, 2H), 2.05 -1.98 (m, 2H), 1.55 -1.51 (m, 4H), 1.25 (br, 64H), 0.88 (t, J = 6.9 Hz, 6H).General procedure for the synthesis of 10a~10c ciTo a two-neck round bottom flask, 9 (2 mmol), p-toluenesulfonic acid (53 mg, 0.28 mmol), 3,7dihydrobenzo[1,2-b:4,5-b']difuran-2,6-dione (0.19 g, 1 mmol) and acetic acid (10 mL) were added. The reaction mixture was stirred at 115 °C for 24 h. Upon cooling to room temperature, the reaction mixture was filtered and the filter cake was washed with methanol. The crude product was then purified by column chromatography using a mixture of hexanes and chloroform as the eluent, affording 10 as a black solid.Di(nonacosan-15-yl) 4,4'-((3E,3'E)-(2,6-dioxobenzo[1,2-b:4,5-b']difuran-3,7(2H,6H)diylidene)bis(6-bromo-2-oxoindoline-1-yl-3-ylidene))dibutyrate (10a). Yield: 0.56 g, 35%. 1 H-NMR (300 MHz, CDCl3) δ 9.06 (s, 2H), 8.90 (d, J = 8.4 Hz), 7.19 (dd, J = 5.1 Hz, 1.6 Hz, 2H),

P- 41 :

 41 74 mg, 95% (from the chloroform fraction). This polymer was purified by Soxhlet extraction using acetone, hexanes and chloroform. Yield: Nowadays one of the most widely studied n-type conjugated polymers are the ones based on the strong electron-deficient building block, naphthalene diimide (NDI) (Figure4-1).[START_REF] Mori | Highly efficient charge-carrier generation and collection in polymer/polymer blend solar cells with a power conversion efficiency of 5.7%[END_REF][START_REF] Guo | Naphthalene Diimide-Based clv Polymer Semiconductors: Synthesis, Structure-Property Correlations, and n-Channel and Ambipolar Field-Effect Transistors[END_REF][START_REF] Kang | Determining the role of polymer molecular weight for high-performance allpolymer solar cells: Its effect on polymer aggregation and phase separation[END_REF][START_REF] Deshmukh | Performance, morphology and photophysics of high open-circuit voltage, low band gap all-polymer solar cells[END_REF][START_REF] Mu | High-Efficiency All-Polymer Solar Cells Based on a Pair of Crystalline Low-Bandgap Polymers[END_REF][START_REF] Sommer | Conjugated polymers based on naphthalene diimide for organic electronics[END_REF][START_REF] Zhan | Rylene and related diimides for organic electronics[END_REF] For example, the copolymer of NDI-20 and bithiophene (Figure4-1), namely P(NDI2OD-T2), was found to show excellent electron charge transport property back in 2009 by He et al., used as channel material in

Figure 4 - 1

 41 Figure 4-1 The chemical structures of NDI and P(NDI2OD-2T).

  et al. used a fluoro-substituted bithiophene units on the backbone of an n-type NDI polymer backbone to increase the electron affinity and electron transport. The resultant polymer, P(NDI2OD-FT2),

Figure 4 - 2

 42 Figure 4-2 The importance of molecular orientation control at the D-A Interface of all-PSCs.[67]

Figure 4 - 3

 43 Figure 4-3 The chemical structure of NDIO moiety.

68. 5 Scheme 4 - 1 Figure 4 - 4 Figure 4 - 5 Figure 4 - 6

 541444546 Scheme 4-1The reaction formulas for Stille coupling polymerization using NDIO20-Br. Reaction conditions: toluene/ Pd(PPh 3 ) 2 Cl 2 /110 °C, 48 h.

Figure 4 -

 4 Figure 4-7 shows the UV-vis-IR absorption spectra of PNDIOT for both the dilute solution (∼10 -6 M

Figure 4 - 7

 47 Figure 4-7 The absorption profile of in PNDIOT solution (chloroform) and thin film.

Figure 4 - 8

 48 Figure 4-8 The cyclic voltammetry diagrams for PNDIOT: reduction process (a) and oxidation process (b).

V - 1 s

 1 -1 at maximum, leading to an 80% drop from what it was in glovebox. It is clear that PNDIOT is a much more robust n-type semiconductor in the air than P(NDI2OD-T2), which can be attributed to the deep-lying LUMO level (~-4.1 eV). Moreover, the p-type charge transport in P(NDI2OD-T2) has become quite prominent leading to a ambipolar charge transport characteristic, while the charge transport in PNDIOT remained unipolar n-type characteristic. The enhanced p-type charge transport in air was also reported by previous studies.Lei et al. found that the oxygen in the air would p-doped the HOMO level of PIBDFBT-40 and led to occurrence of p-type charge transport characteristic.

Figure 4 - 9 Figure 4 - 10

 49410 Figure 4-9 Transfer and output curves of BGBC OTFT devices annealed at 150 C for PNDIOT films tested in a glovebox. Device dimensions: channel length (L) = 30 m; channel width (W) = 1000 m.

Figure 4 - 11

 411 Figure 4-11 The long-term stability of PNDIOT-based OTFT device in ambient conditions.

Figure 4 -

 4 Figure 4-12 The J-V characteristic and EQE spectrum for the solar cell device based on PNDIOT:PTB7-Th. The active layer (1:1 wt ratio) was spin-coated using a solution (11 mg/mL) in chlorobenzene at 1300 rpm and aged for 24 hrs.

Figure 4 - 13 Figure 4 -

 4134 Figure 4-13 The J-V characteristics for the hole-only and electron-only devices for PTB7-Th:PNDIOT blend.

Figure 4 -Figure 4 - 16 Figure 4 - 17

 4416417 Figure 4-15 The XRD pattern and AFM image of the PTB7-Th:PNDIOT blend on PEDOT:PSS/ITO substrate.

1

 1 Materials and CharacterizationAll chemicals were obtained from commercial sources and used as received. 4,9-dibromo-2,7bis((2-octyldodecyl)oxy)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (NDIO20-Br) was provided by our collaborator. High-temperature gel permeation chromatography (HT-GPC) measurements were performed on a Malvern 350 HT-GPC system using 1,2,4trichlorobenzene as eluent and polystyrene as standards at a column temperature of 140 °C.Thermogravimetric analysis (TGA) was carried out on a TA Instruments SDT 2960 at a scan rate of 10 °C min -1 under nitrogen. The UV-Vis absorption spectra of polymers were recorded on a Thermo Scientific model GENESYS™ 10S VIS spectrophotometer. Cyclic voltammetry (CV) data were obtained on a CHI600E electrochemical analyser using an Ag/AgCl reference electrode and two Pt disk electrodes as the working and counter electrodes in a 0.1 M tetrabutylammonium hexafluorophosphate solution in acetonitrile at a scan rate of 50 mV s -1 .

  :PA/LiF/Al. ITO glass substrates were sonicated in water, acetone and IPA. Then the substrates were treated with plasma cleaning. A thin layer of PEDOT:PSS (Al 4083) was deposited through spincoating at 4000 rpm and dried subsequently at 150 °C for 20 min in air. Then the substrates were transferred to a nitrogen glove box, where the polymer blend layer (~90 nm) was spin-coated onto the PEDOT:PSS layer. The active layer was formed using a chlorobenzene solution of PTB7-Th:PA (1:1).Finally, a thin layer of LiF (1 nm) and a layer of Al (100 nm) electrode were deposited in vacuum onto cx xx the substrate at P ≈5.0 × 10 -6 Pa. The active area is 0.07 cm 2 . The current density-voltage (J -V) characteristics of the all-PSCs were measured on an Agilent B2912A Semiconductor Analyser with a ScienceTech SLB300-A Solar Simulator. A 450 W xenon lamp and an air mass (AM) 1.5 filter were used as the light source.Organic semiconductors are under rapid development in the last two decades for their distinguished merits: light weight, low cost and solution processability. It is believed that organic electronics will bring the next generation of electronics, for example, flexible electronics. The development of n-type organic semiconductors is very challenging due to the lack of electron-deficient moieties for the polymer backbone. This thesis presents the synthesis and characterization of several n-type or ambipolar polymer semiconductors that were used for OTFTs and all-PSCs, which demonstrated promising performance.

Figure A- 5 Figure A- 6

 56 Figure A-5 300 MHz 1 H NMR spectrum for 6-bromo-1-(2-decyltetradecyl)indolin-2-one (6) in chloroform-d.

Figure A- 7 Figure A- 8 Figure A- 9

 789 Figure A-7 75 MHz 13 C NMR spectrum for N,N'-(1,4-phenylene)bis(N-dodecyl-2hydroxyacetamide) (2) in chloroform-d.

Figure A-10 75 Figure A- 11 300Figure A- 12 300Figure A- 13 300Figure A- 14 300Figure A- 15 300Figure A- 16 300Figure A- 17 300Figure A- 18 300Figure A- 19 300Figure A- 20 75Figure A- 21 75Figure A- 22 75Figure A- 23 300

 7511121314151617181920212223 Figure A-10 75 MHz 13 C NMR spectrum for (3E,7E)-3,7-bis(6-bromo-1-(2-decyltetradecyl)-2-oxoindolin-3-ylidene)-1,5-didodecyl-5,7-dihydropyrrolo[2,3-f]indole-2,6(1H,3H)-dione (7) in chloroform-d.

Table 3 -

 3 2 OTFT device performance of the polymers annealed at different annealing temperatures.

Table 3 -

 3 3 The summary of all-PSC devices and SCLC devices based on P-37 and P-41. ..................

Table 3 -

 3 [START_REF] Samuel | Increasing the Energy Yield of Generation from New and Renewable Energy Resources[END_REF] The device parameters for the films that were solvent annealed or aged. ...........................

Table 4 -

 4 1 Molecular Weight, thermal stability, electrochemical and photophysical properties of

). PC 71 BM was used to replace PCBM in PSCs to extend the absorption to 380-500 nm and increase the electron affinity.[45] It has been demonstrated PSCs with PC 71 BM showed enhanced device performance compared to those with PCBM.[59] With fullerene

	's excellent electron transport
	properties, high electron affinity and the ability to form favorable nanoscale network with donors,
	almost all reported high efficiency PSCs used these two fullerene derivatives as acceptor. However,
	fullerene derivatives also exhibit several non-trivial drawbacks that hinder its progress in practical
	applications. First, the relatively low lowest unoccupied molecular orbital (LUMO) energy level (~4.1
	to -4.3 eV)[60] set a limitation for the open-circuit voltage of the resultant cells.[61] Second, fullerene
	derivatives fullerenes have strong tendency to crystallize and aggregate causing a performance
	degradation in long term.[62-64] Third, fullerene derivatives have very poor light absorption profile in
	xx
	xvi
	i

  Synthetic route towards the brominated IBDP monomer and IBDP-based polymers.

	+ Scheme 2-1 Reaction conditions: i) Ethanol/reflux.; ii) CH 2 Cl 2 /Et 3 N/0 °C to r.t., THF/MeOH/H 2 O/K 2 CO 3 /r.t.; iii) Route 1
	CH 2 Cl 2 /oxalyl chloride/DMSO/Et 3 N/-78 °C to r.t., CH 2 Cl 2 /thiophenol/TFAA/BF 3 • Et 2 O/r.t.; iv)
	THF/H 2 O/ammonium cerium nitrate/r.t.; v) Toluene/PCl 5 /r.t.; vi) AcOH/Zn/r.t.; vii) AcOH/HCl/reflux;
	viii) Chlorobenzene/P(o-tolyl) 3 /Pd 2 (dba) 3 /120 °C.
	Route 2	Unstable intermediate
	+
	More stable intermediate
	l

Table 2 - 1

 21 The summary of BGBC OTFT performance of P1 and P2.

		Annealing	Hole mobility a	Electron mobility a
	Polymer
		temperature (°C)	(cm 2 V -1 s -1 )	(cm 2 V -1 s -1 )
	P1	100	0.15 0.02 (0.17)	0.063  0.007 (0.071)
		150	0.16  0.03 (0.19)	0.064  0.017 (0.088)
		200	0.040  0.014 (0.057)	0.073  0.013 (0.089)
	P2	100	0.055  0.010 (0.062)	0.023 0.003 (0.027)

Table 2 - 2

 22 The summary of the all-PSC device performance for P1 and P2.All devices used P3HT as the donor and the same device configuration: ITO/PEDOT:PSS/Polymer blend/LiF/Al. The active layer thickness is around 140 nm. The numbers in the parenthesis are average values for 4 devices on the same substrate.

	Acceptor	J SC (mA/cm 2 )	V OC (V)	FF	PCE (%)
	P1	7.47	0.77	0.44	2.56 (2.32)
	P2	6.04	0.76	0.46	2.14 (1.95)

Table 2 -

 2

	D/A ratio	J SC (mA/cm 2 )	V OC (V)	FF	PCE (%)
	1:1	7.47	0.77	0.44	2.56 (2.32)
	1:1.5	8.43	0.77	0.52	3.38 (3.08)
	1:2	7.26	0.77	0.52	2.93 (2.70)
	1.5:1	5.90	0.76	0.41	1.82 (1.55)

3 The D/A optimization of the P1:P3HT blend in all-PSC devices

  2 mL) in CH 2 Cl 2 (50 mL), acetoxyacetyl chloride (2.4 mL, 22.3 mmol) was added dropwise at 0°C. Then the reaction mixture was allowed to warm to room temperature and stirred for 15 h. Then the reaction was quenched with saturated NaHCO 3 Hz, 6H).13 C-NMR (75 MHz, CDCl 3 ) δ 171.35, 140.12, 129.83, 60.57, 49.77, 31.89, 29.60, 29.54, 29.32, 27.72, 26.69, 22.67, 14.11. HR-ESI-MS (M + H) + To a solution of oxalyl chloride (0.85 mL, 9.80 mmol) in CH 2 Cl 2 (30 mL) at -78°C, was added dimethyl sulfoxide (DMSO) (1.3 mL, 18.3 mmol) in CH 2 Cl 2 (10 mL) dropwise. After stirring for 30 min, a solution of 2 (2.50 g, 4.46 mmol) in CH 2 Cl 2(10 mL) was added dropwise. The reaction mixture was stirred for another hour, and then triethylamine (6.0 mL, 43.0 mmol) was added and the solution was allowed to warm to room temperature. After stirring for another 1.5 h, CH 2 Cl 2 (100 mL) and saturated NaHCO 3 aqueous solution (150 mL) were added. The organic phase was separated and washed with saturated NaHCO 3 aqueous solution twice and dried over anhydrous Na 2 SO 4 . Upon removal of sovent, the crude gloyamide intermediate was used immediately without further purification. To a solution of the crude gloyamide in CH 2 Cl 2 (50 mL), thiophenol (0.91 mL, 8.92 mmol) was added. After stirring at room temperature for 15 h, trifluoroacetic acid (TFAA) (5.7 mL, 40.2 mmol) was added. After stirring for another hour, BF 3 • Et 2 O (2.8 mL, 22.0 mmol) was added. The reaction mixture was stirred for another 6 h and quenched by saturated NaHCO 3 aqueous solution cautiously. The organic phase was separated and washed with saturated NaHCO 3 aqueous solution three times and dried over anhydrous Na 2 SO 4 . Removing solvent in vacuo gave the crude product 3, which was used for the next step withoutTo a solution of 3 (1.40 g, 1.89 mmol) in a solvent of THF (50 mL) and water (8.5 mL), ammonium cerium nitrate (8.28 g, 15.1 mmol) was added. The reaction mixture was stirred at room temperature for 24 h. The solution was then concentrated in vacuo and the residue was purified by silica gel column chromatography with hexane : ethyl acetate (2:1) to give 4. Yield: 0.491 g (47%). 1 H-NMR (300 MHz, CDCl 3 ) δ 7.16 (s, 2H), 3.73 (t, J = 7.4 Hz, 4H), 1.70 -1.66 (m, 4H), 0.88 (t, J = 6.3 Hz, 6H).13 C-NMR (75 MHz, CDCl 3 ) δ 183.35, 156.63, 147.33, 123.13, 106.81, 40.84, 31.92, 29.54, 29.48, 29.35, 29.20, 27.10, 26.88, 22.70, 14.14. HR-ESI-MS (M + H) + calc. for C 34 H 53 N 2 O 4

	DMSO-d 6 ) δ 7.36 (s, 4H), 4.58 (t, J = 5.7 Hz, 2H), 3.71 (s, 4H), 3.63 (t, J = 7.0 Hz, 4H), 1.45 -1.30
	(m, 4H), 1.21 -1.19 (m, 36H), 0.85 (t, J = 6.4 calc. for C 34 H 61 N 2 O 4 + : 561.4631; found: 561.4611. further purification. Yield: 1.59 g (48%). 1 H-NMR (300 MHz, CDCl 3 ) δ 7.35 -7.17 (m, 10H), 6.68 (s,
	2H), 4.55 -4.51 (m, 2H), 3.73 -3.37 (m, 4H), 1.35 -1.26 (br, 40H), 0.87 (t, J = 6.4 Hz, 6H).
	Synthesis of 1,5-didodecyl-3,7-bis(phenylthio)-5,7-dihydropyrrolo[2,3-f]indole-2,6(1H,3H)-dione
	(3)
	lxv
	ii

aqueous solution. The organic phase was further washed with brine twice and dried over anhydrous Na 2 SO 4 . Upon removal of solvent in vacuo, the intermediate acetoxy amide was dissolved in a mixture solvent of THF (50 mL), methanol (45 mL), and water (5 mL) before K 2 CO 3 (13.8 g, 100 mmol) was added. The reaction mixture was stirred at room temperature for 24 h. The solid was then filtered off and washed with CH 2 Cl 2 . The combined filtrate was washed with brine, dried over Na 2 SO 4 and concentrated in vacuo to give the crude product, which was further purified by silica gel column chromatography with hexane : ethyl acetate (1:1) to give 2. Yield: 3.04 g (54%). 1 H-NMR (300 MHz, Synthesis of 1,5-didodecylpyrrolo[2,3-f]indole-2,3,6,7(1H,5H)-tetraone (4) lxv iii + : 553.4005; found: 553.4003.

  13 C-NMR(75 MHz, CDCl 3 ) δ 175.05, 146.47, 125.58, 124.80, 123.39, 121.29, 111.97, 44.63, 35.88, 35.34, 31.94, 31.48, 30.01, 29.67, 29.38, 26.38, 22.72, 14.15. HR-ESI-MS (M + H) + calc. for C 32 H 55 BrNO

+ : 548.3467; found: 548.3468.

  Table 3-1). P-41 with the largest side chains can be easily dissolved in all above solvents.

P-37 is less soluble, but can still be dissolved in CF, TCE, CB, and DCB. P-33 with the smallest side chains is only soluble in TCE among the solvents tested. We also tested solubility of PIBDFBT-40, lxx vii which has branched C40 alkyl side chains. This polymer can only be dissolved in TCE and DCB.

Table 3 - 1

 31 The summary of properties of polymers.

			M n
	Polymer	Solubility testing a		PDI λ max	sol/film (nm) E g	opt (eV) E HOMO /E LUMO (eV)
			(kDa)
	P-33	TCE	16	4.0	847/847	1.31	-5.68/-3.85

Table 3 -

 3

	lxx
	xiii

2 OTFT device performance of the polymers annealed at different annealing temperatures.

Table 3 -3

 3 The summary of all-PSC devices and SCLC devices based on P-37 and P-41.

	Acceptor	J SC (mA/cm 2 )	V OC (V)	FF	PCE (%)	µ h (cm 2 /V s) µ e (cm 2 /V s)
	P-37	1.21	0.72	0.39	0.34 (0.29)	2.54×10 -4	7.24×10 -6

Table 3 - 4

 34 The device parameters for the films that were solvent annealed or aged.

	Acceptor	Post-treatment	Duration	J SC (mA/cm 2 )	V OC (V)	FF	PCE (%)
		Solvent-	15 min	1.15	0.72	0.38	0.31 (0.27)
	P-37	annealing Film-aging	30 min 24 hrs 72 hrs	1.02 1.23 1.20	0.70 0.71 0.72	0.39 0.39 0.39	0.28 (0.25) 0.34 (0.29) 0.34 (0.28)
		Solvent-	15 min	0.34	0.55	0.34	0.063 (0.05)
	P-41	annealing Film-aging	30 min 24 hrs 72 hrs	0.12 0.56 0.55	0.33 0.55 0.55	0.30 0.34 0.32	0.011 (0.01) 0.10 (0.08) 0.10 (0.08)

All devices used PTB7-Th as the donor and the same device configuration: ITO/PEDOT:PSS/Polymer blend/LiF/Al. The numbers in the parenthesis are average values for 4 devices on the same substrate.

Tritriacontan-17-yl 4-bromobutanoate (8b). Yield

	). Yield: 15.1 g, 66%. 1 H-NMR (300 MHz, CDCl 3 ) δ 4.92 -
	4.84 (m, 1H), 3.46 (t, J = 6.4 Hz, 2H), 2.48 (t, J = 7.2 Hz, 2H), 2.22 -2.15 (m, 2H), 1.53 -1.50 (m,
	4H), 1.25 (br, 48H), 0.88 (t, J = 6.6 Hz, 6H).
	: 21.3 g, 85%. 1 H-NMR (300 MHz, CDCl 3 ) δ 4.90
	-4.84 (m, 1H), 3.46 (t, J = 6.8 Hz, 2H), 2.48 (t, J = 7.0 Hz, 2H), 2.21 -2.12 (m, 2H), 1.53 -1.45 (m,
	4H), 1.25 (br, 56H), 0.88 (t, J = 6.3 Hz, 6H).
	Heptatriacontan-19-yl 4-bromobutanoate (8c). Yield: 15.4 g, 56%. 1 H-NMR (300 MHz, CDCl 3 ) δ
	4.92 -4.84 (m, 1H), 3.46 (t, J = 6.4 Hz, 2H), 2.48 (t, J = 7.2 Hz, 2H), 2.22 -2.12 (m, 2H), 1.52 -1.50

Di(tritriacontan-17-yl) 4,4'-((3E,3'E)-(2,6-dioxobenzo[1,2-b:4,5-b']difuran-3,7(2H,6H)- diylidene)bis(6-bromo-2-oxoindoline-1-yl-3-ylidene))dibutyrate (10b). Yield

  Hz, 12H).13 C-NMR (75 MHz, CDCl 3 ) δ 172.56, 167.19, 151.97, 146.76, 135.77, 131.72, 129.46, 127.03, 126.75, 126.13, 126.01, 119.90, 112.06, 111.19, 75.21 39.70, 34.26, 32.12, 31.59, 29.90, 29.76, 29.57, 25.57, 22.89, 14.32.

	NMR (300 MHz, CDCl 3 ) δ 9.10 (s, 2H), 8.94 (d, J = 8.7 Hz, 2H), 7.25 (d, J = 7.8 Hz, 2H), 7.09 (s, 2H),
	4.92 (m, 2H), 3.84 (t, J = 6.8 Hz, 4H), 2.44 (t, J = 6.8 Hz, 4H), 2.05 -2.03 (m, 4H), 1.51 -1.24 (br,
	132H), 0.87 (t, J = 6.4
	05 -2.01
	(m, 4H), 1.53(br, 4H), 1.23 (br, 96H), 0.87 (t, J = 6.8 Hz, 12H). 13 C-NMR (75 MHz, CDCl 3 ) δ 172.57,
	167.19, 151.96, 146.76, 135.74, 131.74, 129.47, 127.11, 126.72, 126.07, 126.02, 119.89, 112.10,
	112.23, 75.22 39.76, 34.02, 32.12, 31.60, 29.89, 29.75, 29.57, 25.58, 22.89, 14.30.
	: 0.44 g, 26%. 1 H-
	NMR (300 MHz, CDCl3) δ 9.09 (s, 2H), 8.93 (d, J = 8.7 Hz), 7.22 (dd, J = 5.1 Hz, 1.6 Hz, 2H), 7.08
	(d, J = 1.5 Hz, 2H), 4.94 -4.90 (m, 2H), 3.84 (t, J = 7.2 Hz, 4H), 2.44 (t, J = 6.9 Hz, 4H), 2.06 -2.01
	(m, 4H), 1.53(br, 4H), 1.23 (br, 112H), 0.87 (t, J = 6.6 Hz, 12H). 13 C-NMR (75 MHz, CDCl 3 ) δ 172.57,
	167.19, 151.96, 146.75, 135.74, 131.74, 129.47, 127.01, 126.72, 126.07, 126.02, 119.89, 112.10,
	111.20, 75.22 39.76, 34.26, 32.12, 31.60, 29.89, 29.75, 29.57, 25.58, 22.89, 14.30.
	cii

J = 1.5 Hz, 2H), 4.94 -4.90 (m, 2H), 3.83 (t, J = 7.2 Hz, 4H), 2.44 (t, J = 6.9 Hz, 4H), 2.

Di(heptatriacontan

-19-yl) 4,4'-((3E,3'E)-(2,6-dioxobenzo[1,2-b:4,5-b']difuran-3,7(2H,6H)diylidene)bis(6-bromo-2-oxoindoline-1-yl-3-ylidene))dibutyrate (10c). Yield: 0.44 g, 24%. 1 H-

Table 4 - 1

 41 Molecular Weight, thermal stability, electrochemical and photophysical properties of PNDIOT and PNDIT.

	Polymer	Mn (kDa)	PDI	T d (°C)	HOMO/LUMO (eV)	E g (eV)	λ max (nm) film	sol (nm) λ max
	PNDIOT	68.5	2.3	370	-5.98/-4.08	1.89	341, 598	336, 588
	PNDIT a	23.9	1.3	430	-5.77/-4.0	1.77	341,598	326, 542
	a

Table 4 -

 4 2 A summary of the OTFT performance of PNDIOT and P(NDI2OD-T2).

	Polymer	Environment	T Ann (°C)	µ e,avg (std) (cm 2 V -1 s -1 )	V th (V)	I on /I off	µ h,avg (std) (cm 2 V -1 s -1 )	V th (V)	I on /I off
			100	0.0020 (0.0005)	15	~10 3
	PNDIOT	Nitrogen	150 200	0.0041 (0.0009) 0.0016 (0.0002)	13 21	~10 4 ~10 3
		Air	150	0.0028 (0.0008)	33	~10 4
			100	0.0087 (0.001)	10	~10 3
	P(NDI2O	Nitrogen	150	0.011 (0.002)	9	~10 3
	D-T2)		200	0.010 (0.001)	11	~10 3
		Air	150	0.0022 (0.0008)	20	~10 3	2.6×10 -4 (1.0×10 -4 )	-65

Table 4 -

 4 3 A summary of all-PSCs based on different acceptors

	Acceptor	J SC (mA/cm 2 )	V OC (V)	FF	PCE (%) µ e (cm 2 /V s)	µ h (cm 2 /V s)
	PNDIOT	7.11	0.75	0.61	3.25 (3.03) a	6.93×10 -5	1.35×10 -4
	P(NDI2OD-T2) b	9.4	0.81	0.49	3.70 (3.40) a	2.0×10 -3	2.0×10 -4
	PNDIT c	8.84	0.80	0.43	3.03 (2.87) a	6.7×10 -6	2.0×10 -4

All devices used PTB7-Th as the donor and the same device configuration: ITO/PEDOT:PSS/Polymer Blend/LiF/Al; a The average value was obtained from a batch of 4 devices; b The numbers can be found from the reference.

[START_REF] Li | High Performance All-Polymer Solar Cells by Synergistic Effects of Finecxl v Tuned Crystallinity and Solvent Annealing[END_REF]

c The numbers can be found from the reference.

[START_REF] Choi | Importance of Electron Transport Ability in Naphthalene Diimide-Based Polymer Acceptors for High-Performance, Additive-Free, All-Polymer Solar Cells[END_REF]

Table 4 - 4

 44 The optimization of D/A ratio in the active layer

	D/A ratio	Thickness (nm)	J SC (mA/cm 2 )	V OC (V)	FF	PCE (%)
	1:1	98 ± 6	5.89	0.75	0.59	2.61 (2.50) a
	1.5:1	92 ± 5	6.06	0.75	0.48	2.18 (2.02) a
	1:1.5	97 ± 5	4.12	0.74	0.52	1.58 (1.39) a

Table 4 -

 4

	5 The optimization of active layer thickness
	D/A ratio	Thickness (nm)	J SC (mA/cm 2 )	V OC (V)	FF	PCE (%)

Table 4 - 6

 46 Device performance optimization through post-treatment

	Post-treatment	Duration	J SC (mA/cm 2 )	V OC (V)	FF	PCE (%)
	None	N/A	6.73	0.75	0.59	2.92 (2.68) a
	Solvent-annealing	15 min 30 min	6.77 6.09	0.74 0.74	0.55 0.51	2.73 (2.55) a 2.30 (2.04) a
	Film-aging	24 hrs 72 hrs	7.11 6.88	0.75 0.75	0.61 0.61	3.25 (3.03) a 3.15 (3.00) a

Acknowledgements I would like to express my sincerest gratitude to my two co-advisors, Dr. Yuning Li and Dr. Dario Bassani. Their expertise and insights have helped me greatly during the course of my PhD study. Moreover, their patience and continuous guidance allowed me to develop my research skills and gain knowledge every day in the last four years. I am very much thankful for Dr. Wei Hong, who taught me all the organic synthesis skills that have got me though all those synthetic challenges throughout my PhD study, and Dr. Jesse Quinn, who has been a wonderful colleague and friend on every aspect in both research and life. I would like to thank all the other colleagues Dr. Bin Sun, Dr. Chang Guo, Dr. Yunfeng Deng, Dr.

Mylène LeBorgne and Dr. Shaoyun Chen that I have worked with, for their support and kind friendship.

Appendix