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The work carried out in this thesis systematically investigates the correlation between microstructures and electronic properties of organic semiconductors. The major directions that were pursued in this thesis are: (i) studies on structure-property relationship by rational design and synthesis of monodisperse oligomers with varying chain-lengths (ii) role of electronic properties and aggregation (microstructures) in governing singlet fission (SF). In the first part of the thesis, the optical, structural and charge transport properties of Diketopyrrolopyrrole (DPP)-based oligomers, as a function of the chain length, has been discussed. The energy bands became wider with an increase in chain length and a gain in backbone electron affinity was observed, with an offset in microstructural order. With an increase in chain length, the tendency to form intramolecular aggregates increased as compared to intermolecular aggregates due to the onset of backbone conformational defects and chain folding. An insight into the solid-state packing and microstructural order has been obtained by steady-state and transient spectroscopy, grazing incidence small angle x-ray scattering (GISAXS), atomic force microscopy (AFM) and transmission electron microscopy (TEM) studies. The charge-carrier mobilities varied in accordance with the degree of microstructural order as: dimer > trimer > pentamer. A library of DPP-DPP based trimers was also generated by modifying the donor chromophore (phenyl, thiophene and selenophene) in the oligomer backbone. Highest n-channel mobility of ~0.2 cm2V-1s-1 was obtained which validated that: (a) the effect of solid-state packing predominates the effect of backbone electronic structure on charge carrier mobility. Although oligomers possess lesser backbone defects than polymers in general, their charge carrier mobilities were not comparable to that of 2DPP-OD-TEG polymer, which forms highly oriented and isotropic edge-on crystallites/microstructures in the thin film, shows high n-channel mobility of 3 cm2V-1s-1 and band-like transport ;(b) although delocalized electronic states are achieved at greater chain lengths, the degree of solid-state microstructural order drastically reduces which leads to lower charge carrier mobilities; (c) conformational collapse resulted in lower electron mobilities and an increase in ambipolarity. The later part of the thesis debates on the relative contribution of electronic structure and aggregation (microstructures) in governing singlet fission (SF). Motivated by the recent SF model in carotenoid aggregates, a DPP-DPP based oligomer was synthesized by incorporating a vinylene bridge to imbue “polyene” character in the chromophore. Transient Spectroscopy (TA) measurements were carried out to monitor the formation of triplet states in the oligomer and to probe the occurrence of singlet fission. Although the oligomer exhibits “polyene” character like a typical “carotenoid aggregate”, it did not show singlet fission because of the additional stabilization of the singlet (S1) state which reduces the ∆EST. This study rationalized the importance of judicious control of band structures as well as microstructures to observe the SF phenomenon in this category of chromophores. The novel synthetic protocol provides the scope to tailor DPP-DPP based materials with desired effective conjugation lengths and side chains and can foreshow great prospects for future generation of organic electronics.

Dans leurs usages courants comme isolants électriques, les matériaux solides organiques sont constitutifs aussi bien des câbles de transport d'énergie électrique, des circuits de commande et de conversion de puissance que des composants (micro)électroniques ou des systèmes embarqués (revêtement thermique des satellites, batteries d'accumulateurs...). La diversité des contraintes d'utilisation auxquelles ils sont soumis (champ électrique, rayonnement, température, humidité...) les prédisposent à emmagasiner des charges en leur sein, susceptibles d'affecter la fiabilité des systèmes qui en dépendent. L'un des moyens communément mis en œuvre pour étudier le comportement électrique de ces charges est la mesure de la distribution spatio-temporelle des charges d'espace, en soumettant le diélectrique à une différence de potentiel continue à travers deux électrodes. Cette méthode ne permet cependant pas toujours de distinguer clairement la contribution des charges dues à la génération, d'une part, et celles dues aux phénomènes de transport, d'autre part. Cette étude propose une approche alternative, consistant à déposer sous vide des charges (électrons) au sein de l'isolant par le biais d'un faisceau d'électrons, à une position connue et en quantité maîtrisée, en prenant en compte d'autres processus physiques liés à l'implantation d'électrons afin de prévoir et modéliser le comportement de ces matériaux irradiés. Des films de PolyEthylène basse densité (PEbd), préparés par thermomoulage, ont été irradiés par un faisceau d'électrons de 80 keV avec un flux de 1 nA/cm2. Les mesures de charge d'espace par la méthode Electro-Acoustique Pulsée (PEA), réalisées d'abord in-situ, puis ex-situ sous polarisation électrique DC, confirment une localisation effective de charges au sein du matériau. Les résultats sous polarisation électrique après irradiation mettent en évidence une importante présence de charges positives dans la zone irradiée du diélectrique. Les caractérisations électriques des films PEbd irradiés montrent un comportement complètement différent de celui d'un même matériau non-irradié, laissant penser à une modification de la structure chimique du matériau. Des mesures physico-chimiques (spectroscopie infra-rouge, Photoluminescence et Analyse Enthalpique Différentielle-DSC) sur ces films PEbd irradiés, ne montrent pas une dégradation significative de la structure chimique du diélectrique qui expliquerait le comportement électrique observé sous polarisation post-irradiation. Des mesures complémentaires montrent le comportement réversible du PEbd irradié puis polarisé, qui serait uniquement lié à la présence des charges générées par le faisceau. Les données expérimentales de cette étude ont parallèlement alimenté un modèle numérique de transport de charges, développé pour tenir compte des contraintes sous irradiation. Ce modèle a permis de reproduire les résultats d'implantation de charge par faisceau d'électrons in-situ ainsi que la majorité des processus électriques observés sur du PEbd irradié puis polarisé. Il confirme l'impact de la charge déposée par faisceau d'électrons sur le comportement sous polarisation et permet de conclure quant à l'origine des charges positives observées post-irradiation, qui seraient dues aussi bien aux phénomènes d'injection aux électrodes qu'à la création de paires électrons/trous par le faisceau d'électrons pendant l'irradiation. / In their common uses as electrical insulators, organic solid materials are constitutive of electric power transmission cables, power control and conversion circuits as well as (micro) electronic components or embedded systems (thermal coating of satellites, batteries of accumulators, etc.). Under various constraints of use (electric field, radiation, temperature, humidity ...) they can accumulate charges in their bulk which could affect the reliability of the systems in which they are employed. One of the commonly used means to study the electrical behavior of these charges is to measure the spatiotemporal distribution of charges by subjecting the dielectrics to a continuous potential difference between two electrodes. However, this method does not always allow clearly distinguishing the contribution of charges due to generation on the one hand and the one due to transport phenomena on the other hand. This study proposes an alternative approach, consisting in generating charges (electrons) within the electrical insulation using an electron-beam under vacuum. The charges are hence deposited at a known position and in a controlled quantity. Other physical processes related to the implantation of electrons must then be taken into account in order to predict and model the behavior of these irradiated materials. Low-density polyethylene (LDPE) films, prepared by thermal molding, were irradiated by a 80 keV electron-beam with a current flux of 1 nA/cm2. Space charge measurements using the Pulsed Electro-Acoustic (PEA) method, performed first in-situ and then ex-situ under DC electrical polarization, confirm an effective localization of charges within the material. The results under electrical polarization after irradiation show an important amount of positive charges in the irradiated zone of the dielectric. The electrical characterizations of irradiated LDPE films show a completely different behavior compared to the same non-irradiated material, suggesting a modification of the chemical structure of the material. Physico-chemical measurements (infrared spectroscopy, Photoluminescence and Differential Scanning Calorimetry-DSC) on these irradiated PEbd films do not show a significant degradation of the chemical structure of the dielectric which would explain the observed electrical behavior under post-irradiation polarization. Additional measurements show the reversible behavior of the irradiated then polarized PEbd, which would be only related to the presence of the charges generated by the beam. The experimental data of this study have simultaneously fed a numerical model of charge transport, developed to take into account the irradiation constraints. This model allows reproducing the in-situ results of charge implantation by the electron beam as well as the majority of the electrical processes observed on irradiated and polarized LDPE. It confirms the impact of the electron-beam deposited charge on the behavior under polarization and allows concluding on the origin of the positive charges observed after irradiation, which would be due to injection at the electrodes as well as to the creation of electron-hole pairs by the electron-beam during irradiation.

Comprendre et modéliser le comportement des matériaux sous irradiation électronique est un enjeu important pour l’industrie spatiale. La fiabilité des satellites nécessite de maîtriser et prédire les potentiels de surface s'établissant sur les diélectriques. Ce travail de doctorat a donc pour objectif de caractériser et de modéliser les différents mécanismes physiques (en surface et en volume) gouvernant le potentiel de charges dans les matériaux polymères spatiaux tels que le Téflon® FEP et le Kapton® HN. La mise au point d'un nouveau dispositif et d'un protocole expérimental a permis de corroborer l'existence d’une conductivité latérale des charges, souvent négligée dans les modèles physiques et numériques. Les études paramétriques, révélant l’influence de l’énergie et le flux des électrons incidents, ont permis de brosser un portrait des processus mis en jeu pour le transport (par saut ou par piégeage/dépiégeage) de charges en surface. A la lumière de cette étude, une conductivité équivalente est extraite, assimilant le matériau à un système prenant en compte les mécanismes de transport volumique et surfacique. L'analyse des évolutions non-monotones de potentiel mesurées sur les polymères spatiaux en condition spatiale a permis de révéler une dépendance de la conductivité volumique induite sous irradiation avec la dose reçue. L'étude paramétrique réalisée sur les mécanismes de transport en volume révèle une influence minoritaire du déplacement du barycentre de charges et du vieillissement physicochimique. Un modèle «0D» à un seul niveau de pièges, prenant en compte les mécanismes de piégeage/dépiégeage et recombinaison entre les porteurs de charges, a été développé. Ce modèle simplifié permet de reproduire qualitativement les évolutions de potentiel expérimentales en fonction du débit de dose et lors d'irradiations successives. / Charging behaviours of space dielectric materials, under electron beam irradiation, is of special interest for future spacecraft needs, since this mechanism could induce electrostatic discharges and consequently damages on the sensitive systems on board. In order to assess the risks of charging and discharging, this work aims at understanding the overall charge transport mechanisms and predicting the electrical behaviour of the insulator materials, especially Teflon® FEP and Kapton® HN. For an optimized prediction, the first part of our work is thus to check whether lateral conduction process can take place in the overall charge transport mechanism. Through the definition of a new experimental set-up and protocol, we have been able to discriminate between lateral and bulk conductivity and to reveal the presence of lateral conductivity that is enhanced by radiation ionization processes. We have been able to demonstrate as well that lateral intrinsic conductivity is enhanced with the increase current density and when approaching the sample surface. The second part of our work deals with the characterization of the electrical charging behaviour of Teflon® FEP under multi-energetic electron beam irradiation and the modelling of the overall bulk charge transport mechanisms. An experimental study on charge potential evolution as a function of electron spectrum, electric field, relaxation time, dose and dose rate, was performed. A numerical model has been developed to describe the effect of the different abovementioned mechanisms on the evolution of the surface potential. This model agrees correctly with the experimental phenomenology at qualitative level and therefore allows understanding the physical mechanisms steering charge transport in Teflon® and Kapton®.

Understanding the fundamentals of Organic semiconductors is crucial aspect towards the technological applications. Conjugated polymers have shown many interesting physical properties. Especially the electronic and optical properties of these materials have great impact on the daily life. Much work has been devoted to gain the knowledge on the electrical and photo physical properties of these materials. Despite the large number of studies in fabrication and characterisation on these devices some of the fundamental properties like charge transport, carrier generation and its control by doping are not well accomplished. The Thesis consists of 6 chapters. First chapter is a brief introduction on various properties of semiconducting polymers. Different charge transport models and their basic mechanisms are discussed. Chapter 2 discusses the synthesis, device making and experimental methods used to characterise the polymer devices. Chapter 3 is focused on transport properties in polypyrrole devices and its variation with different experimental conditions. Chapter 4 is aimed to understand the anomalies in the current-voltage characteristics appearing in some of the thiophene based devices. In Chapter 5, the impedance measurement technique is used to characterise the poly (3-hexylthiophene) devices and the outcomes are presented and chapter 6 summarises all the experimental results obtained in this thesis work and presents some future aspects and directions. Chapter 1: Some of the basic properties and recent advancements in the field of organic semiconductors are discussed in this chapter. Organic semiconductor devices based on conjugated polymers are now becoming alternatives to inorganic semiconductors in many fields. Mobility of these conjugated polymers can be increased by adding dopants and also by choosing appropriate metal contacts for charge injection and extraction. The complexity of the metal-polymer interfaces can be better understood by varying the carrier density and studying their transport properties with various experimental tools. Chapter 2: The polymer films prepared in this study are electrochemically deposited on to various conducting substrates. The doping and de-doping of the carriers is done by passing a current and reversing its polarity for different time intervals. Device structures for the measurements are obtained by making a top contact on top of the polymer layer. The current-voltage (I-V) and impedance measurements are carried out in metal-polymer-metal geometry. Temperature dependent studies down to 10 K were performed in a continuous flow cryostat to understand the role of temperature in transport studies. Impedance and light measurements are also carried out in the same geometry. Chapter 3: Transport measurements on polypyrrole devices have shown a space-charge limited (SCLC) conduction, which is also known as bulk property of the materials. I-V curves displayed non-ohmic behaviour at higher voltages and by varying the carrier density the devices show a transition from trap controlled SCLC to trap free/trap filled SCLC. Traps distribution and energies are estimated from the temperature dependent I-V measurements. Poole-Frenkel behaviour, i.e. field dependent mobility is observed in all the polypyrrole samples. The zero field mobility follows Arrhenius behaviour at higher temperatures. Also the temperature variation of mobility displays field dependent and field independent regimes in fully doped and lightly de-doped polypyrrole films. A zero-bias anomaly is observed as the field goes to zero value below 50 K, due to coulomb-blockade transport. Capacitance measurements have shown pseudo inductive behaviour at higher bias, which is also connected with trap-filling regime of PPy devices. Chapter 4: Current-Voltage anomalies are observed in polythiophene (PTh) and poly (3-methylthiophene) [P3MeT] based devices. The origin of this anomaly is not straight forward in polymer devices, so we investigated it in detail. We propose this is a property specific to the above two materials from various experimental studies. The anomalous behaviour appears when the bias is swept from negative to positive keeping the substrate deposited with polymer as anode. The magnitude of peak to valley current ratio (PVCR) which characterises the merit of device can be varied more than two orders of magnitude by varying the carrier density and as well as by varying scan rate. Since the trap states are also one of the reasons for the origin of this anomaly the rate of filling of these states can be helpful in tuning the magnitude of PVCR. Photo generated carriers in the above devices also help in tuning and controlling the magnitude of anomaly, which can make this device as a suitable candidate for opto-electronic studies. Different conductive substrates such as indium tin oxide, platinum, gold and stainless steel are used for deposition of the above polymers. Top contacts (gold, silver and aluminium) have been also varied to understand the origin of this anomaly. Anomalies are observed with all these different substrates and different top contacts. Finally impedance measurements have shown an elongated tail in the Cole-Cole plot in the region of NDR. Chapter 5: Impedance measurements on poly (3-hexylthiophene) devices have shown different relaxation mechanism by varying the doping concentration. For moderately doped devices the relaxation mechanism is classical Debye-type, whereas for highly de-doped samples the relaxation time of the carriers is distributed. Charge transport parameters such as contact resistance, mobility and conduction mechanism details can be obtained by identifying and fitting the data to the equivalent circuit model. The relaxation time of the carriers can give rough estimation of mobility and capacitance. The capacitance variation with applied bias gives the nature of conduction mechanism in the devices. If the capacitance variation is unaffected by the applied bias the transport is bulk limited, if it changes significantly the transport can be considered as either contact limited or depletion layer controlled. Current-Voltage measurements also show that Schottky behaviour is present in all the devices. The rectification ratio varies with doping concentration; at one optimum doping concentration the rectification is very high. I-V measurements on P3MeT devices with varying carrier density have shown a transition in the conduction mechanism from SCLC to contact limited. In the devices with less carrier density the contact limited mechanism is dominating at lower bias voltage and as the bias increases the bulk limited transport takes over. This highlights the role of carrier density in the transport mechanism. Chapter 6: The conclusions from all the works presented in the thesis are summarised in this chapter. Some of the future directions works are presented.

In the recent years area organic photovoltaics is generating a lot of interests because whole process of synthesis and fabrication is less energy intensive process as well as it is cost effective compared to conventional inorganic Si based photovoltaic technology. This work mainly deals with the fabrication and optimization of device fabrication conditions for organic photovoltaic materials. In first part of the work, the solar cell fabrication conditions were optimized for the commonly used system P3HT – PCBM. The fabricated device was optimized for the solvents used for the active material, concentration of the active material solution, donor- acceptor ratio of the active material, annealing conditions of the active layer and the metal evaporation conditions for the cathode. All the optimization procedures were carried out in controlled atmosphere to minimize the environmental effect inference during fabrication of the solar cell devices. All the characterization was carried out at ambient conditions. The efficiency of the solar cell was improved from 0.009% to 6.2%. the environmental stability of the fabricated devices were carried out after encapsulating it with epoxy based resin in both ambient conditions as well as extreme conditions like 85% RH at 25°C inside the humidity chamber. It was observed that both the data matches well with each other indicating proper encapsulation required to safe guard the device for the better performance over the period of time. Second part of this work mainly deals with understanding the structure property relationship of thiophene based donor- acceptor- donor molecule 2,5-dithienyl-3,4-(1,8-naphthylene) cyclopentadienone (DTCPA), which is highly crystalline, low band gap organic molecule which absorbs over entire visible region of the solar spectra. DTCPA crystals of various morphologies were prepared by various recrystallization routes. It was observed that macro scale morphology of these crystals differs from each other. Also depending on the method of recrystallization sizes of the crystals also varies. All the recrystallized DTCPA shows strong orientation toward (001) direction. However, it was observed that lattice parameters of these crystals slightly differ from each other owing to the recrystallization methodology. These variations in crystal parameters are more than 0.02 which is significant. It was also observed that the crystallite sizes depend on the recrystallization routes. Slow evaporation of concentrated solution (SEC) grown crystals has the larger crystallite size of 170nm. It was observed that absorption range of these crystals slightly differ from each other owing to the change in the crystallite sizes and crystal parameters. Third part of this work deals with the fabrication and optimization of thermal evaporation process of DTCPA for photovoltaic applications. DTCPA is stable at higher temperatures as well as has sharp melting point which make it ideal candidate for thermal evaporation. In this work films of DTCPA were fabricated for various evaporation rates by thermal evaporation technique. Chemical integrity of the molecules upon evaporation is found to be intact as observed from FTIR spectroscopy. XRD shows that at lower (25 W/m2) as well as higher (40 W/m2) films are oriented to (001), (400) as well as (311) directions, at 30 W/m2 and 35 W/m2 there is a strong orientation towards (311) and (001) directions respectively. Photo luminescence studies indicate that there is strong 410 nm emission for films deposited at the power of 25 W/m2 and 40 W/m2. Microscopic studies confirm that morphology is dependent on the deposition rates as it changes with the change in deposition rate. This in turn reflects in the device characteristics of these films. It was observed that films deposited at high deposition rates show better device characteristics with high VOC and current density values. All these device fabrication and characterizations were carried out in ambient conditions. Fourth part of this work deals with P3HT - DTCPA composites which exhibit wide range of light absorption. It was observed that DTCPA act as nucleating centers for the P3HT molecules and increases crystallinity in the composite. Furthermore, DTCPA helps in exciton separation because of donor and acceptor moieties present in the molecule. It also helps in charge transportation because of its crystalline nature and further it induces molecular ordering in the P3HT matrix. The band diagram of P3HT- DTCPA suggests that the band edges of both materials are ideal for charge separation. In addition, crystalline nature of the DTCPA molecule helps in effective charge transportation. J-V characteristics shows that there is large built in potential in the devices from these blends leading to large Voc. Composites with lower DTCPA loadings show higher efficiency than with higher loadings. These devices were prepared in ambient conditions and needs to be optimized for obtaining better device properties. In the fifth part of the work two types of system were studied to understand the band edge matching on the photovoltaic properties, carbazole based copolymers and DTCPA based copolymers. In the case of carbazole based copolymers it was observed that by copolymerizing carbazole with thiophene based derivatives lowers the band gap and modifies the HOMO and LUMO levels for better suit for the photovoltaic device fabrication. It was observed that that is two orders of improvements in the efficiency by co polymerizing carbazole with benzothiodizole as improves the JSC and VOC. Also the copolymerization of carbazole with both benzothiodiazole and bithiophene results in better light harvesting as the optical band gap was lowered. In the case of DTCPA copolymers with DTBT and DHTBT as both are random copolymers the solubility was low as well as their HOMO band edge was mismatched with the PEDOT: PSS which is a hole transport layer. However, the alternate polymerization of DTCPA with DTBT improved the band edge matching and also the solubility. As a result there was tenfold improvement in the charge collection and hence the efficiency was improved from 0.02% to 2.4%. Many of the conducting polymers have good material property but poor filmability. In the sixth part of this work deals with fabrication of device quality films by alternate deposition technique like pulsed laser deposition. Two types of system were studied in this work (i) polypyrrole- MWCNT nanocomposites and (ii) Poly DTCPA polymer. In both the cases it was observed that chemical integrity of the polymer retained during ablation. PolyDTCPA films were fabricated by pulsed laser deposition by both IR (Nd-YAG) and UV (KrF) laser source. Morphological studies indicate that IR laser ablated films were particulate in nature whereas UV laser ablated films were grown as continuous layers as polyDTCPA absorbs better in UV region. As a result the IV characteristics indicate that IR laser ablated films are resistive in nature and UV laser ablated films are good rectifiers indicating the suitability of the process for fabrication of device quality films.

Dans cette thèse, des films à base de graphène ont été étudiés comme alternatives viables dans la fabrication d'électrodes transparentes (TCE). Elle met l'accent sur des couches fines de graphène (FLG), sur l'oxyde de graphène réduit (RGO) et leurs hybrides avec des nanotubes de carbone (NTCs) pour être utilisé comme TCE dans les cellule solaires organiques (OSC). Les FLGs et RGO ont été préparés par des méthodes d'exfoliation mécanique ou en phase liquide assistée par micro-ondes. Ces nanomatériaux dilués dans un solvant liquide ont été déposé en couche mince par aérographe. Des caractérisations de transport de charge ont été obtenues grâce à la méthode des 4 pointes. Ces échantillons ont été caractérisés: leur transparence(UV-Visible), leur morphologie et leur topographique (MEB, MET, AFM) ainsi que le travail de sortie (UPS). Pour obtenir des informations sur la qualité structurelle des échantillons, nous avons utilisés les méthodes de spectroscopie XPS, Raman et la photoluminescence. / Graphene mate rials have been researched as viable alternatives of transparent conductors electrodes (TCEs) in this thesis. Current study focuses on few layer graphene (FLG), reduced graphene oxide (rGO) and their hybrids with carbon nanotubes (CNTs) for TCE applications inorganic solar cells (OSCs). FLGs and rGOs have been prepared by mechanical and microwave-assisted exfoliation methods. This films of these materials have been produced by hot-spray method. Results of charge transport characterizations by four-point probes, transparency (UV-Vis), measurements, along with morphological (SEM, TEM) and topgraphic (AFM) studies of films have been presented. UPS studies were performed to determine for a work-function. XPS,Raman and Photoluminescence studies have been employed to obtain the information about the structural quality of the samples.

La structure chimique des semiconducteurs organiques utilisés dans les cellules photovoltaïques à base d’hétérojonction en volume peut fortement influencer les performances du dispositif final. Pour cette raison, une meilleure compréhension des relations structure-propriétés demeure cruciale pour l’amélioration des performances. Dans ce contexte, cette thèse fait état d'études approfondies du transport des charges, de la morphologie et des propriétés photovoltaïques sur de nouveaux copolymères à faible bande interdite. En premier lieu, l'impact de la position des chaînes alkyles sur les propriétés opto-électroniques et morphologiques a été étudié sur une famille de polymères. Les mesures du transport de charges ont montré que la planéité du squelette du copolymère influe sur l’évolution de la mobilité des charges avec la concentration de porteurs libres. Ce comportement suggère que le désordre énergétique électronique est fortement impacté par les angles de torsion intramoléculaire le long de la chaîne conjuguée. Un second copolymère à base d'unités accepteur de [2,1,3] thiadiazole pyridique, dont les niveaux d’énergie des orbitales frontières sont optimales pour l’application photovoltaïque, a ensuite été étudié. Les performances obtenues en cellule photovoltaïque sont très inférieures aux attentes. Des analyses de la morphologie et du transport de charge ont révélé que l’orientation des lamelles cristallines est défavorable au transport perpendiculaire au film organique et empêche ainsi une bonne extraction des charges photo-générées. Enfin, les propriétés opto-électroniques et photovoltaïques de copolymères fluorés ont été étudiées. Dans ce cas, les atomes de fluor favorisent la formation de lamelles orientées favorablement pour le transport. Ces bonnes propriétés nous ont permis d'atteindre un rendement de conversion de puissance de 9,8% avec une simple hétérojonction polymère:fullerène. / The chemical structure of organic semiconductors that are utilized in bulk heterojunction photovoltaic cells may strongly influence the final device performances. Thus, better understanding the structure-property relationships still remains a major task towards high efficiency. Within this framework, this thesis reports in-depth material investigations including charge transport, morphology and photovoltaic studies on various novel low band-gap copolymers. First, the impact of alkyl side chains on the opto-electronic and morphological properties has been studied on a series of polymers. Detailed charge transport investigations showed that a planar conjugated polymer backbone leads to a weak dependence of the charge carrier mobility on the carrier concentration. This observation points out that the intra-molecular torsion angle contributes significantly to the electronic energy disorder. Solar cells using another novel copolymer based on pyridal[2,1,3]thiadiazole acceptor unit have been studied in detail next. Despite the almost ideal frontier molecular orbital energy levels, this copolymer did not perform in solar cells as good as expected. A combined investigation of the thin film microstructure and transport properties showed that the polymers self-assemble into a lamellar structure with polymer chains being oriented preferentially “edge-on”, thus hindering the out-of-plane hole transport and leading to poor charge extraction. Finally, the impact of fluorine atoms in fluorinated polymers on the opto-electronic and photovoltaic properties has been investigated. In this case, the presence of both flat-lying and standing lamellae enabled efficient charge transport in all three directions. As a consequence, good charge extraction was possible and allowed us to achieve a maximum power conversion efficiency of 9.8%.

Organic solar cells have great potential for cost-effective and large area electricity production, but their applicability is limited by the relatively low efficiency. In this dissertation I report investigations of novel materials and the underlying principles of organic solar cells, carried out at the University of St Andrews between 2011 and 2015. Key results of this investigation: • The charge carrier mobility of organic semiconductors in the active layer of polymer solar cells has a rather small influence on the power conversion efficiency. Cooling solar cells of the polymer:fullerene blend PTB7:PC₇₁BM from room temperature to 77 K decreased the hole mobility by a factor of thousand but the device efficiency only halved. • Subphthalocyanine molecules, which are commonly used as electron donor materials in vacuum-deposited active layers of organic solar cells, can, by a slight structural modification, also be used as efficient electron acceptor materials in solution-deposited active layers. Additionally these acceptors offer, compared to standard fullerene acceptors,advantages of a stronger light absorption at the peak of the solar spectrum. • A low band-gap polymer donor material requires a careful selection of the acceptor material in order to achieve efficient charge separation and a maximum open circuit voltage. • Metal structures in nanometer-size can efficiently enhance the electric field and light absorption in organic semiconductors by plasmonic resonance. The fluorescence of a P3HT polymer film above silver nanowires, separated by PEDOT:PSS, increased by factor of two. This could be clearly assigned to an enhanced absorption as the radiative transition of P3HT was identical beside the nanowires. • The use of a processing additive in the casting solution for the active layer of organic solar cells of PTB7:PC₇₁BM strongly influences the morphology, which leads not only to an optimum of charge separation but also to optimal charge collection.

No description available.