<p>In this thesis, the microstructure of organic photovoltaic APFO-3:PC₆₁BM bulk-heterojunction blends was examined. Earlier studies have focused on the microstructure after spin coating. This thesis aims to give a better insight into microstructural degradation as the films are annealed above the glass transition temperature, T_g, and the mixture approaches thermodynamic equilibrium. Electro- and photoluminescence studies indicate that the polymer and PC₆₁BM are intermixed on a scale shorter than the exciton diffusion length of 10 nm, even when annealed above T_g. The temperature stability of APFO-3:PC₆₁BM was also investigated with respect to the molecular weight of the polymer. The photovoltaic performance of these blends was found to be stable up to temperatures approaching the glass transition temperature, especially if a high molecular-weight APFO-3 grade was used.</p><p> </p><p>The crystallization of PC₆₁BM was also investigated. Above T_g, PC₆₁BM crystallization was found to commence, albeit slowly at temperatures close to T_g. At elevated temperatures instead, micrometer sized crystals were observed to form. It was also noted that illumination while annealing APFO-3:PC₆₁BM thin films above T_g affected PC₆₁BM crystallization, the origin of which is so far unclear although chemical degradation could be largely excluded.</p>

In this thesis, the microstructure of organic photovoltaic APFO-3:PC61BM bulk-heterojunction blends was examined. Earlier studies have focused on the microstructure after spin coating. This thesis aims to give a better insight into microstructural degradation as the films are annealed above the glass transition temperature, Tg, and the mixture approaches thermodynamic equilibrium. Electro- and photoluminescence studies indicate that the polymer and PC61BM are intermixed on a scale shorter than the exciton diffusion length of 10 nm, even when annealed above Tg. The temperature stability of APFO-3:PC61BM was also investigated with respect to the molecular weight of the polymer. The photovoltaic performance of these blends was found to be stable up to temperatures approaching the glass transition temperature, especially if a high molecular-weight APFO-3 grade was used.   The crystallization of PC61BM was also investigated. Above Tg, PC61BM crystallization was found to commence, albeit slowly at temperatures close to Tg. At elevated temperatures instead, micrometer sized crystals were observed to form. It was also noted that illumination while annealing APFO-3:PC61BM thin films above Tg affected PC61BM crystallization, the origin of which is so far unclear although chemical degradation could be largely excluded.

Over the last few decades, bulk-heterojunction organic photovoltaic devices comprising an intimately mixed donor-acceptor blend have gained serious attention due to their potential for being cheap, light weight, flexible and environmentally friendly. In this thesis, APFO-3/PCBM bulk-heterojunction based organic photovoltaic devices with an inverted layer sequence were investigated systematically. Doctor blade coating is a technique that is roll-to-roll compatible and cost efficient and has been used to fabricate the solar cells. Initial studies focused on optimization of the electrodes. A thin film of the conductive polymer PEDOT:PSS was chosen to be the transparent anode. Different PEDOT:PSS films with respect to the film thickness and deposition temperature were characterized in terms of conductivity and transmission. Decent conductance and transmittance were obtained in the films deposited with wet film thickness setting of 35 μm, The cathode was fabricated from a metal bilayer comprising Al and Ti with an area about 1 cm2, and the best-working cathodes contained a 70 nm thick Al layer covered by a thin Ti layer of about 10 -15 nm. Optimized coating temperature and wet film thickness settings for the active layer and PEDOT:PSS layer were experimentally determined. The highest efficiency of the APFO-3/PCBM based inverted solar cells fabricated by doctor blading was 0.69%, which exceeded the efficiency of spin-coated inverted cells. A higher efficiency (0.8 %) was achieved by adding a small amount of high molecular weight polystyrene to the active layer. Morphological changes after adding of the polystyrene were observed by optical microscopy and AFM. A coating temperature dependent phase separation of the APFO-3/PCBM/polystyrene blend was found.

In this dissertation novel work is presented showing the performance and device physics of Organic Field Effect Transistors (OFETs) and bulk heterojunction Organic Photovoltaic (OPV) devices fabricated using novel acceptor small molecules. Pentacene and N,N’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN₂) were used as the active layer in p-channel and n-channel Organic Field Effect Transistors (OFETs), respectively, and novel pulsed voltage transient measurements were developed in order to extract transient mobilities and carrier velocities from the transistor response of the device, which were well correlated with the corresponding DC OFET characteristics. A distributed RC network was used to model the OFET’s channel and the transient and DC characteristics of the devices were successfully reproduced. Temperature dependent studies of the DC field effect mobilities and transient mobilities of these two materials were carried out and the results used to extract information on charge carrier transport in the materials at varying time scales. Open-circuit voltages of the OPV devices are correlated with the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) levels various acceptor small molecules and donor polymers comprising the active layers of the devices. / text

The present work addresses issues related to the derivation of reduced models of atomistic systems, their statistical calibration, and their relation to atomistic models of materials. The reduced model, known in the chemical physics community as a coarse-grained model, is calibrated within a Bayesian framework. Particular attention is given to developing likelihood functions, assigning priors on coarse-grained model parameters, and using data from molecular dynamics representations of atomistic systems to calibrate coarse-grained models such that certain physically relevant atomistic observables are accurately reproduced. The developed Bayesian framework is then applied in three case studies of increasing complexity and practical application. A freely jointed chain model is considered first for illustrative purposes. The next example entails the construction of a coarse-grained model for a liquid heptane system, with the explicit design goal of accurately predicting a vapor-liquid transfer free energy. Finally, a coarse-grained model is developed for an alkylthiophene polymer that has been shown to have practical use in certain types of photovoltaic cells. The development therein employs Bayesian decision theory to select an optimal CG potential energy function. Subsequently, this model is subjected to validation tests in a prediction scenario that is relevant to the performance of a polyalkylthiophene-based solar cell. / text

This dissertation focuses on characterizing the nanoscale and surface averaged electrical properties of transparent conducting oxide (TCO) electrodes such as indium tin oxide (ITO) and transparent metal-oxide (MO) electron selective interlayers (ESLs), such as zinc oxide (ZnO), the ability of these materials to rapidly extract photogenerated charges from organic semiconductors (OSCs) used in organic photovoltaic (OPV) cells, and evaluating their impact on the power conversion efficiency (PCE) of OPV devices. In Chapter 1, we will introduce the fundamental principles regarding the need for low cost power generation, the benefits of OPV technologies, as well as the key principles that govern the operation of OPV devices and the key innovations that have advanced this technology. In Chapter 2 of this dissertation, we demonstrate an innovative application of conductive probe atomic force microscopy (CAFM) to map the nanoscale electrical heterogeneity at the interface between an electrode, such as ITO, and an OSC such as the p-type OSC copper phthalocyanine (CuPc).(MacDonald et al. (2012) ACS Nano, 6, p. 9623) In this work we collected arrays of J-V curves, using a CAFM probe as the top contact of CuPc/ITO systems, to map the local J-V responses. By comparing J-V responses to known models for charge transport, we were able to determine if the local rate-limiting step for charge transport is through the OSC (ohmic) or the CuPc/ITO interface (nonohmic). These results strongly correlate with device PCE, as demonstrated through the controlled addition of insulating alkylphosphonic acid self-assembled monolayers (SAMs) at the ITO/CuPc interface. Subsequent chapters focus on the electrical property characterization of RF-magnetron sputtered ZnO (sp-ZnO) ESL films on ITO substrates. We have shown that the energetic alignment of ESLs and the organic semiconducting (OSC) active materials plays a critical role in determining the PCE of OPV devices and the appearance of, or lack thereof, UV light soaking sensitivity. For ZnO and fullerene interfaces, we have shown that either minimizing the oxygen partial pressure during ZnO deposition or exposure of ZnO to UV light minimizes the energetic offset at this interface and maximizes device PCE. We have used a combination of device testing, device modeling, and impedance spectroscopy to fully characterize the effects that energetic alignment has on the charge carrier transport and charge carrier distribution within the OPV device. This work can be found in Chapter 3 of this dissertation and is in preparation for publication. We have also shown that the local properties of sp-ZnO films varies as a function of the underlying ITO crystal face. We show that the local ITO crystal face determines the local nucleation and growth of the sp-ZnO films. We demonstrate that this effects the morphology, the chemical resistance to etching as well as the surface electrical properties of the sp-ZnO films. This is likely due to differences in the surface mobility of sputtered Zn and O atoms on these crystal faces during film nucleation. This affects the nanoscale distribution of electrical and chemical properties. As a result we demonstrate that the PCE, and UV sensitivity of the J-V response of OPVs using sp-ZnO ESLs are strongly impacted by the distribution of ITO crystal faces at the surface of the substrate. This work can be found in Chapter 4 of this dissertation and is in preparation for publication. These studies have contributed to a detailed understanding of the role that electrical heterogeneity, insulating barriers and energetic alignment at the MO/OSC interface play in OPV PCE.

Photovoltaics manufactured using organic materials as a substitute for inorganic materials may provide for cheaper production of solar cells if their efficiencies can be made comparable to existing technologies. Photovoltaic devices are comprised of layered structures where the electrical, chemical, and physical properties at the multiple interfaces play a significant role in the operation of the completed device. This thesis attempts to establish a relationship between interfacial properties and overall device performance by investigation of both the organic/organic heterojunction interface, as well as the interface between the inorganic substrate and the first organic layer with useful insights towards enhancing the efficiency of organic solar cells.It has been proposed that residual chemical species may act as barriers to charge transfer at the interface between the transparent conductor (TCO) and the first organic layer, possibly causing a large contact resistance and leading to reduced device performance. Previous work has investigated the surface of the TCO but no baseline characterization of carbon-free surfaces has previously been given. In this work clean surfaces are investigated to develop a fundamental understanding of the intrinsic spectra such that further analyses of contaminated surfaces can be presented systematically and reproducibly to develop a chemical model of the TCO surface.The energy level offset at the organic/organic heterojunction has been proposed to relate to the maximum potential achievable for a solar cell under illumination, however, few experimental observations have been made where both the interface characterization and device performance are presented. Photovoltaic properties are examined in this work with titanyl phthalocyanine used as a novel donor material for enhancement of spectral absorption and optimization of the open-circuit potential. Characterization of the interface between TiOPc and C60 coupled with characterization of the interface between copper phthalocyanine and C60 shows that the higher ionization potential of TiOPc does correlate to greater open circuit potentials.Examination of photovoltaic behavior using equivalent circuit modeling relates the importance of series resistance and recombination to the homogeneity of the solar cell structure.

Ce travail de thèse concerne l'élaboration de nouveaux copolymères à faible bande interdite de type « push-pull », constitués par une unité donneuse d'électrons (push) et une unité acceptrice d'électrons (pull) en modulant les relations structures-propriétés par stratégie de synthèse. Des copolymères constitués par des unités acceptrices d'électrons (dérivées du benzothiadiazole ou du thienopyrrolodione) et donneuses d'électrons (3,6-carbazole, 2,7-carbazole, dialkoxybezodithiophène) ont été obtenus par différentes méthodes de couplage carbone carbone (C-C). Des études physico-chimiques par des techniques de spectroscopie (UV-visible), d'électrochimie (voltampérométrie cyclique), de diffraction de rayon X et d'analyses thermogravimétriques ont été utilisées pour élucider les propriétés fondamentales des copolymères pour des applications dans le domaine du photovoltaïque organique. Des études de RPE sous éclairement couplées avec de la simulation théorique ont permis l'étude des différents transferts électroniques dans les copolymères push-pull en mélange avec deux types de matériaux accepteurs d'électrons (le PCBM et les nanocristaux de CuInS2). Des calculs de DFT ont mis en évidence une bonne corrélation avec les résultats expérimentaux. Des tests préliminaires en hétérojonctions volumiques sur les (co)polymères ont étés réalisés mettant en évidence les facteurs clés limitant les performances des dispositifs de photovoltaïques organiques. / In this work, the attention was focused on the synthesis of new low-band gap polymers and on the adopted chemical strategy aims on developing the so called push-pull copolymers: formed by a donor (push) and an acceptor (pull) electron unit in the polymer backbone. It was demonstrated that exploited building block approach can lead to copolymers with tunable physical properties. By selecting acceptor (benzothiadiazole or thienopyrrolodione derivatives) and donor (3,6-carbazole, 2,7-carbazole, dialkoxybenzodithiophene) units of different DA strength, it is possible to prepare copolymers through several C-C coupling methods. Detailed physico-chemical studies using complementary spectroscopic, electrochemical, diffraction and thermal techniques enabled the determination of synthesized push-pull copolymers properties, which are crucial for their photovoltaic application. Detailed studies on EPR under illumination and EPR tracing allowed the characterization of various electronic transfers in the presented and particularly designed push-pull copolymers, blended with two types of electron acceptor materials: PCBM and CuInS2 nanocrystals. DFT calculations supported the experimental results. Preliminary tests on synthesized copolymers were carried out taking into account all limiting factors concerning the device fabrication.

Um dos grandes desafios do século XXI está na produção de energia limpa e renovável, já que a demanda mundial por energia continuará crescendo, assim como a necessidade de despoluir o planeta e de diminuir a emissão dos gases do efeito estufa. Nesse contexto, a conversão de energia solar em elétrica coloca-se como uma excelente alternativa, e com isso a dos dispositivos fotovoltaicos. A tecnologia fotovoltaica baseada no silício e em outros semicondutores orgânicos encontra-se em estágio relativamente avançado, porém o custo de produção e de manutenção a proíbe em uso de grande escala. Mais recentemente, iniciaram-se pesquisas com filmes de semicondutores orgânicos, e a rápida melhora na performance dessas células solares a coloca como promissora ao mercado fotovoltaico. Em nosso trabalho, realizamos estudos sobre a performance de dispositivos fotovoltaicos orgânicos baseados na estrutura de heterojunção, estudando a influência de vários parâmetros na performance dos dispositivos. Usamos como camada ativa para nossos dispositivos o poli(3-hexiltiofeno) (P3HT) regiorregular, que é um polímero condutor de gap eletrônico em torno de 1,8 eV misturado ao [6,6]-fenil-C61-ácido butírico-metil ester (PCBM). Essa mistura é apropriada à dissociação dos éxcitons gerados nas cadeias poliméricas pelos fótons absorvidos porque, sendo o PCBM muito eletronegativo, ele captura o elétron do éxciton antes do processo natural de recombinação. Como esse fenômeno ocorre em todo o volume da camada ativa, o dispositivo leva o nome de heterojunção de volume. A estrutura básica que usamos foi de ITO/P3HT-PCBM/Al, isto é, o ITO como eletrodo transparente e bom injetor de buracos e o alumínio como eletrodo injetor de elétrons. Outros dispositivos foram feitos adicionando uma camada transportadora de buracos entre o ITO e o polímero ativo, o Poli(3,4-etileno dióxido-tiofeno):poliestireno-sulfonado (PEDOT:PSS) e/ou cálcio (Ca) entre a camada de alumínio e o polímero. Verificamos que a performance do dispositivo fotovoltaico é bastante alterada quando mediante o contato utilizado, a espessura da camada ativa e a temperatura em que o tratamento térmico é realizado. Investigou-se também, os mecanismos de injeção, transporte e geração de portadores sob variação de temperatura, no intervalo de 90 à 330K. Foi mostrado que, mediante a variação da temperatura, a corrente de curto circuito (JSC), é governada principalmente pela mobilidade dos portadores. A eficiência dos dispositivos desenvolvidos neste trabalho é comparável aos principais valores obtidos na atualidade. Para obtenção destes resultados, foi necessária intensa pesquisa em processamento, principalmente mantendo todas as etapas de fabricação em atmosfera controlada. / One big challenge of the humanity along the 21st Century is to produce energy based on clean and renewable sources. The energy consumption certainly will increase, as well as the necessity in decreasing the emission of greenhouse gases. In this context, solar energy becomes an important alternative for the production of electric energy, in particular, that of photovoltaic devices. Photovoltaics made of silicon and of other inorganic semiconductors are already available, but due to the high cost is not an alternative to produce energy in a large scale. More recently, the organic photovoltaics, due to their quick progress, have becoming as promising technology for the solar energy market. In this work, we studied bulk heterojunction organic photovoltaics, varying several parameters and its influence on the device performance. We used regio-regular poli(3-hexylthiophene) (P3HT), that has an electronic gap close to 1.8 eV, mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). PCBM acts in order to dissociate the photogenerated exciton because, being highly electronegative, it captures the electron form the exciton before the recombination process. We used as basic structure the ITO/P3HT-PCBM/Al. ITO as transparent electrode and injector of holes, and aluminum as the electrons injector electrode. In other devices we added a thin layer of Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), as hole transport layer and/or calcium (Ca) between the Al and the polymer. We verify that the device performance changes considerably with the insertion of such layers, and with the thickness of the active layer and the annealing treatment. We also investigated phenomena related to injection, generation and transport of charge carriers, in the 90-330 K temperature range. We showed that the temperature is the main factor that governs the short-circuit current (JSC). It is important to remark that our devices exhibited similar efficiency compared to that of the literature.

Aujourd'hui, les meilleurs résultats obtenus avec des cellules photovoltaïques organiques atteignent plus de 10 %, et possèdent de nombreux avantages sur les cellules classiques à base de silicium (faibles coûts énergétique et financier, substrats légers et flexibles). Cependant, de nombreux défis technologiques restent à relever. La recherche de nouvelles molécules absorbantes et efficaces pour le photovoltaïque en est un exemple. Nous nous sommes ainsi intéressés à une famille de dipyrrométhènes métallés (Co, Ni, Cu). C'est un système stable, ?-conjugué et fortement absorbant. Après avoir mis au point un protocole de synthèse et caractériser ces complexes, nous avons pu confirmer le caractère semi-conducteur de ces molécules. Malheureusement, les résultats PV sont restés bien en deçà de nos espérances. Un autre aspect limitant du photovoltaïque organique provient des problèmes d'injection de charges à l'interface anodique. Dans le travail que nous présentons, nous avons étudié une famille de molécules, les dipyrannylidènes, comme couche interstitielle anodique pour collecter les trous au sein d'une cellule solaire organique. Une étude en AFM a permis de mettre en évidence une nano-structuration remarquable d'une couche préparée par évaporation sous vide. Les cellules photovoltaïques préparées avec cette couche interstitielle se sont avérées très efficaces. Enfin, des expériences en photoémission ont été réalisées (synchrotron SOLEIL) afin de comprendre le fonctionnement de cette couche et l'importance de sa structuration. / Today, organic photovoltaic solar cells reach more than 10% yield, and have many advantages over conventional silicon cells (low energy and financial cost, lightweight and flexible substrates). However, many technical challenges remain. Finding new and effective absorbing molecules for photovoltaics is one of them. We first studied a new family of molecules, metallated (Co, Ni, Cu) dipyrromethene. These molecule are stable, π-conjugated and highly absorbent. Having developed a protocol synthesis and characterized these complexes, we confirmed the semiconductor behaviour of these molecules. Unfortunately, the PV results were well below our expectations. Another aspect limiting the organic photovoltaic technology comes from problems of charge injection at the anodic interface. In this work we present a family of molecules, dipyrannylidenes as anodic interfacial layer to collect holes in an organic solar cell. An AFM study has highlighted a remarkable nanostructuring prepared by a vacuum evaporation layer. Photovoltaic cells made with this interstitial layer proved very effective. Finally, experiments were performed in photoemission (SOLEIL synchrotron) to understand the functioning of this layer and the importance of its structure.