The cost of the present generation of inorganic silicon solar cells is very high and further breakthroughs in cost and efficiency using traditional materials are becoming less and less likely after over 50 years of development. Next generation organic solar cells offer a solution to the limitations of silicon through the vision of low-cost, liquid-based, large area fabrication technology based on polymer and nanomaterials at room temperature. However, most polymers used in solar cells are dissolved in organic solvents such as xylene, toluene, chloroform, and chlorobenzene. Such solvents are harmful to people and environments, leading to higher costs due to complicated waste disposal processing. This is in conflict with the low cost, green, and renewable energy for which we are aiming. To realize a green organic solar cell, a novel solar cell has been created using an environmentally friendly water-soluble thiophene polymer [(Sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate])] (PTEBS) and nanocrystalline TiO2. This novel system has shown great potential in photovoltaics the work has garnered the attention of the international community.In our innovative solar cells, the water-soluble polythiophene (PTEBS) is used as electron donor. Nanoparticle TiO2 acts as electron acceptor. PTEBS/TiO2 solar cells with various structures including bilayer heterojunctions, bulk heterojunctions and a hybrid of bilayer and bulk heterojunctions have been developed and explored. These results are comparable to the best polymer/metal-oxide solar cells reported by other groups using organic solvents.In summary, this is the first time that green solar cells have been fabricated from environmentally friendly water-soluble polymers. By using water as the solvent and utilizing liquid-based processing, the cost of the energy generated by this type of solar cell will be further lowered. In addition, the flexible polymer offers the ease of fabrication and integration into different devices.

Organic solar cells are one of the most promising candidates for future solar power generation. They are thin and lightweight with several additional advantages such as scalability, environmental sustainability and low cost for processing and installation. However, the low charge carrier mobility of the absorbing material for organic solar cells requires thin absorber layers, limiting photon harvesting and the overall power conversion efficiency. Several attempts, e.g., periodically patterned structures and scattering layers have been tried to enhance the absorption of thin-film solar cells as light trapping elements. However, much effort is required to introduce light trapping structures to conventional rigid metal oxide electrodes and glass substrate. For instance, almost 13 hours are required to fabricate micro structures of 1 m2 area on glass, in contrast, 1 minute on PET using a same laser set-up and an additional scattering layers are demanded for providing light trapping effects to solar cells. In the last years, flexibility is emerging as the one of the major advantages of organic solar cells. To realize flexibility of solar cells, the classically used glass substrates and ITO electrodes are too brittle. Therefore, polymer materials are promising candidates to replace them as flexible electrodes and substrates. In this thesis, the highly transparent conducting polymer, PEDOT:PSS and PET equipped with an AlOx encapsulation layer are used as electrode and substrate, respectively. Besides the flexibility, additional light trapping elements, e.g. scattering particles, nano- and microstructures can be easily applied to the polymer materials since they have the potential for easier shaping and processing. In this study, we apply different light trapping and in-coupling approaches to organic solar cells. First, PET substrates are structured with a direct laser interference patterning system, which is a powerful and scalable one-step technique for patterning polymers. Almost 80 % of the light is diffracted by these patterned PET substrates and thereby the light path in the absorption layer is increased. Optical display films, commercially developed to be used as back light units of liquid crystal displays are also examined as light trapping substrates and exhibit similar enhancement as patterned PET. Moreover, since PEDOT:PSS is prepared by a solution-based process, TiO2 nanoparticles are added as light scattering elements to the PEDOT:PSS electrodes. Consequently, those electrodes provide a dual function as electrical contact and light trapping element. Finally, 2- or 3-dimensional nanostructures are printed by a nano-imprinting technique onto the surface of PEDOT:PSS with PDMS stamps. By controlling the temperature and the time of PEDOT:PSS during an annealing step, nanostructures are transferred from PDMS masks to PEDOT:PSS. To evaluate the effects of light trapping for all above mentioned approaches, flexible organic solar cells are produced by vacuum evaporation using blends of DCV5T-Me and C60 as absorber layer. The substrates are optically characterized using UV-vis spectrometer and goniometer measurements. The topography of the samples is measured by atomic force microscopy, scanning microscopy and optical microscopy. Bending tests with various radii are performed to test the flexibility of the substrates. In summary, light trapping effects are successfully implemented in the electrodes and substrates for OPVs, giving efficiency improvements of up to 16 %. The light trapping mechanisms in our approaches are extensively discussed in this thesis. / Organische Photovoltaik ist einer der vielversprechendsten Kandidaten für die zukünftige Solarstromgewinnung auf flexiblen Substraten. Um diese Flexibilität zu ermöglichen, sind herkömliche Glassubstrate mit ITO-Elektroden zu spröde. Ein vielversprechender Kandidat, um sowohl flexible Elektroden als auch flexible Substrate herzustellen, sind Polymere, da diese sehr biegsam und leicht zu verarbeiten sind. Deshalb wird in dieser Arbeit das hoch transparente, leitfähige Polymer PEDOT:PSS als Elektrode und PET (mit einer AlOx Verkapselungsschicht) als Substrat untersucht. Aufgrund der guten Prozessierbarkeit der Polymere konnten wir zusätzlich zu den eigentlichen Funktionen des Substrates und der Elektrode noch den Mechanismus des Lichteinfangs hinzufügen. Zusätzlich zu ihrer Flexibilität haben organische Solarzellen noch weitere Vorteile: sie sind dünn, leicht, skalierbar und verursachen vergleichsweise geringe Kosten für Herstellung und Installation. Ein Nachteil organischer Solarzellen ist die vergleichsweise geringe Ladungsträgerbeweglichkeit der Absorbermaterialien, welche oft die Schichtdicke der Absorbermaterialien begrenzt. Dies hat weniger absorbierte Photonen, weniger Stromdichte und somit einen geringeren Wirkungsgrad zur Folge. In den letzten Jahren wurden periodisch strukturierte Substrate und streuende Schichten als Lichteinfangelemente eingesetzt, um den Wirkungsgrad organischer Solarzellen mit dünnen Absorberschichten zu erhöhen. Gestaltungsregeln für solche Lichteinfangelemente sind noch weitestgehend unbekannt. Im Rahmen dieser Arbeit strukturieren wir PET Substrate mit einem direkten Laserinterferenzsystem, welches ein leistungsfähiges, skalierbares Einschrittverfahren zur Polymerstrukturierung ist. Da PEDOT:PSS aus der Lösung prozessiert wird, können wir weiterhin Nanopartikel hinzufügen, die der Elektrode zusätzlich noch lichtstreuende Eigenschaften geben. Außerdem können 2- bzw. 3-dimensionale Nanostrukturen leicht mithilfe einer Stempeltechnik eingeprägt werden. Um die Effekte des Lichteinfangs, welcher durch die oben genannten Methoden erzeugt wird, zu untersuchen, werden flexible organische Solarzellen mittels Vakuumverdampfung prozessiert. DCV5T-Me und C60 bilden dabei die photoaktive Schicht. Somit werden die Licht fangenden Eigenschaften dieser flexiblen Solarzellen ausgenutzt und ausführlich in der Arbeit diskutiert.

Organic solar cells have been showing promise as a way of producing renewableenergy with the help of light, flexible, and production effective materials.The efficiencies and lifetimes reached in organic solar cells have steadily beenincreasing over the years as more research in the field is being conducted.One way of increasing the efficiency in organic solar cell devices is introducingan interlayer between the photoactive material and the anode, referred toas the ’hole transport layer’. Most commonly used as a hole transport layer isthe material PEDOT:PSS, which offers desired properties such as transparency,simple processing and good ohmic contact between anode and photoactive material.PEDOT:PSS is also known to be a degradation site in organic solar cells,as it will corrode the electrode in the presence of water.This project has consisted of investigating PEDOT:PSS along with two othercandidates that may one day come to replace PEDOT:PSS as the most commonlyused material, molybdenum trioxide (MoO3) and phosphomolybdic acid(PMA). The aim was to investigate how the different materials energy bandstructure would be affected upon exposure to sunlight, air and annealing, byobserving the work function under different conditions.

A crescente demanda energética mundial vem estimulando pesquisas em novas fontes de energia limpa e renovável e de baixo custo. Nesse contexto, as células solares orgânicas (fotovoltaicos orgânicos – OPVs) destacam-se como uma alternativa promissora no campo dos fotovoltaicos. Por serem fabricadas a partir de soluções eletrônicas, sua fabricação se dá deposição sobre substratos rígidos ou flexíveis, e com isso, também por técnicas de impressão. Muitas moléculas poliméricas têm mostrados excelentes resultados, porém o desempenho das células dependem também da morfologia do filme ultrafino da camada ativa, a qual depende do processamento e sobretudo da atividade dos solventes orgânicos. Até o momento, a maioria dos solventes usados para a fabricação das OPVs de alto desempenho é da classe dos halogenados, como por exemplo, o clorobenzeno e o 1,2-diclorobenzeno. Esses solventes, além de exibirem alto custo de produção, apresentam toxicidade relativamente alta, com impactos adversos à saúde humana e ao meio ambiente. Visto que a tecnologia dos OPVs está próxima de sua comercialização, a procura por solventes alternativos de baixa toxicidade coloca-se como um desafio a essa área. Neste trabalho, identificou-se por meio dos parâmetros de solubilidade de Hansen, um solvente da classe dos não halogenados e não aromáticos com baixa toxicidade ao ser humano e ambientalmente amigável para ser aplicado como solvente de processamento de células solares orgânicas do tipo heterojunção de volume (BHJ). Para isso, os possíveis solventes foram avaliados segundo os parâmetros de solubilidade de Hansen para os polímeros, P3HT e PTB7-Th, e para as moléculas PC61BM e PC71BM, levando em consideração os critérios de riscos com base na ficha de segurança de produtos químicos. Desse modo, a ciclohexanona foi selecionada por conter as características desejadas para o estudo proposto no presente trabalho. As soluções de P3HT e de PTB7- Th em ciclohexanona foram avaliadas quanto à influência da temperatura das soluções na conformação dos polímeros, a partir da técnica de termocromismo. Os resultados revelam necessidade de aquecimento para que haja uma conformação menos agregada tanto do P3HT quanto do PTB7-Th em solução de ciclohexanona. Os filmes de P3HT:PC61BM e PTB7-Th:PC71BM foram depositados pela técnica de spin-coating sobre substratos de vidro e estudos sobre sua morfologia foram realizados por técnicas de imagens de microscopia óptica, AFM e medidas de absorção pela técnica UV-vis, e correlacionadas ao desempenho das células fabricadas. Os resultados obtidos foram promissores uma vez que nesse trabalho conseguimos células de até 5,5 % de eficiência. / The growing global demand for energy has been stimulating research into new sources of clean and renewable energy and low cost. In this context, organic solar cells (organic photovoltaic - OPVs) stand out as a promising alternative in the field of photovoltaics. Because they are manufactured from electronic solutions, it can be deposited on rigid or flexible substrates, facilitating their production by printing techniques. Many polymer molecules have shown excellent results, but the performance of the cells also depends on the morphology of the ultrathin film of the active layer, which depends on the processing and above all the activity of the organic solvents. To date, most of the solvents used in the manufacture of high-performance OPVs belong to the halogen class, for example chlorobenzene and 1,2-dichlorobenzene. These solvents, in addition to exhibiting high cost of production, have relatively high toxicity, with adverse impacts on human health and the environment. Since the technology of OPVs is close to commercialization, the search for low-toxicity alternative solvents poses a challenge in this area. In this work, Hansen\'s solubility parameters were used to identify solvents of non-halogenated and non-aromatic class with low toxicity to humans and environmentally friendly, as substitutes of traditional solvents used to process organic solar cells (BHJ). For this, several solvents were evaluated according to the Hansen solubility parameters for the polymers, P3HT and PTB7-Th, and for the molecules PC61BM and PC71BM, taking into account the risk criteria based on the chemical safety data sheet. The solutions of P3HT and PTB7-Th in cyclohexanone were evaluated taking into account the influence of the temperature of the solutions on the conformation of the polymers, using the thermochromic technique. The solutions of P3HT and PTB7-Th in cyclohexanone were evaluated taking into account the influence of the temperature of the solutions on the conformation of the polymers, using the thermochromic technique. The resulted morphology was then correlated with the devices performance. The results obtained were promising since in this work we obtained cells of up to 5.5% efficiency.

As células solares orgânicas, também conhecidas como (OPVs), fazem parte da terceira geração dos dispositivos fotovoltaicos. Entre outras tecnologias emergentes, a dos OPVs tem a vantagem de ser de fácil processamento e de baixo custo. Ou seja, uma tecnologia comercialmente promissora na área de conversão de energia solar em energia elétrica. No entanto, grandes desafios precisam ser superados para colocar estas células no mercado dos fotovoltaicos. Dentre esses desafios, pode estar incluído, inevitavelmente, a compreensão dos processos físicos envolvidos na fotogeração em OPVs, dentre os quais pode-se destacar o da recombinação de cargas fotogeradas. A recombinação é o principal responsável pela perda de eficiência em OPVs, uma vez que ela elimina uma fração relativamente grande de portadores de carga, diminuindo consideravelmente a potência de saída da célula. Para estudar este efeito indesejado em células orgânicas, desenvolvemos um modelo analítico para fotocorrente em OPVs do tipo bulk heterojunction (BHJ), assumindo uma recombinação bimolecular de cinética de segunda ordem. O modelo é representado por uma expressão analítica obtida a partir das equações fundamentais da eletrodinâmica clássica, onde despreza-se a contribuição da corrente de difusão e as mobilidades dos elétrons e dos buracos são consideradas iguais. Essa expressão foi de grande valia na análise dos resultados experimentais, sobretudo os de corrente-tensão (J-V) sob iluminação, e além disso, ela permitiu extrair parâmetros intrínsecos do transporte de carga, como mobilidade e coeficiente de recombinação. Neste sentido, foram fabricados dispositivos cuja estrutura foi ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al, e com eles foram realizados inúmeros experimentos. As técnicas usadas na parte experimental foram: medidas J-V, no escuro e sob iluminação, medidas de transiente de fotovoltagem (TPV), de transiente de fotocorrente (TPC), e de Foto-CELIV (Charge Extraction Linear Increasing Voltage). Usamos como parâmetros experimentais a temperatura e intensidade de iluminação. Das medidas J-V sob iluminação, foram extraídos os parâmetros essenciais da célula: corrente de curto (Jsc), potencial de circuito aberto (Voc), fator de preenchimento (FF) e a eficiência (PCE). A partir das abordagens experimental e teórica, exploramos a influência da recombinação bimolecular no comportamento fotovoltaico dos dispositivos. O desenvolvimento do modelo teve contribuição de trabalhos que se basearam em modelagem numérica a partir de condições físicas semelhantes às usadas em nosso tratamento e que foram levadas em consideração no processo de análise dos resultados experimentais. / Organic solar cells, also known as (OPVs), are part of the third generation of photovoltaic devices. Among other emerging technologies, OPVs have the advantage of being easy to process and exhibits low cost of production. That is, it is a promising commercial technology in the area of converting solar energy into electricity. However, major challenges need to be overcome to put these cells in the photovoltaic market. Among them, it can be included, inevitably, the comprehension of the physical processes involved in photogeneration in OPVs, of which, the recombination of photogenerated carriers is included. Recombination is primarily factor responsible for the loss of efficiency in OPVs, since recombination eliminates a large fraction of the carriers, considerably reducing the output power of the cell. To study this undesirable effect in organic cells, we developed an analytical model for the photocurrent in bulk heterojunction cells (BHJ), which assumes the bimolecular recombination of second order kinetics. The model is represented by an analytical expression obtained by the equations of the classical electrodynamics, where we neglected the contribution of the diffusion current and assumed that electrons and holes have equal mobilities. The expression was of great value for the analysis of the experimental results, especially the current-voltage (J-V) measurements under illumination, and it allowed to extract intrinsic parameters of charge transport effects, such as mobility and recombination coefficient. For this, it were fabricated devices whose structure was ITO/PEDOT:PSS/P3HT:PCBM/Ca-Al, and with them were performed numerous experiments. The techniques used in the experimental part were: J-V measurements, in the dark and under illumination, transient photovoltage (TPV), transient photocurrent (TPC), and of Charge Extraction Linear Increasing Voltage (Photo-CELIV). We used as experimental parameters the temperature and the intensity of. From J-V measurements under illumination we extracted the essential cell parameters: short current (Jsc), open circuit potential (Voc), fill factor (FF) and efficiency (PCE). From the experimental and theoretical approaches, we explored the influence of bimolecular recombination on the photovoltaic behavior of the devices. The development of the model had contributions of works based on numerical modelings from physical conditions similar to those used in our treatment and that were taken into account in the process of analysis of the experimental results.

Face à la croissance de la demande énergétique, les énergies alternatives, telles que l’énergie photovoltaïque, représentent des solutions réalistes. Cette dernière nécessite des matériaux efficaces pour la capture des photons et leur conversion en électricité.Les cellules solaires organiques (CSOs) sont basées sur les propriétés semiconductrices de certaines molécules ou de certains polymères π-conjugués. Dans le domaine des CSOs, les efforts de recherche actuels se concentrent selon trois axes : la réduction des coûts, l’augmentation de la durée de vie des cellules solaires et l’augmentation des rendements de conversion photovoltaïque. Les récentsdéveloppements ont conduit à une complexification des architectures des CSOs ainsi que des semi-conducteurs organiques utilisés, induisant une augmentation des coûts de fabrication. Dans une logique de développement économiquement efficace et écologiquement soutenable, il est nécessaire aujourd’hui de se concentrer sur des semi-conducteurs organiques viables économiquement et dont la synthèse est respectueuse de l’environnement. Ce travail doctoral a pour but de développer de nouveaux matériaux semi-conducteurs organiques bio-inspirés et bas coût. Les molécules étudiées présentent une structure donneur-accepteur-donneur. Leur squelette est celui de la curcumine, molécule qui donne sa couleur au curcuma. Le groupement accepteur est un difluorure de bore. Les groupements donneurs quant à eux varient suivant les semi-conducteurs. Les propriétés optoélectroniques de dix-sept dérivés curcuminoïdes ont été étudiées. Plusieurs d’entre eux se sont démarqués : ceux avec des groupements anthracène, ceux avec des dérivés thiophènes, enfin et impact sur les performances photovoltaïques de la formulation de l’encre utilisée pour le dépôt de la couche a été étudié en détail. Différents matériaux accepteurs ont été testés, de même que l’utilisation de mélanges ternaires. Pour l’un de dérivés curcuminoïde en combinaison avec du PC61BM, des rendements supérieurs à 4 % ont été obtenus avec des tensions de circuit ouvert supérieures à 1,0 V. Au regard de la simplicité structurale de ce semi-conducteur, ces résultats figurent à notre connaissance parmi les meilleurs reportés dans la littérature. Les phénomènes photophysiques ont également été étudiés par spectroscopie d’absorption des espèces transitoires. Enfin, le procédé de fabrication a été rapproché des conditions industrielles en éliminant les solvants halogénés utilisés et en travaillant à l’air ambiant. Finalement, bien qu’intéressantes, les propriétés photovoltaïques restent limitées pour une application industrielle du fait de la faible mobilité des trous de ces matériaux. / To face the growing needs in energy, renewable energies like solar photovoltaic represent realistic solutions. Photovoltaic energy requires efficient materials to absorb photons and to convert them into electricity. Organic solar cells (OSCs) are based on semiconducting π-conjugated polymer or small molecules. Current research in this field focuses on three main topics: the reduction of costs, the increase of device lifetime and the increase of power conversion efficiency. This last issue led to an increase in the complexity of OSCs architecture as well as organic semi-conductors, leading to anincrease in manufacturing costs. In order to develop sustainable and eco-friendly processes, it is now important to work on cost effective semi-conductors obtained fromgreen synthetic methodology. The aim of this thesis was to develop new bio-inspired organic semi-conductors. These materials are potentially low cost. Molecules studied present donor-acceptor-donor structure. They have the skeleton of curcumine. Curcumine is a natural yellow dye present in curcuma. Acceptor group is boron difluoride. Donor groups vary depending on the curcuminoid derivative. Optoelectronicproperties of seventeen semi-conductors were studied. Several of them stood out: those with anthracene groups, those with thiophene derivatives, finally and especially, those with triphenylamine groups. For this last family, the impact on the photovoltaic performances of the ink formulation used for deposition has been deeply studied. Several acceptor materials were tested, as well as ternary blend. For one curcuminoid derivative combined with PC61BM, efficiency above 4% has been achieved with open circuit voltage up to 1.0 V. Due to the very simple chemical structure of the donor, this represents one of the best result reported in literature to our knowledge. Transient species were also studied by ultrafast spectroscopy. The fabrication process was also changed to eliminate halogenated solvent and to enable processing in ambient air like in industrial process. Finally, photovoltaic properties observed are interesting. Nevertheless, they are not sufficient for industrial application due to low hole mobility in these materials.

Ce travail est consacré à l‟étude de la stabilité photochimique de nanocomposites polymère / argile en vue de leur insertion dans un système multicouche organique / inorganique pour l‟encapsulation des cellules solaires organiques. L‟objectif est d‟obtenir des films de nanocomposites polymère / argile flexibles, transparents, pouvant être mis en oeuvre par voie liquide, et photochimiquement stables. Dans une première partie, la caractérisation de nanocomposites à base d‟alcool polyvinylique (PVA) a montré leur aptitude à être insérés dans un système multicouche, notamment en ce qui concerne les propriétés barrière aux gaz. L‟étude du comportement photochimique du PVA basée sur l‟identification des produits de dégradation a permis de proposer un mécanisme de photooxydation du PVA et de déterminer les effets du photovieillissement sur les propriétés du film (rugosité, perméabilité, transparence). L‟insertion de nanocharges lamellaires (Montmorillonite, Laponite ou Hydroxydes Doubles Lamellaires) dans le PVA induit des effets différents (prodégradant ou stabilisant) en fonction de la nature de l‟argile (naturelle ou synthétique). Cependant, lors d‟irradiations en absence d‟oxygène, le PVA et les nanocomposites PVA / argile sont très stables. Enfin, l‟encapsulation alternant couche inorganique SiOx et couche organique PVA ou nanocomposite PVA / argile, permet d‟atteindre les niveaux de perméation requis pour les cellules solaires organiques pour des applications nomades. / This work was devoted to the study of the photochemical behavior of polymer / clay nanocomposites with the aim to use these nanocomposites in a multilayer organic / inorganic coating for organic solar cells encapsulation. The goal of this work was to obtain polymer / clay nanocomposite films that are flexible, transparent, which can be processed by solution, and that are photochemically stable. In the first part, the characterization of nanocomposites based on polyvinyl alcohol (PVA) has shown their ability to be inserted into a multilayer system, particularly for gas barrier properties. The study of the photochemical behavior of PVA with the identification of photodegradation products allows us to propose a photooxidation mechanism of PVA and to determine the effects of photoageing on the film properties (roughness, permeability, transparency). The insertion of lamellar nanofillers (Montmorillonite, Laponite or Layered Double Hydroxide) in PVA induces different effects (prodegradant or stabilising) depending on the nature of the clay (natural or synthetic). However, in absence of oxygen, the PVA and PVA / clay nanocomposites are very photostable. Finally, encapsulation alternating inorganic SiOx layer and PVA or PVA / clay nanocomposite layer permits to obtain the permeability levels required for organic solar cells in niche markets (consumer electronics).

Dans une cellule solaire, il existe un compromis entre l’efficacité d’absorption des photons et le rendement quantique de collection des charges électriques. Dans les semi-conducteurs organiques, la longueur de diffusion des porteurs est limitée à une centaine de nanomètres, si bien qu’il est nécessaire de travailler avec des couches photo-actives ultraminces (< 100 nm). Pour limiter l’épaisseur physique des matériaux utilisés tout en maintenant une absorption élevée, il est possible d’utiliser les propriétés des cristaux photoniques (CP), pour allonger la durée d’interaction des photons avec le milieu absorbant. Cela consiste à former un CP dans la couche active ou à son voisinage et d’exciter des modes résonants de la structure. Ce travail de thèse est divisé en plusieurs parties. Dans un premier temps, à l’aide d’outils numériques, nous nous intéressons aux phénomènes qui régissent le gain d’absorption lors du couplage d’une onde plane avec un mode résonant d’une membrane à CP. Ensuite, nous étudions une cellule à CP, où l’électrode d’ITO est nano-structurée, et nous optimisons le gain d’absorption d’une couche photo-active ultramince (50 nm). Enfin, dans un travail expérimental, nous fabriquons des cristaux colloïdaux bidimensionnels à base de microsphères diélectriques par différentes méthodes d’auto assemblage. / In a solar cell, there is a trade-off between light absorption capacity and internal quantum efficiency. In organic semi conductors, charge carrier diffusion is limited to a few hundred nanometers, which implies to work with very thin active layers (< 100 nm). In order to limit the thickness of the material while keeping high light absorption, it is possible to use the properties of photonic crystals (PC) to enhance light matter interaction duration. It consists in forming a PC in or around the active layer, and to excite a resonant mode of the formed photonic structure. The work of this thesis is divided into several parts. In a first approach, using numerical tools, we investigate the phenomena that give rise to absorption enhancement when a plane wave is coupled to a resonant mode of a PC membrane. We then study a nano-structured cell architecture, in which the ITO electrode is periodically patterned, and we optimize absorption enhancement in the thin active layer (50 nm). Finally, in an experimental work, we fabricate two dimensional colloidal crystals formed by dielectric microsphere self assembly.

<p>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.</p><p>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.</p><p>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.</p><p>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.</p><p> </p>

<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>