Donor-acceptor (D-A) copolymers have revolutionized the field of organic electronics over the last decade. Comprised of a electron rich and an electron deficient molecular unit, these copolymers facilitate the systematic modification of the material's optoelectronic properties. The ability to tune the optical band gap and to optimize the molecular frontier orbitals as well as the manifold of structural sites that enable chemical modifications has created a tremendous variety of copolymer structures. Today, these materials reach or even exceed the performance of amorphous inorganic semiconductors. Most impressively, the charge carrier mobility of D-A copolymers has been pushed to the technologically important value of 10 cm^{2}V^{-1}s^{-1}. Furthermore, owed to their enormous variability they are the material of choice for the donor component in organic solar cells, which have recently surpassed the efficiency threshold of 10%. Because of the great number of available D-A copolymers and due to their fast chemical evolution, there is a significant lack of understanding of the fundamental physical properties of these materials. Furthermore, the complex chemical and electronic structure of D-A copolymers in combination with their semi-crystalline morphology impede a straightforward identification of the microscopic origin of their superior performance. In this thesis, two aspects of prototype D-A copolymers were analysed. These are the investigation of electron transport in several copolymers and the application of low band gap copolymers as acceptor component in organic solar cells. In the first part, the investigation of a series of chemically modified fluorene-based copolymers is presented. The charge carrier mobility varies strongly between the different derivatives, although only moderate structural changes on the copolymers structure were made. Furthermore, rather unusual photocurrent transients were observed for one of the copolymers. Numerical simulations of the experimental results reveal that this behavior arises from a severe trapping of electrons in an exponential distribution of trap states. Based on the comparison of simulation and experiment, the general impact of charge carrier trapping on the shape of photo-CELIV and time-of-flight transients is discussed. In addition, the high performance naphthalenediimide (NDI)-based copolymer P(NDI2OD-T2) was characterized. It is shown that the copolymer posses one of the highest electron mobilities reported so far, which makes it attractive to be used as the electron accepting component in organic photovoltaic cells.par Solar cells were prepared from two NDI-containing copolymers, blended with the hole transporting polymer P3HT. I demonstrate that the use of appropriate, high boiling point solvents can significantly increase the power conversion efficiency of these devices. Spectroscopic studies reveal that the pre-aggregation of the copolymers is suppressed in these solvents, which has a strong impact on the blend morphology. Finally, a systematic study of P3HT:P(NDI2OD-T2) blends is presented, which quantifies the processes that limit the efficiency of devices. The major loss channel for excited states was determined by transient and steady state spectroscopic investigations: the majority of initially generated electron-hole pairs is annihilated by an ultrafast geminate recombination process. Furthermore, exciton self-trapping in P(NDI2OD-T2) domains account for an additional reduction of the efficiency. The correlation of the photocurrent to microscopic morphology parameters was used to disclose the factors that limit the charge generation efficiency. Our results suggest that the orientation of the donor and acceptor crystallites relative to each other represents the main factor that determines the free charge carrier yield in this material system. This provides an explanation for the overall low efficiencies that are generally observed in all-polymer solar cells. / Donator-Akzeptor (D-A) Copolymere haben das Feld der organischen Elektronik revolutioniert. Bestehend aus einer elektronen-reichen und einer elektronen-armen molekularen Einheit,ermöglichen diese Polymere die systematische Anpassung ihrer optischen und elektronischen Eigenschaften. Zu diesen zählen insbesondere die optische Bandlücke und die Lage der Energiezustände. Dabei lassen sie sich sehr vielseitig chemisch modifizieren, was zu einer imensen Anzahl an unterschiedlichen Polymerstrukturen geführt hat. Dies hat entscheidend dazu beigetragen, dass D-A-Copolymere heute in Bezug auf ihren Ladungstransport die Effizienz von anorganischen Halbleitern erreichen oder bereits übetreffen. Des Weiteren lassen sich diese Materialien auch hervorragend in Organischen Solarzellen verwenden, welche jüngst eine Effizienz von über 10% überschritten haben. Als Folge der beträchtlichen Anzahl an unterschiedlichen D-A-Copolymeren konnte das physikalische Verständnis ihrer Eigenschaften bisher nicht mit dieser rasanten Entwicklung Schritt halten. Dies liegt nicht zuletzt an der komplexen chemischen und mikroskopischen Struktur im Film, in welchem die Polymere in einem teil-kristallinen Zustand vorliegen. Um ein besseres Verständnis der grundlegenden Funktionsweise zu erlangen, habe ich in meiner Arbeit sowohl den Ladungstransport als auch die photovoltaischen Eigenschaften einer Reihe von prototypischen, elektronen-transportierenden D-A Copolymeren beleuchtet. Im ersten Teil wurden Copolymere mit geringfügigen chemischen Variationen untersucht. Diese Variationen führen zu einer starken Änderung des Ladungstransportverhaltens. Besonders auffällig waren hier die Ergebnisse eines Polymers, welches sehr ungewöhnliche transiente Strom-Charakteristiken zeigte. Die nähere Untersuchung ergab, dass in diesem Material elektrisch aktive Fallenzustände existieren. Dieser Effekt wurde dann benutzt um den Einfluss solcher Fallen auf transiente Messung im Allgemeinen zu beschreiben. Zusätzlich wurde der Elektronentransport in einem neuartigen Copolymer untersucht, welche die bis dato größte gemesse Elektronenmobilität für konjugierte Polymere zeigte. Darauf basierend wurde versucht, die neuartigen Copolymere als Akzeptoren in Organischen Solarzellen zu implementieren. Die Optimierung dieser Zellen erwies sich jedoch als schwierig, konnte aber erreicht werden, indem die Lösungseigenschaften der Copolymere untersucht und systematisch gesteuert wurden. Im Weiteren werden umfangreiche Untersuchungen zu den relevanten Verlustprozessen gezeigt. Besonders hervorzuheben ist hier die Beobachtung, dass hohe Effizienzen nur bei einer coplanaren Packung der Donator/Akzeptor-Kristalle erreicht werden können. Diese Struktureigenschaft wird hier zum ersten Mal beschrieben und stellt einen wichtigen Erkenntnisgewinn zum Verständnis von Polymersolarzellen dar.

The present thesis is focused on rational design and synthesis of π-conjugated donoracceptor-donor (D-A-D) type oligomers and D-A type copolymers. Thesis is organized in seven chapters, apart from introduction remaining six chapters are grouped into two parts (A and B). Part A deals with Chapters 2, 3, 4 and Part B contains chapters 5, 6 and 7. A brief discussion on the content of individual chapters is provided below. Chapter 1 discusses the introduction to organic solar cell with operating principles and effect of spinodal decomposition on stability of the devices is presented. The status and literature related to the improvement of life time of the organic solar cells by self-assembly approach has been explored. In addition, design and synthesis of the fluorine substituted π-conjugated organic semiconductors for n-type OFETs and OLED has been discussed. Part A This part of the thesis attempt to address some of the challenges listed below (1) Investigation of miscibility of binary components in bulk heterojunction solar cells through H-bonding approach. (2) Synthesis of new low band gap molecular semiconductors having H-bonding sites. (3) Fabrication of bulk heterojunction solar cell devices using these new molecules and exploring the photovoltaics performance. Chapter 2, donor-acceptor-donor (D-A-D) concept has been employed to design low band gap oligomers named as TTB. Barbiturate functional group has been utilized to explore the concepts of supramolecular chemistry. It is shown that, TTB molecule self-organizes via intermolecular H-bonding between barbituric acid units. Interactions between the oligothiophene subunits were also found to be important, affording nanoribbons that were observed by atomic force and transmission electron microscopy. The applicability of TTB for organic electronic applications was investigated by fabricating organic field-effect transistors (OFETs) and organic photovoltaic device. The crystalline nanoribbons were beneficial in understanding the phase morphology of PCBM and TTB blend. Chapter 3, the self-assemble property of TTB was disrupted by the substitution of methyl group on the nitrogen of the barbituric acid moiety. The optical and electrochemical properties of the new derivative have been investigated by UV-Visible spectroscopy, photoluminescence spectroscopy and cyclic voltammetry. Further investigations on the effect of self-assembly on organic solar cells were carried out by fabricating BHJ and OFET. The results proved that the self-assembly within the donor moieties led to complete phase separation between the donor and acceptor which had an adverse effect on the photovoltaic performance. Chapter 4, the conjugation of TTB was extended by the synthesis of two new copolymers by polymerizing with two oliogothiophene (terthiophene and benzobithiophene) derivatives with different donating strength. The investigation of photophysical and electrochemical properties of copolymers were studied by varying the donating strength. As we increase the donating strength of oligothiophenes, the intramolecular charge transfer band of DA copolymers was red shifted. Further, density functional theory (DFT) calculation of these materials was carried out to get insight into their photophysical properties. Part B This part of the thesis attempt to address some of the challenges listed below (1) Investigation of fluorine substituted organic semiconductos like 2,2’ bithiazole and pheanthroimidazole. (2) Synthesis of pentafluoro phenyl appended derivatives of 2,2’ bithiazole and pheanthroimidazole. (3) Fabrication of OFETs and OLEDs using these new molecules and elucidated the device performance with molecular structure. Chapter 5, pentafluorophenyl appended 2,2’-bithiazole derivatives were synthesized. The single crystal x-ray diffraction studies shows the unusual strong type-II F•••F interactions within the distance of 2.668 Å, at an angle of 89.14° and 174.15°. It also shows the usual type-I F•••F interaction within the distance of 2.825Å, at an angle of 137.38° and 135.93°. Upon bromination type-II Br•••Br interaction was observed and the packing was further stabilized by S•••Br interactions. The conjugation was further extended with different aromatic and heteroaromatic substituents and synthesized the star shaped structure. The band gap as well as the electronic energy levels was tuned by substituting various aromatic and heteroaromatic substituents. These star shaped derivatives shows electron mobilities in the order of 10-4 to 10-3cm2/Vs. Chapter 6, Novel D-A copolymers were synthesized by Stille condensation of electron acceptor fluorinated phenanthroimidazole with electron donors like terthiophene and benzobithiophene. Prior to that insoluble pentafluoro phenyl phenanthroimidazole was Nalkylated in presence of DMF which concurrently resulted in C-F activation of the pentafluoro phenyl moiety. As we increase the donor strength from benzobithiophene to terthiophene the absorbance spectra was red shifted from 446 nm to 482 nm in solution and 455 nm to 484 nm in solid state. The band gap of these copolymers was found to be 2.4 eV for PIBDT and 2.2 eV for PIDHTT from the absorbance spectra. The photoluminescence data shows that these materials are promising for the yellow colour as well as orange colour displays, of narrow wavelength range (FWHM 40 nm for PIBDT and 35 nm for PIDHTT), which can be achieved just by the manipulation of donor moieties in the copolymers. The preliminary electroluminiscence data shows high brightness of 888cd/m2 (orange luminescence) for PIDHTT and 410cd/m2 (yellow luminescence) for PIBDT. Chapter 7, Acenaphtho[1,2-b]quinoxaline based donor–acceptor type low band gap conjugated copolymers were synthesized by Stille coupling reaction with the corresponding oligothiophene derivatives. The optical properties of the copolymers were characterized by ultraviolet-visible spectrometry while the electrochemical properties were determined by cyclic voltammetry. The band gap of these polymers was found to be in the range of 1.8-2.0 eV as calculated from the optical absorption band edge. The intense charge transfer band in absorption spectra shows the significant effect of acceptor in the copolymers. X-ray diffraction measurements show weak π–π stacking interactions between the polymer chains. The OFET devices fabricated using these co-polymers showed dominant p-channel transistor behavior with the highest mobility of 1×10-3cm2/Vs.

Diese Arbeit beschäftigt sich mit der Untersuchung der Langzeitstabilität organischer Solarzellen. Die Solarzellen als Gegenstand dieser Untersuchungen sind dabei aus Materialien aufgebaut, die mittels thermischer Gasphasenabscheidung im Vakuum hergestellt werden. Das unterscheidet diese von vielen in der Literatur vorgestellten Alterungsstudien, die Polymersolarzellen behandeln. Als Standardsystem werden einfache pii-Bauelemente ausgewählt, die in ZnPc und C60 zwei gut untersuchte Materialien in der aktiven Donor-Akzeptor-Schicht nutzen. Die Ergebnisse dieser Arbeit sind dabei in drei Kapiteln zusammengefasst. In Kapitel 5 wird untersucht, wie sich verschiedene Faktoren auf die Lebensdauer der Solarzellen auswirken. Für verkapselte Solarzellen mit MeO-TPD in der Lochtransportschicht wird die thermische Beschleunigung der Degradation mit einem Arrhenius’schen Verhalten beschrieben und eine Aktivierungsenergie EA=712 meV gefunden. Aus dieser Beschreibung wird für verkapselte Solarzelle bei 100 mW cm-2 und 45°C eine Lebensdauer von 62.000 h extrapoliert, die experimentell nicht verifiziert werden kann. Auch der Einfluss der Beleuchtungsintensität auf die Degradationsgeschwindigkeit wird untersucht und kann systematisch erklärt werden: Die Beschleunigung, die sich aus einer Erhöhung der Intensität weißen Lichtes ergibt, kann beschrieben werden, indem man die Anzahl extrahierter Ladungsträger berechnet. Bei Alterungen unter verschiedene Intensitäten ist diese Zahl identisch, wenn man die Messung bei gleichem Grad der Degradation betrachtet. Diese Modell kann auch auf monochromatische Beleuchtung ausgedehnt werden und es zeigt sich bei einem Vergleich über alle untersuchten Wellenlängen, dass der Anstieg der fallenden Kurven umso steiler wird, je höher die kürzeste Wellenlänge des jeweiligen Spektrums ist. Der zweite Teil dieses Kapitels ist der Degradation unverkapselter Solarzellen mit BF-DPB als Lochtransportmaterial gewidmet. Durch Variation von Temperatur und relativer Luftfeuchte konnten beide Einflussfaktoren in einem kombinierten Modell, dem Peck-Modell, zusammengefasst werden. Dieses wurde bisher nicht zur Beschreibung des Degradationsverhaltens von Solarzellen verwendet. Eine Vorhersage der Lebensdauer bei beliebigen Werten für beide Parameter ist somit möglich. Deutlich sticht in diesem Experiment hervor, dass die Anwesenheit von Wasser die Degradation klar dominiert. Darauf folgen Messungen, bei denen die Wasserpermeationsrate (WVTR) der Verkapselung variiert wird. Dabei stellt sich heraus, dass diese besser als 10-3 g m-2 d-1 sein muss, um die Stabilität zu verbessern. Durch eine Trennung der WVTR der äußeren Barriere und jener der Aluminiumelektrode ist es möglich, den Wert WVTR(Al) zu ermitteln. Dieser beträgt 8 x10-4 g m-2 d-1. Zusätzlich kann die Wassermenge, die benötigt wird, um die untersuch-ten Solarzelle auf 50% des Startwertes zu degradieren, zu 10 mg m2 bestimmt werden. Kapitel 6 zeigt eine umfassende Charakterisierung von an Luft degradierten Solarzellen. Mit den chemischen Analysemethoden TOF-SIMS und LDI-TOF-MS können mehrere Reaktionen der verwendeten Materialien mit Luft identifiziert werden. Dabei sticht die Oxidation der BPhen-Aluminium-Grenzschicht, die zur Bildung von Al2O3 führt, hervor. Weitere Reaktionsprodukte, vor allem in Verbindung mit Fluor, welches aus der Zersetzung von C60F36 stammt, werden gezeigt. Die Oxidation der Organik-Aluminium-Grenzschicht kann mit Hilfe von Elektrolumineszenzmessungen untersucht werden. Bei diesen zeigt sich, dass die Abnahme der aktiven Fläche in vollem Umfang Ursache für die Reduktion der Kurzschlussstromdichte ist. Als Eintrittskanäle für Sauerstoff und Wasser werden neben pinholes auch die Seitenkanten der Solarzelle identifiziert. Hinweise für die flächige Diffusion von Wasser werden zusätzlich erbracht. Erster Ansatz zur Verbesserung der Langzeitstabilität ist der Austausch von BPhen durch ein dotiertes Elektronen-transportmaterial. Eine Variation von fünf Materialien zeigt, dass ein Zusammenhang zwischen Rauigkeit dieses Materials und der Lebensdauer besteht: So werden die besten Stabilität für Materialien wie C60 und Bis-HFl-NTCDI gezeigt, die mit einer geringen Rauigkeit aufwachsen. Die Lebensdauer beträgt am Beispiel von Bis-HFl-NTCDI bei [T=65°C; rH=2,2%] T50=762 h und ist damit etwa viermal so groß wie bei Verwendung von BPhen. Weitere Optimierungsversuche, zum Beispiel durch Variation der Elektrode, des p-Dotanden, oder der Invertierung der Struktur zeigen zwar zusätzliche Degradationspfade auf, führen aber zu keiner Verbesserung der Stabilität. Auf Basis zuvor durchgeführter Überlegungen zu Redoxreaktionen (organischer) Materialien mit Wasser und Sauerstoff wird zum Abschluss der Arbeit ein möglicher Aufbau für luftstabile organische Solarzellen vorgeschlagen.

This study focuses on the architectural modification of pin-type small-molecule organic solar cells, in particular on the absorption layer and its influence on the key solar cell parameters, such as short circuit current density, fill factor and open circuit voltage. Three different approaches have been applied to improve the match between the solar spectrum and the spectral sensitivity of organic solar cells. In the first part, deposition parameters such as substrate temperature, gradient strength and (graded) absorption layer thickness are evaluated and compared to organic solar cells with homogeneously deposited absorption layers. Moreover, the gradient-like distribution of the absorption layer is characterized optically and morphological effects have been extensively studied. In order to isolate the origin of the efficiency improvement due to the graded architecture, voltage-dependent spectral response measurements have been performed and gave new insights. The second part concentrates on the efficient in-coupling of converted UV light, which is usually lost because of the cut off properties of organic light in-coupling layers. Via Förster resonance energy transfer, the absorbed UV light is re-emitted as red light and contributes significantly to higher short circuit current densities. The correlation between doping concentration, simple stack architecture modifications and the performance improvement is duly presented. In the third and last part, the impact of tri-component bulk heterojunction absorption layers is investigated, as these have potential to broaden the sensitivity spectrum of organic solar cells without chemical modification of designated absorber molecules. Along with the possibility to easily increase the photocurrent, an interesting behavior of the open circuit voltage has been observed. Knowledge about the impact of slight modifications within the solar stack architecture is important in order to be able to improve the device efficiency for the production of cheap and clean energy.

The objective of this research is to determine if it is possible to reduce the number of organic solar cells required to power a load using a DC to DC converter thereby reducing the cost of the organic solar array system. An organic solar power system designer may choose an organic implementation of a DC to DC converter to go along with the organic solar cell array. Common DC to DC converters include the buck converter, boost converter, buck/boost converter, and Cuk converter, all of which are not good candidates for organic implementation due to their use of inductors. Organic inductors are relatively more lossy than organic capacitors. So, an inductor-less DC to DC converter, such as the Dickson charge pump, would be a better candidate for organic implementation. Solar cells connected in an array configuration usually do not perform up to their full potential due to current and voltage mismatches between solar cells. These mismatches can be related to each solar cell's circuit model parameters such as the photon current density, diode ideality factor, diode reverse saturation current density, parallel resistance, and series resistance. This research varies these circuit model parameters as dependent variables, and observes the loads and power levels that make the Dickson charge pump a feasible option. The results show that current mismatch does produce an opportunity to use a DC to DC converter to save the use of a few solar cells. However, the Dickson charge pump was found to be infeasible due to an input voltage requirement that could not be met using the tested organic solar cells.

Un défi mondial fondamental est de développer des technologies peu coûteuses et stables pour récolter efficacement l'énergie solaire et la transformer en formes pratiques. Ainsi pour la conversion photovoltaïque plusieurs générations de cellules solaires ont émergé. En général, on peut diviser les types existants de cellules solaires en deux classes distinctes: les photovoltaïques inorganiques conventionnels (IPV), comme les jonctions silicium p-n, et les cellules solaires excitoniques (XSCs). Selon le type de matériaux utilisés les cellules solaires excitoniques sont classées en deux catégories: les cellules solaires à colorant (DSC) et les cellules organiques (OPV) développées en couche unique, ou en bi-couche, et les hétérojonction en volume (BHJ). Les cellules solaires à base de points quantiques (QDSC) sont un autre type de cellules solaires qui ont une configuration similaire aux DSCs ou OPVs.Bien que la performance des cellules solaires excitoniques ait été un thème central de la communauté scientifique pendant de nombreuses années, des approches théoriques facilitant sa compréhension sont nécessaires. Les théories semi-classiques son inadaptées pour traiter les phénomènes quantiques dans les cellules solaires nano-structurées. De plus, en raison de l'attraction coulombienne entre les porteurs photo-générés, l'application du formalisme de la fonction de Green hors équilibre (NEGF) pose certaines difficultés. Par conséquent, dans cette thèse, nous développons un nouveau formalisme quantique, basé sur la théorie de la diffusion quantique et sur l'équation de Lippmann-Schwinger, pour fournir un cadre complet pour comprendre les processus fondamentaux intervenant dans le fonctionnement des cellules solaires excitoniques.En particulier, nous nous concentrons sur des aspects qui ont été peu pris en compte dans le passé et nous abordons, au travers d’un modèle à deux niveaux, l'interaction Coulombienne électron-trou à courte et à longue portée, la recombinaison électron-trou, l'existence de canaux d'évacuation supplémentaires, le couplage électron phonon et la formation de bandes polaroniques.Ici, les cellules solaires excitoniques à deux niveaux sont considérées dans les régimes permanents et transitoires d'injection de charge. Les photocellules moléculaires où le processus de conversion de l'énergie se déroule dans un seul complexe donneur-accepteur moléculaire attaché aux électrodes sont considérées comme étant représentatives des XSC dans le régime permanent. A titre d'exemple pour les dispositifs photovoltaïques dans le régime transitoire, nous considérons les cellules photovoltaïques organiques hétéro-jonctions massives (BHJ OPV) qui sont l'approche la plus courante des OPV et se composent d'espèces mixtes donneuses et accepteuses. Dans ces systèmes, l'exciton créé par l'absorption des photons dans le côté donneur doit atteindre d'abord l'interface donneur-accepteur. A partir de ce moment, seulement un régime transitoire commence où les charges peuvent être séparées et injectées dans leurs côtés respectifs.Nous démontrons que la séparation du porteur de charge est un processus complexe qui est affecté par différents paramètres, tels que la force de l'interaction électron-trou et le taux de recombinaison non radiative. En outre, en fonction de la structure de la cellule, l'interaction électron-trou peut normalement diminuer ou augmenter anormalement l'efficacité. Le modèle proposé aide à comprendre les mécanismes des cellules solaires excitoniques, et il peut être utilisé pour optimiser leur rendement. / A fundamental global challenge is to develop an inexpensive, stable and scalable technology for efficiently harvesting solar photon energy and converting it into convenient forms. Photovoltaic energy conversion is attracting great attention such that several generations of solar cells have emerged. The existing types of solar cells roughly fall into two distinct classes: conventional inorganic photovoltaics (IPVs), such as silicon p-n junctions, and excitonic solar cells (XSCs). The mechanistic distinction of IPVs and XSCs results in fundamental differences in their photovoltaic behavior.According to the type of materials used in their structure, excitonic solar cells are classified into two categories: dye-sensitized solar cells (DSC) and organic photovoltaics (OPV) developed in single-layer and bi-layer including planar and bulk hetero--junction configurations. Quantum dot solar cells (QDSC) are another type of solar cells that have a similar configurations to DSCs or OPVs.While understanding the performance of excitonic solar cells has been a central effort of the scientific community for many years, theoretical approaches facilitating the understanding of electron-hole interaction and recombination effects on the cell performance are needed. Semiclassical theories are inefficient tools to treat quantum phenomena in nano-structured solar cells, and on the other hand, due to the Coulomb attraction between the photo generated carriers, the application of standard Non-Equilibrium Green Function (NEGF) formalism presents some difficulties although some specific methods allow to circumvent this problem.In this thesis we develop a new quantum formalism, which is based on quantum scattering theory and on the Lippmann-Schwinger equation, to provide a comprehensive framework for understanding the fundamental processes taking place in the operation of excitonic solar cells. Considering simple two-level models we address important effects such as the short--range and long--range electron--hole Coulomb interaction, the electron--hole recombination, the existence of extra evacuation channels, and the electron--phonon coupling and polaronic bands formation.Here, the two-level excitonic solar cells are considered in the permanent and transitory regimes of charge injection. The molecular photocells where the energy conversion process takes place in a single molecular donor-acceptor complex attached to electrodes are considered as a representative of XSCs in the permanent regime. As an example for the photovoltaic devices in the transitory regime, we consider the bulk hetero--junction organic photovoltaic cells (BHJ OPVs) which are the most common approach to OPVs and consists of mixed donor and acceptor species that form interpenetrating connective networks. In these systems the exciton created by the photon absorption in the donor side must reach first the donor--acceptor interface. From this moment only a transitory regime begins where the charges can be separated and injected in their respective sides.We demonstrate that the charge carrier separation is a complex process that is affected by different parameters, such as the strength of the electron--hole interaction and the non--radiative recombination rate. Furthermore, depending on the cell structure, the electron-hole interaction can normally decrease or abnormally increase the cell efficiency. The proposed model helps to understand the mechanisms of excitonic solar cells, and it can be used to optimize their yield.

Cette thèse se propose d'explorer les mérites d'une famille d'approches de simulation quantique ab initio, les théories de perturbation à N-corps, pour l'exploration des propriétés électroniques et optiques de systèmes organiques. Nous avons étudié en particulier l'approximation dite de GW et l'équation de Bethe-Salpeter, très largement utilisées dès les années soixante pour les semiconducteurs de volume, mais dont l'utilisation pour les systèmes organiques moléculaires est très limitée. L'étude de quelques cas d'intérêt pour le photovoltaïque organique, et en particulier de petites molécules pour lesquelles sont disponibles des données expérimentales ou des résultats issus d'approches de chimie quantique, nous ont permis de valider ces approches issues de la physique du solide.Ce doctorat s'inscrit dans le cadre du développement d'un outil de simulation quantique spécifique (le projet FIESTA) dont l'objectif est de combiner les formalismes GW et Bethe-Salpeter avec les techniques de la chimie quantique, c'est-à-dire en particulier l'utilisation de bases localisées analytiques (bases gaussiennes) et des approches de type «résolution de l'identité» pour le traitement des intégrales Coulombiennes. Ce code est aujourd'hui massivement parallélisé, permettant, au delà des études de validation présentées dans ce travail de thèse, l'étude de systèmes complexes comprenant plusieurs centaines d'atomes. En cours de développement, l'incorporation d'approches hybrides combinant mécanique quantique et écrantage à longue portée par des approches modèles de milieu polarisable m'a permis d'une part de me familiariser avec le code et le développement méthodologique, et permet d'autre part d'envisager l'étude de systèmes réalistes en couplage avec leur environnement.Le manuscrit s‘ouvre sur une introduction au photovoltaïque organique afin de mettre en lumière les questionnements spécifiques qui requièrent le développement de nouveaux outils théoriques à la fois fiables en terme de précision et suffisamment efficaces pour traiter des systèmes de grande taille. Le premier chapitre est d'ordre méthodologique et rappelle les fondements des techniques ab initio de type champ-moyen (Hartree, Hartree-Fock et théorie de la fonctionnelle de la densité). En partant des principes de la photoémission, les théories de perturbation à N-corps et la notion de quasi-particule sont ensuite introduites, conduisant aux équations de Hedin et aux approximations GW et COHSEX. De même, à partir de la compréhension d'une expérience d'optique, le traitement des interactions électron-trou est présenté, menant à l'équation de Bethe-Salpeter. Le chapitre 2 introduit brièvement les spécificités techniques liées à l'implémentation des formalismes GW et Bethe-Salpeter. Les propriétés analytiques des bases gaussiennes et les principes mathématiques derrière les techniques de type «résolution de l'identité» et «déformation de contour», sont brièvement décrites. Le troisième chapitre présente les résultats scientifiques obtenus durant cette thèse. Le cas paradigmatique d'un polypeptide model nous permettra de discuter des spécificités de l'approche GW appliquée à des systèmes moléculaires afin d'obtenir des énergies de quasiparticule de bonne qualité. De même, l'utilisation de l'équation de Bethe-Salpeter pour l'obtention du spectre optique de ce système sera présentée, ainsi que le cas d'une famille de colorants d'importance pour les cellules de Graetzel (les coumarines). Finalement, nous explorons dans le cas du fullerène C60 et du graphène le calcul des termes de couplage électron-phonon dans le cadre de l'approche GW, c'est-à-dire au delà des approches standards de type théorie de la fonctionnelle de la densité. Notre étude vise à vérifier si une approximation statique et à écrantage constant au premier ordre permet de garder la qualité des résultats GW pour un coût numérique réduit. Après la conclusion, les appendices donnent le détail de certaines dérivations. / The present thesis aims at exploring the properties and merits of the ab initio Green's function many-body perturbation theory (MBPT) GW and Bethe-Salpeter formalisms, in order to provide a well-grounded and accurate description of the electronic and optical properties of condensed matter systems. While these approaches have been developed for extended inorganic semiconductors and extensively tested on this class of systems since the 60 s, the present work wants to assess their quality for gas phase organic molecules, where systematic studies still remain scarce. By means of small isolated study case molecules, we want to progress in the development of a theoretical framework, allowing an accurate description of complex organic systems of interest for organic photovoltaic devices. This represents the main motivation of this scientific project and we profit here from the wealth of experimental or high-level quantum chemistry reference data, which is available for these small, but paradigmatic study cases.This doctoral thesis came along with the development of a specific tool, the FIESTA package, which is a Gaussian basis implementation of the GW and Bethe-Salpeter formalisms applying resolution of the identity techniques with auxiliary bases and a contour deformation approach to dynamical correlations. Initially conceived as a serial GW code, with limited basis sets and functionalities, the code is now massively parallel and includes the Bethe-Salpeter formalism. The capacity to perform calculations on several hundreds of atoms to moderate costs clearly paves the way to enlarge our studies from simple model molecules to more realistic organic systems. An ongoing project related to the development of discrete polarizable models accounting for the molecular environment allowed me further to become more familiar with the actual implementation and code structure.The manuscript at hand is organized as follows. In an introductory chapter, we briefly present the basic mechanisms characterizing organic solar cells, accentuating the properties which seek for an accurate theoretical description in order to provide some insight into the factors determining solar cell efficiencies. The first chapter of the main part is methodological, including a discussion of the principle features and approximations behind standard mean-field techniques (Hartree, Hartree-Fock, density functional theory). Starting from a description of photoemission experiments, the MBPT and quasiparticle ideas are introduced, leading to the so-called Hedin's equations, the GW method and the COHSEX approach. In order to properly describe optical experiments, electron-hole interactions are included on top of the description of inter-electronic correlations. In this context, the Bethe-Salpeter formalism is introduced, along with an excursus on time-dependent density functional theory. Chapter 2 briefly presents the technical specifications of the GW and Bethe-Salpeter implementation in the FIESTA package. The properties of Gaussian basis sets, the ideas behind the resolution of the identity techniques and finally the contour deformation approach to dynamical correlations are discussed. The third chapter deals with the results obtained during this doctoral thesis. On the electronic structure level, a recent study on a paradigmatic dipeptide molecule will be presented. Further, also its optical properties will be explored, together with an in-depth discussion of charge-transfer excitations in a family of coumarin molecules. Finally, by means of the Buckminster fullerene C60 and the two-dimensional semi-metal graphene, we will analyze the reliability of two many-body formalisms, the so-called static COHSEX and constant-screening approximation, for an efficient calculation of electron-phonon interactions in organic systems at the MBPT level. After a short conclusion, the Appendix containing details and derivations of the formalisms presented before closes this work.

Ce travail a été consacré à la caractérisation, l’optimisation et l’étude du comportement photochimique de couches actives de cellules photovoltaïques organiques. L’objectif était d’identifier les mécanismes de photovieillissement dans le but de proposer des stratégies pour améliorer la stabilité de cellules solaires organiques à base de Poly(3-hexylthiophène) (P3HT) et de [6,6]-phényl-C61-butanoate de méthyl (PCBM). Le premier axe de travail a été consacré à l’étude de l’influence des paramètres structuraux du P3HT (masse molaire, régiorégularité, pureté …) sur sa stabilité photochimique dans un premier temps, puis sur l’optimisation des performances des cellules dans un second temps. Pour ce faire, quatre P3HT commerciaux différents ont été étudiés. Ils ont tout d’abord été caractérisés avec précision grâce à différentes techniques analytiques. La relation entre microstructure du P3HT et sa photostabilité a ensuite été étudiée, et les paramètres pertinents influençant la photodégradation identifiés. Le deuxième axe de travail a été consacré à la stabilité photochimique de mélanges P3HT:PCBM sur substrat inerte, puis sur des couches tampons. Dans une dernière partie, la dégradation de couches actives au sein de dispositifs complets a été effectuée en faisant « l’autopsie » de cellules vieillies. / This work was devoted to optimization, characterization and photochemical behavior of active layers of organic solar cells. The objective was to identify photo-ageing mechanisms in order to propose strategies to improve the stability of Poly (3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butanoic acid methyl ester (PCBM) organic solar cells. The first axe of this work was devoted to the study of the influence of structural parameters of P3HT (molecular weight, regioregularity, purity …) firstly on its photochemical stability and secondly on the optimization of the performances of devices. In this goal, four different commercial P3HT were studied. They were first characterized accurately with different analytical techniques. The relationship between P3HT microstructure and photostability has been studied and relevant parameters influencing photodegradation have been identified. The second axe of this work was devoted to the photochemical stability of P3HT:PCBM blends on inert substrate, and then on buffer layers. Finally, the degradation of active layers in complete devices has been performed by making the “autopsy” of degraded solar cells.

Morphological architecturing is one of the smart and efficient ways to maximize the number of excitons harvested from the known photoactive materials and existing fabrication technologies. Surfaces and interfaces play a vital role in absorbing light and therefore when patterned regularly, aid in the improvement of light absorption. This thesis deals with the study of light management by morphologically architecturing the organic electroactive materials. Here, morphological architecturing is carried out using electrospinning technique by optimizing various parameters. In the first part, organic photovoltaic system is tailored by morphologically modifying the conjugated polymer active layer and analyzing the enhancement in light collection and hence performance of photovoltaic devices. In the second part, the prospects of using free standing buffer layer instead of thin film buffer layer in a solar cell is evaluated. Furthermore, the study on morphological engineering of conjugated small molecule is carried out, by varying the solvents and derivatives, in order to control morphologies by understanding the underlying mechanism. Overall this thesis attempts to understand the fundamentals in morphological architecturing, by physical architecturing of the small molecules in a device for light management applications as well as demonstrating improvement in light absorption in existing organic photovoltaic systems. In the introduction chapter, a brief description of organic photovoltaics is given followed by highlighting the importance of processing methods in light management and in organic photovoltaics. The significance of structured architecture in improving the device characteristics is presented. The issues and challenges in existing architecturing techniques available in literature are discussed. Electrospinning as a tool for morphological modification for organic photovoltaics is demonstrated. This is followed by an outline of the thesis. In Chapter 2, brief description of procedures carried out for fabrication, characterization and optimization of electrospinning process parameters are discussed. The description of fabrication procedures including electrospinning, spincoating and thermal evaporation are given. Characterization techniques used in this thesis for surface and feature analysis, structural, compositional, optical and opto-electrical analyses are described. Optimization of electrospinning process parameters in obtaining various morphologies are evaluated. In Chapter 3, enhancement of device characteristics of poly (3-hexylthiophene): phenyl C61-butyric acid methyl ester (P3HT: PCBM) by changing active layer film morphology into network structure is elucidated. Network structure is provided by electrospraying assisted hierarchical assembly of short fibrils. Effect of electrospraying parameters such as solvent, polymer blend concentration, applied voltage, tip to collector distance, flow rate and deposition time are analyzed. Solvent and applied voltage are observed to be the major parameters governing the formation of network structure. The optimized conditions are used to investigate the optical and structural properties. Percent reflectance studies showed improvement in light absorption due to increase in surface area. Structural characterization studies indicate an increase in orientation of crystallites and crystallinity as compared to spincoated samples. The optimized conditions along with additional spincoated layer of P3HT:PCBM are used to fabricate bulk heterojunction device. Device characteristics exhibited an increase in short circuit current and thus increase in efficiency from 2.18% to 3.66%. There is a enhancement of 37.5% going from maximum external quantum efficiency of 40%-55% for electrosprayed and spincoated devices. It is anticipated that network morphology could be the next possible structure to be explored in organic photovoltaic materials. In Chapter 4, photonic structure is analyzed and compared. A photonics device requires uniform periodic structural arrangement. Various techniques are used to fabricate these types of structures, employing several steps of fabrication. This work proposes single step hierarchical array of equal submicron size porous structure fabricated by tuning electrospinning processing parameters. The dictating process parameters on evolving structure are high voltage, tip to collector distance and solvent. Morphological and optical investigations suggest that uniform periodic topography helps in light scattering leading to multi reflection and thus enhancement in light absorption. This structure is evaluated as active layer in organic photovoltaic devices using poly (3 hexyl thiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blend and its device characteristics are analyzed. Consistent and reliable device characteristics obtained through photonic structure is demonstrated. Finally, comparison is drawn to network structure to assess the advantages and limitations of both morphologies as active layer in organic photovoltaics. In Chapter 5, instead of architecturing active layer the next polymer film layer in the organic solar cells, that is the hole transport layer is transformed into free standing nanofiber mats. Morphological, structural and surface wetting properties are assessed for these nanofiber mats followed by fabrication of inverted organic solar cell. The free standing nanofibers mats are obtained by electrospinning the blend of Poly(3,4-ethylenedioxythiophene) Polystyrene sulfonate (PEDOT:PSS) a conducting water soluble polymer with other water soluble polymers such as poly vinyl alcohol (PVA) and poly ethylene oxide (PEO). The study is further extended by employing two batches of PEDOT:PSS of varying conductivity that are analyzed side by side for six ternary and two binary blends each. Electrospinning parameters such as applied voltage and flow rate are optimized and fibers of diameter 150-200 nm are obtained. Maximum content of PEDOT:PSS with which free standing fiber mats could be achieved are 98 and 99%. Subsequent increase in PEDOT:PSS results in formation of beads. Surface wetting behavior showed that hydrophillicity increases with increase in PEDOT:PSS content. Devices are fabricated and the variation in characteristics and charge collection with respect to addition of PEO and PVA are discussed. In Chapter 6, a conjugated small molecule is taken as case study unlike the use of the conjugated polymer studies in previous chapters. A mechanism is proposed for tuning the sphere-spike morphology and also to control the crystallite size through solvent management using a conjugated small molecule. Electrospraying of an organic molecule is carried out using various solvents, obtaining fibril structures along with a range of distinct morphologies. Solvent characteristics play a major role in achieving the morphology of the organic material. A thiophene derivative (7, 9-di (thiophen-2-yl)-8H-cyclopenta [a]acenaphthylen-8-one) (DTCPA) of donor-acceptor-donor (DAD) architecture is used to study this solvent effect. Seven solvents with decreasing vapor pressure are selected for experiments. Electrospraying is conducted at a solution concentration of 1.5 wt % and a constant applied voltage of 15 kV. Gradual transformation in morphology of the electrospun product from spikes-sphere to only spikes is observed. A mechanism describing this transformation is proposed based on the electron micrograph analysis and XRD analysis. These data indicate that the morphological change is due to the synergistic effect of both vapor pressure and dielectric constant of the solvents. Through a reasonable control over the crystallites size and morphology along with supporting transformation mechanism theory, the work in this chapter elucidates electrospraying as a prospective method for designing the architectures in organic electronics. In Chapter 7, light management studies are carried out by morphologically architecturing the carbazole derivatives through electrospraying. The effect of derivatives on morphology is analyzed. The two carbazole derivatives; carbazole-benzothiadiazole (Cz-Bz) resulted in 2D structures and carbazole-benzothiadiazole-bithiophene (Cz-Bz-Bt) resulted in 3D structures after electrospraying. These structures are further analyzed to study the effect of vapor pressure of solvents and solution concentration. Structural characteristics indicate that electrospraying imparts change in molecular structure orientation. Optical studies showed 19 – 31% enhancement in light absorption. Further, three types of organic photovoltaic devices are fabricated and the opto-electrical properties are evaluated. Also, the effect of substrate on morphological formation is assessed. In Chapter 8, the major contributions and conclusions drawn from the morphological architecturing of both conjugated polymers and small molecules are summarized, along with few recommendations for future research.

In recent years, organic photovoltaics (OPVs) have attracted considerable attention as a potential source of renewable energy over traditional materials due to their light weight, low production cost, mechanically stability and compatibility with flexible substrates in roll to roll processing for high volume production. In the OPVs interface plays an important role in determining the performance of the device. Interface signifies formation of efficient contact with electrode, film, and transport of free charge carrier, which results in better performance in the device. Interface engineering also helps in improving mechanical robustness of the device. Hence, understanding of interface, modification and its evaluation is important in fabrication of efficient device. In this thesis interface is modified such that the performance of the device can be improved (chapter 3 and chapter 4). In Chapter 5 and chapter 6 interface is modified such that device can be fabricated on uncommon substrate. Fabrication of device on uncommon substrates (fiber reinforced plastic and flexible glass substrate), has unique challenges. In chapter 5 and chapter 6, we look at how interface is modified to overcome the challenges associated and also understand the role of interface in improving the performance of device on such substrates is discussed. In Chapter 1 we discuss about working of organic solar cells and the challenges associated in device fabrication. Understanding of interface to overcome challenges associated is explained. It also covers brief introduction to the succeeding chapters discussed in the thesis and its recent developments. To understand the properties of interface and to analyze device performance various characterization techniques have been used are discussed in chapter 2. This chapter also covers the materials and general device fabrication techniques used in this thesis. In chapter 3, a narrow bandgap (NBG) polymer used as a near IR sensitizer in P3HT: PCBM blend. Since, P3HT with a band gap of ~1.9 eV, the commonly used p-type material absorbs approximately ~25 % of incident light. Hence, MP2 (NBG polymer) is used along with P3HT: PCBM in active layer to form a ternary blend, which helps in increased absorption. Basic properties of MP2 are evaluated using UV-visible spectroscopy, differential scanning calaorimetry(DSC), thermogravimetric analyser (TGA), gel permeation chromatography (GPC) and photoluminescence (PL) techniques. To evaluate enhanced absorption of ternary UV-visible spectroscopy is carried out. Charge transfer from one moiety to other in ternary blend is evaluated using PL and Ttime resolved microwave conductivity (TRMC). Morphology of the ternary is assessed using atomic force microscope (AFM) and structural characterization is carried out by X-ray diffraction (XRD). Performance of the device is evaluated by current-voltage (J-V) characterizations. Further improved performance is supported by external quantum efficiency (EQE). Charge extraction with linear increasing voltage (CELIV) of the device is done to evaluate the recombination mechanism in the device and to assess the performance of the device. One-dimensional (1D) ZnO nanostructures provide direct paths for charge transport, and also offer large interfacial area to make them an ideal electron transport layer. In chapter 4 highly aligned ZnO nanorods is used as electron transport layer in OPV. Growth of ZnO nanorods is two-step processes, growing seed layer and growing ZnO nanorods from hydrothermal process using an appropriate seed layer. Two different soft-chemical solution- growth methods (upward and downward) are developed to fabricate self-assembled, oriented ZnO nanorods. Substrate mounting, surface properties and optical transmittance are optimized by varying the nanorods growth conditions. Further the ZnO nanorods are UV ozone treated and its effect on performance of nanostructured buffer layer based device is evaluated. In Chapter 5 OPV is fabricated on an opaque FRP substrate. Fabrication of OPV device on opaque substrate plastic is unique and hence understanding various properties is vital. Such devices fabrication require bottom up approach, with transparent electrode as the top electrode and metal electrode on the surface of FRP. FRP has inherent rough surface of about few microns RMS roughness. In order to reduce the roughness of the substrate FRP was planarized. The planarized layer is chosen, such that it chemically binds with the substrate. The chemical interaction between substrate and planarizing coating is evaluated by FTIR and Raman spectroscopy. The binding of planarized layer and FRP is evaluated using nanoscratch technique and surface energies are studied using contact angle measurements. In addition, adhesion properties of the metal electrodes, which are deposited on planarized FRP are evaluated using nanoscratch technique. Fabrication of OPV requires a top transparent electrode. Simple spin coating technique is used to optimize the top electrode. The property of top electrode is evaluated using UV-visible spectroscopy for transmittance, and sheet resistance of the electrode is characterized. OPV device is fabricated on planarized FRP substrate using optimized top transparent electrode and its PV properties is evaluated. Performance of the device is evaluated for two different bottom electrodes and further performance of device is enhanced using buffer layers. Usually flexible OPVs are fabricated on plastic substrate such as PET, PEN. However they are not structurally stable at high temperatures and have high oxygen and moisture Permeability. In Chapter 6 Organic based photovoltaic devices were fabricated on flexible glass. Flexible glass has high strength and it is also known for low oxygen and moisture permeability. Fabrication of device on flexible glass has never been done before and hence, generation of data is necessary for commercialization of the technology. Device fabrication is optimized by using two different transparent conducting layers (ITO- sputter deposited, PEDOT: PSS-solution processed) and device performance was evaluated for both. Since the substrate is flexible in nature understanding the performance of the device during flexing is important. For this 2-parallel plate flexural apparatus is fabricated for in-situ measurements along with current voltage measurements. These devices are flexed cyclically and performance of device is evaluated. Therefore, work discussed in the thesis show by modifying the interface of the device, and understanding various interfaces of the device is crucial for improving the performance of the device. Also by engineering the interface, devices can be fabricated on various types of substrate.