Une alternative aux énergies fossiles est le domaine du photovoltaïque organique qui a récemment commencé son transfert technologique des laboratoires de recherche vers l’industrie. De nombreux efforts de recherche sont réalisés sur les matériaux et les procédés pour augmenter les performances des cellules solaires organiques. Dans ce contexte, ce travail présente une étude complète allant de la conception de nouveaux polymères donneurs d’électrons à grande largeur de bande interdite à leur caractérisation en dispositifs photovoltaïques. La principale caractéristique recherchée a été de diminuer le niveau énergétique HOMO des polymères pour augmenter la tension en circuit ouvert des dispositifs photovoltaïques. L’approche « donneur-accepteur » a été utilisée pour obtenir les propriétés désirées. Des polymères comportant des unités pauvres en électrons, quinoxaline ou dithienoquinoxaline, et des unités riches en électrons, dibenzosilole ou carbazole, ont été synthétisés par couplage de Suzuki ou par hétéroarylation directe. Des masses molaires allant jusqu’à 56 kg.mol-1 ont été obtenues. Le motif quinoxaline a été décliné sous forme de plusieurs molécules substituées par des atomes de fluor sur le benzène ou par des groupements thiophènes, bithiophènes et terthiophènes sur la partie pyrazine. Des espaceurs thiophènes ou thiazoles ont été utilisés pour relier l’unité riche en électrons et l’unité pauvre en électrons. Les relations entre les modifications structurales et les propriétés structurales et optoélectroniques des polymères ont été analysées. Les propriétés optiques ont été étudiées par spectroscopie UV-visible et par spectroscopie de fluorescence et ont montré une absorption allant jusqu’à 550 nm pour les polymères à motifs dithienoquinoxaline-dibenzosilole, 650 nm pour les polymères à motifs quinoxaline-dibenzosilole et 700 nm pour la famille quinoxaline-carbazole. Ces valeurs correspondent à des largeurs de bande interdite comprises entre 1,8 eV et 2,3 eV. Les niveaux énergétiques HOMO et LUMO des polymères ont été déterminés par électrochimie. Tous les polymères possèdent des niveaux énergétiques HOMO inférieurs à -5,0 eV. Les atomes de fluor et les espaceurs thiazoles ont permis d’abaisser les niveaux énergétiques HOMO des polymères jusqu’à -5,69 eV. Les structures des polymères ont été modélisées par DFT et étudiées par diffraction des rayons X. Les mobilités des trous des polymères ont été mesurées en transistor organique à effet de champ, des valeurs atteignant 9,0. 10 3 cm.V 1.s 1 ont été atteintes. Les polymères ont été testés en dispositifs photovoltaïques selon une architecture standard à hétérojonction volumique en mélange binaire et en mélange ternaire. En mélange avec le PC71BM ou l’IC61BA, ces polymères ont permis d’atteindre des tensions en circuit ouvert entre 0,65 V et 1,05 V et des rendements de conversion photovoltaïque jusqu’à 5,14 % sur une surface active de 0,28 cm2. Les morphologies des couches actives ont été étudiées par AFM afin de comprendre en détail les paramètres de fonctionnement des cellules obtenues. Les polymères présentés dans cette étude ont été utilisés dans des cellules solaires à mélange ternaire présentant de bonnes performances. Certains polymères ont été testés dans des photocathodes pour la production d’hydrogène et ont permis d’obtenir une amélioration du potentiel de réduction par rapport à celui obtenu avec les photocathodes à base de P3HT. Enfin, compte tenu de leurs propriétés optoélectroniques et de leurs performances photovoltaïques certains de ces polymères devraient pouvoir être employés de manière avantageuse en sous cellules de dispositifs tandem en remplacement du P3HT par exemple. / An alternative to fossil fuels are the organic photovoltaic cells which have recently started their technological transfer from research laboratories to industry. Many research efforts have been made on the modification of materials and processes to increase the performance of organic solar cells. In this context, this work presents a comprehensive study from the design of new electron-donor high band gap polymers to their characterisation in photovoltaic devices. The main requirement was to decrease the HOMO energy level of the polymers in order to increase the open circuit voltage of the solar cells. The "push-pull" approach was used to obtain the desired properties. Polymers with quinoxaline or dithienoquinoxaline as electron-deficient units and dibenzosilole or carbazole as electron-rich units were synthesized by Suzuki coupling or by direct heteroarylation. Molecular weights up to 56 kg.mol 1 were obtained. The electron-withdrawing unit quinoxaline was substituted by fluorine atoms on the benzene moiety and by thiophene, bithiophene and terthiophene group on the pyrazine moiety. Thiophenes or thiazoles were used as spacers to link the electron-donating and the electron-withdrawing units. The relationship between the structural modification of the polymers and their optoelectronic properties were analysed. The optical properties were studied by UV-visible spectroscopy and fluorescence spectroscopy. Whereby it appears that polymers with dithienoquinoxaline-dibenzosilole units showed an absorption up to 550 nm and polymers with both quinoxaline-dibenzosilole units and quinoxaline-carbazole units showed an absorption up to 650-700 nm respectively. The corresponding optical band gaps were found to range from 1.8 eV to 2.3 eV. The HOMO and LUMO energy levels of the polymers were determined by electrochemistry. All polymers exhibited HOMO energy levels below -5.0 eV. Fluorine atoms and thiazole spacers significantly lowered the HOMO energy levels of the polymers up to -5.69 eV. DFT was used to model the polymer structures. X-ray diffraction was used to analyse the distances between the polymer chains. Hole mobilities were measured in organic field effect transistors and values of up to 9.0 x 10 3 cm2.V-1.s-1 were obtained. The polymers were tested in organic photovoltaic devices according to a standard bulk heterojunction structure in binary and ternary mixtures. In a blend with PC71BM or IC61BA, these polymers have led to open circuit voltages ranging from 0.65 V to 1.05 V and to power conversion efficiencies of up to 5.14 % on a surface area of 0.28 cm2. The active layer morphologies were studied by AFM. The polymers presented in this work were used in ternary blend solar cells. Some polymers were tested in photocathodes for hydrogen evolution and showed an improvement of the reduction potential compared to that of the photocathodes based on P3HT. Owing to their optoelectronic properties and their photovoltaic properties in standard device configurations, some of the materials developed in this study appear as valuable materials for future developments of organic tandem solar cells.

Organic solar cells (OSC) are interesting as low cost alternative to conventional solar cells. Unsubstituted Metal-phthalocyanines (Pc) are excellent electron donating molecules for heterojunction OSC. Usually organic solar cells with Pcs are produced by vapor deposition, although solution based deposition (like spin casting) is cheaper and offers more possibilities to control the structure of the film. With solution based deposition several parameters (like temperature, solvent and etc.) affect the self-organized structure formation via nucleation and growth. The reason why vapor deposition is typically used is the poor solubility of the metal-phthalocyanines in most common solvents. Furthermore the process of nucleation and growth of Pc aggregates from solution is not well understood. For preparation of Pc films from solution, it is necessary to find the appropriate solvents, assess the solution deposition techniques, such as dip coating, and spin casting. It is necessary to understand the nucleation and growth process for aggregation/precipitation and to use this knowledge to produce nanostructures appropriate for OSC. This is important because the nanostructure of the films determines their performance. In this thesis, optical absorption and the stability of 8 different unsubstituted metal Pc’s were studied quantitatively in 28 different solvents. Among the several solution based deposited thin films produced based on this study, copper phthalocyanine (CuPc) dissolved in trifluoroacetic acid (TFA) is chosen as a model system for an in-depth study. CuPc has sufficient solubility and stability in TFA and upon solution processing forms appropriate structures for OSCs. CuPc molecules aggregate into layers of nanoribbons with a thickness of ~ 1 nm and an adjustable width and length. The morphology and the number of deposited layers in the thin films are controlled by different parameters, like temperature and solution concentration. Material properties of CuPc deposited from TFA are studied in detail via x-ray diffraction, UV-Vis and FT-IR spectroscopy. Atomic force microscopy was used to study the morphology of the dried film. The mechanism of the formation of CuPc nanoribbons from spin casted CuPc/TFA solution in ambient temperature is investigated and explained. The parameters (e.g. solution concentration profile) governing nucleation and growth are calculated based on the spin casting theory of a binary mixture of a nonvolatile solute and evaporative solvent. Based on this and intermolecular interactions between CuPc and substrate a nucleation and growth model is developed explaining the aggregation of CuPc in a supersaturated TFA solution. Finally, a solution processed thin film of CuPc is applied as a donor layer in a functioning bilayer heterojunction OSC and the influence of the structure on OSC performance is studied. / In den vergangenen Jahren wurden kosteneffiziente nasschemische Beschichtungsverfahren für die Herstellung organischer Dünnfilme für verschiedene opto-elektronische Anwendungen entdeckt und weiterentwickelt. Unter anderem wurden Phthalocyanin-Moleküle in photoaktiven Schichten für die Herstellung von Solarzellen intensiv erforscht. Aufgrund der kleinen bzw. unbekannten Löslichkeit wurden Phthalocyanin-Schichten durch Aufdampfverfahren im Vakuum hergestellt. Des Weiteren wurde die Löslichkeit durch chemische Synthese erhöht, was aber die Eigenschaften von Pc beeinträchtigte. In dieser Arbeit wurde die Löslichkeit, optische Absorption und Stabilität von 8 verschiedenen unsubstituierten Metall-Phthalocyaninen in 28 verschiedenen Lösungsmitteln quantitativ gemessen. Wegen ausreichender Löslichkeit, Stabilität und Anwendbarkeit in organischen Solarzellen wurde Kupferphthalocyanin (CuPc) in Trifluoressigsäure (TFA) für weitere Untersuchungen ausgewählt. Durch die Rotationsbeschichtung von CuPc aus TFA Lösung wurde ein dünner Film aus der verdampfenden Lösung auf dem Substrat platziert. Nach dem Verdampfen des Lösungsmittels, die Nanobändern aus CuPc bedecken das Substrat. Die Nanobänder haben eine Dicke von etwa ~ 1 nm (typische Dimension eines CuPc-Molekül) und variierender Breite und Länge, je nach Menge des Materials. Solche Nanobändern können durch Rotationsbeschichtung oder auch durch andere Nassbeschichtungsverfahren, wie Tauchbeschichtung, erzeugt werden. Ähnliche Fibrillen-Strukturen entstehen durch Nassbeschichtung von anderen Metall-Phthalocyaninen, wie Eisen- und Magnesium-Phthalocyanin, aus TFA-Lösung sowie auf anderen Substraten, wie Glas oder Indium Zinnoxid. Materialeigenschaften von aufgebrachten CuPc aus TFA Lösung und CuPc in der Lösung wurden ausführlich mit Röntgenbeugung, Spektroskopie- und Mikroskopie Methoden untersucht. Es wird gezeigt, dass die Nanobänder nicht in der Lösung, sondern durch Verdampfen des Lösungsmittels und der Übersättigung der Lösung entstehen. Die Rasterkraftmikroskopie wurde dazu verwendet, um die Morphologie des getrockneten Films bei unterschiedlicher Konzentration zu studieren. Der Mechanismus der Entstehung der Nanobändern wurde im Detail studiert. Gemäß der Keimbildung und Wachstumstheorie wurde die Entstehung der CuPc Nanobänder aus einer übersättigt Lösung diskutiert. Die Form der Nanobändern wurde unter Berücksichtigung der Wechselwirkung zwischen den Molekülen und dem Substrat diskutiert. Die nassverarbeitete CuPc-Dünnschicht wurde als Donorschicht in organischen Doppelschicht Solarzellen mit C60-Molekül, als Akzeptor eingesetzt. Die Effizienz der Energieumwandlung einer solchen Zelle wurde entsprechend den Schichtdicken der CuPc Schicht untersucht.

Organic solar cells have the potential to be portable power sources that are light-weight, flexible, and inexpensive. However, the highest power conversion efficiency for organic solar cells to date is ~8%, and most high-efficiency solar cells have an area of less than 1 cm². This thesis advances the field of organic solar cells by studying the physics and engineering of the devices to understand the reverse saturation current, which is related to efficiency, and the effects of area scaling. The most commonly accepted models to describe the physics of organic photovoltaic devices are reviewed and applied to planar heterojunction solar cells based on pentacene / C60 as a model system. The equivalent circuit model developed for inorganic solar cells is shown to work well to describe the behavior of organic devices and parameterize their current-voltage characteristics with five parameters. Changes in the parameters with different material combinations or device structures are analyzed to better understand the operation of the presented organic solar cells. A one-dimensional diffusion model for the behavior of excitons and treatment of the organic layers as planes is demonstrated to adequately model the external quantum efficiency and photocurrent in pentacene / C60 solar cells. The origin of the open-circuit voltage is studied using cells with different electrodes and different donor materials. While changing the electrodes does not affect open-circuit voltage, it is greatly modified by changes in the donor. Tests with additional semiconductors show the change in open-circuit voltage is not consistent from donor to donor as the acceptor is varied, suggesting a more complex relation than just the difference in energy levels. Study of the temperature dependence of the equivalent circuit parameters shows that the reverse saturation current, which has a significant role in determining the open-circuit voltage, has a thermally activated behavior. From this behavior, the reverse saturation current is related back to charge transfer at the donor / acceptor heterojunction to suggest that both the effective energy barrier presented by the energy levels and the electronic coupling are important in determining the reverse saturation current and open-circuit voltage. This marks a shift from just considering a built-in voltage or the energy levels to also considering the electronic coupling of the donor and acceptor materials. Temperature-dependent performance characteristics are also used to show key differences between organic and inorganic devices. Finally, the effect of area scaling is explored with pentacene / C60 solar cells having areas of 0.11, 7, and 36.4 cm². Analysis with the equivalent circuit model shows that performance decreases as area increases because of an increasing series resistance presented by the transparent electrode. A metal grid, to provide low resistance pathways for current, fabricated on top of the transparent electrode is proposed to reduce the effective resistance. The grid is unique in that it is placed between the electrode and the semiconductor layer and must be passivated to prevent shorts through the thin semiconductor to the back metal electrode. Analysis of the grid predicts greatly reduced series resistance, and experimental results show reduced resistance and improved performance for the 7 cm² and 36.4 cm² devices when including the grid.

In organischen Solarzellen resultiert die photovoltaische Aktivität aus dem das Sonnenlicht absorbierenden Donor-Akzeptor-Heteroübergang. Die Grenzfläche zwischen den beiden organischen Materialien dient der effizienten Ladungsträgertrennung. Die vorliegende Arbeit leistet einen Beitrag im Verständnis zum Wirkungsmechanismus und der zu optimierenden Parameter in diesen Solarzellen. In Bezug auf die Anpassung des Donor-Akzeptor-Heteroübergangs wird neben dem Mechanismus der Ladungsträgergeneration an der Grenzfläche die erzielbare Leerlaufspannung in den Solarzellen diskutiert. Ein wesentliches Kriterium zur Erhöhung der Leerlaufspannung ist die Anpassung der Energieniveaus am Heteroübergang. Eine effiziente Ladungsträgertrennung wird durch eine hinreichende Stufe im Ionisationspotenzial sowie in der Elektronenaffinität am Heteroübergang erreicht. Zur Maximierung der Leerlaufspannung muss diese Überschussenergie, d.h. die Energiedifferenz zwischen photogeneriertem Exziton und freiem Ladungsträgerpaar, auf das notwendige Minimum reduziert werden. Eine Reihe von Dicyanovinyl-Oligothiophenen (DCVnT, n=3-6) wurden als Donor im Heteroübergang zu Fulleren C60 verwendet. Das Ionisationspotenzial der DCVnT nimmt mit zunehmender Kettenlänge ab, während die Elektronenaffinität, die weitestgehend durch die Dicyanovinyl-Endgruppen bestimmt wird, von der Kettenlänge nahezu unabhängig ist. Mittels photoinduzierter Absorptionsspektroskopie und zeitaufgelöster Fluoreszenzmessung wurde der Energie- und Elektronentransfer zwischen DCVnT und C60 entlang der homologen Reihe der DCVnT untersucht. Eine wesentliche Feststellung ist die Korrelation zwischen Rekombination in den Triplettzustand und der Leerlaufspannung. So konnte unter anderem gezeigt werden, dass durch die Verwendung angepasster Heteroübergänge unter bestimmten energetischen Voraussetzungen die indirekte Triplettbesetzung einen bislang nicht beachteten Verlustmechanismus für organische Solarzellen darstellt. Für organische Solarzellen ist demnach ein Kompromiss zwischen möglichst hoher Leerlaufspannung und effizienter Ladungsträgerdissoziation unter Vermeidung dieser Triplettrekombination zu erzielen. Weiterhin wird ein Konzept zur Nutzung dieser indirekten Triplettrekombination diskutiert. Dieses basiert auf der Tatsache, dass die Lebensdauer der Exzitonen im Triplettzustand gegenüber denen im Singulettzustand um 3-6 Größenordnungen höher ist. Damit wird eine höhere Diffusionslänge erwartet, was in einer dickeren und damit stärker absorbierenden aktiven Schicht genutzt werden könnte.

Molecular doping is a key technique for realizing high efficient organic light-emitting diodes (OLEDs) and photovoltaics (OPV). Furthermore, its most recent application in organic field-effect transistors (OFETs) marks a milestone on the roadmap towards flexible organic CMOS technology. However, in contrast to silicon based devices, the understanding of the fundamental processes of molecular doping is still controversially discussed. This work aims at the detailed analysis of the molecular doping process by employing Photoelectron spectroscopy (PES) on various doped thin-films prepared by co-evaporation in vacuum. Here, the focus is on explanation of the experimental findings by a statistical description in order to contribute to the fundamental understanding of the doping mechanism. First, the Fermi level shifts in thin-films of the common hole transport materials MeO-TPD, ZnPc, and pentacene p-doped by the acceptors C60F36 and F6-TCNNQ are studied. The precise control of molar doping ratios as low as 1e−5 is demonstrated, allowing analysis of the doping properties in a much broader range as previously accessible. Characteristic kinks and slopes in the Fermi level vs. doping concentration diagrams are found. Furthermore, the doping efficiency is found to decrease with increasing doping concentrations to just a few percent at molar ratios above 0.1. By numerically solving the charge neutrality equation using a classical semiconductor physics approach, these findings are explained by trap-limitation, dopant saturation, and reserve regimes as known from inorganic semiconductor physics. Using the example of p-doped MeO-TPD thin-films, it is finally demonstrated that the density of deep gap states depends on the purity degree of the host material. Similar studies are conducted on thin-films of C60, ZnPc, and pentacene n-doped by the di-metal complex W2(hpp)4. The corresponding Fermi level plots possess also host material specific kinks and slopes, which however, can be explained by application of the statistical doping description and assuming just dopant saturation and trap-limitation. Furthermore, it is demonstrated that electron traps with defined density can intentionally be introduced in pentacene by co-evaporation of C60 and gradually filled-up by n-doping with W2(hpp)4. In contrast to p-dopants, the highly efficient n-dopant W2(hpp)4 is prone to degradation in air due to its low IP of just 2.4eV. Therefore, the degradation of pure films of W2(hpp)4 as well as of n-doped films applying various host materials is studied under air exposure by conductivity measurements and PES. An unexpected (partial) passivation of W2(hpp)4 molecules against oxidation is found, however, this effect is identified to depend on the energy levels of the used host material. This finding is explained by a down-shift of the W2(hpp)4 energy levels upon charge transfer to a host material with deeper lying energy levels and thus allows for new conclusions on the relative alignment of the energy levels of dopant and host molecules in doped films in general. The maximum open-circuit voltage Voc of BHJ solar cells is limited by the effective HOMO(donor)-LUMO(acceptor) gap of the photo-active absorber blend. Therefore, the relative energy levels within ZnPc:C60 blend layers are furthermore investigated by PES, identifying an increase of the HOMO(ZnPc)-LUMO(C60) gap by 0.25 eV when varying the blend stoichiometry from 6:1 to 1:6. The trend in this gap correlates with observed changes in Voc of respective BHJ solar cells as well as with measured charge transfer energies. As physical origins for the changed energy levels, a suppressed crystallization of the C60 phase due to presence of donor molecules as well as concentration-dependent growth modes of the ZnPc phase are discussed.

In this work, the capacitive response of organic electronic devices is analysed. Particular attention is given to small-molecule organic solar cells, with the purpose of deriving an equivalent circuit for the small-signal response of these devices. The different components characterising the solar cells electrical response are individuated and discussed and a specific physical meaning is associated with each element of the equivalent circuit. In the experimental section, the capacitive elements of the equivalent circuit are characterised by analysing organic diodes and solar cells. It is found that the capacitance of an organic solar cell is a combination of four components: the dielectric response of the materials, the depletion regions formed at the interfaces, the accumulation of free and trapped charge carriers. The depletion regions formed in organic doped semiconductors are characterised by analysing organic p/n homojunction diodes composed of Zinc-Phtalocyanine (ZnPc). The results demonstrate that the mechanisms involved in the formation of depletion zones in organic semiconductors can be described by the classical Mott-Schottky theory. This allows to estimate the free charge carrier density of doped layers with capacitance measurements. In addition, the current-voltage characteristics of organic p/n homojunctions are found not to obey the classical Shockley theory. It is demonstrated that charge carrier tunnelling is the cause of this discrepancy and an analytic model is used to describe the current-voltage characteristics. The accumulation of free charge carriers is found to induce capacitance effects typical of relaxation semiconductors. In presence of unbalanced charge carriers injection, negative capacitance values are observed. It is shown that in different organic semiconductor devices, the injection of minority charge carriers induces a depletion in the majority concentration, resulting in a negative value of the accumulation capacitance. Finally, the capacitance associated to trap states in ZnPc:C60 organic solar cells is analysed. The spatial position and occupation mechanisms of the traps are estimated. The trapping mechanism in small-molecule organic solar cells is clarified and the energetic distribution of these trap states is estimated being a Gaussian function with 55 meV width, a density of 3.5 × 1016 cm−3 and centred 0.458 eV below the electron transport level. Trap states are also found to act as recombination centres, limiting the efficiency of organic solar cells.

As the Worlds energy demand is increasing we need more of our energy to be generated from resources that affect the climate as little as possible. Solar power could be the solution if there were solar panels with a less energy demanding production than the established silicon based solar modules. Printable organic solar cells will enable a cheap production process, thus they are mainly made out of polymers in solution. However, to be able to decrease the total cost of the solar modules the commonly used indium tin oxide (ITO) for the transparent electrode needs to be replaced by a less expensive material. If the cheap, high conductive and transparent polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) could replace ITO the cost of organic solar modules would significantly decrease. For PEDOT:PSS to be able to replace ITO there are requirements that have to be met. The transparent electrode needs to be apart from transparent, highly conductive, have a low contact resistance to the other materials in the organic solar cell and be printable. In this study it has been shown that the PEDOT:PSS film with Zonyl and Diethylene Glycol (DEG) as an secondary dopant, is capable of laminating to thin films made out of PEDOT:PSS, metal or a polymer fullerene blend. The contact resistances between two PEDOT:PSS films and PEDOT:PSS film and a metal film proved to be low. When laminating to a metal film an interlayer of Silver Nano Wires (AgNW) was needed to achieve a low contact resistance.

This work investigates fabrication methods of nickel oxide thin films and their use in organic electronics. Two fabrication techniques were studied: UV-ozone oxidation of pure nickel films and reactive RF magnetron sputtering. The former was used to produce Ni/Ni2O3 bi-layer anodes to use as a substitute for the de facto standard ITO anode. OLEDs fabricated using Ni/Ni2O3 bi-layer anodes exhibited comparable device performance to standard ITO devices. UV-ozone oxidation was also used to fabricate Ni2O3 buffer layers for OPVs. Solar cells fabricated using Ni2O3 coated ITO exhibited an enhanced power conversion efficiency of up to 90%. RF magnetron sputtering was used to produce NiOx buffer layers with tunable conductivity and optical transparency for OPVs. Solar cells fabricated using NiOx coated ITO exhibited an enhanced power conversion efficiency of up to 60%. Nickel oxide films have been characterized with various techniques: sheet resistance measurements, optical transmission, XPS, UPS, AFM, and TEM.

This work investigates fabrication methods of nickel oxide thin films and their use in organic electronics. Two fabrication techniques were studied: UV-ozone oxidation of pure nickel films and reactive RF magnetron sputtering. The former was used to produce Ni/Ni2O3 bi-layer anodes to use as a substitute for the de facto standard ITO anode. OLEDs fabricated using Ni/Ni2O3 bi-layer anodes exhibited comparable device performance to standard ITO devices. UV-ozone oxidation was also used to fabricate Ni2O3 buffer layers for OPVs. Solar cells fabricated using Ni2O3 coated ITO exhibited an enhanced power conversion efficiency of up to 90%. RF magnetron sputtering was used to produce NiOx buffer layers with tunable conductivity and optical transparency for OPVs. Solar cells fabricated using NiOx coated ITO exhibited an enhanced power conversion efficiency of up to 60%. Nickel oxide films have been characterized with various techniques: sheet resistance measurements, optical transmission, XPS, UPS, AFM, and TEM.

Thesis (Ph.D.); Submitted to Iowa State Univ., Ames, IA (US); 12 Dec 2003. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2100" Gang Li. 12/12/2003. Report is also available in paper and microfiche from NTIS.