No description available.

This dissertation describes the study of organic thin-film solar cells in pursuit of affordable, renewable, and environmentally-friendly energy sources. Particular emphasis is given to the molecular ordering found in liquid crystalline or polycrystalline films as a way to leverage the efficiencies of these types of cells. Maximum efficiencies estimated based on excitonic character of organic solar cells show power conversion efficiencies larger than 10% are possible in principle. However, their performance is often limited due to small exciton diffusion lengths and poor transport properties which may be attributed to the amorphous nature of most organic semiconductors.Discotic liquid crystal (DLC) copper phthalocyanine was investigated as an easily processible building block for solar cells in which ordered molecular arrangements are enabled by a self-organization in its mesophases. An increase in photocurrent and a reduction in series resistance have been observed in a cell which underwent an annealing process. X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements suggest that structural and morphological changes induced after the annealing process are related to these improvements.In an alternative approach, p-type pentacene thin films prepared by physical vapor deposition were incorporated into heterojunction solar cells with C60 as n-type layers. Power conversion efficiencies of 2.7 % under broadband illumination (350-900 nm) with a peak external quantum efficiency of 58 % have been achieved with the broad spectral coverage across the visible spectrum. Analysis using an exciton diffusion model shows this efficient carrier generation is mainly due to the large exciton diffusion length of pentacene films. Joint XRD and AFM studies reveal that the highly crystalline nature of pentacene films can account for the observed large exciton diffusion length. In addition, the electrical characteristics are studied as a function of light intensity using the equivalent circuit model used for inorganic pn-junction solar cells. Dependences of equivalent-circuit parameters on light intensity are further investigated using a modified equivalent circuit model, and their effects on the overall photovoltaic performance are discussed.

xvi, 117 p. ; ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / High efficiency thin film solar cells have the potential for being a world energy solution because of their cost-effectiveness. Looking to the future of solar energy, there is the opportunity and challenge for thin film solar cells. The main theme of this research is to develop a detailed understanding of electronically active defect states and their role in limiting device performance in copper indium gallium diselenide (CIGS) solar cells. Metastability in the CIGS is a good tool to manipulate electronic defect density and thus identify its effect on the device performance. Especially, this approach keeps many device parameters constant, including the chemical composition, grain size, and interface layers. Understanding metastability is likely to lead to the improvement of CIGS solar cells. We observed systematic changes in CIGS device properties as a result of the metastable changes, such as increases in sub-bandgap defect densities and decreases in hole carrier mobilities. Metastable changes were characterized using high frequency admittance spectroscopy, drive-level capacitance profiling (DLCP), and current-voltage measurements. We found two distinctive capacitance steps in the high frequency admittance spectra that correspond to (1) the thermal activation of hole carriers into/out of acceptor defect and (2) a temperature-independent dielectric relaxation freeze-out process and an equivalent circuit analysis was employed to deduce the dielectric relaxation time. Finally, hole carrier mobility was deduced once hole carrier density was determined by DLCP method. We found that metastable defect creation in CIGS films can be made either by light-soaking or with forward bias current injection. The deep acceptor density and the hole carrier density were observed to increase in a 1:1 ratio, which seems to be consistent with the theoretical model of V Cu -V Se defect complex suggested by Lany and Zunger. Metastable defect creation kinetics follows a sub-linear power law in time and intensity. Numerical simulation using SCAPS-1D strongly supports a compensated donor- acceptor conversion model for the experimentally observed metastable changes in CIGS. This detailed numerical modeling yielded qualitative and quantitative agreement even for a specially fabricated bifacial CIGS solar cell. Finally, the influence of reduced hole carrier mobility and its role in limiting device performance was investigated. / Adviser: J. David Cohen

Le comportement mécanique de films minces métalliques déposés sur des substrats souples joue un rôle déterminant dans les performances de l'électronique flexible et des micro- systèmes électromécaniques (MEMS).Dans un premier temps, une nouvelle méthode est présentée pour caractériser le module d'élasticité de films minces submicroniques. Avec deux couches déposées de chaque côté et sur la moitié du substrat polymère, la corrélation d'image numérique (CIN) a été utilisée pour mesurer simultanément la déformation du film et du substrat in situ au cours d'un essai de traction. La différence entre les déformations mesurées sur la partie vierge et le composite permet d'extraire les propriétés élastiques de films minces de manière simple et avec grande précision. Comme attendu, la distribution des déformations est uniforme au travers de l'épaisseur du film ce qui indique une adhésion parfaite entre le film et le substrat. Dans le cas de films minces de tungstène, de chrome, de nickel et de cuivre, les valeurs de module obtenues sont proches de celles des mêmes matériaux à l'état massif.Dans un deuxième temps, une nouvelle méthode expérimentale utilisant une machine de déformation uniaxiale est présentée pour étudier l'effet Bauschinger dans des films minces métalliques déposés sur des substrats étirables. Grâce à un dispositif original, les films minces sont déposés sur des substrats prétendus et peuvent donc être déformés alternativement en tension et en compression dans un large domaine de déformations. La déformation élastique intra granulaire des films minces polycristallins et la déformation macroscopique du substrat sont mesurées in situ par diffraction des rayons X et CIN respectivement. A partir des courbes « déformation élastique – déformation macroscopique », la réponse mécanique de l'ensemble film / substrat est analysée au vu de l'histoire complète du chargement et de la microstructure (contraintes résiduelles, texture) des films minces. / The mechanical behavior of metallic thin films deposited on soft substrates plays a crucial role in the performance of flexible electronics and MicroElectroMechanical Systems (MEMS).At first, a novel method is presented to characterize the in-plane elastic modulus of sub micrometer thin films. With two coating layers bonded symmetrically to half polyimide substrates, Digital Image Correlation (DIC) has been employed to measure time-resolved full-field strain maps of film and substrate during in situ tensile testing. The strain differences between virgin and composite parts allowed to extract the elastic properties of the thin films in a simple way with high precision. As expected, the strain distribution is uniform through the film thickness which indicates a perfect adhesion between the film and the substrate. In the case of tungsten, chromium, nickel and copper films, the values obtained are close to the bulk one.In a second step, a new experimental method using uniaxial tensile testing is presented to study Bauschinger effect in thin metallic films deposited on stretchable substrates. Thanks to our new pre-tensile setup (specific grips), the thin films were deposited on pre-stretched substrates and thus could be deformed alternately in tension and compression within a large strain domain. The elastic intra-granular strain of polycrystalline thin films and true strain of substrates are measured in situ by X-Ray Diffraction (XRD) and DIC. From lattice strain-true strain curves, the mechanical response of copper and nickel /substrate sets is analyzed in view of the complete loading history and the presence of residual stresses and crystallographic texture in thin films.