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The effect of light exposure in ambient air on thin films made from an electron acceptor polymer poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (N2200), an electron donor polymer Poly[[2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl] (TQ1) and their blends, has been studied using UV-vis spectroscopy and Atomic Force Microscopy (AFM). For solutions of TQ1, N2200 and blends, the linearity of the Beer-Lambert law for absorption spectroscopy has been verified. The measured UV-vis spectra show that TQ1 thin films are more sensitive to degradation by simulated sunlight than N2200 films. They also show that among the polymer blends, the N2200-rich blend with volume ratio 1:2 (TQ1:N2200) was less sensitive to degradation by simulated sunlight than blends of ratio 1:1 and 2:1. The AFM images showed a change in roughness between the undegraded and degraded films, where the TQ1, 1:1 and 1:2 films obtained lower roughness after 45 hours of degradation, and the N2200 and the 2:1 films obtained higher roughness. / Effekten av simulerad solljusexponering i omgivande luft på tunna filmer gjorda av en elektronaccepterande polymer poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (N2200), en elektrondonerande polymer Poly[[2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl] (TQ1) och deras blandningar, har undersökts genom ultraviolett-synlig-spektroskopi (UV-vis-spektroskopi) och atomkraftsmikroskopi (AFM). Genom lösningar av TQ1, N2200 och blandningar, har det linjära förhållandet i Beer Lamberts lag för absorptionsspektroskopi verifierats. De mätta UV-vis-spektrumen visar att tunna TQ1-filmer är känsligare mot degradering genom simulerat solljus än tunna N2200-filmer. De visar också att den N2200-rika blandningen med ett volymförhållande av 1:2 (TQ1:N2200) var mindre känslig för degradering av simulerat solljus än blandningar med volymförhållandet 1:1 och 2:1. AFM-bilderna visade en förändring i råhet mellan degraderade och icke-degraderade filmer, där TQ1-, 1:1-, och 1:2-filmerna fick en lägre ytråhet efter 45 timmar av degradering, och N2200- och 2:1-filmera fick en högre ytråhet.

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This thesis is divided into four parts with organic chromophores for dye sensitized solar cells as the common feature and an introduction with general concepts of the dye sensitized solar cells. The first part of the thesis describes the development of an efficient organic chromophore for dye sensitized solar cells. The chromophore consists of a triphenylamine moiety as an electron donor, a conjugated linker with a thiophene moiety and cyanoacrylic acid as an electron acceptor and anchoring group. During this work a strategy to obtain an efficient sensitizer was developed. Alternating the donor, linker or acceptor moieties independently, would give us the tool to tune the HOMO and LUMO energy levels of the chromophores. The following parts of this thesis regard this development strategy. The second part describes the contributions to the HOMO and LUMO energy levels when alternating the linker moiety. By varying the linker the HOMO and LUMO energy levels was indeed shifted. Unexpected effects of the solar cell performances when increasing the linker length were revealed, however. The third part describes the investigation of an alternative acceptor group, rhodanine-3-acetic acid, in combination with different linker lengths. The HOMO and LUMO energy level tuning was once again successfully shifted. The poor electronic coupling of the acceptor group to the semiconductor surface proved to be a problem for the overall efficiency of the solar cell, however. The fourth part describes the contributions from different donor groups to the HOMO and LUMO energy levels and has so far been the most successful in terms of reaching high efficiencies in the solar cell. A top overall efficiency of 7.1 % was achieved. / QC 20101108

<p> In the current research, use of p-n junction GaAs nanowires (NWs) grown by gas source molecular beam epitaxy on GaAs (111) B substrates for the fabrication of flexible solar cells are reported. The solar cells were fabricated by embedding the NWs in a polymer matrix (SU8 2), followed by ohmic contact formation to the tops of the NWs as well as the rear side of the substrate. I-V characteristic curves were obtained by illuminating the solar cells using a solar simulator, indicating a photovoltaic effect. NWs were also detached from the substrate by different methods and successfully transferred onto a flexible substrate for potential use as solar cells. Scanning electron microscopy was used throughout the research for characterization and optimization of the fabrication processes including NW embedment, removal from the substrate, and contact formation.</p> / Thesis / Master of Applied Science (MASc)

Efficient optical-to-electrical conversion is a fundamental requirement of a range of silicon devices such as those which employ photodetection, solid-state-imaging and photovoltaic power generation. This thesis investigates the effects of using indium, a deep-level acceptor in silicon, as a dopant for thin film single crystalline silicon solar cells and UV-A detectors. Indium acts as a p-type dopant in silicon and has been proposed previously as a substitutional lattice defect that would enable sub-band gap transitions as described by the so-called impurity photovoltaic (IPV) effect. The physical mechanisms responsible for operation of the devices presented in this work are described. Models for electrical performance, optical absorbance and device fabrication are used as methods to interpret data and optimize device parameters. Specifically, a two-diode model is used to account for the electrical loss mechanisms within a device, while modeling optical absorption by a multilayer structure consisting of Silicon-On-Insulator (SOI) is approached using a novel multi-wavelength numerical model that describes the reflections and transmissions at each of the device’s layers. Additionally, Technology Computer Aided Design (TCAD) simulations were used to optimize the critical fabrication parameters associated with the ion implantation and thermal annealing techniques used during the device fabrication process. Selected from multiple devices fabricated during the course of this work, the most efficient solar cells in SOI (2.5 μm thick active layer) exhibited a maximum conversion efficiency of 4.74 % for indium-doped and 4.16 % for boron-doped layers. The most efficient UV-A detector fabricated in SOI (100 nm thick) exhibited a maximum responsivity to 365 nm light of 20 mA/W for indium-doped and 16 mA/W for boron-doped devices. In both types of devices, indium doping consistently resulted in a relative increase in efficiency when compared to equivalently fabricated, boron doped devices, despite experimental carrier decay measurements confirming the action of the indium as a recombination centre. External and internal quantum efficiency measurements confirm a relative enhancement in absorption, for solar cells and detectors doped with indium, which is correlated with the p-type dopant concentration and the ratio of n-type to p-type concentrations. The origin of the enhancement is postulated to be caused by a relaxation of the momentum-space restrictions associated with undoped silicon, a postulate supported by previously reported absorption data. This thesis presents the first comprehensive data from indium doped silicon devices designed for optical-to-electrical conversion. The implications for a range of widely deployed devices may be significant. / Thesis / Doctor of Philosophy (PhD)

In the first part of this dissertation, an analytical approach to n⁺/p solar cells is developed. Based on this model, the short circuit current, open circuit voltage and energy conversion efficiency of the cells as a function of doping concentration, junction depth, minority carrier surface recombination velocity and diffusion length are discussed. This model simplifies the analysis of the solar cell, and the calculated results agree closely with both experimental results and numerical analysis. The second part of the dissertation deals with grain boundary (GB) recombination, passivation, and characterization. A simple GB model is developed, and an expression for the GB barrier height under illumination is derived by introducing a quasi Fermi-function. By using this model, the dependencies of the minority carrier transport parameters on the illumination level, grain size, depth from the surface and trap state density are derived. The model is compared to the experimental results for GB capacitance and electron lifetime measurements. Hydrogen passivation of poly-Si solar cells was accomplished experimentally by hydrogen implantation. A systematic study of implantation parameters was conducted and an optimum condition was found for the samples under examination. Under this condition, 18 % improvement in cell efficiency was achieved for the cells, with initial efficiencies of about 7%. A transmission electron microscope (TEM) study of the hydrogen-dislocation interaction and hydrogen ion bombardment effects on the surface were also conducted. The GB recombination velocity and the hydrogen passivation effect were characterized by the electron beam induced current (EBIC) technique. A nonuniform passivation depth was observed. The nature of the passivation mechanism was addressed. It was found that hydrogen saturation of dangling bonds in Si grain boundaries is not the only mechanism for hydrogen passivation. / Ph. D.

With looming energy crisis across the globe, achieving high efficiency and low cost solar cells have long been the key objective for photovoltaic researchers. III-V compound semiconductor based multijunction solar cells have been the dominant choice for space power due to their superior performance compared to any other existing solar cell technologies. In spite of unmatched performance of III-V solar cells, Si cells have dominated the terrestrial market due to their lower cost. Most of the current III-V solar cells are grown on Ge or GaAs substrates, which are not only smaller in diameter, but are also more expensive than Si substrate. Direct integration of high efficiency III-V solar cells on larger diameter, cheaper and readily available Si substrate is highly desirable for increased density, low-cost and lightweight photovoltaics. However, the polar-on-nonpolar epitaxy, the thermal mismatch and the 4% lattice mismatch makes the direct growth of GaAs on Si challenging, rendering the metamorphic cell sensitive to dislocations. The focus of this work is to investigate and correlate the impact of threading dislocation density on the performance of lattice-mismatched single-junction (1J) GaAs and dual-junction (2J) InGaP/GaAs solar cells on Si substrate. Utilizing our calibrated dislocation-assisted modeling process, we present the design methodology to optimize the structure of 2J InGaP/GaAs solar cell on Si substrate. Our modeling results suggest an optimistic future for integrating III-V solar cell technology on Si substrate and will be useful for future design and prediction of metamorphic III-V solar cell performance on Si substrate. / Master of Science

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