This dissertation is a summary of the research effort for the theoretical study, modeling, design, and optimization of solid state photovoltaic devices. The effect of grating structures on the back reflector is studied using electromagnetic modeling and an optimized structure is proposed to enhance the optical absorbance of these devices. Solar cells with optimized arbitrarily shaped gratings exhibit a 29% improvement over planar cells and 9.0% improvement over the optimal cell with periodic gratings. A new model incorporating carrier transport and recombination is proposed and simulation result shows the significance of this model in the modeling of intermediate band solar cell. The material ZnTeO is used as a numerical example for the intermediate band solar cell model. The optimal impurity concentration is determined to be 1018 cm-3 for an optical absorption cross section of 10-14 cm2. The conversion efficiency of a ZnTe solar cell with a total recombination lifetime of 10 ns is calculated to increase from 14.39 % to 26.87 % with the incorporation of oxygen. Fully coupled solution to partial differential equations provides insight into the operation of intermediate band solar cell. A doping compensation scheme is proposed to mitigate the space charge effects, and the device achieves conversion efficiencies of approximately 40%, similar to the maximum expected values from prior 0-D models. A spectrally decoupled scheme for subbandgap photovolatics is proposed in which, device structures with non-uniform occupation of intermediate electronic states are employed to reduce the dependence of conversion efficiency on spectral overlap. Solar cell conversion efficiencies are calculated for structures where absorption bands are spatially decoupled due to defined occupation of intermediate states. The spectrally-decoupled device provides a means to achieve high theoretical efficiency independent of spectral overlap that approaches the detailed balance efficiency limit of 63.2 % for intermediate state devices without spectral overlap. The analysis of experimental work using the model developed for intermediate band solar cell is conducted and ZnTeO alloy is chosen to be the material for intermediate band solar cell, where oxygen states are served as intermediate sites in the fundamental bandgap
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