Physical models of thin film polycrystalline solar cells based on measured grain-boundary and electronic-parameter properties. Quarterly report

Solar cells fabricated on polycrystalline silicon, either bulk or thin-film, can potentially be cost-effective when used in terrestrial photovoltaic energy-conversion systems. To achieve this goal, the polysilicon cell efficiency must be increased considerably from its present values. A severe limitation to the cell efficiency is due to the grain boundaries and their influence on carrier recombination. To remove this limitation, an understanding of the fundamental physics underlying the effects of the grain boundaries on cell performance is helpful. This fundamental physics is discussed, and models are developed for recombination currents in polysilicon pn-junction solar cells. Several analytic approximations, suggested by physical insight, are used and checked ultimately for self-consistency with the results of the analysis. The models are defined such that their parameters can be related directly to measurements, and the models are hence useful in interpreting experimental results. They also can be used to study, in a systematic way, cell-design modifications to improve the efficiency, e.g., grain-boundary passivation techniques