Modeling of grain boundary effects on photovoltaic properties of metal-insulator thin-film polycrystalline semiconductor solar cells

A model has been developed to characterize the photovoltaic properties of a thin-film polycrystalline semiconductor solar cell with a metal-insulator-semiconductor (MIS) structure and an accumulated back contact which forms a back surface field. Semiconductor doping density, thickness, grain size, and grain boundary trap density are the parameters used to characterize the semiconductor. Collection velocity, diode ideality factor, effective junction barrier height, and light transmittance are the parameters used to characterize the MIS interfacial region. The three-dimensional grain size and grain boundary problem is simplified by transforming it to an equivalent one-dimensional single-level bulk trap problem. The majority-carrier density and the built-in potential are assumed to be affected by light intensity, doping and trap densities. The effects of deep-level grain boundary traps and of surface passivation on the short-circuit current, open-circuit voltage, fill factor, collection and conversion efficiencies of these cells are calculated based on the potential distribution in the thin-film semiconductor. Recombination current and series and shunt resistance are not included and the superposition principle is not used in the calculation. Sample calculations are included for the specific case of electrodeposited n-type CdTe MIS solar cells. Methods to improve the conversion efficiency and the application of modeling are discussed.