Surface photovoltage analysis of thin CdS layers on polycrystalline chalcopyrite absorber layers by Kelvin probe force microscopy

An extensive Kelvin probe force microscopy study in an ultrahigh vacuum has been undertaken to examine the influence of growth modifications of a few nm thick CdS buffer layers in thin film chalcopyrite solar cells. In regions around the grain boundaries of the polycrystalline Cu(In, Ga)Se-2 substrate a lowering of the work function extending to about 200 nm away from this vertical interface was observed. This electrical inhomogeneity depends strongly on the Cu(In, Ga)Se-2 surface condition and is interpreted by a diffusion process along the substrate grain boundaries. Our results contribute to the understanding of the crucial role of the several nm thick CdS layer for improving the photovoltaic performance of chalcopyrite thin film solar cells.