Reduction of the infrared response is found in the CdS/CdTe thin-film solar cells with a decrease in the thicknesses of the photovoltaic active layers from 16 to 3 mum. The reduction is concluded to be mainly due to an increase in recombinations of photogenerated electrons in the p-layer caused by the unintentionally increased Cu acceptor concentration. A simple model for the depth profile of the donor and acceptor densities is proposed to explain that the acceptor concentration at the n-p junction can be unintentionally increased with a decrease in the thicknesses of the photovoltaic active layers, even if the process parameters for the Cu-doping are unchanged. (C) 2001 Elsevier Science B.V. All rights reserved.