Models of the photovoltaic cell proposed by various authors are subjected to a critical examination to isolate their underlying assumptions and to devise possible experimental tests of the models. The results of an investigation of a model in which the Cu
2S layer plays an active role are presented. These include a calculation of spectral response and a comparison of calculated and measured spectral response curves; an electron microprobe study whose conclusion is that the Cu
2S layer is thinner than 0.5 µm; a discussion of the crossover of dark and light

characteristics and a test of whether this shift is adequately explained by an internal series resistance; and a study of the effects of low-energy proton irradiation on the properties of Cu-Cd-S cells.