Comparison study of close-spaced sublimated and chemical bath deposited CdS films: effects on CdTe solar cells

Close-spaced-sublimated (CSS) CdS films exhibit strong fundamental edge luminescence, high optical absorption and a bandgap of ~2.41 eV. Structurally, these films show good crystallinity with thickness-dependent grain sizes that vary between 100-400 nm. In contrast, chemical-bath-deposited (CBD) CdS exhibits subband luminescence, lower absorption and a thickness-dependent bandgap. These films have CdS grains typically less than 50 nm in size and poorer crystallinity. However, CdTe devices fabricated with these lower “quality” CBD CdS films yield higher V_ocs and fill factors. Carrier lifetimes in finished CSS CdS devices measured between 100 and 200 ps while lifetimes in CBD CdS devices were much higher (>500 ps). Compositional differences in the Cd/(S+Te) ratio at the interface suggest the possibility of lower CdS doping and higher CdTe compensation as one reason for lower V_ocs in CSS CdS devices