Comparison 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 filmshave 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 Voc's 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 interfacesuggest the possibility of lower CdS doping and higher CdTe compensation as one reason for lower Voc's in CSS CdS devices.