Electrical characterization of Cu composition effects in CdS/CdTe thin-film solar cells with a ZnTe:Cu back contact

We study the effects of Cu composition on the CdTe/ZnTe:Cu back contact and the bulk CdTe. For the back contact, its potential barrier decreases with Cu concentration while its saturation current density increases. For the bulk CdTe, the hole density increases with Cu concentration. We identify a Cu-related deep level at ∼0.55 eV whose concentration is significant when the Cu concentration is high. The device performance, which initially improves with Cu concentration then decreases, reflects the interplay between the positive influences (reducing the back contact potential barrier while increasing the saturation current density of the back contact and hole density in CdTe bulk) and negative influences (increasing deep levels in CdTe) of Cu.