Cu K-edge EXAFS Studies of CdCl_2 Effects on CdTe Solar Cells

摘要

The highest performance CdS/CdTe thin film solar cells are generally completed with a Cu-containing back contact. The copper appears to be critical for achieving heavy p-type doping of the CdTe at the contact to permit the formation of a low resistance contact. In previous extended x-ray absorption fine structure (EXAFS) work we have inferred that most of the Cu in CdTe films resides as Cu_2O at the boundaries of CdTe grains in films that have received a chloride treatment in the presence of oxygen, a critical step needed to improve the performance of all CdTe thin-film cells. This has suggested a mechanism for grain boundary passivation in thin-film CdTe solar cells. We believe most of the diffused Cu decorates grain boundaries as oxides, consistent with the low doping densities typically observed in CdTe solar cells. The significance for grain boundary passivation will be discussed. We also find evidence that the grain-boundary Cu_2O in CdCl_2 treated CdTe films is unstable and tends to transform to CuO under some stress conditions.