Correlation of surface phases with electrical behavior in thin-film CdTe devices

摘要

Glancing incidence X-ray diffraction (GIXRD) analysis of crystalline surface phases in cadmium telluride solar cells is correlated with device behavior for different processing conditions. GIXRD sampling depths are calculated for different phases and X-ray sources. Air heat treatment of CdTe films forms the native oxide CdTeO_3, while treatment in CdCl_2/air vapor forms CdTeO_3 and CdO. Air treatment followed by CdCl_2/air treatment forms surface oxides, reduces CdS diffusion into CdTe and improves junction performance. The CdTeO_3 surface oxide limits CdCl_2 concentration in the CdTe film, yet allows sufficient CdCl_2 species for CdTe/CdS junction activation. In devices, residual oxide phases contribute to series resistance and current-voltage retrace hysteresis. These effects are mitigated by oxide removal and formation of a Te layer. Addition of Cu to the contact followed by thermal treatment increases open circuit voltage. Optimal cell performance is obtained for Cu/(Cu + Te) atomic ratio = 0.7.