Stable Cu-Based Back Contacts for CdTe Thin Film Photovoltaic Devices

Cadmium telluride (CdTe) thin film photovoltaic devices fabricated in $a$-line process developed at Colorado State University (CSU) have shown stability during long-term (over a 5 year period) accelerated stress testing. These devices have a copper (Cu) containing back contact. The Cu profile as measured by secondary ion mass spectrometry characterization shows, for the maximum stressed device (23,399 h), that there is a significant (two times) change in the concentration of secondary Cu ions in the bulk of the material; however, the Cu concentration gradient at the back of the device has no significant change, and the CdS layer has no significant Cu concentration increase at open-circuit bias and $65°C$ temperature conditions. This indicates that with a proper $CdCl2$ treatment, Cu can be used to form the back contact for CdTe devices with acceptable stability. These devices have a projected field lifetime of greater than 60 years.