Structural and chemical characterization of the back contact region in high efficiency CdTe solar cells

Cadmium telluride (CdTe) is the leading commercialized thin-film photovoltaic technology. Copper is commonly used in back contacts to obtain high efficiency, but has also been implicated as a harmful factor for device stability. Thus it is critical to understand its composition and distribution within complete devices. In this work the composition and structure of the back contact region was examined in high efficiency devices (~16%) contacted using a ZnTe:Cu buffer layer followed by gold metallization. The microstructure was examined in the as-deposited state and after rapid thermal processing (RTP) using high resolution transmission electron microscopy and EDX chemical mapping. After RTP the ZnTe exhibits a bilayer structure with polycrystalline, twinned grains adjacent to Au and an amorphous region adjacent to CdTe characterized by extensive Cd-Zn interdiffusion. The copper that is co-deposited uniformly within ZnTe is found to segregate dramatically after RTP activation, either collecting near the ZnTe/Au interface or forming CuxTe clusters in CdTe at defects or grain boundaries near the interface with ZnTe. Chlorine, present throughout CdTe and concentrated at grain boundaries, does not penetrate significantly into the back contact region during RTP activation.