Effects of Cd1-xZnxS alloy composition and post-deposition air anneal on ultra-thin CdTe solar cells produced by MOCVD

Ultra-thin CdTe:As/Cd_1-xZn_xS photovoltaic solar cells with an absorber thickness of 0.5 µm were deposited by metal-organic chemical vapour deposition on indium tin oxide coated boro-aluminosilicate substrates. The Zn precursor concentration was varied to compensate for Zn leaching effects after CdCl₂ activation treatment. Analysis of the solar cell composition and structure by X-ray photoelectron spectroscopy depth profiling and X-ray diffraction showed that higher concentrations of Zn in the Cd_1‑xZn_xS window layer resulted in suppression of S diffusion across the CdTe/Cd_1‑xZn_xS interface after CdCl₂ activation treatment. Excessive Zn content in the Cd_1-xZn_xS alloy preserved the spectral response in the blue region of the solar spectrum, but increased series resistance for the solar cells. A modest increase in the Zn content of the Cd_1-xZn_xS alloy together with a post-deposition air anneal resulted in an improved blue response and an enhanced open circuit voltage and fill factor. This device yielded a mean efficiency of 8.3% over 8 cells (0.25 cm² cell area) and best cell efficiency of 8.8%.