Crystal and electronic structures of CuxS solar cell absorbers

The crystal and electronic band structures of CuxS(1.25 < x ≤ 2) are systematically studied using the density-functional theory method. For Cu2S, all the three chalcocite phases, i.e., the low-chalcocite, the high-chalcocite, and the cubic-chalcocite phases have direct band gaps around 1.3–1.5 eV, with the low-chalcocite being the most stable one. However, Cu vacancies can form spontaneously in these compounds, causing instability of Cu2S. We find that under Cu-rich condition, the anilite Cu1.75S is the most stable structure. It has a predicted band gap of 1.4 eV and could a promising solar cell absorber.