Stability and electronic structures of CuxS solar cell absorbers

Cu_xS is one of the most promising solar cell absorber materials that has the potential to replace the leading thin-film solar cell material Cu(In,Ga)Se₂ for high efficiency and low cost. In the past, solar cells based on Cu_xS have reached efficiency as high as 10%, but it also suffers serious stability issues. To further improve its efficiency and especially the stability, it is important to understand the stability and electronic structure of Cu_xS. However, due to the complexity of their crystal structures, no systematic theoretical studies have been carried out to understand the stability and electronic structure of the Cu_xS systems. In this work, using first-principles method, we have systematically studied the crystal and electronic band structures of Cu_xS (1.25 <; x≤ 2). For Cu₂S, we find that all the three chalcocite phases, i.e., the low-chalcocite, the high-chalcocite, and the cubic-chalcocite phases, have direct bandgaps 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 Cu_1.75S is the most stable structure. It has a predicted bandgap of 1.4 eV and could be a promising solar cell absorber.