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The high p-type conductivity in Cu-excess CuAlS2 reported from a recent experimental paper is not well understood as it is not supported by earlier theoretical studies. We found that Cu can be heavily doped at the Al site and can form compensated defect bands to shift up the valence band, leading to shallow acceptor levels under a Cu-excess condition. In particular, Zn doping at the Al site in Cu-excess CuAlS2 has been suggested as a promising approach to improving the p-type conductivity in Cu-excess CuAlS2.

Most reported p-type transparent conductive materials are Cu-based compounds such as CuAlO2 and CuCrO2. Here, we report that compounds based on ns2 cations with low binding energy can also possess high valence band maximum, which is crucial for the p-type doping according to the doping limit rules. In particular, CsGeCl3, a compound with valence band maximum from ns2 cations, is predicted as a promising p-type transparent conductive material by first-principles calculations. Our results show that the p-type defect Ge vacancy dominates its intrinsic defects with a shallow transition level, and the calculated hole effective masses are low in CsGeCl3.