First-Principles Study of Cu₉S₅: A Novel p‑Type Conductive Semiconductor

Cu₉S₅ (digenite) is a p-type semiconductor with excellent electrical conductivity, high mobility of copper ions, and high work function. When used as the back electrode of CdTe solar cells, a high power conversion efficiency (PCE) is obtained. Density functional theory (DFT) method was used to study the structural and electronic properties of Cu₉S₅ in this work. From the calculated band structures, we find that the Fermi level of the Cu₉S₅ slightly crosses the valence band by about 0.08 eV below the valence band maximum (VBM), indicating a high hole concentration and potential high electrical conductivity as a p-type semiconductor. It is also found that the crystal structure of Cu₉S₅ remained stable with a few Cu atoms diffused away, which introduces a p-type doping effect. Finally, we give a quantitative discussion on why CdTe solar cells with Cu₉S₅ as the back electrode shows the high PCE.