First-principles study of indium-free photovoltaic compounds Ag2ZnSnSe4 and Cu2ZnSnSe4

We have theoretically evaluated the phase stability and electronic structure of Ag₂ZnSnSe₄ (AZTSe) and compared the results with those of Cu₂ZnSnSe₄ (CZTSe). The enthalpies of formation for the kesterite (KS)-, stannite (ST)-, and wurtz-stannite (WST)-type AZTSe were calculated by first-principles calculations. For AZTSe, the kesterite (KS) phase is the most stable among these three phases. The valence band maximum (VBM) of KS-type AZTSe consists of the antibonding orbital of Ag 4d and Se 4p, while the conduction band minimum (CBM) consists of the antibonding orbital of Sn 5s and Se 4p. The band gap energy of KS-type AZTSe (0.94 eV) is wider than that of KS-type CZTSe (0.63 eV). This is because the energy level of VBM in AZTSe is lower than that in CZTSe.