PHASE STABILITY AND ELECTRONIC STRUCTURE OF NEW PHOTOVOLTAIC COMPOUNDS, Cu_2II-IVSe_4 (II: Zn, Cd, Hg, IV: Si, Ge, Sn)

摘要

We have theoretically evaluated phase stability and electronic structure of Cu_2II-IVSe_4 (II ＝ Zn, Cd, Hg, IV ＝ Si, Ge, Sn) and compared the results with those of Cu_2ZnSnSe_4 (CZTSe). The enthalpies of formation for kesterite (KS), stannite (ST) and wurtz-stannite (WST) phases of Cu_2II-IVSe_4 were calculated by first principles calculation. In Cu_2ZnIVSe_4 (IV ＝Si, Ge, and Sn), the KS phase is more stable than the ST and WST phases. On the other hand, in Cu_2IISnSe_4, (II ＝ Cd and Hg), the ST phase is more stable than the KS and WST phases. The theoretical band gaps of KS-type Cu_2ZnSiSe_4 (1.48 eV) and Cu_2ZnGeSe_4 (1.10 eV) are wider than that of KS-type Cu_2ZnSnSe_4 (0.63 eV). The theoretical band gaps of ST-type Cu_2CdSnSe_4 (0.52 eV) and ST-type Cu_2HgSnSe_4 (0.07 eV) are smaller than that of KS-type Cu_2ZnSnSe_4 (0.63 eV). The valence band maximum (VBM) of Cu_2ZnIVSe_4 consists of an antibonding orbital of Cu 3d and Se 4p, while the conduction band minimum (CBM) consists of an antibonding orbital of IV ns and Se 4p. The VBMs of Cu 3d ＋ Se 4p in Cu_2IISnSe_4 (II = Cd and Hg) and Cu_2ZnIVSe_4 (IV = Ge and Si) are similar to those in Cu_2ZnSnSe_4. Therefore, the energy levels of VBMs in Cu_2II-IVSe_4 do not change so much compared with those of CZTSe. On the other hand, the energy levels of CBMs of IV ns ＋ Se 4p in Cu_2ZnSiSe_4 and Cu_2ZnGeSe_4 become higher than those in Cu_2ZnSnSe_4, while the energy levels of CBMs in Cu_2IISnSe_4 become lower than those in CZTSe.