FIRST-PRINCIPLES STUDY OF IN-FREE PHOTOVOLTAIC COMPOUNDS, I_2-II-IVSe4 (I= Cu, Ag; II= Zn, Cd, Hg; IV= Si, Ge, Sn; VI= S, Se)

摘要

We have systematically studied phase stability and electronic structure of I2-II-IV-VI4 (I=Cu, Ag; II = Zn, Cd, Hg; IV =Si, Ge, Sn; VI= S, Se) by first-principles calculations. The enthalpies of formation for kesterite (KS), stannite (ST) andwurtz-stannite (WST) phases of I2-II-IV-VI4 were calculated. In I2-ZnSnSe_4 (I=Cu, Ag) and 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 (CZTSe) (0.63 eV). The band gaps of ST-typeCu_2CdSnSe_4 (0.52 eV) and ST-type Cu_2HgSnSe_4 (0.07 eV) are smaller than that of KS-type CZTSe (0.63 eV). Theband gap of KS-type Ag_2ZnSnSe_4 (AZTSe) (0.94 eV) is wider than that of KS-type CZTSe (0.63 eV). The valenceband maximum (VBM) of I2-II-IV-VI4 consists of an antibonding orbital of I (Cu, Ag) d and VI (S, Se) p, while theconduction band minimum (CBM) consists of an antibonding orbital of IV (Si, Ge, Sn) s and VI (S, Se) p. The energylevels of VBMs in Cu_2II-IVSe_4 do not change much compared with those of CZTSe. On the other hand, the energylevels of CBMs of IV ns + Se 4p in Cu_2ZnSiSe_4 and Cu_2ZnGeSe_4 become higher than those in Cu_2ZnSnSe_4, while theenergy levels of CBMs in Cu_2IISnSe_4 become lower than those in CZTSe. The band gap energy of AZTSe is wider thanthat of CZTSe because the energy level of VBM (Ag 4d +Se 4p) in AZTSe is lower than that (Cu 3d +Se 4p) in CZTSe.