The structural, mechanical, and optoelectronic properties of Cu2-II-Sn-VI4 (II = Mg, Zn, Cd; VI = S, Se): A DFT study

Abstract

Quaternary compound Cu2ZnSnX4 (X = S, Se) is of great interest to many researchers for photovoltaic applications in recent years. In the present study, the first-principles calculations are performed to investigate the influence of cation substitution on the different physical properties of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe). The difference in stability between two Kesterite and Stannite structures is examined for each compound. The results show that the pristine Kesterite structure will be transformed into the Stannite structure when the Zn atoms are replaced by the Mg or Cd atoms. Our results are consistent well with the available literature. The mechanical stability of six compounds are proved and they are ductile materials. The allowed direct electronic transition is found for all compounds at the Γ point. The band gap variation is analyzed in detail. The band gaps calculated from hybrid functional PBE0 of the studied compounds can agree well with the experimental data. The suitable band gap values of 1.12 and 1.28 eV are obtained for Cu2MgSnSe4 (CMTSe) and Cu2CdSnS4 (CCTS) in single-junction solar cells, whereas the band gaps of Cu2MgSnS4 (CMTS, 1.70 eV) and Cu2CdSnSe4 (CCTSe, 0.77 eV) are too large or small. The band gap energy difference between two tetragonal structures is shown to be negligible for CMTS, CMTSe, CCTS, and CCTSe. Furthermore, the optical properties are compared, and CMTSe and CCTS show strong light absorption capacity from 300 to 800 nm. Our study indicates that both CMTSe and CCTS are the promising materials for solar cell applications.