First‐principles calculation of Cu2SnS3 and related compounds

We evaluate the electronic structures of Cu2SnS3 (CTS) and related compounds, Cu2GeS3 (CGS), Cu2SiS3 (CSS), Cu2SnSe3 (CTSe), Cu2GeSe3 (CGSe), and Cu2SiSe3 (CSSe), using a first‐principles calculation. To determine band‐gap energy, Eg, without use of the reported crystal structure, we used the Heyd–Scuseria–Ernzerhof (HSE) screened hybrid density functional after geometry optimization by a DFT‐D2 approach in consideration of the van der Waals (vdW) forces. The Eg values of the monoclinic CTS and related compounds with a space group of Cc were calculated using the HSE functional with α = 0.32. The determined Eg values were 0.88 eV for CTS, 1.54 eV for CGS, 2.87 eV for CSS, 0.45 eV for CTSe, 0.72 eV for CGSe, and 1.72 eV for CSSe. Additionally, we discuss the reason for the difference in Eg between CTS and related compounds from density of states (DOS).