First-principles investigation of the optical properties of CuIn(SxSe1–x)2

We optimized the lattice structure of sulfur-doped CuInSe2 using first principles. The lattice constants for CuIn(SxSe1–x)2 vary linearly with x according to a(x)=–0.02828x+0.58786 nm and c(x)=–0.05692x+1.1834 nm, which agree well with experimental data. The optical properties of CuIn(SxSe1–x)2 were then systematically investigated using first-principles calculations with the HSE06 functional. We present data for the complex dielectric function, refractive index, extinction coefficient, reflectivity index, absorption coefficient, and optical bandgap for CuIn(SxSe1–x)2. The optical bandgap Eg obtained from the absorption coefficient is 1.07 eV for CuInSe2 and 1.384 eV for CuInS2. These values are very close to experimental results, indicating that first-principles calculations can yield accurate bandgap values. The optical bandgap of CuIn(SxSe1–x)2 increases linearly with the sulfur concentration according to Eg=0.3139x+1.0825 eV.