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.