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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.

Two kinds of exchange–correlation functional GGA–PBE and HSE06 were used in the first principle method to calculate the complex dielectric function of CuInSe 2 and CuAlSe 2 . Compared with experimental data, GGA–PBE functional cannot properly predict dielectric function, while calculated results by HSE06 quantitatively agree with experimental data. With HSE06 functional, optical properties of CuIn 1− x Al x Se 2 ( x =0, 0.25, 0.5, 0.75 and 1) were calculated. As x increases from 0 to 1, within the solar spectrum, the real and the imaginary parts of the dielectric functions reduce, the static dielectric constant decreases from 7 to 5, the absorption coefficient and refractive indices decrease, while the optical band gap increases from 1.07 to 2.61 eV.