First principle insights and experimental investigations of the electronic and optical properties of CuInS2 single crystals

The function of the Perdew-Burke-Ernzerhof (PBE) approach is employed in density functional calculations to investigate copper indium disulfide (CuInS2) single crystals. The utilization of the generalized gradient approximation (GGA + U) method enhances the fluency and accuracy of the computation. This enabled the evaluation of band structure and density of states (DOS), including optical features like conductivity, reflectivity, refractive index, loss function, dielectric function, and absorption coefficient. The density of states (DOS) revealed that the valence bands at 0.8 eV are mostly a result of hybridisation between the In(5p) and S(3p) levels with slight input from Cu(3d). The CuInS2 single crystals are synthesised using a chemical vapour transport technique. Optical parameters such as optical density, skin depth, Urbach's parameters, refractive index, extinction coefficient, and dielectric constant are evaluated. The optical direct and indirect bandgap values are found as 1.60 eV and 1.51 eV, respectively. The band tailing or Urbach energy is calculated as 0.056 eV, allowing an accurate indirect bandgap of 1.454 eV to be predicted. The wavelength corresponding to the intersection of the real and imaginary dielectric functions is 819 nm, closer to the absorption band edge (797 nm).