Engel-Vosko generalized gradient approximation within DFT investigations of optoelectronic and thermoelectric properties of copper thioantimonates(III) and thioarsenate(III) for solar-energy conversion

The electronic structure and optical properties of Rb2Cu2Sb2S5, Cs2Cu2Sb2S5, and Rb8Cu6As8S19 were investigated from first principles based on Engel–Vosko generalized gradient approximation (EV‐GGA). The calculated band structures and density of states confirm that these compounds have an indirect bandgap. The bands near the Fermi level were mainly contributed from Cu‐3d states along with a small participation of S‐3p states. The partial density of states shows that Cs and Rb elements are bonded ionically to sulfur, whereas Cu, Sb, As, and S element show covalent bonds with each other. From the electronic structure and using EV‐GGA, the frequency‐dependent optical parameters such as the real and imaginary parts of the dielectric functions, energy loss function, and reflectivity were calculated. These optical parameters prove that these compounds are potentially interesting for the field of optoelectronic and optical devices. The temperature‐dependent thermoelectric properties such as the electrical conductivity, Seebeck coefficient, thermal conductivity, and power factor were calculated based on combination of DFT output and Boltzmann transport theory.