Impact of anionic system modification on the desired properties for CuGa(S1−xSex)2 solid solutions

Abstract

One of promising directions of the modern solar cells’ development is related to the use of the ternary chalcopyrite crystals (CuInS2, CuGaS2 etc.) and their solid solutions as efficient light absorbing layers. Unfortunately, so far there is no systematic research linking chemical composition to useful properties allowing their optimization to increase the efficiency of solar cells. Therefore, we report the results of the detailed theoretical studies of the structural, electronic, and optical properties for the series of CuGa(S1−xSex)2 solid solutions (x = 0, 0.25, 0.5, 0.75, 1) in the framework of the density functional theory. For this purpose, crystal structures are analyzed within the second coordination environment approach. Effects of changes of chemical composition on the crystal structure and physical properties are presented. We found, that an increase of Se concentration leads to the decrease of the band gap, which remains to be direct over all the anion concentrations x. Analysis of the calculated dielectric function showed that substitution of S by Se is accompanied by an increase of the refractive indices na and nc and decrease of the optical birefringence. Then, based on the results of the DFT calculations, we derived formulas that allows for the easy estimation of the values of the lattice constants, band gap and the birefringence coefficient depending only on the concentration of S (and Se). The elastic constants and the Debye temperature were calculated for all considered solid solutions. These findings may be of special importance to design smart chalcopyrite materials with desired properties.