Strain effects on the electronic and optical properties of kesterite Cu₂ ZnGeX₄ (X = S, Se): First-principles study

Following the chronological stages of calculations imposed by the WIEN2K code, we have performed a series of density functional theory calculations, from which we were able to study the effect of strain on the kesterite structures for two quaternary semiconductor compounds Cu₂ ZnGeS₄ and Cu₂ ZnGeSe₄. Remarkable changes were found in the electronic and optical properties of these two materials during the application of biaxial strain. Indeed, the band gap energy of both materials decreases from the equilibrium state, and the applied strain is more pronounced. The main optical features are also related to the applied strain. Notably, we found that the energies of the peaks present in the dielectric function spectra are slightly shifted towards low energies with strain, leading to significant refraction and extinction index responses. The obtained results can be used to reinforce the candidature of Cu₂ ZnGeX₄ (X = S, Se) in the field of photovoltaic devices.