Structural, Electronic and Optical Properties in Earth- Abundant Photovoltaic Absorber of Cu2ZnSnS4 and Cu2ZnSnSe4 from DFT calculations

DFT analyses of band structure dispersion and of contribution of different anionic sub-groups to the studied electronic structure together with the anisotropy of optical functions were performed for the two promising solar cell crystals Cu2ZnSnS4 and Cu2ZnSnSe4. We have applied the state-of-the-art full potential linear augmented plane wave (FPLAPW) method in a scalar relativistic version. Exchange and correlation potential were introduced within a framework of the local density approximation (LDA) and gradient approximation (GGA). We show that Cu2ZnSnS4 as well as Cu2ZnSnSe4 crystals possess a direct energy band gap situated around the center of the BZ. Careful analysis of the total density of states together with the partial contribution of the particular orbital were performed for evaluations of contribution of corresponding bonds to the origin of the chemical bonds. Role of replacing of S by Se is analyzed in the details for the electronic density of states with respect to the nature of chemical bonds. The principal analysis is performed for the dispersion of the optical constants. The influence of the different chemical bonds into the dispersion of the optical functions is analyzed in order to optimize the optical features with respect to the requirements of the solar cell elements.