Electronic and optical properties of $Cu2ZnSnS4$ and $Cu2ZnSnSe4$

The electronic structure as well as the optical response of kesterite and stannite structures of $Cu2ZnSnS4$ and $Cu2ZnSnSe4$ are analyzed by a relativistic full-potential linearized augmented plane wave method. The energy dispersion of the conduction-band edge reveals larger effective electron mass of the two $Cu2ZnSnS4$ compounds $(mc1≈0.18m0)$ compared with $Cu2ZnSnSe4$ $(mc1≈0.07m0)$⁠. Whereas the effective electron mass tensor is fairly isotropic, the effective hole masses show strong anisotropy. The fundamental band-gap energy is estimated to be $Eg≈1.5eV$ for $Cu2ZnSnS4$ and $Eg≈1.0eV$ for $Cu2ZnSnSe4$⁠. The larger band gap results in a smaller high-frequency dielectric constant: $ε∞≈6.7$ for $Cu2ZnSnS4$ whereas $ε∞≈8.6$ for $Cu2ZnSnSe4$⁠. The characteristic anisotropy of the dielectric function $ε(ω)$ in the stannite compounds allows for a complementary identification of the crystalline structure type. Overall, however, all four compounds show similar atomic-resolved density-of-states, dielectric function, and optical absorption coefficient $α(ω)$⁠.