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≈1.5 eV for Cu2ZnSnS4 and Eg≈1.0 eV 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).