A DFT study of the electronic and optical properties of a photovoltaic absorber material Cu2ZnGeS4 using GGA and mBJ exchange correlation potentials

The electronic and optical properties of Cu2ZnGeS4 are calculated by means of the full potential linearized augmented plane wave method (FP-LAPW) using the generalized gradient approximation (GGA) and the modified Becke-Johnson (mBJ) as exchange correlation functionals. The band structures, total and partial densities of states as well as optical properties are presented and discussed in the context of the available experimental data and theoretical calculations. Our calculations show that the conduction bands minima (CBM) is composed of contributions mainly from Ge-p states while the valence bands maxima (VBM) is dominated by Cu-d states. We find that Cu2ZnGeS4 possesses a direct energy band gap situated at the center (Γ point) of the Brillouin Zone (BZ). The calculated energy gaps of 0.50 eV (GGA) and 1.21 eV (mBJ) are much smaller than the experimental value of around 2.2 eV. We have shown anisotropic behavior of the compound using optical constants and birefringence plots and found that the Δn(0) in GGA and mBJ is about 0.06 and 0.08, respectively when applied scissors correction. We find that applying the scissors correction leads to better agreement with the experiment.