This paper presents a study related to methods of estimation of the band gap for kesterite-type Cu2ZnSnS(Se)4 based
on their electronic structure and optical properties driven by the first-principles calculations within the hybrid
functional. The estimations have been performed by four different methods. The first one is based on the ...
This paper presents a study related to methods of estimation of the band gap for kesterite-type Cu2ZnSnS(Se)4 based
on their electronic structure and optical properties driven by the first-principles calculations within the hybrid
functional. The estimations have been performed by four different methods. The first one is based on the band structure
estimated as the difference of the energy corresponding to the conduction band and valence band edges. The rest three
methods are based on the spectral distribution of the absorption coefficient a(ħw) driven from the first-principles
calculations: (i) Tauc plot, (ii) first derivative of a(ħw) on photon energy ħw in the energy range around the
fundamental absorption edge, and (iii) linear combination of the methods (i) and (ii). We show that the band gap
estimated by different methods can deviate each from other drastically and the large difference is not only relayed to
defects, impurities and other lattice imperfections. We show also, that the influence of lattice anisotropy on optical
properties of CZTS and CZTSe is negligible. We have pointed out importance of the dependence of dielectric constant
on surface texture. The equation describing dependence of dielectric constant on the experimentally observed surface
texture is derived.