  Tail state formation in solar cell absorbers leads to a detrimental effect on
solar cell performance. Nevertheless, the characterization of the band tailing
in experimental semiconductor crystals is generally difficult. In this article,
to determine the tail state generation in various solar cell materials, we have
developed a quite general theoretical scheme in which the experimental Urbach
energy is compared with the absorption edge energy derived from density
functional theory (DFT) calculation. For this purpose, the absorption spectra
of solar cell materials, including CdTe, CuInSe2 (CISe), CuGaSe2 (CGSe),
Cu2ZnSnSe4 (CZTSe), Cu2ZnSnS4 (CZTS) and hybrid perovskites, have been
calculated by DFT particularly using very-high-density k meshes. As a result,
we find that the tail state formation is negligible in CdTe, CISe, CGSe and
hybrid perovskite polycrystals. However, coevaporated CZTSe and CZTS layers
exhibit very large Urbach energies, which are far larger than the theoretical
counterparts. Based on DFT analysis results, we conclude that the quite large
tail state formation observed in the CZTSe and CZTS originates from extensive
cation disordering. In particular, even a slight cation substitution is found
to generate unusual band fluctuation in CZT(S)Se. In contrast, CH3NH3PbI3
hybrid perovskite shows the sharpest absorption edge theoretically, which
agrees with experiment.
