Crystal and electronic band structure of $Cu2ZnSnX4$ (⁠$X=S$ and Se) photovoltaic absorbers: First-principles insights

The structural and electronic properties of $Cu2ZnSnS4$ and $Cu2ZnSnSe4$ are studied using first-principles calculations. We find that the low energy crystal structure obeys the octet rule and is the kesterite (KS) structure. However, the stannite or partially disordered KS structures can also exist in synthesized samples due to the small energy cost. We find that the dependence of the band structure on the (Cu,Zn) cation ordering is weak and predict that the band gap of $Cu2ZnSnSe4$ should be on the order of 1.0 eV and not 1.5 eV as was reported in previous absorption measurements.