Cu2Zn(Si,Ge)Se4 quaternary semiconductors as potential photovoltaic materials

Abstract The geometric and electronic structures of Cu2ZnSixGe1-xSe4 alloys are studied using density functional theory calculations. The variation of lattice constants is less than 1.2%. Negative formation enthalpies are obtained, indicating these alloys are completely miscible. The conduction band minimum gradually shifts upward with the Si concentration, resulting in a tunable bandgap from 1.27 to 2.26 eV. The bandgap of the Cu2ZnSi0.25Ge0.75Se4 alloy is very close to the optimal value. Compared with other two well-studied alloys with the optimal bandgap (Cu2ZnSi0.5Sn0.5Se4 and Cu2ZnSnS4), the Cu2ZnSi0.25Ge0.75Se4 has a red-shift in the optical absorption edge, highest absorption coefficient and smallest transmittance.