Cu2Zn(Sn,Ge)Se4and Cu2Zn(Sn,Si)Se4alloys as photovoltaic materials: Structural and electronic properties

As alternatives to the mixed-anion Cu${}_{2}$ZnSn(S,Se)${}_{4}$ alloys, the mixed-cation Cu${}_{2}$Zn(Sn,Ge)Se${}_{4}$ and Cu${}_{2}$Zn(Sn,Si)Se${}_{4}$ alloys can also span a band gap range that fits the requirement of the solar cell light absorber. However, material properties of these alloys as functions of alloy composition $x$ are not well known. In this paper, using the first-principles calculations, we study the structural and electronic properties of these alloys. We find that (i) the Cu${}_{2}$Zn(Sn,Ge)Se${}_{4}$ alloys are highly miscible with low formation enthalpies, while the Cu${}_{2}$Zn(Sn,Si)Se${}_{4}$ alloys are less miscible; (ii) the band gap of Cu${}_{2}$Zn(Sn,Ge)Se${}_{4}$ increases almost linearly from 1.0 eV to 1.5 eV as the Ge composition $x$ increases from 0 to 1, whereas the band gap of Cu${}_{2}$Zn(Sn,Si)Se${}_{4}$ spans a larger range from 1.0 eV to 2.4 eV and shows a slightly larger bowing; and (iii) the calculated band offsets shows that the band gap increase of the alloys with the addition of Ge or Si results primarily from the conduction band upshift, whereas the valence band shift is less than 0.2 eV. Based on these results, we expect that the component-uniform and band-gap-tunable Cu${}_{2}$Zn(Sn,Ge)Se${}_{4}$ and Cu${}_{2}$Zn(Sn,Si)Se${}_{4}$ alloys can be synthesized and have an improved photovoltaic efficiency.