Bandgap Engineering of Amorphous ZnO–SnO _x Alloys and Their Favorable Band Offsets with Wide-Gap Cu(In,Ga)Se ₂

ZnO–SnO x alloys have garnered significant attention in recent years as a Cd-free buffer candidate for wide-gap chalcopyrite Cu(In,Ga)Se 2 solar cells because of its earth-abundant constituents and tunable electronic band structure to obtain desirable band offsets with the Cu(In,Ga)Se 2 absorber. Numerical simulations have shown that recombination at the ZnO–SnO x buffer and Cu(In,Ga)Se 2 absorber interface can be mitigated with a moderate positive conduction band offset (CBO) (0.1–0.3 eV), leading to an optimized solar cell efficiency of >32%. In this work, Zn 1– x Sn x O 1+δ alloy thin films across the entire composition x were grown by magnetron sputtering at room temperature. Optoelectronic properties of Zn 1– x Sn x O 1+δ alloys were optimized via manipulating the growth and postgrowth annealing conditions. We find that Zn 1– x Sn x O 1+δ films with mid-compositions of 0.3 < x < 0.4 remain amorphous and exhibit a relatively high electron mobility of ∼10 cm 2 /(V s) with a high transparency of > 80% in a wide spectral window from 350 to >2500 nm after annealing at >400 °C. The bandgap of these films varies from 3.5 to 3.75 eV. As such, we fabricated a thin heterojunction using Zn 0.68 Sn 0.32 O 1+δ grown on a Cu(In,Ga)Se 2 film with a wide gap of ∼1.4 eV and obtained a favorable type I band alignment with CBO of ∼0.1 eV. This experimentally achievable favorable band offset opens an avenue toward high-performance Cu(In,Ga)Se 2 solar cells.