Bandgap Engineering of Amorphous ZnO–SnO _<i>x</i> Alloys and Their Favorable Band Offsets with Wide-Gap Cu(In,Ga)Se ₂

ZnO–SnOx alloys have garnered significant attention in recent years as a Cd-free buffer candidate for wide-gap chalcopyrite Cu(In,Ga)Se2 solar cells because of its earth-abundant constituents and tunable electronic band structure to obtain desirable band offsets with the Cu(In,Ga)Se2 absorber. Numerical simulations have shown that recombination at the ZnO–SnOx buffer and Cu(In,Ga)Se2 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, Zn1–xSnxO1+δ alloy thin films across the entire composition x were grown by magnetron sputtering at room temperature. Optoelectronic properties of Zn1–xSnxO1+δ alloys were optimized via manipulating the growth and postgrowth annealing conditions. We find that Zn1–xSnxO1+δ films with mid-compositions of 0.3 < x < 0.4 remain amorphous and exhibit a relatively high electron mobility of ∼10 cm2/(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 Zn0.68Sn0.32O1+δ grown on a Cu(In,Ga)Se2 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)Se2 solar cells.