Studies of Atomic-Layer-Deposited Zn_1–<i>X</i>Sn_<i>X</i>O_<i>Y</i> as an Alternative Buffer Layer for Cu₂Zn(Sn_1–<i>X</i>,Ge_<i>X</i>)S₄ Thin-Film Solar Cells

Thin-film solar cells based on Cu2Zn(Sn1-X,Ge-X)S-4 (CZTGS) absorbers are emerging technologies for solar energy conversion with low-cost, nontoxic components. In this work, we investigated CZTGS-based solar cells with varying Ge contents and an alternative buffer layer of Zn1-XSnXOY (ZTO) by atomic layer deposition. The results are compared to those of devices with the traditional CdS buffer layer. Overall, higher efficiency and open-circuit voltage (V-OC) are observed for the devices with ZTO compared to those with the CdS buffer layer. Moreover, the results show that the CZTGS/ZTO device performance may further be improved by varying the ZTO properties (band gap or thickness) or by applying a surface treatment on the CZTGS absorber. An air annealing treatment of the chemically etched absorber surface improved the CZTGS device performance; however, nonetched devices show poor efficiency due to the presence of secondary phases (GeO2) at the absorber/buffer interface, shown by hard X-ray photoelectron spectroscopy measurements, as previously reported for the full-germanium Cu2ZnGeS4-based devices.