Studies of Atomic-Layer-Deposited\nZn<sub>1–<i>X</i></sub>Sn<sub><i>X</i></sub>O<sub><i>Y</i></sub> as an Alternative Buffer Layer for\nCu<sub>2</sub>Zn(Sn<sub>

Thin-film solar cells\nbased on Cu<sub>2</sub>Zn(Sn<sub>1−<i>X</i></sub>,Ge<sub><i>X</i></sub>)S<sub>4</sub> (CZTGS)\nabsorbers are emerging technologies for solar energy conversion with\nlow-cost, nontoxic components. In this work, we investigated CZTGS-based\nsolar cells with varying Ge contents and an alternative buffer layer\nof Zn<sub>1–<i>X</i></sub>Sn<sub><i>X</i></sub>O<sub><i>Y</i></sub> (ZTO) by atomic layer deposition.\nThe results are compared to those of devices with the traditional\nCdS buffer layer. Overall, higher efficiency and open-circuit voltage\n(<i>V</i><sub>OC</sub>) are observed for the devices with\nZTO compared to those with the CdS buffer layer. Moreover, the results\nshow that the CZTGS/ZTO device performance may further be improved\nby varying the ZTO properties (band gap or thickness) or by applying\na surface treatment on the CZTGS absorber. An air annealing treatment\nof the chemically etched absorber surface improved the CZTGS device\nperformance; however, nonetched devices show poor efficiency due to\nthe presence of secondary phases (GeO<sub>2</sub>) at the absorber/buffer\ninterface, shown by hard X-ray photoelectron spectroscopy measurements,\nas previously reported for the full-germanium Cu<sub>2</sub>ZnGeS<sub>4</sub>-based devices.