Studies of Atomic-Layer-Deposited Zn_1–XSn_XO_Y as an Alternative Buffer Layer for Cu₂Zn(Sn_1–X,Ge_X)S₄ Thin-Film Solar Cells

Abstract

Thin-film solar cells based on Cu 2 Zn(Sn 1−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 Zn 1−X Sn X O Y (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 (GeO 2 ) at the absorber/buffer interface, shown by hard X-ray photoelectron spectroscopy measurements, as previously reported for the full-germanium Cu 2 ZnGeS 4 -based devices.KEYWORDS: atomic-layer-deposited Zn 1−X Sn X O Y (ALD ZTO), hard X-ray photoelectron spectroscopy (HAXPES), Cu 2 Zn(Ge,Sn)S 4 (CZTGS) absorbers