Properties of the Interface Between the Atomic Layer Grown Zinc–Tin–Oxide and Cu₂ZnGeS₄ Relevant for Kesterite Thin‐Film Solar Cells

Abstract

Recently, a record open‐circuit voltage (1.1 V) is obtained for wide‐bandgap Cu2ZnGeS4 (CZGS) thin‐film solar cells with a Zn1−xSnxOy (ZTO) buffer grown by atomic layer deposition (ALD) (N. Saini et al. Sol. RRL 2021, 2100837). However, a low short‐circuit current and a small variation in device performance with ZTO properties are observed, suggesting similar interface properties. Since the CZGS absorber cannot be etched due to peeling, it is suggested that the presence of an oxide interlayer can influence the device performance. Here, the chemical and electronic properties of the near‐surface region of CZGS absorber and its interface with ZTO buffers with varying properties (bandgap and thickness) are studied nondestructively by employing excitation‐dependent X‐ray photoelectron spectroscopy. The formation of Ge oxide species is indicated at the absorber surface during device fabrication, which is preserved during ALD. Further, it is shown that the ZTO/CZGS interface properties are similar and not influenced by variations in ZTO properties, which can explain the small variation in device performance. Thus, attention shall be given to possible absorber surface cleaning treatment typically applied before the buffer deposition for Ge‐containing CZGS, and an optimization of the ALD ZTO buffer layer growth shall be considered for a non‐etched absorber.