Discovery of two new Cu–Sn chalco–halides for potential solar absorber applications

Abstract

We explore multiple-cation chalco–halide phase fields evaluated by their synthetic accessibility using machine learning models. Exploratory synthesis guided by computational tools leads to the discovery of two new compounds; CuSn₂SI₃ and Cu_0.35Sn_5.29S₂I₇, their structures, and electronic and optical properties are reported herein. This is the first report of a stable quaternary compound in the Cu–Sn–S–I phase field. The two new compounds show related crystal structures where Sn₄S₂I₄ layers are a common structural motif in both. These Sn₄S₂I₄ layers are connected by Cu₂I₂ layers and disordered Cu–Sn–I layers, forming the three-dimensional structures of CuSn₂SI₃ and Cu_0.35Sn_5.29S₂I₇ respectively. Electronic band structure calculations using density functional theory show the presence of a direct band gap in CuSn₂SI₃ and suggest anisotropic transport, in line with the layered structure of the compound. A mixture of the two compounds with ∼86% CuSn₂SI₃, shows a band gap in the visible region, close to 2.1 eV and a significant photo-induced charge carrier mobility of ∼1.3 cm² V⁻¹ s⁻¹. This demonstrates Cu–Sn chalco–halides can form a promising phase space to explore for solar absorber materials, with further design and tuning of band gap.