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

Abstract

In search of new inorganic solar absorber candidates, 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, and 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 CuI layers and disordered Cu-Sn-I layers, forming the three-dimensional structures of CuSn₂SI₃ and Cu_0.35Sn_5.29S₂I₇ respectively. Electronic band structure DFT calculations show the presence of a direct band gap in CuSn₂SI₃ and suggests anisotropic transport with the existence of flat bands along one of the directions, 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 s, making it a promising candidate solar absorber material.