Evaluating the Potential of CsBiSCl2 as a Solar Absorber

Abstract

Efforts to develop lead-free and stable alternatives to halide perovskites have thus far mostly yielded materials with power conversion efficiencies (PCEs) well below 10% in solar cells. Recently, photovoltaics based on CsBiSCl₂ were reported to achieve 10.38% PCE. Still, the crystal structure is unknown, and it is unclear whether the reported thin film synthesis method could realize thin films with the desired phase and stoichiometry. Herein, we use ab-initio Random Structure Searching (AiRSS) with a bespoke machine learned interatomic potential to explore the potential energy surface of CsBiSCl₂ , finding the previously-proposed cubic perovskite structure to be implausible. The lowest-energy structure we find is a four formula unit orthorhombic structure (Pnma space group) that lies 2.4 meV/atom above the convex hull. There is strong competition in the Cs-Bi-S-Cl family, which can lead to phase impurities. By examining the reported solution synthesis method, we find that it is challenging to obtain the dimethylammonium bismuth sulfide intermediate product, and that Bi₂S₃ with dimethylammonium iodide on the surface likely forms instead. The significant I-containing residues in this intermediate results in Cs₃Bi₂I₉ being preferentially formed in thin films instead of CsBiSCl₂ . Solid state synthesis without I present leads to phase impurities, consistent with the lowest-energy CsBiSCl2 phase being metastable. Taking these experimental and computational results together, it is unlikely that >10%-efficient CsBiSCl₂ solar cells have been achieved.