Hidden spontaneous polarisation in the chalcohalide photovoltaic absorber SnSbSI.

Perovskite-inspired materials aim to replicate the optoelectronic performance of lead-halide perovskites, while eliminating issues with stability and toxicity. Chalcohalides of group IV/V elements have attracted attention due to enhanced stability provided by stronger metal-chalcogen bonds, alongside compositional flexibility and ns lone pair cations - a performance-defining feature of halide perovskites. Following the experimental report of solution-grown tin-antimony sulfoiodide (SnSbSI) solar cells, with power conversion efficiencies above 4%, we assess the structural and electronic properties of this emerging photovoltaic material. We find that the reported centrosymmetric crystal structure represents an average over multiple polar 2 configurations. The instability is confirmed through a combination of lattice dynamics and molecular dynamics simulations. We predict a large spontaneous polarisation of 37 μC cm that could be active for electron-hole separation in operating solar cells. We further assess the radiative efficiency limit of this material, calculating > 30% for film thicknesses > 0.5 μm.