Hidden spontaneous polarisation in the chalcohalide photovoltaic absorber Sn₂SbS₂I₃

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 ns² lone pair cations - a performance-defining feature of halide perovskites. Following the experimental report of solution-grown tin-antimony sulfoiodide (Sn₂SbS₂I₃) 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 Cmcm crystal structure represents an average over multiple polar Cmc2₁ 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^-2 that could be active for electron-hole separation in operating solar cells. We further assess the radiative efficiency limit of this material, calculating η_max > 30% for film thicknesses t > 0.5 μm.