Hidden spontaneous polarisation in the chalcohalide photovoltaic absorber Sn2SbS2I3

Perovskite-inspired materials aim to replicate the optical and electronic performance of the famous lead-halide perovskites, while eliminating issues with stability and toxicity. In this work, we focused on an emerging material in the class of mixed-metal chalcohalides; Sn₂Sb_S2I₃, which recently attracted attention after a first experimental synthesis produced solar cells with promising efficiencies (>4%)(Nie et al. Matter 2020). We found that the experimentally-reported centrosymmetric (Cmcm) crystal structure in fact represents an average over multiple polar symmetry-broken Cmc2₁ configurations. We confirmed this instability through a combination of modelling techniques, including thermodynamic analysis with both DFT and the Random Phase Approximation (RPA), as well as lattice dynamics and molecular dynamics simulations. The close resemblance of this dynamic crystal structure and polar behaviour to that of MAPI begs the question of its importance in high-performance defect-tolerant solar materials. Our results shine a spotlight on the largely-unexplored class of mixed-metal chalcohalides, and provides insight regarding both structure-property relationships in perovskite-inspired materials and the potential success of mixed-metal chalcohalides as high-performance optoelectronic materials.

S. R. Kavanagh, C. N. Savory, D. O. Scanlon and A. Walsh, Mater. Horiz., 2021, 8, 2709–2716. https://doi.org/10.1039/D1MH00764E

Seán Kavanagh is a PhD student in the research groups of Professors David Scanlon (Chemistry, UCL) and Aron Walsh (Materials, Imperial), studying emerging solar photovoltaic materials and defect processes.