Unravelling the Impact of Disorder on the Electronic Properties Of Mixed-Metal Chalcohalides

Mixed-metal mixed-anion systems have seen a significant rise in interest as ‘perovskite-inspired materials’ as are expected to combine the excellent stability seen in metal chalcogenide solar cells with the well-known performance of hybrid halide perovskite solar cells.^[1]<br/><br/>Sn₂SbS₂I₃ is a promising solution-processed photovoltaic absorber having achieved efficiency above 4% in initial devices.^[2] Theoretical work predicts that the material family of A₂BCh₂X₃ mixed-metal chalcohalides could also be ferroelectric, with Sn₂SbS₂I₃ having strong lattice polarization and large dielectric constants.^[3]<br/><br/>However, this family has not been rigorously explored resulting in confusion in the literature regarding the structure of these materials, with some works observing disorder in room temperature crystals.^[4] Without a proper description of the structure, prediction of the electronic properties cannot be accurately performed. Understanding the extent of the disorder in these systems is of key importance due to its tendency to quench favourable properties such as macroscopic polarisation.<br/><br/>Using Density Functional Theory, Cluster expansion and Monte Carlo techniques, we have systematically examined the cation disorder in Sn₂SbS₂I₃ for the first time and will discuss its likely impact on the potential for this material family to produce ferroelectric and photovoltaic devices.^[5]<br/><b>References</b><br/>[1] Nie, R.; Sumukam, R. R.; Reddy, S. H.; Banavoth, M.; Seok, S. I. Lead-Free Perovskite Solar Cells Enabled by Hetero-Valent Substitutes. <i>Energy Environ. Sci.</i> <b>2020</b>, <i>13</i> (8), 2363–2385.<br/>[2] Nie, R.; Lee, K. S.; Hu, M.; Paik, M. J.; Seok, S. I. Heteroleptic Tin-Antimony Sulfoiodide for Stable and Lead-Free Solar Cells. Matter <b>2020</b>, 3 (5), 1701–1713.<br/>[3] Kavanagh, S. R.; Savory, C. N.; Scanlon, D. O.; Walsh, A. Hidden Spontaneous Polarisation in the Chalcohalide Photovoltaic Sn2SbS2I3. Materials Horizons <b>2021</b><br/>[4] Doussier, C.; Moëlo, Y.; Léone, P.; Meerschaut, A.; Evain, M. Crystal Structure of Pb2SbS2I3, and Re-Examination of the Crystal Chemistry within the Group of (Pb/Sn/Sb) Chalcogeno-Iodides. <i>Solid State Sciences</i> <b>2007</b>, <i>9</i> (9), 792–803.<br/>[5] Nicolson, A. T. J.; Morita, K; Kavanagh, S. R.; Walsh, A; Scanlon, D. O. Submitted (2022)