Hidden Spontaneous Polarisation in the ns2-Cation Sn2 SbS2 I3 Chalcohalide Photovoltaic Absorber Seán R. Kavanagh, Christopher N. Savory, David O. Scanlon, Aron Walsh [email protected] EL02.10.02 Cmcm b c Ibanez et al. (S, Se) S I 

Sn2 SbS2 I3 – ‘Perovskite-Inspired’ • Novel ns2-cation chalco-halide. • Mixed anions (and cations) allows mixed ionic-covalent character. • Strong dielectric screening? • Lattice polarity? • Defect Tolerance? • Nie et al. (Group of Sang Il Seok), demonstrated >4% efficiency in the first experimental device fabrication (Nov 2020). 1. Huang, Y.-T., Kavanagh, S. R., Scanlon, D. O., Walsh, A. & Hoye, R. L. Z. Perovskite-Inspired Materials for Photovoltaics -- From Design to Devices. 2021 Nanotechnology 32 132004 2. R. Nie, K. S. Lee, M. Hu, M. J. Paik and S. I. Seok, Heteroleptic Tin-Antimony Sulfoiodide for Stable and Lead-free Solar Cells. 2020 Matter, S2590238520304471. 

Sn2 SbS2 I3 – Structural Relaxation A. Ibanez, J.-C. Jumas, J. Olivier-Fourcade and E. Philippot, Journal of Solid State Chemistry, 1984, 55, 83–91. 

Sn2 SbS2 I3 – Structural Relaxation Sn Sb S I Cmcm Cmc2 1 b c ΔE(Cmc21 /Cmcm) = -35.8 meV/atom (RPA w/ HSE06 orbitals) 

Sn2 SbS2 I3 – Structural Relaxation Sn Sb S I Cmcm Cmc2 1 b c ΔE(Cmc21 /Cmcm) = -35.8 meV/atom (RPA w/ HSE06 orbitals) 

Sn2 SbS2 I3 – Dynamic Stability Cmcm Cmc21 

Sn2 SbS2 I3 – Structural Relaxation A. Ibanez, J.-C. Jumas, J. Olivier-Fourcade and E. Philippot, Journal of Solid State Chemistry, 1984, 55, 83–91. 

Sn2 SbS2 I3 – Spontaneous Polarisation Cmcm ⟹ Cmc21 ΔP(Cmc21 /Cmcm) = 37 μC/cm2 (optB86b-vdW) c.f. BaTiO3 (∼27 μC/cm2), KNbO3 (∼30 μC/cm2), MAPbI3 (4.4 μC/cm2), SbSI (11 μC/cm2) 

Sn2 SbS2 I3 – Molecular Dynamics ΔP = 37 μC/cm2 

Sn2 SbS2 I3 – Electronic & Optical Properties Eg = 1.08 eV (HSE06 + SOC) 

Sn2 SbS2 I3 – Electronic & Optical Properties 

Sn2 SbS2 I3 – Potential Defect Tolerance Eg = 1.08 eV (HSE06 + SOC) - Small band gap - Anti-bonding character, high- energy VBM (Sn 5s2 – anion p) - Mixed ionic-covalent bonding: - Strong dielectric screening - Wide conduction & valence bands - Atomic-chain structure (⟹ benign grain boundaries?) PL lifetime >7 ns recorded by Nie et al. R. Nie, K. S. Lee, M. Hu, M. J. Paik and S. I. Seok, Heteroleptic Tin-Antimony Sulfoiodide for Stable and Lead-free Solar Cells. 2020 Matter, S2590238520304471. 

Conclusions & Acknowledgements Spontaneous symmetry breaking and lattice polarization, hidden by macroscopic averaging, unveiled in Sn2 SbS2 I3 . o Potential benefits for charge separation and PV efficiency. Promising outlook for the application in high- efficiency solution-processed solar cells. Ongoing Molecular Dynamics calculations to further probe polarization switchability. @Kavanagh_Sean_ kavanase [email protected]