Frenkel excitons in vacancy-ordered titanium halide perovskites (Cs 2 TiX 6 ) Seán Kavanagh 1,2 , Shanti Liga 3 , Christopher N. Savory 1 , Gerasimos Konstantatos 3 , Aron Walsh 2 , David O. Scanlon 1 1 Thomas Young Centre and Department of Chemistry, University College London, London WC1H 0AJ, U.K. 2 Thomas Young Centre and Department of Materials, Imperial College London, London SW7 2AZ, U.K. 3 ICFO-Insitut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Casteldefels, 08860 Barcelona, Spain. Perovskite-inspired materials have emerged as promising candidates for a range of material applications, including radiation detectors, solar photovoltaics, LEDs and thermoelectrics. 1–31 In particular, chemical substitution while retaining the perovskite crystal structure has been a popular materials design strategy, leading to the emergence of vacancy-ordered halide perovskites (A 2 BX 6 ; A = CH 3 NH 3 , Cs, Rb, K; B = Sn, Ti, Zr, Te, Hf; X = I, Br, Cl) in recent years. Theoretical investigations of the electronic structure of these materials, however, consistently and severely overestimate the energy band gaps of these materials 4–6 – a key property for their proposed optoelectronic and thermoelectric applications. In this study, 7 we use first-principles GW+BSE+SOC calculations to reveal the presence of anomalously strong excitonic binding in these bulk inorganic semiconductors, explaining the origin of this discrepancy between theory and experiment. We predict giant exciton binding energies in the range of 0.5 - 2 eV, despite electronic bandgaps in the range of 1 – 5 eV, and show that these are charge-transfer Frenkel excitons, closely resembling those typically seen in molecular crystals and organic compounds. We show these evolving properties to be the result of low effective structural dimensionality and band localisation, highlighting the key roles of structural connectivity and frontier-orbital interactions within materials design strategies – which has oft been overlooked in the discovery of perovskite-inspired materials. Moreover, this work demonstrates the importance of advanced theoretical characterisation methods for accurately predicting optoelectronic behaviour and thus designing suitable candidate materials. Beyond the vital implications for device applications of these emerging materials, this ultra-strong exciton binding (greater than 2D materials with similar bandgaps) in a nominally-3D compound also provides a playground for studying exotic excitonic interactions in bulk semiconductors.
References 1. Y.-T. Huang, S. R. Kavanagh, D. O. Scanlon, A. Walsh and R. L. Z. Hoye, Nanotechnology, 2021, 32, 132004. 2. Y. Wang‡ & S. R. Kavanagh‡, I. Burgués-Ceballos, A. Walsh, D. Scanlon and G. Konstantatos, Nat. Photon., 2022, 16, 235–241. 3. S. M. Liga and G. Konstantatos, J. Mater. Chem. C, 2021, 9, 11098–11103. 4. M. Chen, M.-G. Ju, A. D. Carl, Y. Zong, R. L. Grimm, J. Gu, X. C. Zeng, Y. Zhou and N. P. Padture, Joule, 2018, 2, 558–570. 5. J. Euvrard, X. Wang, T. Li, Y. Yan and D. B. Mitzi, J. Mater. Chem. A, 2020, 8, 4049–4054. 6. B. Cucco, G. Bouder, L. Pedesseau, C. Katan, J. Even, M. Kepenekian and G. Volonakis, Appl. Phys. Lett., 2021, 119, 181903. 7. S. R. Kavanagh, C. N. Savory, S. M. Liga, G. Konstantatos, A. Walsh and D. O. Scanlon, J. Phys. Chem. Lett., 2022, 13, 10965–10975.
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