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Frenkel Excitons in Vacancy-ordered Titanium Halide Perovskites (Cs₂TiX₆)

Frenkel Excitons in Vacancy-ordered Titanium Halide Perovskites (Cs₂TiX₆)

Poster presented at the 2022 EPFL SeeFuturePV conference in Lausanne, Switzerland.

Preprint here: https://chemrxiv.org/engage/chemrxiv/article-details/62f51937e78f70b71237c131
Under review at the Journal of Physical Chemistry Letters.

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Seán R. Kavanagh

September 17, 2022

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  1. M = Sn, Ti X = Br, Cl, I Cs

    Seán R. Kavanagh, Christopher N. Savory, Shanti M. Liga, Gerasimos Konstantatos, Aron Walsh, David O. Scanlon Scan me for a YouTube talk on this work! Figures: (a) Electronic band structures calculated with hybrid DFT including spin-orbit coupling (HSE06+SOC), alongside charge densities at the (b) CBM and (c) VBM. Theoretical investigations dramatically and consistently overestimate the electronic bandgaps of the Cs2 TiX6 family of compounds (X = I, Br, Cl), as well as incorrectly predicting the relative bandgap trends between Cs2 TiX6 and Cs2 SnX6 – despite accurately predicting the electronic structure of Cs2 SnX6 . Here we reveal strong excitonic effects as the origin of this major discrepancy between theory and experiment; a consequence of both low structural dimensionality and orbital localisation. @Kavanagh_Sean_ sean.kavanagh.19@ucl.ac.uk Y.-T. Huang, S.R. Kavanagh, D.O. Scanlon, A. Walsh, and R.L.Z. Hoye, Nanotechnology (2021) B. Cucco, G. Bouder, L. Pedesseau, C. Katan, J. Even, M. Kepenekian, G. Volonakis, Appl Phys Lett (2021) S.R. Kavanagh, C. N. Savory, S.M. Liga, G. Konstantatos, A. Walsh and D.O. Scanlon (In submission) Revealing strongly-bound excitons in vacancy- ordered 5tanium perovskites (Cs2 TiX6 )  L W X  −6 −4 −2 0 2 4 6 Energy (eV) Total DOS I (p) Ti (d) Ti (s) a b c  L W X  −6 −4 −2 0 2 4 6 Energy (eV) Total DOS I (p) Sn (s) Sn (p) a b c 0 1 2 3 h (eV) (Arbitrary Units) Hybrid DFT GW0+BSE Experiment Cs 2 TiBr 6 0 1 2 3 h (eV) (Arbitrary Units) Hybrid DFT GW0+BSE Experiment Cs 2 TiI 6 E ex (dark) E ex (dark) E ex (dark) 0 1 2 3 4 h (eV) 2 3 4 5 6 7 8 9 real 0 1 HSE06 GW0 + BSE GW0 (RPA) Cs 2 TiI 6 0 1 2 3 4 h (eV) (Arbitrary Units) Cs 2 SnBr 6 Hybrid DFT GW0+BSE Experiment a 0 1 2 3 4 5 h (eV) (Arbitrary Units) Hybrid DFT GW0+BSE Experiment Cs 2 SnCl 6 b E ex (dark) E ex (dark) Beyond-DFT calculations including electron-hole interactions (GW+BSE) reveal strong excitonic binding for Cs2 TiX6 , with band contributions to the exciton wavefunction across the Brillouin zone. ⬅ Figures left/right ➡ This delocalisation in reciprocal space corresponds to a strong real-space localisation, and thus a strongly-bound charge-transfer Frenkel exciton. Cs2 SnX6 exhibits opposite behaviour, with delocalised Wannier-Mott excitons (though intermediate behaviour for Cs2 SnCl6 ). Exciton binding significantly redshifts the absorption onset, resolving the discrepancy between theory/experiment and reproducing the measured absorption spectra. ⬅ Figures left/right ➡ Cs2 SnI6 Cs2 TiI6 0 1 2 3 4 5 h (eV) (Arbitrary Units) Hybrid DFT GW0+BSE Experiment Cs 2 TiCl 6 E ex (dark) Remaining Issues: - Major overestimation of quasiparticle bandgaps within GW (Cs2 TiX6 & Cs2 SnX6 ) - Consequence of significant under- screening within the RPA Coulomb potential W. - Known to worsen for localised (e.g. d/f- orbital) and low-dimensional systems. - Requires state-of-the-art self-consistent vertex corrections within the GW calculation. M = Sn, Ti X = Br, Cl, I Cs Conclusions: - Strongly-bound, localised excitons are present in Cs2 TiX6 , explaining long-standing experiment/theory disagreement. - Strong exciton binding can reduce charge separation and open-circuit voltages (Voc ) in solar cells, likely a key origin of the poor photovoltaic performance achieved thus far in this material class. - Importance of frontier orbital character and structural connectivity & dimensionality when employing atomic substitution in materials design strategies -- here resulting in qualitatively different electronic behaviour despite equal cation valence and similar bandgaps.