Frenkel Excitons in Vacancy-Ordered Perovskites (Cs₂MX₆)

My poster on 'Frenkel Excitons in Vacancy-Ordered Perovskites (Cs₂MX₆)' for APS March 2023, at Las Vegas, USA.

Paper discussed available here (open-access):
https://pubs.acs.org/doi/full/10.1021/acs.jpclett.2c02436

Questions welcome! For other computational photovoltaics, defects and disorder talks, have a look at my YouTube channel!

If you're interested in this work, you can check out our recent review on these and other perovskite-inspired materials:
https://iopscience.iop.org/article/10.1088/1361-6528/abcf6d

For more info about me and my research articles see:
https://seankavanagh.com

Transcript

1. Seán R. Kavanagh*, Christopher N. Savory, Shanti M. Liga, Gerasimos Konstantatos, Aron Walsh*, David O. Scanlon* Scan me for the paper and a YouTube talk on this study! Figure 1: (Left) Band structures from hybrid DFT including spin-orbit coupling (HSE06+SOC). (Right) Band contributions to the lowest-energy bright exciton (GW+BSE) Theoretical investigations dramatically and consistently overestimate the electronic bandgaps of vacancy-ordered perovskites: Cs2 TiX6 (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 . We reveal ultra-strong excitonic effects (Eb >1 eV), despite relatively small bandgaps (1-3 eV), as the origin of this major experiment-theory discrepancy; a consequence of both low structural dimensionality and band localization. @Kavanagh_Sean_ sean.kavanagh.19@ucl.ac.uk S.R. Kavanagh, C. N. Savory, S.M. Liga, G. Konstantatos, A. Walsh and D.O. Scanlon J Phys Chem Lett (2022) 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) Frenkel Excitons in Vacancy-Ordered Perovskites (Cs2 MX6 ) 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 (Figure 1). This delocalisation in reciprocal space corresponds to a strong real-space localisation, and thus a strongly-bound charge-transfer Frenkel exciton. Exciton binding significantly redshifts the absorption onset, resolving the discrepancy between theory/experiment and reproducing the measured absorption spectra: Cs2 SnX6 Cs2 TiX6 Conclusions: Highly localised, isolated MX6 octahedra yield ‘molecular salt’ behaviour: - Strongly-bound, localised Frenkel excitons with enormous binding energies revealed in Cs2 TiX6 , explaining long-standing experiment-theory disagreement. - Strong van der Waals dispersion interactions are found in these vacancy- ordered compounds, decreasing volumes by >10% with ΔEg ~ 0.1-0.3 eV. - 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. El o (Ti ) p Ho ( F e e Exc d 0 1 2 3 4 (Arbitrary Units) hν (eV) Hybrid DFT G0W0+BSE Experiment Cs 2 TiI 6 E ex (dark) E ex (dark) E ex (dark) (Arbitrary Units) Hybrid DFT G0W0+BSE Experiment Cs 2 TiBr 6 0 1 2 3 4 hν (eV) Hybrid DFT G0W0+BSE Experiment (This Work) Experiment (Grandhi et al.) Cs 2 TiCl 6 0 1 2 3 4 5 (Arbitrary Units) hν (eV) E ex (dark) E ex (dark) 0 1 2 3 4 (Arbitrary Units) hν (eV) Hybrid DFT G0W0+BSE Experiment Cs 2 SnI 6 0 1 2 3 4 5 6 (Arbitrary Units) hν (eV) Hybrid DFT G0W0+BSE Experiment (This Work) Experiment (Karim et al.) (hν - 0.5 eV) Cs 2 SnCl 6 0 1 2 3 4 (Arbitrary Units) Cs 2 SnBr 6 Hybrid DFT G0W0+BSE Experiment hν (eV) E ex (dark) E ex (dark) E ex (dark)  L W X  −6 −4 −2 0 2 4 6 Energy (eV) Total DOS I (p) Ti (d) Ti (s) a b  L W X  −6 −4 −2 0 2 4 6 Energy (eV) Total DOS I (p) Sn (s) Sn (p) a b Ti d I p I p Sn s - I p t2g eg Exciton binding Cs2 SnX6 exhibits opposite behaviour, with delocalised Wannier-Mott excitons (though intermediate behaviour for Cs2 SnCl6 ). Figure 2: Optical absorption spectra of Cs2 TiX6 , calculated with hybrid DFT (no exciton binding), G0 W0 +BSE (exciton binding) and compared to experiment. Figure 3: Optical absorption spectra of Cs2 SnX6 , calculated with hybrid DFT (no exciton binding), G0 W0 +BSE (exciton binding) and compared to experiment. Table 1: Calculated electronic bandgaps Eg (lowest energy vertical excitations), using G0 W0 with and without solving BSE (exciton binding). Formally the difference Δ|EGW – EGW+BSE | is defined as the exciton binding energy Eb . J. Phys. Chem. Lett. 2022, 13, 10965–10975