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
   

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