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Frenkel Excitons in Vacancy-Ordered Perovskites (Cs₂MX₆)

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

Seán R. Kavanagh

March 19, 2023
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  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_
    [email protected] 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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