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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.

For other research articles and updates, check out my website at:
https://seankavanagh.com/

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_
    [email protected] 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.

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