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/

Transcript

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.
   

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