We report a study on the optical properties of the layered polymorph of vacancy-ordered triple perovskite Cs3Bi2Br9. The electronic structure, determined from density functional theory calculations, shows the top of the valence band and bottom of the conduction band minima are, unusually, dominated by Bi s and p states, respectively. This produces a sharp exciton peak in the absorption spectra with a binding energy that was approximated to be 940 meV, which is substantially stronger than values found in other halide perovskites and, instead, more closely reflects values seen in alkali halide crystals. This large binding energy is indicative of a strongly localized character and results in a highly structured emission at room temperature as the exciton couples to vibrations in the lattice

Bass, KK

Bradforth, SE

Buckeridge, J

Djurovich, PI

Estergreen, L

Melot, BC

Savory, CN

Scanlon, DO

Thompson, ME

UCL Discovery

Vibronic Structure in the Room TemperaturePhotoluminescence of the Halide PerovskiteCs3Bi2Br9Kelsey K. Bass,† Laura Estergreen,† Christopher N. Savory,‡ John Buckeridge,‡David O. Scanlon,‡,¶ Peter I. Djurovich,† Stephen E. Bradforth,† Mark E.Thompson,† and Brent C. Melot∗,†Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA,University College London, Kathleen Lonsdale Materials Chemistry, 20 Gordon Street, LondonWC1H 0AJ, United Kingdom, and Diamond Light Source Ltd., Diamond House, Harwell Scienceand Innovation Campus, Didcot, Oxfordshire OX11 0DE, United KingdomE-mail: melot@usc.edu∗To whom correspondence should be addressed†Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA‡University College London, Kathleen Lonsdale Materials Chemistry, 20 Gordon Street, London WC1H 0AJ,United Kingdom¶Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot, OxfordshireOX11 0DE, United Kingdom1AbstractWe report a study on the optical properties of the layered polymorph of vacancy-orderedtriple perovskite Cs3Bi2Br9. The electronic structure, determined from Density FunctionalTheory calculations, shows the top of the valence band and bottom of the conduction bandminimum are, unusually, dominated by Bi-s and Bi-p states respectively. This produces asharp exciton peak in the absorption spectra with a binding energy that was approximated tobe 940 meV, which is substantially stronger than values found in other halide perovskites and,instead, more closely reflects values seen in alkali halide crystals. This large binding energy isindicative of a strongly localized character and results in a highly structured emission at roomtemperature as the exciton couples to vibrations in the lattice.2Interest in developing new solution processable light-absorbing layers for solar panels hassurged since the first reports on the perovskite CH3NH3PbI3.1 Despite the extremely high pho-tovoltaic efficiency these materials have demonstrated,2–4 the potential environmental and public-health consequences that could result from the wide-scale distribution of solar panels based onwater soluble compounds containing Pb presents a huge hurdle to the eventual commercializa-tion of perovskite solar cells.5 To address these concerns, perovksites based on Sn2+ have beensuggested as less-toxic alternatives, but in most cases these analogues have proven too unstabletowards oxidation to be realistic replacements.6Figure 1: Room temperature crystal structure of Cs3Bi2Br9. The gray atoms are Bi, blue atoms areBr, and purple atoms are Cs.More recently, emphasis has shifted away from maintaining a perfectly isostructural frameworktowards simply identifying new materials that can efficiently convert sunlight into electrical charge.In this respect, bismuth halides offer a promising new avenue because their strong absorption co-efficients make them very effective at capturing the solar spectrum.7 Unfortunately, the trivalentoxidation state of Bi ions preclude their direct incorporation into the prototypical ABX3 perovskitestructure. Instead, structural derivatives like double and triple perovskites, which partially sub-stitute some of the bismuth with lower valent metals or vacancies, offer the best opportunity toevaluate the photophysical properties of Bi-based absorbing layers.8–11Here, we investigate the optical properties of Cs3Bi2Br9, a layered form of the vacancy-ordered3perovskites, which can be viewed as a tripling of the traditional perovskite unit cell with only twothirds of the octahedral positions fully occupied. The remaining octahedral sites remain vacantand segregate so as to produce corrugated layers of corner-sharing BiBr6 octahedra as illustratedin Figure 1. The resulting local coordination environment of the octahedra is irregular and exhibitsa trigonal distortion that produces three short and three long Bi-Br bonds with the shorter linkagesshared between neighboring octahedra in each layer.Previous work on Cs3Bi2Br9 found no evidence for photoluminescence above liquid heliumtemperatures aside from a weak red emission that quenched above 160 K. The authors attributedthis signal to small amounts of oxygen defects resulting from the use of a Bi2O3 precursor.12,13To avoid these potential problems, Cs3Bi2Br9 was prepared by dissolving BiBr3 and CsBr in hot,aqueous HBr. Bright yellow powders were collected on cooling through simple vacuum filtration.Thin films were prepared by spin casting a 40 wt% solution of the powder dissolved in dimethyl-sulfoxide (DMSO). Considering the recent reports that CH3NH3PbI3 and other halide perovskitestend to exhibit large concentrations of vacancies in halide sublattice,14 the as-cast films were sub-sequently annealed in a Br2-rich atmosphere in order to mitigate any influence from defects onthe intrinsic optical properties. The annealed films show a significant increase in the diffractedintensity compared with the as-cast films [see Figure 3 (a)], which suggests a substantial improve-ment to crystal quality with no appreciable change to particle shape or size during the process asdemonstrated by the SEM images in Figure 3 (b) and (c).The X-ray diffraction pattern of the annealed films indicates the crystallites are highly orientedwith the (00l) facets exposed parallel to the plane of the substrate. Films prepared in this wayexhibited a strong absorption and, in contrast to previous reports, a weak blue emission at roomtemperature as shown in Figure 3 (b) and (c). The onset of optical absorption for the films agreeswell with the diffuse reflectance data collected on the polycrystalline powder, which shows a directonset of optical absorption at 2.65 eV (468 nm) (see SI Figure 2). The annealed films display asharp exciton peak (λmax=440 nm) when measured in transmission [Figure 3 (b)], similar in shapeto features observed in layered organic-inorganic hybrid perovskites.15–17 To a first approximation,410 20 30 40 50 60 702θ (deg) [λ = 1.79812Å]050100150200250Counts as-castannealed(b)1μm 1μm(c)(a)Figure 2: (a) Comparison of the X-ray diffraction patterns for the as-cast (black) films of Cs3Bi2Br9and those annealed in Br2 atmosphere (red). SEM images of the (b) as-cast and (c) annealed films,showing no change in the morphology of the particles.the binding energy of the electron-hole pair can be estimated as the difference between the maximaof the exciton peak and the leading edge of the subsequent plateau, which for Cs3Bi2Br9 is roughly940 meV. Compared to the 50 meV exciton binding energy for CH3NH3PbI3 18 or the 200-300 meVseen for similar layered perovskites,19,20 this represents a very strongly localized state that is morecommonly found in wide bandgap alkali halides like NaCl.21Although the photoluminescence was very weak, effectively invisible to the naked eye, it stillproved possible to resolve five well-defined peaks at 463 nm, 468 nm, 473 nm, 481 nm, and 492 nm[Figure 3 (c)]. It is highly unusual to observe this kind of structured emission at room temperaturefor non-molecular solids, but is well known to occur at liquid helium temperatures in high puritysemiconductors.22 At these low temperatures, the thermal energy available is insufficient to dis-sociate the electron-hole pair and, as a consequence, the excitons have more time to couple withphonons in the lattice during the recombination process.As seen from SI Figure 3, the phonon replicas in the emission spectra show no systematicspacing in energy and therefore seem to suggest that the exciton couples to more than one phonon.5200 300 400 500 600 700 800wavelengthsxnmc00.20.40.60.81NormalizedsAbsorption450 460 470 480 490 500 510wavelengthsxnmcsEmissionsInensitysxarb.sunitsc[λexc =s330snm]xacxbcFigure 3: (a) absorption and (b) emission (λexc=330 nm) spectra of film of Cs3Bi2Br9 after anneal-ing in a Br2 atmosphere.Similar luminescent properties have been reported when small concentrations of absorbing ionscontaining a pair of s2 electrons (such as Bi3+, Sb3+, or Pb2+) were substituted into non-absorbingmatrices like Cs2NaYCl6.23 In this dilute limit, the vibronic structure was attributed to a pseudosecond order Jahn-Teller distortion where the Bi ions shifted away from the center of the regularoctahedra to create one short and one long axial bond length.24 These previous reports foundseveral sharp emission lines on top of a broader band, much like in the spectra seen here forCs3Bi2Br9, albeit only at liquid helium temperatures.Γ A L M K H A-6-4-20246Energy (eV)ΓFigure 4: HSE06+SOC DFT band structure of the room temperature form, P3m1, of Cs3Bi2Br9.6Density Functional Theory (DFT) calculations were employed to further elucidate the natureof the electronic excitations. The band structure, shown in Figure 4, reveals the existence of a lowlying 2.52 eV indirect transition from the Γ point to A as well as a slightly larger direct gap of2.64 eV. The shape of the experimental absorption spectra, which is in close agreement with thecalculated spectra (see SI Figure 4), suggests that, despite the presence of a lower lying indirectexcitation, the primary mechanism for optical excitation occurs at the Γ-point. The weak emissionintensity seems consistent with relaxation from the indirect gap and is expected as consequence ofthe excitons coupling to one or more phonons to emit a photon.Projecting the atomic orbital character onto the bands reveals that the top of the valence bandconsists of an admixture of Br 4p and Bi 6s orbitals with the bottom of the conduction bandconsisting primarily of Bi p (SI Figures 5-8). Thus it appears the excited electron-hole pair isstrongly localized on Bi centers in the lattice, and has a pronounced Frenkel (localized) character,rather than the typical Wannier (delocalized) behavior typical of semiconductors.(b) (c)(a)Wave vector-10123456Frequency (THz)Γ A ΓL M K H AFigure 5: (a) Phonon band structure for the room temperature trigonal structure of Cs3Bi2Br9.Local bismuth coordination environment in the room temperature (a) and low temperature (b)structure form of Cs3Bi2Br9The calculated phonon dispersion curve also offers in-sight into why the recombination of theexciton shows the amount of structure that it does. As seen in Figure 5 (a), two soft modes (one7of which is doubly degenerate) associated with a rocking motion of the BiBr6 octahedra exist atthe zone-centre, which suggests the structure is unstable with respect to some energy-loweringstructural distortion. This kind of dynamical instability is in keeping with previous temperature-dependent structural studies that identified a phase transition below 95K that distorts the structureinto a monoclinic phase.25 The symmetry of these unstable modes (Eg and A1u) are consistent witha distortion from the trigonal to monoclinic form that results from a rocking motion of bromine ionsto produce an off-centering of the Bi within the octahedral cage as illustrated in Figure 5 (b) and (c).These Eg and A1u modes are replicated at the zone-boundary at A, but alternating in phase betweenthe layers, further in keeping with the doubled periodicity along c in the monoclinic phase. It,therefore, appears that optical excitation of the films triggers a similar Jahn-Teller distortion of theBiBr6 octahedra in the same way as was previously observed for isolated Bi ions.24 An imaginarymode of Bg symmetry exists at the zone-boundary q-points M, corresponding to a rotation of theBiBr6 octahedra about the [211¯] direction, and L, consisting of the same rotation with alternatingphase along c.Unlike other halide perovskites, Cs3Bi2Br9 exhibits unusually strong exciton-phonon coupling.In this aspect, the strongly localized electron-hole pairs created in Cs3Bi2Bi9 are similar to excitonsfound in organic chromophores.26 This difference offers a unique counter-example to the weaklybound excitons in CH3NH3PbI3 that are easily dissociated into free carriers in photovoltatic cells.27The unusual optical properties of Cs3Bi2Br9 make it an interesting new platform to study theinterplay of excitons with lattice vibrations in this new class of light-absorbing materials and offersfundamental insight into how to design new, less-toxic solution processable halide perovskites. Inorder for materials like Cs3Bi2Br9 to find utility in photovoltaic devices, future work will focus ondeveloping structural or compositional modifications that decrease the exciton binding energy andreduce the band gap in order to increase the amount of light collected in the solar spectrum.8AcknowledgementsComputational work in this article used the UCL Legion HPC Facility (Legion@UCL) and theArcher UK National Supercomputing Service, which was accessed through the UK’s HEC Mate-rials Chemistry Consortium, funded by EPSRC (EP/L000202). CNS is grateful to the Departmentof Chemistry at UCL for the provision of a DTA studentship. DOS acknowledges support fromthe SUPERSOLAR Solar Energy Hub (EP/J017361/1) for the provision of a flexible funding callaward and membership of the Materials Design Network.Supporting Information AvailableThe diffraction and optical properties of the unannealed films, scanning electron micrographs,direct comparisons between the optical properties of the powder and films, as well as the projecteddensities-of-state for each elements can be found in the supporting information. This material isavailable free of charge via the Internet at http://pubs.acs.org/.9References(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Journal of the American Chemical Society2009, 131, 6050–6051.(2) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.;Yang, Y. Science 2014, 345, 542–546.(3) Jeon, N. J.; Lee, H. G.; Kim, Y. C.; Seo, J.; Noh, J. H.; Lee, J.; Seok, S. I. 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Journal of Applied Physics 2003, 93, 3605–3614.(27) Herz, L. M. Annual Review of Physical Chemistry 2016, 67.12Graphical TOC Entry200 400 600 800wavelengthE3nm]00.20.40.60.81NormalizedAbsorption460 480 500wavelengthE3nm]EmissionEInensity3arb.Eunits][λexc =E330Enm]The unusual electronic structure of Cs3Bi2Br9 produces a sharp exciton peakin the absorption spectra with a binding energy that was approximated to be940 meV, which is substantially stronger than values found in other halide per-ovskites and, instead, closely reflects the behavior of alkali halide crystals.13

Vibronic Structure in Room Temperature Photoluminescence of the Halide Perovskite Cs3Bi2Br9

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Vibronic Structure in Room Temperature Photoluminescence of the Halide Perovskite Cs3Bi2Br9

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