From 0D Cs3Bi2I9 to 2D Cs3Bi2I6Cl3: Dimensional Expansion

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From 0D CsBiI to 2D CsBiICl: Dimensional Expansion Induces Direct Bandgap but Enhances Electron-Phonon Coupling Kyle M McCall, Constantinos C. Stoumpos, Oleg Y. Kontsevoi, Grant C. B. Alexander, Bruce W. Wessels, and Mercouri G. Kanatzidis Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00636 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 11, 2019

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Chemistry of Materials

From 0D Cs3Bi2I9 to 2D Cs3Bi2I6Cl3: Dimensional Expansion Induces Direct Bandgap but Enhances Electron-Phonon Coupling Kyle M. McCall,†,‡ Constantinos C. Stoumpos,†,§ Oleg Y. Kontsevoi,§ Grant C. B. Alexander,† Bruce W. Wessels,‡ Mercouri G. Kanatzidis*,† †Department

of Chemistry, ‡Department of Materials Science and Engineering, §Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, United States ABSTRACT: Alternative all-inorganic halide perovskites are sought to replace the hybrid lead halide perovskites due to their increased stability. Here, the (111)-oriented defect perovskite family A3M2X9 based on trivalent M3+ is expanded through the use of mixed halides, resulting in Cs3Bi2I6Cl3. This compound shares the (111)-oriented 2D bilayer structure of α-Cs3Sb2I9 (space group P-3m1), with Cl occupying the bridging positions of the bilayers and I in the terminal sites, in contrast to the parent compound Cs3Bi2I9 which consists of 0D molecular [Bi2I9]3- dimers. The increased dimensionality induces a direct band gap as calculated by DFT, but has an absorption edge of 2.07 eV, nearly identical to the indirect band gap compound Cs3Bi2I9. Intriguingly, there is a remarkable lack of Cl orbital contribution to the band edge states of Cs3Bi2I6Cl3, despite Bi-Cl bonds binding all octahedra together. This highlights the importance of interlayer interactions in the defect perovskite family, which enhances the effective dimensionality of these 2D and 0D materials and may improve their optoelectronic performance. However, these changes in the excitonic absorption do not reflect free excitons, as Cs3Bi2I6Cl3 exhibits broad photoluminescence (PL) as a result of self-trapped excitons (STE), which appear to be universal in (111)-oriented defect perovskites.

INTRODUCTION Halide perovskites have impressive optoelectronic properties which have achieved solar cell efficiencies greater than 22% in just a few years.1-2 The 3D perovskite structure has the formula AMX3, (A = Cs+, CH3NH3+ = MA+, or CH(NH2)2+ = FA+; M = Sn2+ or Pb2+; and X = Cl–, Br–, or I–,), and consists of corner-sharing octahedra MX6 with A cations in the voids. Initial works focused mainly on hybrid organicinorganic perovskites, led by the flagship compound MAPbI3.2 However, the interest has now expanded to alternative perovskites,3-4 including hybrid 2D perovskites such as (100)-oriented Ruddlesden-Popper perovskites,5-6 and the (110)-oriented perovskites with self-trapped exciton (STE) photoluminescence (PL).7-8 Further extensions incorporate other main-group metals that retain the ns2 lone pair, such as Sb3+ and Bi3+,9 in allinorganic compounds for increased stability. The double perovskites A2MIMIIIX6 permit the substitution of a trivalent MIII in the 3D perovskite structure by incorporating a suitable MI cation,10-11 while the defect perovskites A3MIII2X9 compensate the additional M-site charge with ordered vacancies which reduce the structural dimensionality to form (111)-oriented 2D bilayers of corner-connected octahedra as in α-Cs3Sb2I9 (space group P-3m1) or the 0D bioctahedral dimers of Cs3Bi2I9 (P63/mmc).12-13 Our previous work on the iodide defect perovskites A3M2I9 (A = Cs, Rb; M = Bi, Sb) reported that these compounds are

indirect gap semiconductors and have strong electronphonon coupling that results in broadband STE PL emission.14 We also showed that self-trapping of charge carriers may be responsible for low mobility measured via α-particle response.15 Interestingly, the 0D Cs3Bi2I9, which contains discrete molecular [Bi2I9]3- clusters and is not a proper perovskite, showed the best charge transport characteristics despite its lower dimensionality. Given the importance of structural and electronic dimensionality,16 we sought to increase the dimensionality of Cs3Bi2I9 through halide mixing, which has been quite effective in tuning the structures and properties of halide compounds and halide perovskites in particular. Mixing nearby halides (Br-I or Cl-Br) can tune the band gaps of perovskites with compatible structures,17-20 while mixing dissimilar Cl and I has dramatic effects on optoelectronic properties21-22 and can even force formation of new structures or compounds, such as the recently discovered Cs2PbI2Cl2.23-25 The 2D defect perovskite Cs3Bi2I6Br3 was very recently reported to have nearly complete ordering of I and Br atoms.26 Here we report the newest member of the (111)-oriented defect perovskite family, the 2D Cs3Bi2I6Cl3. This structure adopts the 2D bilayer structure of Cs3Bi2Br9 with I atoms capping the bilayers and Cl in the bridging positions. The absorption edge of this compound is unchanged from the 0D parent Cs3Bi2I9, though the prominent excitonic peak of Cs3Bi2I9 is absent in the 2D structure. Density functional

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theory calculations show that the band gap of Cs3Bi2I6Cl3 is now direct and that Cl does not contribute to the band edge states, despite its place as the bridging ligand which binds the octahedral bilayers together. Photoluminescence measurements reveal self-trapped exciton emission characterized by an enhancement in the electron-phonon coupling, demonstrating that dimensional enhancement cannot free the characteristic self-trapped exciton in this defect perovskite family.

EXPERIMENTAL METHODS Reagents. The halide salts RbCl (99.8%, Sigma Aldrich), CsCl (99.999%, Alfa Aesar), and BiI3 (99.999%, Sigma Aldrich) were used as purchased without additional purification. A3Bi2I6Cl3 (A = Cs, Rb) synthesis. The A3Bi2I6Cl3 compounds were synthesized via a stoichiometric melt reaction of ACl with BiI3. In a typical reaction, 4.5 mmol of ACl (0.76 g and 0.54 g for CsCl and RbCl, respectively) was mixed thoroughly with 3 mmol of BiI3 (1.769 g) and placed into a fused silica ampule (10 mm o.d.), which was evacuated to 3 x 10-3 mbar and flame-sealed. The ampule was placed in a tube furnace which was heated in ten hours to 750 ºC, held for ten hours, and cooled over ten hours. The resulting A3Bi2I6Cl3 ingots were dark cherry red and polycrystalline, though small (1 mm) single crystals could be obtained by cleaving along the lamellar surfaces. The resulting crystals are air-stable but will decompose in water over a matter of hours. Single crystal growth. The vertical Bridgman method was used to grow large single crystals of Cs3Bi2I6Cl3 for optoelectronic characterization. Stoichiometric amounts of the binary halides CsCl and BiI3 were mixed thoroughly and flame-sealed under 3 x 10-3 mbar vacuum in an ampule (10 mm o.d.) with a pointed tip. The ampoule was placed in a two-zone Bridgman furnace with the hot zone set at 725 ºC and the cold zone at 300 ºC. The ampule was held stationary for 12 hours to ensure a full melt, then dropped at a rate of 1 mm/h for crystal growth until it reached 300 ºC, when it was annealed at 300 ºC for several days and cooled slowly to relieve thermal stress. The resulting Cs3Bi2I6Cl3 ingots were dark cherry red and had several large (maximum size of 18 x 7 x 1 mm3) single crystalline grains. The obtained crystals cleaved quite easily along the layer, producing clean surfaces which required no mechanical polishing. Physical characterization. High-resolution synchrotron PXRD patterns were collected from powdered samples using beamline 11-BM at the Advanced Photon Source, Argonne National Laboratory. The samples were ground into fine powder and sealed in Kapton capillaries. The beamline operated at an average wavelength of 0.412736 Å in transmission geometry, with the sample rotating at 5400 rpm to minimize the effects of preferred orientation. 12 discrete detectors (spaced 2º in 2 apart) were swept from 0 to 6º 2 to cover a 28º 2 range, with data points acquired in 0.001º steps at a scan speed of 0.1º/s. Le Bail analysis was carried out using the Jana2006 crystallography package.27 Differential thermal analysis was conducted on powdered

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samples (~50 mg) flame-sealed in evacuated fused silica ampoules using a NETZSCH STA 449 F3 Jupiter operating in TGA-DTA mode. Samples were measured in the range of 25 ºC to 700 ºC with heating and cooling rates of 10 ºC/min, and a similarly prepared Al2O3 sample was used for comparison. Single crystal X-ray diffraction. Single crystals were selected for X-ray diffraction and mounted to glass fiber tips with superglue, all measurements were conducted at room temperature. Cs3Bi2I6Cl3 was measured on a Bruker Duo diffractometer (with Mo Kα radiation, λ = 0.71073 Å) with an Apex II CCD area detector. The crystal-to-detector distance was 60 mm, and the exposure time was 20 s/frame. Data collection and integration were performed using the Bruker Apex3 software suite. Rb3Bi2I6Cl3 was measured on a STOE IPDS II diffractometer (Mo Kα radiation, λ = 0.71073 Å) operated at 50 kV and 40 mA. The data collection, integration, and absorption correction were carried out using the STOE programs X-Area, X-Red, and XShape, respectively. The crystal structures were solved using the ShelXT package28 operating in JANA200627 (Rb3Bi2I6Cl3) or Olex229 (Cs3Bi2I6Cl3), and the CIF files prepared using EnCIFer.30 Optical characterization. Diffuse-reflectance measurements were conducted from 200 to 2000 nm on powdered A3Bi2I6Cl3 samples using a Shimadzu UV-3600 PC double-beam, double-monochromator spectrophotometer, with BaSO4 as a non-absorbing reference. The generated reflectance data was transformed to absorbance via the Kubelka-Munk equation31 α/S = (1 – R)2/2R, where R is the reflectance and α and S are the absorption and scattering coefficients, respectively, and the band gap is estimated by extrapolation of the linear region. Raman measurements at room temperature were conducted on A3Bi2I6Cl3 single crystals with a confocal Horiba LabRAM HR Evolution spectrometer. The excitation wavelength for Raman measurements was 785 nm (1.58 eV) and the laser intensity was 100 mW. Room-temperature photoluminescence measurements were conducted using the same confocal Horiba LabRAM HR Evolution spectrometer with an excitation wavelength of 473 nm (2.62 eV). The laser intensity ranged from 2.5 mW to 12.5 mW but no reproducible photoluminescence response was observed, signal could only be obtained above 10 mW and resulted in beam damage to the crystal. Low temperature photoluminescence was measured on Cs3Bi2I6Cl3 single crystals using a previously described set-up.14 In short, a 405nm laser was used for excitation with the power (tuned both digitally and using neutral density filters) between 0.05 mW and 25 mW, and the temperature was controlled by a cryostat between 13 K and 150 K. Electronic band structure calculations. First-principles electronic band structure calculations were carried out within the density functional theory (DFT) formalism using the Projector Augmented Wave method32 implemented in the Vienna Ab-initio Simulation Package.33-34 For exchangecorrelation function, the generalized gradient approximation (GGA) was employed within revised Perdew-Burke-Ernzerhof formalism for solids (PBEsol) and

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Chemistry of Materials the spin−orbit coupling (SOC) was included in the calculation.35 The energy cut off for plane wave basis was set to 350 eV and 7×7×5 k-point mesh was chosen for Brillouin zone (BZ) sampling. To obtain the ground states for the compound, the crystal structure, the lattice parameters and the positions of atoms in the cell were relaxed until the atomic forces on each atom were less than 0.01 eV/Å. The hole and electron effective mass components were obtained as the inverse of the eigenvalues of the tensor of second derivatives of the band dispersions calculated numerically using the finite difference method for valence band maximum and conduction band minimum bands, respectively.

RESULTS AND DISCUSSION In order to generate the layered 2D defect perovskite structure, we pursued the addition of Cl in the ratio of 1 Cl atom per 2 I atoms, targeting full substitution of the bridging iodides in the formula of Cs3Bi2I9. We hypothesized that the I-Cl-I anisotropy could induce the formation of the (111)-layered structure from the 0D cluster structure of Cs3Bi2I9 because the interposition of the smaller Cl atoms between the two Bi-sites strongly polarizes the octahedra, making them more “flexible” and allowing them to freely rotate about the trigonal axis. A 90º rotation would then bind the octahedra together, thus transforming the structure into the (111)-layer type (Figure 1a). The resulting heteroleptic mixed halide Cs3Bi2I6Cl3 was synthesized via direct solid-state reaction of CsCl and BiI3 at 750 ºC.

Figure 1. Cs3Bi2I6Cl3 structure: (a) Structure comparison of 0D Cs3Bi2I9 and 2D Cs3Bi2I6Cl3 viewed down the b-axis. The latter is a bilayered defect perovskite structure (b) Cs3Bi2I6Cl3 viewed along the c-axis, and (c) coordination environment of Bi3+ in [BiCl6/2I3]3- octahedron

Cs3Bi2I6Cl3 crystallizes in the trigonal space group P-3m1 with lattice parameters a = b = 8.243(2) Å and c = 10.021(2) Å (Figure S1a, Tables S1-5). The 2D structure of Cs3Bi2I6Cl3

is isostructural to those of α-Cs3Sb2I9 and Cs3Bi2Br9,36 consisting of corner-connected [BiCl6/2I3]3- octahedra. Unlike the parent structure where each octahedron has homoleptic coordination, in Cs3Bi2I6Cl3 each octahedron is comprised of three capping I atoms that terminate the layers and three bridging Cl atoms that connect to octahedra in the opposite layer (Figure 1a,b). This ordering is due to the large size difference between Cl (ionic radius 1.81 Å) and I (2.20 Å),37 a similar ordering occurs in Cs2PbI2Cl2 in which Cl occupies the inter-octahedral positions with I at the terminal sites of each octahedron.24 A recent study on the Br-I series Cs3Bi2IxBr9-x also found that Br preferentially occupied the bridging site, particularly in the composition Cs3Bi2I6Br3, though some halide mixing was observed due to the reduced size difference of Br (1.96 Å)37 and I.26 The Bi-X bond lengths in Cs3Bi2I6Cl3 are nearly identical (2.9034(11) Å for I, 2.9262(6) Å for Cl) due to the shared Cl atom which lengthens the Bi-Cl-Bi bonds (Figure 1c). The resulting length difference (0.0228 Å) is less than 10% of the difference in the same bond lengths in Cs3Bi2Br9 (2.979 Å for bridging and 2.713 Å for terminal Bi-Br bonds, difference of 0.266 Å),38 shifting the Bi atom significantly closer to the octahedral center. Cs3Bi2I6Cl3 melts incongruently, with two thermal events observed via differential thermal analysis at 520 ºC and 548 ºC (Figure S1b), the first of which likely corresponds to dissociation of BiI3 as found in the case of Cs3Bi2I9,14 but this does not prohibit growth of large single crystals via the Bridgman method (Figure S1c). The Raman spectrum of Cs3Bi2I6Cl3 is characteristic of the 2D structure rather than the 0D structure of Cs3Bi2I9, which exhibits extra vibrational modes near 100 cm-1 related to the different frequencies of the bridging and terminal Bi-I stretches of the complex [Bi2I9]3- bioctahedral unit.14 Instead, the Raman spectrum of Cs3Bi2I6Cl3 is similar to that of Cs3Sb2I9, with two major peaks at 134 cm-1 (16.6 meV) and 154 cm-1 (19.1 meV) corresponding to the asymmetric and symmetric stretching modes of the octahedra, respectively (Figure 2a).14, 39 Additionally, there are several lower energy modes below 100 cm-1 (12.4 meV), tentatively assigned to bending transitions, and one high-energy mode at 251 cm-1 (31.1 meV) that likely corresponds to a phonon mode involving the lighter Cl atom.

Figure 2. (a) Raman spectrum under 785 nm excitation and (b) UV-Vis absorbance spectra (derived from diffuse reflectance measurements) of Cs3Bi2I6Cl3, with Cs3Bi2I9 for comparison.

Excitonic effects dominate the band-edge of the A3M2I9 defect perovskites, obscuring the electronic band gap in

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these materials.14 In particular, the absorption edge of Cs3Bi2I9 is at 2.06 eV while the band gap is near 2.86 eV,40 as evidenced by the presence of an excitonic peak (which should lie below the band gap) at 2.6 eV (Figure 2b).14 Hence, excitonic effects must also be considered when evaluating the UV-Vis absorption of Cs3Bi2I6Cl3. The enhanced dimensionality of the 2D Cs3Bi2I6Cl3 should enhance orbital overlap and lower the band gap compared to the 0D Cs3Bi2I9,16 but the exciton binding energy should also be reduced due to the weakened quantum confinement, shifting the absorption edge to higher energy.41 The reduced oscillator strength of the excitonic absorption in Cs3Bi2I6Cl3 is apparent from the lack of the prominent excitonic peak, as in the 0D parent this peak lies far above the absorption edge and is strong enough to dominate the band-edge absorption. Hence the 0D character of the excitonic absorption of Cs3Bi2I9 is exchanged for the 2D excitonic absorption of Cs3Sb2I9 which is not clearly distinguishable from the electronic band gap,14 this may be the result of reduced exciton binding energy in Cs3Bi2I6Cl3. The prominent Cl-Bi-Cl bonding which bridges the octahedral connections should further open up the band gap relative to an all-iodine composition, as the Bi-I bonds are isolated to the terminal sides of each octahedron and should play a relatively small role at the band edge. In spite of all these effects, the ingot is deep red (Figure S1c), strikingly similar in color to Cs3Bi2I9. Accordingly, the absorption edge of the two compounds is nearly identical, with a value of 2.07 eV for Cs3Bi2I6Cl3 and 2.06 eV for Cs3Bi2I9 (Figure 2b).14 This is also consistent with the 2.05 eV band gap of Cs3Bi2I6Br3.26 This seeming lack of absorption edge shift motivated a first-principles calculation of the electronic structure, to better disentangle these effects. Density functional theory (DFT) calculations of the electronic band structure of Cs3Bi2I6Cl3 were carried out using the revised Perdew-Burke-Ernzerhof functional for solids (PBEsol) with spin-orbit coupling (SOC) included.35 The results show a direct band gap of 1.38 eV at the A point (Figure 3a), in contrast to the indirect gaps of the A3M2I9 defect perovskites.15 The calculated gap of 1.38 eV is underestimated relative to the absorption edge of 2.07 eV, but their difference is quite similar to that of the isostructural Cs3Sb2I9 using the same functional (1.2 eV versus 1.89 eV absorption edge).15 This similarity with an all-iodine structure implies that chlorine in Cs3Bi2I6Cl3 does not influence the band edges, despite the large role expected from its coordination as the only bridging ligand in the structure.

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Figure 3. (a) Electronic band structure and (b) projected density of states of Cs3Bi2I6Cl3.

The projected density of states more clearly shows the lack of Cl orbital role (Figure 3b), as the valence band maximum (VBM) of Cs3Bi2I6Cl3 is dominated by I p states with some contribution from I p-Bi s hybridized orbitals, while the conduction band minimum (CBM) is comprised of strongly hybridized Bi p and I p states. Importantly, there are no Cl states at the CBM and only a small contribution near the VBM. The near lack of Cl orbital contribution goes against the intuitive view of bonding in this structure, as every inter-octahedral connection is a Bi-Cl-Bi bridging bond. Instead, the electron transport is dominated by the I p Bi p antibonding orbitals through terminal I sites, and the Cl atoms effectively disrupt the transport. Calculations with the same structure with I in place of Cl (“Cs3Bi2I6I3”), show that out-of-plane electron and hole effective masses are reduced significantly, from 0.63 me to 0.39 me for electrons and from 1.8 me to 0.41 me for holes (see Supporting Information for further discussion) as Cl is not present to amplify the anisotropy of transport. This highlights the importance of interlayer interaction, pointing to significant orbital overlap within each layer and across the interlayer space. The interlayer space between octahedral planes is quite small at 3.37(1) Å and the nearest interlayer I-I distance (4.11(1) Å along the diagonal) is slightly shorter than the I-I distance (4.17(1) Å) for I atoms in the same octahedra, close enough that interlayer interactions should be present. Similar effects should exist for Cs3Bi2I9, which also possesses an ordered vacancy layer along the c-axis separating the layers of 0D dimers, as the interlayer spacing (3.41(1) Å) and interlayer I-I distance (4.16(1) Å) are essentially identical to those of Cs3Bi2I6Cl3, resulting in Bi-Bi distances of 8.15(1) Å for both compounds.38 Such interactions belie a higher effective electronic dimensionality than expected from the quasi-2D and 0D crystal structures,16 and may explain why the 0D molecular Cs3Bi2I9 maintains a reasonable electron mobility of 4.3 cm2V-1s-1 despite its lack of inter-octahedral connections between molecular dimers.15 To further explore the optoelectronic properties of Cs3Bi2I6Cl3, photoluminescence at low temperatures was measured under 405 nm excitation (Figure 4). The emission is nearly identical to that of Cs3Sb2I9,14 with a single broad peak at 1.94 eV with a low-energy tail which is well-fit by an exponentially modified Gaussian (Figure S3a). This shape is characteristic of phonon-assisted STE recombination, which consists of a series of phonon replica peaks with intensities following a Poisson distribution, which results in

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Chemistry of Materials a low-energy tail corresponding to high numbers of coupled phonons.14 At 13 K, the PL peak maximum does not shift under a wide range of excitation intensities from 0.05 mW to 25 mW (Figure 4a), and has a power law coefficient of 0.935 ± 0.055 (Figure S3b) similar to that of Cs3Sb2I9 (0.89).14

Figure 4. (a) Excitation intensity-dependent photoluminescence of Cs3Bi2I6Cl3 at 13 K and (b) temperaturedependent PL of Cs3Bi2I6Cl3 at 3 mW.

Temperature-dependent PL of Cs3Bi2I6Cl3 was measured from 13 K until it quenched around 80 K (Figure 4b), with a corresponding redshift in the peak maximum from 1.94 eV to 1.92 eV (Figure S3c). The evolution of the peak width with temperature, as defined by the full width at half maximum (FWHM), is well-described by both the Toyozawa model42 used to describe the A3M2I9 perovskites (Equation S2)14 as well as the Fröhlich longitudinal optical (LO) phonon broadening model developed by Rudin et al. and applied to the 3D hybrid perovskites by Wright et al. (Equation S3).43-44 A least-squares fit of the experimental FWHM using the Toyozawa model (Equation S2) finds an effective phonon energy Eph of 4.0 ± 0.4 meV with a high Huang-Rhys parameter S = 212 ± 12 (Figure S3d), which is a measure of the electron-phonon coupling that describes the mean number of phonons involved in the STE transition. This value is indicative of an extremely strong electronphonon coupling similar to that of the alkali halides, for instance LiI has a Huang-Rhys parameter of 12045 while those of the A3M2I9 compounds are between 21.2 and 79.5.14 A recent report finds a simulated value of 188 in the double perovskite Cs2NaInCl6,46 indicating that such a strong electron-phonon coupling is not unreasonable. Indeed, substituting I for the more electronegative Cl should make this compound more polar, enhancing the electron-phonon coupling compared to Cs3Bi2I9. Similarly, fitting Equation S3 to the FWHM produces excellent agreement (Figure S3d), with derived values of 8.52 ± 0.94 meV for the LO phonon energy ELO and a high coupling constant LO of 291.8 ± 61.9 meV. The phonon energy of 8.52 meV is in agreement with the broad Raman peaks observed near 66 cm-1 (8.2 meV). Furthermore, the high coupling constant of 291.8 meV dwarfs those of the 3D hybrid perovskites APbX3 (A = MA,FA; X = Br,I; with LO of 40-61 meV)44 and is higher than the 226 meV LO value of the double perovskite Cs2AgBiBr6,47 highlighting the strength of the electron-phonon coupling in Cs3Bi2I6Cl3. Together, these findings support the assignment of the

broad PL emission of Cs3Bi2I6Cl3 to phonon-assisted recombination of self-trapped excitons. STEs may be a universal feature of the (111)-oriented defect perovskites, as they are found here and in the iodide defect perovskites A3M2I9,14 despite differences in the electronic band structure.15 These compounds lack the dynamic disorder of the 3D perovskites,48 which encourages the formation of protective large polarons49 around mobile carriers. The defect perovskites instead have self-trapping of charge carriers due to the formation of small polarons50 which strongly localize charges. This propensity for small polaron formation may be a result of either the increased charge of the M3+ cation, which encourages localization of free electrons to the highly positive M site, or due to the constraints imposed by the unique connectivity of the (111)-oriented 2D structure. The bilayered defect perovskite structure has no straight lines connecting more than two octahedra, giving charge carriers continual opportunities to scatter by requiring them to turn within the bilayers or pass through the interlayer space. This dimensional enhancement is not limited to Cs3Bi2I9, as a previous study of MA3Sb2I9 showed that Cl addition could induce the 2D structure, however the doping level was below the 2:1 ratio required for complete halide substitution.23 Thus, we believe that MA3Sb2I9 and MA3Bi2I9, which crystallize in the 0D dimer structure,51 can be similarly transformed into the 2D compounds MA3M2I6Cl3. A cursory examination of the Rb3Bi2I6Cl3 system confirms that the structure returns to the trigonal P-3m1 system from the parent monoclinic structure, though crystallographically the halide segregation is incomplete (Figure S4, Tables S6-10). Mixed halides may also help stabilize higher layer (111)-oriented 2D perovskites like the recently discovered Cs4CuSb2Cl12 and Cs4MnSb2Cl12.52-53

CONCLUSION The mixed halide Cs3Bi2I6Cl3, is an ordered halide variant of the 2D Cs3Bi2Br9 structure type rather than a solid solution version of the 0D Cs3Bi2I9 structure, presumably because the disparate size of the Cl and I atoms. The presence of Cl in Cs3Bi2I6Cl3 does not significantly impact the absorption edge, as the value of 2.07 eV is essentially identical to that of Cs3Bi2I9. While the increased dimensionality induces a direct band gap, excitonic absorption dominates the absorption edge and obscures the electronic band gap. The expected band gap decrease due to increased dimensionality appears to be offset by a reduced exciton binding energy, as evidenced by the absence of an excitonic peak above the absorption edge like that observed in 0D Cs3Bi2I9. DFT calculations show no band-edge contribution from Cl orbitals despite the inter-octahedral Bi-Cl bonding, highlighting the significance of interlayer interaction in the defect perovskites. Such interactions reduce the impact of the Cl addition and increase the effective dimensionality, which may enhance optoelectronic performance. Though the excitonic absorption changes from the 0D excitonic peak to the broader 2D excitonic absorption, the exciton is not free as Cs3Bi2I6Cl3 exhibits photoluminescence attributed to

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phonon-assisted recombination of self-trapped excitons with an extremely strong electron-phonon coupling. This work highlights an underexplored flexibility of the defect perovskites, and will guide future tuning of these compounds towards functional perovskites.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental methods, crystallographic tables, Cs3Bi2I6Cl3 effective mass calculations, analysis of Cs3Bi2I6Cl3 photoluminescence, and Rb3Bi2I6Cl3 characterization (PDF) Cs3Bi2I6Cl3 crystallographic information file (CIF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Present Addresses §Department of Materials Science and Technology, University of Crete, Heraklion, Greece

ORCID Kyle M. McCall: 0000-0001-8628-3811 Constantinos C. Stoumpos: 0000-0001-8396-9578 Grant C. B. Alexander: 0000-0002-6041-4737 Oleg Y. Kontsevoi: 0000-0001-6075-630X Bruce W. Wessels: 0000-0002-8957-7097 Mercouri G. Kanatzidis: 0000-0003-2037-4168

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the Department of Homeland Security ARI program under the grant 2014-DN-077-ARI08601. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This work made use of the IMSERC at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the State of Illinois and International Institute for Nanotechnology (IIN). This work made use of the SPID facility of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN.

REFERENCES 1. Stoumpos, C. C.; Kanatzidis, M. G., Halide Perovskites: Poor Man's High-Performance Semiconductors. Adv. Mater. 2016, 28 (28), 5778-5793. 2. Stranks, S. D.; Snaith, H. J., Metal-halide perovskites for photovoltaic and light-emitting devices. Nature Nanotechnology 2015, 10, 391.

Page 6 of 9

3. Niu, G.; Guo, X.; Wang, L., Review of recent progress in chemical stability of perovskite solar cells. J. Mater. Chem. A 2015, 3 (17), 8970-8980. 4. Saparov, B.; Mitzi, D. B., Organic–Inorganic Perovskites: Structural Versatility for Functional Materials Design. Chem. Rev. 2016, 116 (7), 4558-4596. 5. Stoumpos, C. C.; Cao, D. H.; Clark, D. J.; Young, J.; Rondinelli, J. M.; Jang, J. I.; Hupp, J. T.; Kanatzidis, M. G., Ruddlesden– Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors. Chem. Mater. 2016, 28 (8), 2852-2867. 6. Cao, D. H.; Stoumpos, C. C.; Yokoyama, T.; Logsdon, J. L.; Song, T.-B.; Farha, O. K.; Wasielewski, M. R.; Hupp, J. T.; Kanatzidis, M. G., Thin Films and Solar Cells Based on Semiconducting TwoDimensional Ruddlesden–Popper (CH3(CH2)3NH3)2(CH3NH3)n−1SnnI3n+1 Perovskites. ACS Energy Lett. 2017, 2 (5), 982-990. 7. Cortecchia, D.; Yin, J.; Bruno, A.; Lo, S.-Z. A.; Gurzadyan, G. G.; Mhaisalkar, S.; Bredas, J.-L.; Soci, C., Polaron self-localization in white-light emitting hybrid perovskites. J. Mater. Chem. C 2017, 5 (11), 2771-2780. 8. Hu, T.; Smith, M. D.; Dohner, E. R.; Sher, M.-J.; Wu, X.; Trinh, M. T.; Fisher, A.; Corbett, J.; Zhu, X. Y.; Karunadasa, H. I.; Lindenberg, A. M., Mechanism for Broadband White-Light Emission from Two-Dimensional (110) Hybrid Perovskites. J. Phys. Chem. Lett. 2016, 7 (12), 2258-2263. 9. Ganose, A. M.; Savory, C. N.; Scanlon, D. O., Beyond Methylammonium Lead Iodide: Prospects for the Emergent Field of ns2 Containing Solar Absorbers. Chem. Commun. 2016, 53 (1), 20-44. 10. McClure, E. T.; Ball, M. R.; Windl, W.; Woodward, P. M., Cs2AgBiX6 (X = Br, Cl): New Visible Light Absorbing, Lead-Free Halide Perovskite Semiconductors. Chem. Mater. 2016, 28 (5), 1348-1354. 11. Slavney, A. H.; Hu, T.; Lindenberg, A. M.; Karunadasa, H. I., A Bismuth-Halide Double Perovskite with Long Carrier Recombination Lifetime for Photovoltaic Applications. J. Am. Chem. Soc. 2016, 138 (7), 2138-2141. 12. Peresh, E. Y.; Sidei, V. I.; Zubaka, O. V.; Stercho, I. P., K2(Rb2,Cs2,Tl2)TeBr6(I6) and Rb3(Cs3)Sb2(Bi2)Br9(I9) perovskite compounds. Inorg. Mater. 2011, 47 (2), 208-212. 13. Chang, J.-H.; Doert, T.; Ruck, M., Structural Variety of Defect Perovskite Variants M3E2X9 (M = Rb, Tl, E = Bi, Sb, X = Br, I). Z. Anorg. Allg. Chem 2016, 642 (13), 736-748. 14. McCall, K. M.; Stoumpos, C. C.; Kostina, S. S.; Kanatzidis, M. G.; Wessels, B. W., Strong Electron–Phonon Coupling and SelfTrapped Excitons in the Defect Halide Perovskites A3M2I9 (A = Cs, Rb; M = Bi, Sb). Chem. Mater. 2017, 29 (9), 4129-4145. 15. McCall, K. M.; Liu, Z.; Trimarchi, G.; Stoumpos, C. C.; Lin, W.; He, Y.; Hadar, I.; Kanatzidis, M. G.; Wessels, B. W., α-Particle Detection and Charge Transport Characteristics in the A3M2I9 Defect Perovskites (A = Cs, Rb; M = Bi, Sb). ACS Photonics 2018, 5 (9), 3748-3762. 16. Xiao, Z.; Meng, W.; Wang, J.; Mitzi, D. B.; Yan, Y., Searching for promising new perovskite-based photovoltaic absorbers: the importance of electronic dimensionality. Mater. Horiz. 2017, 4 (2), 206-216. 17. Sutton, R. J.; Eperon, G. E.; Miranda, L.; Parrott, E. S.; Kamino, B. A.; Patel, J. B.; Hörantner, M. T.; Johnston, M. B.; Haghighirad, A. A.; Moore, D. T.; Snaith, H. J., Bandgap-Tunable Cesium Lead Halide Perovskites with High Thermal Stability for Efficient Solar Cells. Adv. Energy Mater. 2016, 6 (8), 1502458. 18. Mao, L.; Wu, Y.; Stoumpos, C. C.; Traore, B.; Katan, C.; Even, J.; Wasielewski, M. R.; Kanatzidis, M. G., Tunable White-light Emission in Single Cation Templated Three-layered 2D Perovskites (CH3CH2NH3)4Pb3Br10-xClx. J. Am. Chem. Soc. 2017, 139 (34), 1195611963.

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Page 7 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials 19. Kovalenko, M. V.; Protesescu, L.; Bodnarchuk, M. I., Properties and potential optoelectronic applications of lead halide perovskite nanocrystals. Science 2017, 358 (6364), 745. 20. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V., Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15 (6), 3692-3696. 21. Colella, S.; Mosconi, E.; Fedeli, P.; Listorti, A.; Gazza, F.; Orlandi, F.; Ferro, P.; Besagni, T.; Rizzo, A.; Calestani, G.; Gigli, G.; De Angelis, F.; Mosca, R., MAPbI3-xClx Mixed Halide Perovskite for Hybrid Solar Cells: The Role of Chloride as Dopant on the Transport and Structural Properties. Chem. Mater. 2013, 25 (22), 4613-4618. 22. Leblanc, N.; Bi, W.; Mercier, N.; Auban-Senzier, P.; Pasquier, C., Photochromism, Electrical Properties, and Structural Investigations of a Series of Hydrated Methylviologen Halobismuthate Hybrids: Influence of the Anionic Oligomer Size and Iodide Doping on the Photoinduced Properties and on the Dehydration Process. Inorg. Chem. 2010, 49 (13), 5824-5833. 23. Jiang, F.; Yang, D.; Jiang, Y.; Liu, T.; Zhao, X.; Ming, Y.; Luo, B.; Qin, F.; Fan, J.; Han, H.; Zhang, L.; Zhou, Y., ChlorineIncorporation-Induced Formation of the Layered Phase for Antimony-Based Lead-Free Perovskite Solar Cells. J. Am. Chem. Soc. 2018, 140 (3), 1019-1027. 24. Li, J.; Yu, Q.; He, Y.; Stoumpos, C. C.; Niu, G.; Trimarchi, G.; Guo, H.; Dong, G.; Wang, D.; Wang, L.; Kanatzidis, M. G., Cs2PbI2Cl2, All-Inorganic Two-Dimensional Ruddlesden-Popper Mixed Halide Perovskite with Optoelectronic Response. J. Am. Chem. Soc. 2018, 140 (35), 11085-11090. 25. Leblanc, N.; Mercier, N.; Zorina, L.; Simonov, S.; AubanSenzier, P.; Pasquier, C., Large Spontaneous Polarization and Clear Hysteresis Loop of a Room-Temperature Hybrid Ferroelectric Based on Mixed-Halide [BiI3Cl2] Polar Chains and Methylviologen Dication. J. Am. Chem. Soc. 2011, 133 (38), 14924-14927. 26. Hodgkins, T. L.; Savory, C. N.; Bass, K. K.; Seckman, B. L.; Scanlon, D. O.; Djurovich, P. I.; Thompson, M.; Melot, B., Anionic Order and Band Gap Engineering in Vacancy Ordered Triple Perovskites. Chem. Commun. 2019, 55, 3164-3167. 27. Petříček, V.; Dušek, M.; Palatinus, L., Crystallographic Computing System JANA2006: General features. Zeitschrift für Kristallographie - Crystalline Materials 2014, 229 (5), 345-352. 28. Sheldrick, G., SHELXT - Integrated space-group and crystal-structure determination. Acta Crystallogr. Sect. A 2015, 71 (1), 3-8. 29. Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H., OLEX2: a complete structure solution, refinement and analysis program. J. Appl. Crystallogr. 2009, 42 (2), 339-341. 30. Allen, F. H.; Johnson, O.; Shields, G. P.; Smith, B. R.; Towler, M., CIF applications. XV. enCIFer: a program for viewing, editing and visualizing CIFs. J. Appl. Crystallogr. 2004, 37 (2), 335-338. 31. Kortüm, G.; Braun, W.; Herzog, G., Prinzip und Meßmethodik der diffusen Reflexionsspektroskopie. Angew. Chem. 1963, 75 (14), 653-661. 32. Blöchl, P. E., Projector augmented-wave method. Phys. Rev. B 1994, 50 (24), 17953-17979. 33. Kresse, G.; Furthmüller, J., Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54 (16), 11169-11186. 34. Kresse, G.; Hafner, J., Ab initio molecular-dynamics simulation of the liquid-metal--amorphous-semiconductor transition in germanium. Phys. Rev. B 1994, 49 (20), 14251-14269. 35. Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E.; Constantin, L. A.; Zhou, X.; Burke, K., Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces. Phys. Rev. Lett. 2008, 100 (13), 136406.

36. Bass, K. K.; Estergreen, L.; Savory, C. N.; Buckeridge, J.; Scanlon, D. O.; Djurovich, P. I.; Bradforth, S. E.; Thompson, M. E.; Melot, B. C., Vibronic Structure in Room Temperature Photoluminescence of the Halide Perovskite Cs3Bi2Br9. Inorg. Chem. 2017, 56 (1), 42-45. 37. Shannon, R., Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr. Sect. A 1976, 32 (5), 751-767. 38. Lazarini, F., Caesium enneabromodibismuthate(III). Acta Crystallogr. Sect. B 1977, 33 (9), 2961-2964. 39. Nakamoto, K., Applications in Inorganic Chemistry. In Infrared and Raman Spectra of Inorganic and Coordination Compounds, John Wiley & Sons, Inc.: 2008; pp 149-354. 40. Machulin, V. F.; Motsnyi, F. V.; Smolanka, O. M.; Svechnikov, G. S.; Peresh, E. Y., Effect of temperature variation on shift and broadening of the exciton band in Cs3Bi2I9 layered crystals. Low Temp. Phys. 2004, 30 (12), 964-967. 41. Koutselas, I. B.; Ducasse, L.; Papavassiliou, G. C., Electronic properties of three- and low-dimensional semiconducting materials with Pb halide and Sn halide units. J. Phys. Condens. Matter 1996, 8 (9), 1217. 42. Toyozawa, Y., Further Contribution to the Theory of the Line-Shape of the Exciton Absorption Band. Prog. Theor. Phys. 1962, 27 (1), 89-104. 43. Rudin, S.; Reinecke, T. L.; Segall, B., Temperaturedependent exciton linewidths in semiconductors. Phys. Rev. B 1990, 42 (17), 11218-11231. 44. Wright, A. D.; Verdi, C.; Milot, R. L.; Eperon, G. E.; PerezOsorio, M. A.; Snaith, H. J.; Giustino, F.; Johnston, M. B.; Herz, L. M., Electron-phonon coupling in hybrid lead halide perovskites. Nat. Commun. 2016, 7, 11755. 45. Dawson, R. K.; Pooley, D., F Band Absorption in Alkali Halides as a Function of Temperature. Phys. Status Solidi B 1969, 35 (1), 95-105. 46. Luo, J.; Wang, X.; Li, S.; Liu, J.; Guo, Y.; Niu, G.; Yao, L.; Fu, Y.; Gao, L.; Dong, Q.; Zhao, C.; Leng, M.; Ma, F.; Liang, W.; Wang, L.; Jin, S.; Han, J.; Zhang, L.; Etheridge, J.; Wang, J.; Yan, Y.; Sargent, E. H.; Tang, J., Efficient and stable emission of warm-white light from lead-free halide double perovskites. Nature 2018, 563 (7732), 541-545. 47. Steele, J. A.; Puech, P.; Keshavarz, M.; Yang, R.; Banerjee, S.; Debroye, E.; Kim, C. W.; Yuan, H.; Heo, N. H.; Vanacken, J.; Walsh, A.; Hofkens, J.; Roeffaers, M. B. J., Giant Electron–Phonon Coupling and Deep Conduction Band Resonance in Metal Halide Double Perovskite. ACS Nano 2018, 12 (8), 8081-8090. 48. Yaffe, O.; Guo, Y.; Tan, L. Z.; Egger, D. A.; Hull, T.; Stoumpos, C. C.; Zheng, F.; Heinz, T. F.; Kronik, L.; Kanatzidis, M. G.; Owen, J. S.; Rappe, A. M.; Pimenta, M. A.; Brus, L. E., Local Polar Fluctuations in Lead Halide Perovskite Crystals. Phys. Rev. Lett. 2017, 118 (13), 136001. 49. Miyata, K.; Meggiolaro, D.; Trinh, M. T.; Joshi, P. P.; Mosconi, E.; Jones, S. C.; De Angelis, F.; Zhu, X. Y., Large polarons in lead halide perovskites. Sci. Adv. 2017, 3 (8), e1701217. 50. Shluger, A. L.; Stoneham, A. M., Small polarons in real crystals: concepts and problems. J. Phys. Condens. Matter 1993, 5 (19), 3049. 51. Jakubas, R.; Zaleski, J.; Sobczyk, L., Phase transitions in (CH3NH3)3Bi2I9 (MAIB). Ferroelectrics 1990, 108 (1), 109-114. 52. Vargas, B.; Ramos, E.; Pérez-Gutiérrez, E.; Alonso, J. C.; Solis-Ibarra, D., A Direct Bandgap Copper–Antimony Halide Perovskite. J. Am. Chem. Soc. 2017, 139 (27), 9116-9119. 53. Vargas, B.; Torres-Cadena, R.; Rodríguez-Hernández, J.; Gembicky, M.; Xie, H.; Jiménez-Mier, J.; Liu, Y.-S.; MenéndezProupin, E.; Dunbar, K. R.; Lopez, N.; Olalde-Velasco, P.; SolisIbarra, D., Optical, Electronic and Magnetic Engineering of

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Layered Halide Perovskites. Chem. Mater. 2018, 30 (15), 53155321.

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Chemistry of Materials

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From 0D Cs3Bi2I9 to 2D Cs3Bi2I6Cl3: Dimensional Expansion

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