White-Light Emission from the Structural Distortion Induced by Control

2 days ago - Synopsis. The inorganic framework of the 2D perovskite (BZA)2PbCl4 exhibits a highly distorted structure. It gives rise to self-trapped e...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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White-Light Emission from the Structural Distortion Induced by Control of Halide Composition of Two-Dimensional Perovskites ((C6H5CH2NH3)2PbBr4−xClx) Mi-Hee Jung*

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 05/01/19. For personal use only.

Department of Nanotechnology and Advanced Materials Engineering, Sejong University, 209, Neungdong-ro, Gwangjin-gu, Seoul 05006, Republic of Korea S Supporting Information *

ABSTRACT: Two-dimensional (2D) perovskites, which have a 2D orientation of the inorganic framework that determines largely the electronic characteristics and an organic cation in the interlayer that leads to a quantum well structure, have attracted a great deal of attention due to their superior stable optoelectronic properties. Especially, some of the greatest interest in 2D perovskites is their application in broad-band white-light emission for solid state lighting. We prepared (BZA)2PbBr4−xClx (BZA= benzylammonium and x = 0, 1.5, 2, 3, 3.5, 4) to tune the white-light emission. In the (BZA)2PbBr4 perovskite structure, the bond lengths of Pb−Br for PbBr62− exhibit the same lengths of 2.98 and 3.00 Å, respectively. However, in PbCl6 octahedra the bridging Pb−Cl distances were 2.83/2.88 Å and 2.85/2.88 Å, respectively. The 2D perovskite (BZA)2PbCl4 exhibits a turquoise light emission due to its highly distorted structure, whereas (BZA)2PbBr4 emits a narrow blue emission. We controlled the white emission by mixing the two compounds in proportion and changed the color from blue to white using the intermediate compound (BZA)2PbBr4−xClx (x = 1.5, 2, 3, 3.5). The intermediate compound (BZA)2PbBr4−xClx (x = 1.5, 2, 3, 3.5) shifted in the white space of Commission Internationale de l’É clairage coordinates, which were (0.324, 0.383), (0.312. 0.369), (0.319, 0.374), and (0.338, 0.396), respectively. The correlated color temperature of all compounds was observed above 5000 K, which suggests that these perovskites could be utilized as “cold” white-light-emitting materials.

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INTRODUCTION Organic−inorganic hybrid perovskites (OIHPs) have gained considerable attention as efficient light-absorbing materials in photovoltaic devices due to their high optical absorption coefficient,1 long electron and hole diffusion length,2,3 and low exciton binding energy.4 The power conversion efficiency has been boosted from 3% to a certified 23.7% in just a few years,5,6 which surpasses those of commercial photovoltaic technologies such as amorphous Si, GaAS, and CdTe solar cells. We expect that suppressing the recombination in the solar cell reduces the difference between the open circuit voltage (Voc) and the band gap energy, resulting in a decrease in the voltage loss. This leads to the photocurrent reaching the theoretical value, further increasing the efficiency of the perovskite solar cell. In addition, these hybrid perovskite materials are promising light emitters for application in light-emitting diodes,7−9 lasers,10 and photodetectors11 because of their narrowband emission (full width at half-maximum (fwhm) of 20 nm), high photoluminescence (PL) quantum efficiency, which is over 90%,12 and facile color tunability by the quantum well effect of their inorganic framework.13 However, a three-dimensional (3D) lead iodide based perovskite (i.e., CH3NH3PbI3), which is most commonly used as an OIHP, is easily decomposed into its composition in humid conditions, which is a critical issue in © XXXX American Chemical Society

long-term stability for commercial applications. However, recently, two-dimensional (2D) OIHPs, which were classified by the slicing of the 3D perovskite lattice along the (100), (110), and (111) directions, have attracted a great deal of attention due to their superior stable optoelectronic properties.14,15 Especially, one of the emerging applications for 2D perovskites is solid-state lighting16 due to their long carrier diffusion length and low density of deep trap states.3,17,18 Solidstate lighting gives an attractive solution for the disadvantages of traditional incandescent and fluorescent lighting sources.19 Under near-ultraviolet photoexcitation, these 2D layered perovskites emit a broad-band white-light emission, covering the entire visible-light region, which produces the near-sunlight. This broad-band PL is usually associated with structural deformation in the crystal lattice,16 which induces the self-trapped excitons (STEs) generated from recombination of excited electron−hole pairs through strong electron−phonon coupling20,21 due to the lattice deformability. Thus, it is possible to systematically adjust the white-light emission according to the synthetic design of the layered hybrid perovskite. Most white-light-emitting perovskite materials reported so far are Received: January 17, 2019

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DOI: 10.1021/acs.inorgchem.9b00145 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

solution was continuously stirred to prevent crystal formation. The heat was removed, and then the clear solution was slowly cooled to room temperature. Colorless crystals begin to accumulate at the bottom of the beaker. The crystals were collected by vacuum filtration and washed with diethyl ether, followed by further drying in a vacuum oven at 60 °C for 24 h. The crystal was stored in the desiccator before measurements of single-crystal X-ray diffraction and optical properties. Syntheis of (BZA)2PbBr4−xClx (x = 1.5, 2, 3, 3.5, 4). Using mechanochemical synthesis, (BZA)2PbBr4−xClx perovskite powders were synthesized by grinding an presynthesized organic salt (BZABr, BZACl), PbBr2, and PbCl2 in an electrical ball mill for 30 min at 500 rpm. To prepare the (BZA)2PbBr0.5Cl3.5 perovskite, 0.431 g (3 mmol) of BZACl, 0.188 g (1 mmol) of BZABr, and 0.556 g (2 mmol) of PbCl2 were mixed inside a ball mill under an ambient atmosphere to prepare a large amount of polycrystalline powder. The rest of the perovskite series was prepared by the same protocol with a stoichiometric molar ratio for each compound (Table S1). All samples were sealed in an evacuated Pyrex tube at a pressure below 1 × 10−3 Torr and then annealed at 200 °C for 12 h in the furnace. The samples were cooled to room temperature and then stored in a vacuum desiccator. High-Resolution PXRD and Single-Crystal X-ray Diffraction. PXRD measurements were carried out at room temperature using a Empyrean PANalytical instrument with Cu Kα radiation (40 kV, 30 mA, λ = 1.54056 Å) and a Ni filter with a graphite monochromator. The single-crystal X-ray diffraction data were collected with synchrotron radiation (λ = 0.61000 Å) on an ADSC Quantum210 detector at BL2D SMC with a silicon (111) double-crystal monochromator (DCM) at the Pohang Accelerator Laboratory in Korea. The data collection was performed using the PAL BL2DSMDC program27 under the following conditions: detector distance 63 mm, ω scanning (Δω) at 3° intervals, and exposure time of 1 s per frame. The cell refinement, data reduction, and adsorption correction were carried out using HKL3000sm (Ver. 703r).28 The space groups were determined by XPREP (Bruker, version 2013/3 for windows). The crystal structures for (BZA)2PbBr4 and(BZA)2PbCl4 were solved by direct methods using the SHELXT-2014 program29 and refined using full-matrix least-squares calculations based on F2 with the SHELXL-2014 program package.29 The crystallographic data have been deposited at the Cambridge Crystallographic Data Centre (CCDC) with the deposition numbers 1879416 and 1879417 for (BZA)2 PbCl4 and (BZA) 2 PbBr 4, respectively. Optical Absorption Spectroscopy. Optical diffuse reflectance measurements were carried out using a Varian Cary 5000 UV/vis/ NIR spectrophotometer in the wavelength region of from 200 to 1500 nm. The band gap of the compound was calculated by conversion of reflectance to absorption data for photon energy using the Kubelka−Munk equation:30 α/S = (1 − R)2(2R)−1, where R is the reflectance and α and S are the absorption and scattering coefficients, respectively. Static and Time-Resolved PL. The measurement of PL excitation and emission spectra of (BZA)2PbBr4−xClx (x = 0, 1.5, 2, 3, 3.5, 4) was performed with a fluorescence spectrophotometer (FluoroMate FS-2, Scinco, Republic of Korea). A time-resolved PL (TRPL) measurement was carried out with an inverted-type scanning confocal microscope (MicroTime-200, Picoquant, Germany) with a 60× (air) objective. The lifetime measurements were carried out at the Korea Basic Science Institute (KBSI), Daegu Center, Korea. A 375 nm laser was used as a source of excitation to the sample, and a dichroic mirror (Z375RDC, AHF), a long-pass filter (HQ405lp, AHF), a 75 μm pinhole, a band-pass filter, and an avalanche photodiode detector (PDM series, MPD) were assembled to collect the PL emission from the excited sample. Under irradiation from a 470 nm laser, the PL emission was obtained with assembly of a dichroic mirror (490 DCXR, AHF), a long-pass filter (HQ500lp, AHF), a 75 μm pinhole, and a single photon avalanche diode (PDM series, MPD). The count of fluorescence photons was estimated by the time-correlated single-photon counting (TCSPC) technique. The image mapping of

single-layer 2D organolead halide perovskites. For example, a (110)-oriented (C6H13N3)PbBr4 perovskite22 with broad emission was reported in 2006. Dohner et al.23,24 reported two families of hybrid perovskites, (N-MEDA)PbBr4−xClx (N-MEDA = N1-methylethane-1,2-diammonium) and (EDBE)PbX4 (EDBE = 2,2′-(ethylenedioxy)bis(ethylammonium)) with X = Cl, Br, which exhibited a stable photoluminescence quantum efficiency of 9%. Yangui et al.25 presented an in-depth experimental investigation of broad-band white-light emission using the self-assembled (C6H11NH3)2PbBr4 perovskite. The phenylethylammonium lead chloride (C6H5C2H4NH3)2PbCl426 and (AEA)PbBr4 (AEA = 3-(2-ammonioethyl)anilinium)16 were investigated for structural distortion to optimize the broad white-light emission. In this work, we studied hybrid halide perovskites with tunable white-light emission prepared with the use of the benzylammonium (C6H5CH2NH3+ = BZA) cation: (BZA)2PbBr4,(BZA)2PbCl4 and (BZA)2PbBr4−xClx (x = 3.5, 3, 2, 1.5) solid powders were prepared. (BZA)2PbBr4 and (BZA)2PbCl4 have similar structures, but the chloride motif is heavily distorted with respect to the bromine motif. The 2D perovskite (BZA)2PbCl4 exhibits white-light emission due to a highly distorted geometry of the inorganic PbCl6 sheet, whereas (BZA)2PbBr4 emits a narrow blue emission. To obtain efficient broad-band white-light emission, we tuned the halide composition in a solid precursor and found intermediate compounds, especially (BZA)2PbBr2Cl2, have greater optimized broad-band white-light emission in comparison to their parent compounds. These correlations suggest that the PL of the hybride perovskite system could be tuned through the local deformation of crystal structure and new design rules can be suggested for the synthesis of perovskite white-light-emitting materials by utilizing the correlation between the structural distortion and PL properties.

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EXPERIMENTAL SECTION

Starting Materials. Lead oxide (PbO, ACS reagent, ≥99.0%), benzylamine hydrochloride (C6H5CH2NH2·HCl), hydrobromic acid (HBr, reagent grade, 48%), hydrochloric acid (HCl, ACS reagent, 37%), and hypophosphorous acid (H3PO2, 50 wt % in H2O) were purchased from Sigma-Aldrich and used without further purification. Synthesis of (BZA)2PbBr4. PbO (0.22565 g, 3 mmol) was added to 5.0 mL of an HBr solution (48% aqueous solution), which was stabilized with 1 mL of a hypophosphorous acid solution (50 wt % in H2O) as a reducing agent. The mixture was stirred at 120 °C until the PbO powder was completely dissolved in the solution. In a separate beaker, benzylamine hydrochloride (0.18407 g, 3 mmol) was dissolved in 1 mL of an HBr solution at 120 °C. After both solutions turned clear, the solution of benzylamine hydrochloride was added slowly to the first solution with vigorous stirring at 120 °C. The solution was continuously stirred to prevent crystal formation. The heat was removed, and then the clear solution was slowly cooled to room temperature. Colorless crystals begin to accumulate at the bottom of the beaker. The crystals were collected by vacuum filtration and washed with diethyl ether, followed by further drying in a vacuum oven at 60 °C for 24 h. The crystal was stored in a desiccator before measurements of single-crystal X-ray diffraction and optical properties. Synthesis of (BZA)2PbCl4. PbO (0.22565 g, 3 mmol) was added to 5.0 mL of an HCl solution (37% aqueous solution), which was stabilized with 1 mL of a hypophosphorous acid solution (50 wt % in H2O) as a reducing agent. The mixture was stirred at 120 °C until the PbO powder was completely dissolved in the solution. In a separate beaker, benzylamine hydrochloride (0.18407 g, 3 mmol) was dissolved in 1 mL of HCl solution at 120 °C. After both solutions turned clear, the solution of benzylamine hydrochloride was added slowly to the first solution with vigorous stirring at 120 °C. The B

DOI: 10.1021/acs.inorgchem.9b00145 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Details for (BZA)2PbBr4 and (BZA)2PbCl4 empirical formula formula wt temp (K) wavelength (Å) cryst syst space group unit cell dimensions V (Å3) Z calcd density (Mg/m3) abs coeff (mm−1) F(000) cryst size (mm3) θ range for data collection (deg) index ranges no. of rflns collected no. of indep rflns completeness to θ = 24.835° (%) refinement method no. of data/restraints/params goodness of fit on F2 final R indices (I > 2σ(I)) R indices (all data) absolute structure param extinction coeff largest diff peak and hole (e Å−3)

(BZA)2PbBr4

(BZA)2PbCl4

(C6H5CH2NH3)2PbBr4 743.15 296(2) 0.700 orthorhombic Cmc21 a = 33.406(2) Å, α = 90°, b = 8.1528(6) Å, β = 90°, c = 8.1385(5) Å, γ = 90° 2216.6(3) 4 2.227 14.221 1360 0.128 × 0.045 × 0.012 2.402−25.496 −36 ≤ h ≤ 40, −10 ≤ k ≤ 10, −10 ≤ l ≤ 10 12228 2185 (R(int) = 0.0503) 99.4 full matrix least squares on F2 2185/125/102 1.099 R1 = 0.0282, wR2 = 0.0547 R1 = 0.0444, wR2 = 0.0616 0.51(4) 0.00111(9) 0.953 and −0.739

(C6H5CH2NH3)2PbCl4 565.31 296(2) 0.700 orthorhombic Cmc21 a = 33.6826(9) Å, α = 90°, b = 7.8282(2) Å, β = 90°, c = 7.7382(2) Å, γ = 90° 2040.36(9) 4 1.840 8.391 1072 0.193 × 0.045 × 0.012 1.191−27.827 −44 ≤ h ≤ 44, −10 ≤ k ≤ 10, −10 ≤ l ≤ 10 13035 2563 (R(int) = 0.0504) 99.9 2563/1/102 0.723 R1 = 0.0286, wR2 = 0.0796 R1 = 0.0348, wR2 = 0.0950 0.150(16) 0.0078(5) 1.100 and −0.952

TRPL (200 × 200 pixels) was obtained by the time-tagged timeresolved (TTTR) data acquisition method. The PL decay collected every 16 ps was fitted with Symphotime-64 software (Ver. 2.2) with the exponential decay model I(t) = ∑Ai exp(−t/τi), where I(t) is the time-dependent PL intensity, A is the amplitude, and τ is the PL lifetime. Absolute Photoluminescence Quantum Yield (PLQY). The PLQY was determined using an absolute quantum yield measurement (Hitachi, F-7000 Fluorimeter) with an integrating sphere. The perovskite was excited by a 330 nm laser (CrystaLaser, GCL532005-L) source. The procedure of data acquisition followed a previous protocol31 and is explained in detail in Tables S2 and S3 in the Supporting Information.

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RESULTS AND DISCUSSION (BZA)2PbBr4 and (BZA)2PbCl4 crytallized in the polar orthorhombic space group Cmc21, which is in agreement with previous reports.32−34 Both of them have colorless platelike crystals (Figure S1). For (BZA)2PbBr4, the crystal structure was previously reported as being in centrosymmetric space group Cmca.34 However, we determined the noncentrosymmetric space group as Cmc21, as reported by Du et al.,35 because disordering of organic cations in (BZA)2PbBr4 was not severe in this study. The detailed crystallographic data are given in Table 1. Both compounds consisted of inorganic layers of corner-sharing PbX6 (X = Br, Cl) octahedra that were separated from one another with two univalent (BZA)+ organic cations, which is seen in Figure 1. The (BZA)+ cation bilayers moves a distance between the inorganic layers, which resulted in long interlayer halide−halide distances of 11.44 Å for (BZA)2PbBr4 and 11.77 Å for (BZA)2PbCl4, respectively (Figure 1a,c). There is a weak stabilizing force between the BZA ions in the interalyer due to a “point-to-face” π−σ attraction between the C−H vector and the ring centroid, with

Figure 1. Crystal structures of (a) (BZA)2PbBr4 and (c) (BZA)2PbCl4 perovskties. The (BZA)+ cation bilayers resulted in a long interlayer halide−halide distance of 11.44 Å for (BZA)2PbBr4 and 11.77 Å for the (BZA) 2 PbCl 4 , respectively. Structure fragments of (b) (BZA)2PbBr4 and (d) (BZA)2PbCl4 to show the hydrogenbonding network between the ammonium group of the BZA ion and the adjacent halide ions in the perovskite. The distances between the C−H vector and the ring centroid of BZA cations (“point-to-face” π−σ attraction) are 3.87 and 3.60 Å for (BZA)2PbBr4 and (BZA)2PbCl4 perovskites, respectively. C

DOI: 10.1021/acs.inorgchem.9b00145 Inorg. Chem. XXXX, XXX, XXX−XXX

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emission, whereas the (BZA)2PbBr4 perovskite emitted a narrow blue emission. To control the white emission, we prepared an intermediate perovskite by mixing the (BZA)2PbBr4 and (BZA)2PbCl4 perovskites. These results will be useful to study the relationship between the perovskite structure and the PL. We prepared the intermediate (BZA)2PbBr4−xClx perovskite series using a solid-state grinding method at 200 °C. To confirm the intermidate phase between the (BZA)2PbBr4 and the (BZA)2PbCl4 perovskites, we collected data from highresolution PXRD for the (BZA)2PbBr4−xClx perovskite series (Figure 4a). As we can see in Figure 4b,c, the (400) and (600) peaks were shifted from high angle to low angle but did not linearly decrease due to the serious distortion with the b and c axes in the intermediate compositions. This indicates that the structural distortion predominantly occurred in the adjoining PbX62− octahedra in the inorganic layer rather than in the framework of the organic layer. This is consistent with the result that the (111) and (311) peaks were shifted toward a higher angle with a linear trend (Figure 4c). Consequently, the cell volume was decreased as the intermediate compound approached toward the (BZA)2PbCl4 perovskite (Figure 5b). We calculated the cell parameters using Rietveld refinement (Figure S2). The b and c axes of unit cell parameters decreased from (BZA)2PbBr4 to (BZA)2PbCl4, whereas the long axis, a, gradually increased from (BZA)2PbBr4 to (BZA)2PbCl4 (Figure 5a). The cell parameters of the intermediate compound were between those of (BZA)2PbBr4 and (BZA)2PbCl4. As x became closer to 4, the distortion of the structure increased, which resulted in a decrease in cell volume (Figure 5b). The optical absorption spectra of all perovskites showed that the increasing chlorine composition shifts the band gap to a high energy value, and it increased proportionally with x value (Figure 5c,d). The optical parameter of the lead hybrid perovskite was mainly determined by the overlapping of s/p antibonding orbitals between the lead and halide ions.36,37 This overlapping was related to the integral electron transfer between the valence band maximum (VBM) and the conduction band minimum (CBM). The bandwidth narrowing was attributed to the decreasing dimensionality and the angle (ϕ) of structural distortion in the Pb−X−Pb bond between the consecutive inorganic PbX62− octahedra layers. As the (BZA)2PbCl4 group was incorporated into the structure, a smaller band dispersion was produced because of the structural distortion and, as a consequence, the widths of the CBM and VBM narrowed, which led to an increased experimental optical band gap.38,39 All of the present perovskites exhibited a sharp band edge in the absorption spectra, in which the band gap was determined, suggesting a direct band semiconductor. The band gap ranged from 3.51 eV for (BZA)2PbCl4 to 2.92 eV to (BZA)2PbBr4, and for the intermediate materials the band gap increased from 3.1 to 3.37 eV for the x values varying from 1.5 to 3.5 (Figure 5d). The (BZA)2PbBr4 and (BZA)2PbCl4 perovskites had a secondary excitonic peak near the sharp absorption edge, which is a general feature of 2D perovskites. In order to see the band edge emission of the perovskites, we measured the PL using a fluoresence spectrometer. Figure S3 shows the PL emission and the excitation spectra of the (BZA)2PbBr4−xClx series at room temperature. (BZA)2PbCl4 exhibited a broad emission spectrum from 320 to 610 nm, whereas the emission spectrum of (BZA)2PbBr4 had two very sharply split peaks at 406 and 426 nm, which matched the band gap energy (2.86 and 3.02 eV) from the absorption

(BZA)2PbBr4 representing 3.87 Å and(BZA)2PbCl4 representing 3.60 Å (Figure 1b,d). The protonated ammonium ion of (BZA)+ anchored to the four halide ions of the lead halide octahedral structure with a hydrogen bond. In (BZA)2PbBr4, the hydrogen bonds between the N−H of the BZA cation and two of the adjacent apical bromides were stronger than those to the other bromide atoms and exhibited donor−acceptor distances of 2.63 and 2.68 Å, respectively (Figure 1b). (BZA)2PbCl4 showed the same coordination environment as for (BZA)2PbBr4 and showed the donor−acceptor distances of 2.35 and 2.35 Å, respectively (Figure 1d). The hydrogen bonds (N−H···X, X = Br, Cl) between the end of the organic cation and the terminus of the inorganic layer affect the configuration of inorganic layers. As shown in Figure 2a,b, the BZA cations of (BZA)2PbBr4 and

Figure 2. Magnified views of the hydrogen bridging geometries of (a) (BZA)2PbBr4 and (b) (BZA)2PbCl4, showing the more tilted octahedra of (BZA)2PbCl4 in comparison to (BZA)2PbBr4 and the position of the cations relative to the corner-sharing octahedra. Top-down views of (c) (BZA)2PbBr4 and (d) (BZA)2PbCl4. Both show octahedral layers stacked with a staggered conformation in the a axis direction.

(BZA)2PbCl4 are positioned in the center of the short diagonal of the parallelogram, even though that of (BZA)2PbCl4 is closer to an acute angle position (Figure 2b), which results in the staggered conformation of the inorganic lattice in both (BZA)2PbCl4 and (BZA)2PbBr4 (Figure 2c,d). The luminescence property of an organic−inorganic hybrid perovskite is associated with the lattice distortion of the inorganic layer. Figure 3 shows the lead halide framework between the adjoining PbX62− octahedra in the inorganic layer. The Pb−Br−Pb bond angles for (BZA)2PbBr4 were 149.9 and 150° (Figure 3a), which have almost the same value. In contrast, the Pb−Cl−Pb bond angles for (BZA)2PbCl4 were 142.2 and 154.4°, which indicated structural distortion inside the inorganic framework (Figure 3b). This result is consistent with the bond length of the PbX62− octahedra in the inorganic layer. The structural distortion resulted in different distances between Pb−X bonds in the coordination geometry of the X−Pb−X unit. In the (BZA)2PbBr4 perovskite structure, all Pb−Br bonds show the same lengths of 2.98 and 3.00 Å, respectively (Figure 3c). However, for (BZA)2PbCl4, the bridging Cl−Pb−Cl distances consist of short and long bond length combinations, with Pb−Cl bond lengths of 2.85 and 2.88 Å for the b axis and 2.88 and 2.83 Å for the c axis, respectively (Figure 3d). The high distortion level of the inorganic layer in the (BZA)2PbCl4 perovskite resulted in a broad-band white-light D

DOI: 10.1021/acs.inorgchem.9b00145 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Structural geometry of (a) (BZA)2PbBr4 and (b) (BZA)2PbCl4. Bond distances of (c) Pb−Br and (d) Pb−Cl for PbBr62− and PbCl62− octahedra, respectively.

Figure 4. (a) High-resolution PXRD for the (BZA)2PbBr4−xClx perovskite series. (b, c) Shift of the peak from lower angle to higher angle, indicating that the cell volume decreased from x = 0 to x = 4. E

DOI: 10.1021/acs.inorgchem.9b00145 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a, b) Unit cell parameters, (c) diffuse reflectance spectra, and (d) band gaps of the solid powder of the (BZA)2PbBr4−xClx perovskite (x = 4, 3.5, 3, 2, 1.5, 0) series.

Inc. (BZA)2PbBr 4 and (BZA)2PbCl4 had chromaticity coordinates of (0.274, 0.273) and (0.282, 0.310), respectively, in which both regions correspond to bright blue emissions. The intermediate compounds (BZA)2PbBr4−xClx (x = 1.5, 2, 3, 3.5) moved to the center with CIE coordinates of (0.324, 0.383), (0.312, 0.369), (0.319, 0.374), and (0.338, 0.396), respectively (Table 3). The CCTs of the series were all above 5000 K, which indicated that these materials emitted a cold white light. The color rendering indexes (CRIs) were also changed from 80.803 (x = 1.5) to 83.515 (x = 3), as seen in Table 3. Among all compounds, (BZA)2PbBr2.5Cl1.5 and (BZA)2PbBr2Cl2 have PLQYs of 10.41% and 10.76%, respectively, which are much higher than those of the other compounds. The reason for the high PLQY can be ascribed to further stabilized STEs through the strong electron−phonon coupling due to a more distorted structure in comparison to the rest. It almost reaches the highest PLQY value ever reported for organic−inorganic hybrid perovskites as white-light-emitting materials.42 The static PL emission result indicates that the intermediate (BZA)2PbBr4−xClx (x = 1.5, 2, 3, 3.5) series had a broad band emission in comparison to the pure perovskite compound (Figure 7b), which could be due to the structural distortion that was discussed in the previous paragraph. As shown in Figure 7c, (BZA)2PbBr4−xClx (x = 1.5, 2, 3, 3.5, 4) perovskites emit white photoluminescence under an 254 nm ultraviolet (UV) light at ambient temperature, whereas (BZA)2PbBr4 perovskite emits blue emission under a 365 nm UV light.

spectrum. The emission of the PL bandwidth increased with an increase in the fraction of the (BZA)2PbCl4 compound, and it underwent a red shift, which was a trend different from that of the absorption, as seen in Figure 5c. As the intermediate state approached (BZA)2PbCl4, it had much more structural distortion, which resulted in more STE states and a broader emission.40,41 In Figure 6a, the TRPL shows the averge lifetime of the (BZA)2PbBr4−xClx perovskite (x = 0, 1.5, 2, 3, 3.5, 4) series. We summarize the emission lifetimes of the series in Table 2. The average lifetime of the series sharply increased from being the shortest in the (BZA)2PbBr4 perovskite (τavg = 0.71 ns) to the (BZA)2PbBr2Cl2 perovskite (τavg = 7.58 ns), which had the longest lifetime among the samples. This can be attributed to the association of the band edge emission and the STE emission.41 However, as the series approached the (BZA)2PbCl4 member, the average lifetime decreased, which indicated that the intermediate perovskite had a more distorted structure in comparison to the (BZA)2PbCl4 perovskite. This result was consistent with the fluorescence PL emission results (Figure S3). In addition, the image mapping of the PL emission of the (BZA)2PbBr2Cl2 perovskite exhibited a much longer lifetime among the perovskites (Figure 6b), which indicated that most of the PL emission was predominantly from the band edge emission and the STE emission. The light emission properties of the (BZA)2PbBr4−xClx (x = 0, 1.5, 2, 3, 3.5, 4) series are readily tunable through the modulation of halide composition, as seen in Figure 7. The Commission Internationale de l’É clairage (CIE) color coordinates of these compounds exhibits a range from the region of very weak blue emission to the center of the white emission in the 1931 color space chromaticity diagram in Figure 7a. The chromaticity coordinates and the correlated color temperatures (CCTs) were calculated using a ColorCalculator by OSRAM,

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CONCLUSIONS

Through the structural distortion of the inorganic framework in a 2D hyrbrid perovskite, we demonstrated white-light emission materials by using the (BZA)2PbBr4−xClx (x = 0, 1.5, 2, 3, 3.5, 4) perovskite series. In the PbBr6 octahedral geometry of F

DOI: 10.1021/acs.inorgchem.9b00145 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. (a) Transient photoluminescence decay profile of the single-crystal (BZA)2PbBr4−xClx perovskite (x = 4, 3.5, 3, 2, 1.5, 0) series at room temperature. The fitting results (solid curves) are also included for comparison. (b) Image mapping of TRPL of the (BZA)2PbBr4−xClx perovskite (x = 4, 3.5, 3, 2, 1.5, 0) series.

of the two perovskite compounds according to the appropriate halide composition, a greater optimized white-light emission was observed in the intermediate compounds (BZA)2PbBr4−xClx (x = 1.5, 2, 3, 3.5). In comparison to their parent perovskites, the chromaticity coordinates of intermediate compounds (BZA)2PbBr4−xClx (x = 1.5, 2, 3, 3.5) moved into the center from the (0.324, 0.383), (0.312, 0.369), (0.319, 0.374), and (0.338, 0.396) coordinates, respectively. Especially, (BZA)2PbBr2.5Cl1.5

the (BZA)2PbBr4 perovskite, all bridging Pb−Br bond lengths exhibit the same lengths of 2.98 and 3.00 Å, respectively. In comparison with that, the Pb−Cl bond distances of (BZA)2PbCl4 perovskite are 2.85 and 2.88 Å for the b axis and 2.88 and 2.83 Å for the c axis, respectively. Thus, the 2D perovskite (BZA)2PbCl4 exhibited a broad-band blue emission that was attributed to its highly distorted structure, whereas (BZA)2PbBr4 emitted a narrow-band blue emission. By mixing G

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Table 2. Emission Decay Analysis of the (BZA)2PbBr4−xClx Perovskite Series (x = 4, 3.5, 3, 2, 1.5, 0) from Fluorescence Lifetime Measurementsa perovskite

A1

τ1

A2

τ2

(BZA)2PbBr4 (BZA)2PbBr2.5Cl1.5 (BZA)2PbBr2Cl2 (BZA)2PbBrCl3 (BZA)2PbBr0.5Cl3.5 (BZA)2PbCl4

4.9 6.4 4.11 3.57 0.37 0.33

0.48 8.01 7.58 6.03 8.7 8.48

0.2 3.2

2.04 3.8

1.41 2 1.89

1.82 3.2 2.90

A3

2.62 2.79

τ3

⟨τ⟩

τamp

0.97 0.75

0.71 7.21 7.58 5.59 4.17 3.96

0.54 6.63 7.58 4.84 2.43 2.06

a The emission decay was analyzed using the equation I(t) = A1 exp(−t/τ1) + A2 exp(−t/τ2) + A3 exp(−t/τ3) + ..., where ∑iAi/(A1 + A2 + A3 + ···) = 1, the average lifetime ⟨τ⟩, is defined by ⟨τ⟩ = ∑iAiτi2/∑iAiτi, and τamp is the amplitude of average lifetime.

Figure 7. (a) CIE color coordinates of the (BZA)2PbBr4−xClx perovskites series with x = 0 (●), 1.5 (■), 2 (●), 3 (▼), 3.5 (▲), 4 (◀) in a 1931 color space chromaticity diagram. (b) Steady-state PL of the (BZA)2PbBr4−xClx perovskite series. (c) Blue-light emission from (BZA)2PbBr4 from a powdered polycrystalline sample and white-light emission from the (BZA)2PbBr4−xClx (x = 1.5, 2, 3, 3.5, 4) perovskite series under UV light with λex = 365 or 254 nm.

Table 3. Commission Internationale de l’É clairage (CIE) Color Coordinates (x, y), Correlated Color Temperatures (CCTs), Color Purities, Color Rendering Indexes (CRIs), and Photoluminescence Quantum Yields (PLQYs) of the (BZA)2PbBr4−xClx (x = 0, 1.5, 2, 3, 3.5, 4) Perovskite Series CIE coordinate perovskite

x

y

CCT (K)

color purity (%)

CRI

PLQY (%)

(BZA)2PbBr4 (BZA)2PbBr2.5Cl1.5 (BZA)2PbBr2Cl2 (BZA)2PbBrCl3 (BZA)2PbBr0.5Cl3.5 (BZA)2PbCl4

0.274 0.324 0.312 0.319 0.338 0.282

0.273 0.383 0.369 0.374 0.396 0.310

12034 5805 6297 5981 5327 8842

82.88 91.06 91.36 91.67 88.79 89.70

91.467 80.803 81.902 83.515 81.858 91.470

4.98 10.41 10.76 3.87 2.83 3.57

and (BZA)2PbBr2Cl2 perovskites give PLQYs of more than 10%, which is comparable to those for previously reported white-emitting perovskite materials. Therefore, we expect that this study will provide basic principles for the design of optimized crystal structures of perovskites to develop white-lightemitting perovskite materials.

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Synthetic ratio for (BZA)2PbBr4−xClx using a grinding method, images of optical microscopy for (BZA)2PbBr4 and (BZA)2PbCl4 single crystals, Rietveld refinement results and static photoluminescence spectra of (BZA)2PbBr4−xClx perovskite (x = 0, 1.5, 2, 3, 3.5, 4) compounds, and procedure of data acquisition of PLQY (PDF)

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AUTHOR INFORMATION

Corresponding Author

*M.-H.J.: fax, +82-2-3408-4342; tel, +82-2-6935-2597; e-mail, [email protected]. ORCID

Mi-Hee Jung: 0000-0003-4997-9742 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B04931751) and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2019R1A2C1003108).

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J

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