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White-light Emission and Structural Distortion in New Corrugated 2D Lead Bromide Perovskites Lingling Mao, Yilei Wu, Constantinos C. Stoumpos, Michael R. Wasielewski, and Mercouri G. Kanatzidis J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01312 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017
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Journal of the American Chemical Society
White-light Emission and Structural Distortion in New Corrugated 2D Lead Bromide Perovskites Lingling Mao1, Yilei Wu1, Constantinos C. Stoumpos1, Michael R. Wasielewski1,2 and Mercouri G. Kanatzidis1,2 1
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
2
Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208, United States
ABSTRACT: Hybrid inorganic-organic perovskites are developing rapidly as high performance semiconductors. Recently, two-dimensional (2D) perovskites were found to have white-light, broadband emission in the visible range and was attributed mainly to the role of self-trapped excitons (STEs). Here, we describe three new 2D lead bromide perovskites incorporating a series of bifunctional ammonium di-cations as templates which also emit white light: 1) α-(DMEN)PbBr4 (DMEN = 2-(Dimethylamino)ethylamine), which adopts a unique corrugated layered structure in space group Pbca with unit cell a =18.901(4) Å, b = 11.782(2) Å and c = 23.680(5) Å; 2) (DMAPA)PbBr4 which crystalizes in P21/c with a = 10.717(2) Å, b = 11.735(2) Å, c = 12.127(2) Å, β = 111.53(3)°and 3) (DMABA)PbBr4 (DMAPA = 3-(Dimethylamino)-1-propylamine, DMABA = 4-dimethylaminobutylamine) which adopts Aba2 with a = 41.685(8) Å, b = 23.962(5) Å, c = 12.000(2) Å. Photoluminescence (PL) studies show a correlation between the distortion of the “PbBr6” octahedron in the 2D layer and the broadening of PL emission, with the most distorted structure having the broadest emission (183 nm full width at half maximum) and longest lifetime (τavg = 1.39 ns). The most distorted member α-(DMEN)PbBr4 exhibits white-light emission with a color rendering index (CRI) of 73 and correlated color temperature (CCT) of 7863K, producing “cold” white light similar to a fluorescent light source.
these “roof-like” (110)-oriented 2D perovskites as “n×n”,
INTRODUCTION Hybrid inorganic-organic halide perovskites exhibit exceptional performance in photovoltaic and optoelectronic device applications.1-8 In addition to the 3D perovskites, which are the most extensively investigated materials, the two-dimensional (2D) perovskites also show promising results for applications such as stable solar cells and light emitting diodes (LEDs).9-12 White-light emitting 2D perovskites are attracting strong interest for solid-state lighting applications.13-14 Having the advantages of easyprocessing, low cost, high tunability and color stability, white-light emitting 2D perovskites are promising as single-component light emitters.15-16 The 2D perovskites derive from the 3D structure by slicing the inorganic lattice along different crystallographic planes, which can be categorized as the (100)-oriented, (110)-oriented and (111)oriented.17 The 2D materials have greater room for property tuning (than the 3D perovskites) by changing widely the functional organic cations or increasing the perovskite layer thickness.18 Different from the (100)-oriented 2D perovskites, which consist of flat perovskite sheets, (110)-oriented perovskites have corrugated layers.19 Depending on where the corrugation occurs in the layers these structures can be defined as 2×2, 3×3, 3×4, 4×4 etc. as shown in Figure 1. We name
Figure 1. Schematic representation of different structural types of corrugated (110)-oriented members of the 2D perovskite family.
which n stands for the number of the octahedra composing half of the roof as seen in Figure 1. Because of the more distorted nature of the (110)-oriented perovskites, a broadband photoluminescence (PL) emission presumably arises from self-trapped excitons (STEs) states. STEs are photo-generated transient defects formed by large lattice deformations associated with strong electron-phonon
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coupling.20-21 The first (110)-oriented lead bromide perovskite with broad emission (C6H13N3)PbBr4 was reported in 2006.22 Recently, (N-MEDA)[PbBr4]13 (N-MEDA = N1methylethane- 1,2-diammonium and (EDBE)[PbBr4]14 (EDBE = 2,2′- (ethylenedioxy)bis(ethylammonium) were both reported to have “warm” white-light emission. A (100)-oriented 2D perovskite, (C6H11NH3)2PbBr4, was also found to have white-light emission.21 White-light emission has been observed in chloride systems such as (EDBE)PbCl4,14 (FC2H4NH3)2PbCl4,23 and 3D lead chloride clusters.24 Here, we report three new 2D lead bromide perovskites α-(DMEN)PbBr4, (DMAPA)PbBr4 and (DMABA)PbBr4 (Table 1). The α-(DMEN)PbBr4 features a new corrugated “3×3” layer and belongs to the (110)-oriented family (Figure 1), while (DMAPA)PbBr4 and (DMABA)PbBr4 belong to the conventional (100)-oriented type with flat layers as shown in Figure 3. The corrugated 2D perovskite α(DMEN)PbBr4 exhibits white light emission attributed to its highly distorted structure. The optical properties of the three 2D compounds described here exhibit a strong correlation between the PL emission bandwidth with the degree of distortion in the “PbBr6” octahedra in the crystal structure.
EXPERIMENTAL SECTION Synthesis. All chemicals were purchased from SigmaAldrich and unless otherwise stated, were used as received. α-(DMEN)PbBr4 and β-(DMEN)PbBr4. An amount of 0.669 g (3 mmol) PbO powder was dissolved in 6.0 ml of 48% hydrobromic acid and 1.0 ml of 50% aqueous H3PO2 by heating under stirring for 10 min at 150°C until all PbO dissolved. 0.264 g (5 mmol) of 2(Dimethylamino)ethylamine was added drop wise to the previous solution under heating and stirring. Pale yellow and colorless plate-like crystals formed during slow cooling. After leaving the pale yellow plates in solution for 1014 days, all crystals transformed to colorless rhombic shaped crystals of α-(DMEN)PbBr4 (Figure 2). Yield 0.632g (34.1% based on total Pb content). Pale yellow crystals of β-(DMEN)PbBr4 can be separated through immediate filtration. Though PXRD shows pure the β phase is pure at times, it is not guaranteed for every experiment. (DMAPA)PbBr4 and (DMABA)PbBr4. An amount of 0.892 g (4 mmol) PbO powder was dissolved in 6.0 ml of 48% hydrobromic acid and 1.0 ml of 50% aqueous H3PO2 by heating under stirring for 10 min at 150°C until all PbO dissolved. 1.0 ml of 48% hydrobromic acid was added to protonate 0.408 g (4 mmol) of 3-(Dimethylamino)-1propylamine. Mixing both solution under heating at 150°C for 5 min, pale yellow plate-like crystals form during slow cooling. Yield 0.973 g (38.5% based on total Pb content). The same method was used to prepare (DMABA)PbBr4, using 0.500 g (4.3 mmol) 4-dimethylaminobutylamine instead. Yield 1.042 g (40.4% based on total Pb content).
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Single Crystal X-ray Diffraction. Single crystals of appropriate size were selected for X-ray diffraction experiments. After screening a few diffracted frames to ensure crystal quality, full sphere data were collected using either a STOE IPDS 2 or IPDS 2T diffractometer with graphitemonochromatized Mo Kα radiation (λ = 0.71073 Å), operating at 50 kV and 40 mA under N2 flow. Integration and numerical absorption corrections on the data were performed using the STOE X-AREA programs. Crystal structures were solved by direct methods and refined by fullmatrix least-squares on F2 using the OLEX2 program package.25 Powder X-ray Diffraction (PXRD). PXRD analysis was performed using a Rigaku Miniflex600 or STOE’s STADI MPpowder X-ray diffractometer (Cu Kα graphite, λ = 1.5406 Å) operating at 40 kV/15 mA with a Kβ foil filter. Optical Absorption Spectroscopy. Optical diffuse reflectance measurements were performed using a Shimadzu UV-3600 UV-VIS-NIR spectrometer operating in the 200 –1200 nm region at room temperature. BaSO4 was used as the reference of 100% reflectance for all measurements. The reflectance versus wavelength data generated were used to estimate the band gap by converting reflectance to absorption according to the Kubelka–Munk equation26: α/S = (1–R)2(2R)−1, where R is the reflectance, α and S are the absorption and scattering coefficients, respectively. Steady state and Time-resolved Photoluminescence. Steady-state and time-resolved photoluminescence (TRPL) spectra were acquired using HORIBA Fluorolog-3 equipped with a 450-W xenon lamp and a TCSPC module (diode laser excitation at λ = 375 nm) and an integrating sphere (Horiba Quanta–φ) for absolute photoluminescence quantum yield determination. The spectra were corrected for the monochromator wavelength dependence and photomultiplier response functions provided by the manufacturer. The measurements were performed using dried, powdered polycrystalline samples. No filters were used during the TRPL measurements.
RESULTS AND DISCUSSION Unlike the relatively common “2×2” structural type,13-14, 22 α-(DMEN)PbBr4 represents the first example of the “3×3” type of roof-like corrugated structure. Table 1. Structural characteristics and bandgaps of the new compounds reported in this work. Compound
2D structural type
Space group
bandgap
α-(DMEN)PbBr4
“3×3” (110)
Pbca
3.00 eV
(DMAPA)PbBr4
(100)
P21/c
2.88 eV
(DMABA)PbBr4
(100)
Aba2
2.85 eV
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Journal of the American Chemical Society Table 2. Crystal data and structure refinement for α-(DMEN)PbBr4 and β-(DMEN)PbBr4. Empirical formula
α-[(CH3)2NH(CH2)2NH3]PbBr4
β-[(CH3)2NH(CH2)2NH3]PbBr4
Formula weight
617.00
617.00
Crystal system
Orthorhombic
Monoclinic
Space group
Pbca
P21/c
Unit cell dimensions
a = 18.901(4) Å, α = 90° b = 11.782(2) Å, β = 90° c = 23.680(5) Å, γ = 90°
a = 17.625(4) Å, α = 90° b = 11.982(2) Å, β = 90.44(3)° c = 18.724(4) Å, γ = 90°
Volume, Z
5273.3(18) Å3, 16
3953.9(14) Å3, 12
3
Density (calculated)
3.109 g/cm
3.110 g/cm3
Absorption coefficient
24.878 mm-1
24.885 mm-1
F(000)
4384
3288
θ range for data collection
1.720 to 29.225°
2.761 to 29.182°
Index ranges
-25
