CsPbBr3 Perovskite Composites with Near-Unity


10 hours ago - All-inorganic perovskites have emerged as a new class of phosphor materials owing to their outstanding optical properties. Zero-dimensi...
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Functional Inorganic Materials and Devices

Cs4PbBr6/CsPbBr3 Perovskite Composites with Near-Unity Luminescence Quantum Yield: Large-Scale Synthesis, Luminescence and Formation Mechanism, and White Light-Emitting Diode Application Yameng Chen, Yang Zhou, Qing Zhao, Junying Zhang, Ju-Ping Ma, Tong-Tong Xuan, ShaoQiang Guo, Zi-Jun Yong, Jing Wang, Yoshihiro Kuroiwa, Chikako Moriyoshi, and Hong-Tao Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04556 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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Cs4PbBr6/CsPbBr3

Perovskite

Composites

with

Near-Unity

Luminescence Quantum Yield: Large-Scale Synthesis, Luminescence and Formation Mechanism, and White Light-Emitting Diode Application Ya-Meng Chen†,||, Yang Zhou†,||, Qing Zhao‡,||, Jun-Ying Zhang§,||, Ju-Ping Ma†, Tong-Tong Xuan ⊥ , Shao-Qiang Guo§, Zi-Jun Yong†, Jing Wang ⊥ , Yoshihiro Kuroiwa‡, Chikako Moriyoshi‡, and Hong-Tao Sun*,†



College of Chemistry, Chemical Engineering and Materials Science, Soochow University,

Suzhou 215123, P. R. China ‡

Department of Physical Science, Hiroshima University, Higashihiroshima, Hiroshima

739-8526, Japan §



Department of Physics, Beihang University, Beijing 100191, P. R. China State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry,

Sun Yat-sen University, Guangzhou 510275, P. R. China

KEYWORDS: Zero-dimensional perovskites; Cs4PbBr6; Phosphors; Luminescence; Luminescence mechanism

ABSTRACT: All-inorganic perovskites have emerged as a new class of phosphor materials owing to their outstanding optical properties. Zero-dimensional inorganic perovskites, in 1

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particular the Cs4PbBr6-related systems, are inspiring intensive research owing to the high photoluminescence quantum yield (PLQY) and good stability. However, synthesizing such perovskites with high PLQYs through an enviromentally friendly, cost-effective, scalable, and high-yield approach remains challenging, and their luminescence mechanisms has been elusive. Here, we report a simple, scalable, room-temperature self-assembly strategy for the synthesis of Cs4PbBr6/CsPbBr3 perovskite composites with near-unity PLQY (95%), high product yield (71%) and good stability, using low-cost, low-toxicity chemicals as precursors. A broad range of experimental and theoretical characterizations suggest that the high-efficiency PL originates from CsPbBr3 nanocrystals well passivated by the zero-dimensional Cs4PbBr6 matrix that forms based on a dissolution-crystallization process. These findings underscore the importance in accurately identifying the phase purity of zero-dimensional perovskites by synchrotron X-ray technique to gain deep insights into the structure-property

relationship.

Additionally,

we

demonstrate

that

green-emitting

Cs4PbBr6/CsPbBr3, combined with red-emitting K2SiF6:Mn4+, can be used for the construction of WLEDs. Our work may pave the way for the use of such composite perovskites as highly luminescent emitters in various applications such as lighting, displays, and other optoelectronic and photonic devices.

1. INTRODUCTION Lead halide perovskites, which have made significant successes in the applications of high-performance optoelectronic devices such as solar cells and photodetectors,1-9 are emerging

as

one

of

the

most

promising

phosphors

for

phosphor-converted

white-light-emitting diodes (pc-WLEDs) owing to their outstanding physical properties.10-23 In particular, colloidal cesium lead halide perovskite nanocrystals (CsPbX3 NCs, X=halide 2

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ion) have been explored extensively because of their bright photoluminescence (PL) with high PL quantum yields (PLQYs), extremely narrow full width at half maximum (FWHM), and a wide color gamut.16-21 Furthermore, compared with traditional Cd-based chalcogenide NCs, CsPbX3 NCs can be synthesized with much lower temperatures and simpler procedures. Despite these advantages, the use of CsPbX3 NCs for pc-WLEDs still suffers from a series of drawbacks. Firstly, most approaches reported are suitable for the synthesis of CsPbX3 NCs on the lab-scale, thus greatly limiting their industrial scale-up production. Secondly, the stability of CsPbX3 NCs are much poorer than that of the commercially available phosphors. Although many post-synthesis methods have been developed to circumvent this issue, they are costly, time-consuming, and hard to large-scale optimization of the properties of as-synthesized CsPbX3 NCs.24-29 Thirdly, the PLQYs of CsPbX3 NCs are notably decreased when they are in the form of agglomerates with respect to well-dispersed colloidal counterparts. Therefore, much effort is required to make inorganic perovskites suitable for pc-WLED application. Based on the connectivity of [PbX6] octahedra, perovskites can be classified into four crystal

structures

including

three-dimensional

(3-D),

two-dimensional

(2-D),

one-dimensional (1-D), and zero-dimensional (0-D) lattices.30-35 Previous work has revealed that 2-D hybrid perovskites possess excellent stability over 3-D cousins owing to the shielding effect of organic layers.36 0-D perovskites allow a complete isolation of individual [PbX6] octahedra in crystal lattices, leading to stability superior to those with other dimensionalities.30,31,33-35 Some 0-D inorganic perovskites, in particular the Cs4PbBr6-related systems, have received wide attention owing to the high PLQYs and good stability.30,31,33-35 Over the past several years, significant advances for the synthesis of Cs4PbBr6-related 3

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perovskite phosphors have been achieved.30,31,33-35 However, to fully realize its potential for pc-WLED application, the following criteria need to be simultaneously fulfilled: high PLQYs and good stability of perovskites, and environmentally friendly, low-cost, scalable, and high-yield synthesis strategy. Obviously, at present synthesizing Cs4PbBr6-related perovskite phosphors that satisfies these criteria remains a significant challenge. Additionally, it is noteworthy that the emission mechanism of Cs4PbBr6-related systems remains elusive. Some researchers have attributed the emission to the intrinsic emission of Cs4PbBr6,30,31 whereas others have assigned the emission to green-luminescent species, such as CsPbBr3 NCs and defects, existing in the matrix of Cs4PbBr6.34 Herein, we meet these challenges through a novel synthesis that produces micrometer-sized Cs4PbBr6/CsPbBr3 perovskite composites with a near-unity PLQY (up to 95%) and high product yield (up to 71%). Such highly luminescent composites can be readily synthesized by mixing judiciously-selected precursors in dimethyl sulfoxide (DMSO), followed by addition of the HBr water solution. This room-temperature self-assembly method is highly attractive in that it is simple, environment benign, cost-effective, scalable, and high reaction yield. A broad range of experimental characterizations, along with density functional theory (DFT) calculations, help us identify that the formation of Cs4PbBr6/CsPbBr3 composites is based on a dissolution-crystallization process in a mixed solvent of water and DMSO, and that high-efficiency luminescence stems from CsPbBr3 NCs passivated by the Cs4PbBr6 matrix. Additionally, benchmarking of these composites against colloidal CsPbBr3 NCs reveals greatly improved stability. Finally, we demonstrate that green-emitting Cs4PbBr6/CsPbBr3, combined with red-emitting K2SiF6:Mn4+, can be employed for the 4

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construction of WLEDs. Our work offers a facile route for the synthesis of stable, highly luminescent perovskite composites based on the combination of 0-D and 3-D inorganic perovskites.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis Cesium acetate (CsAc, 99.999%, Alfa), PbBr2 (99%, Aladdin), HBr water solution (48% by weight in H2O, J&K), HBr/HAc (33% by weight in HAc, J&K), dimethyl sulfoxide (DMSO, +99.7%, Acros), octadecene (ODE, 90%, Alfa), oleic acid (OA, 90%, Alfa), oleylamine (OLA, 80–90%, Acros), hexane ( ≥98.0%(GC), Aladdin), and toluene (AR, QS) were used without purification unless otherwise noted. Synthesis of Cs4PbBr6/CsPbBr3 composites: For the synthesis of S4, PbBr2 (0.22 mmol) and CsAc (0.88 mmol) were dissolved in dimethyl sulfoxide (DMSO, 0.5 mL), which were stirred for 1 h at room temperature to form precursor solution in a nitrogen-filled glovebox. Then, 0.1 mL aqueous HBr solution was slowly injected into the precursor solution with stirred. Immediately, yellow-green precipitates were formed in the mixed solution and the solution was stirred for 12 h at room temperature. The precipitation was separated from the mixed solution by centrifugation and then washed by 0.2 mL DMSO. After this purification process, the powders were collected and dried at 50 °C under vacuum overnight. We also investigated the amount of DMSO and water used to explore their influence on the properties (i.e., PLQYs and reaction yields) of the products. We note that the volume of H2O was adjusted by changing the amounts of HBr water solution, deionized water and HBr/HAc to keep the Br- amount to be constant. For the synthesis of S1, S1-1, S2.5, S2.5-1, S4-1 and S5, 5

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we only adjusted the amount of CsAc used while kept other factors the same as S4. The DMSO amounts used for the purification of S1, S1-1, S2.5, S2.5-1, S4-1 and S5 are 0.05, 0.20, 0.125, 0.20, 0.40 and 0.25 mL, respectively. The scale-up synthesis of Cs4PbBr6/CsPbBr3 composites was carried out by following the procedure of S4, except that the amounts of all reagents were increased by 50 times. Synthesis of colloidal CsPbBr3 nanocrystals: CsPbBr3 Nanocrystals were synthesized by a strategy as developed by Protesescu et al..16 1.25 mmol Cs2CO3, 1.25 mL OA, and ODE (20 mL) were loaded into a 100 mL three-neck flask and dried under vacuum for 1 h at 120 ºC. The reaction solution was then kept at 150 ºC under N2 until it became clear. ODE (5 mL) and PbBr2 (0.188 mmol) were added into a 50 mL three-neck flask and dried under vacuum for 1 h at 120 ºC. Then, dried OLA (0.5 mL) and dried OA (0.5 mL) were injected under N2. Following complete dissolution of the PbBr2 salt, the temperature was raised to 170 ºC and the Cs-oleate solution (0.4 mL, 0.117 M) was quickly injected. The reaction mixture was kept at this temperature for 5 s and then was cooled to room temperature by an ice-water bath. The final NCs were purified by centrifugation and redispersed in toluene to form a stable solution. The NCs exhibit bright photoluminescence (PLQY=86%). For CsPbBr3 thin films, the toluene dispersion solution was dropped onto the glass substrate, followed by drying at room temperature to completely evaporate any residual solvent. 2.2. Characterization The laboratory XRD patterns were carried out at room temperature using a Bruker D2 PHASER (λ = 1.5418 Å). The high-resolution synchrotron XRD measurement were recorded on the BL02B2 beam line of SPring-8 to obtain high-quality diffraction patterns at room temperature. All samples for high-resolution synchrotron XRD were sealed into Hilgenberg glass capillaries with an inner diameter of 0.3 mm and 0.5 mm. The capillary was rotated 6

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during the measurement to reduce the preferred orientation effect and to average the intensity. The X-ray wavelength used is 0.413162 Å. Rietveld structural refinements were performed against XRD data utilizing the GSAS program.37 The absorption behaviors of the samples were studied by a double-beam UV-Vis-NIR spectrophotometer (Cary 5000, Agilent) equipped with an integrated sphere. The obtained reflectance spectra were converted to pseudo-absorbance spectra using the Kubelka-Munk transformation. PL and PL excitation spectra were taken by using an FLS 980 spectrofluorometer (Edinburgh Instruments Ltd.) or a monochromator (iHR550, Horiba) equipped with a photomultiplier tube (Hamamatsu, R928). The spectrum response of the detection system was corrected using a standard sample. The photostability of Cs4PbBr6/CsPbBr3 composites and CsPbBr3 nanocrystal films were evaluated under the excitation of a 453 nm laser diode; the power density is 6.11 mW/mm2. The long-term stability of Cs4PbBr6/CsPbBr3 and CsPbBr3 nanocrystals was examined by measuring their PLQYs at different intervals after exposure of them at ambient conditions (Humidity: 50%; temperature: 19.5 ºC). Room-temperature PLQY were measured using an integration sphere incorporated into a spectrofluorometer (FluoroLog, Horiba) equipped with a 450 W xenon lamp. Single-particle emission microscopy measurement were taken by using a laser scanning confocal microscopy (LSM 880, Zeiss). Scanning electron microscopy (SEM) images were acquired on an S-4700 microscopy (Hitachi, Japan) operating at 10 kV. Time-resolved PL measurements monitored at 520 nm were acquired on a Lifespec II setup (Edinburgh Instrument, U.K.) with the excitation of a picosecond-pulsed 477 nm laser. Green-emitting Cs4PbBr6/CsPbBr3 composites and red-emitting K2SiF6:Mn4+ phosphors were blended with silicone gel A and B to form phosphor in silicone, which was directly painted on the surface of a blue chip and then dried in a vacuum oven. The optical properties of the fabricated devices were determined using an integrating sphere with an analyzer system. 2.3. DFT calculations 7

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The band structure and density of state of Cs4PbBr6 were simulated using the Vienna ab initio package (VASP), based on the local density-functional approximation, plane-wave basis (Ecut=450 eV), and noncollinear projector-augmented waves (PAW) method.38 We use Perdew Burke-Ernzerhof (PBE) version of generalized gradient approximation to describe the electron exchange-correlation potential.39 The Brillouin zone is sampled with the Gamma centered meshes of the K points with 2×2×2, the ions were relaxed until the maximum forces were less than 0.01 eV· Å-1 and the total energy was converged to 1×10-4 eV/atom.

3. RESULTS AND DISCUSSION 3.1 Large-Scale Synthesis and Structural Characterization of Cs4PbBr6/CsPbBr3 Composites. The preparation process for the product is shown in Figure 1a. Two precursors, cesium acetate (CsAc) and PbBr2 (0.22 mmol), were added into DMSO (0.5 mL), resulting in white turbid dispersion with a large amount of precipitates that signifies an incomplete dissolution of the precursors (Figure S1, Supporting Information). Then, 0.1 mL HBr water solution (48% by weight in H2O) was added under stirring, which leads to a quick change in color from white to green. After the completion of the reaction, the yellow-green precipitate, with an irregular shape (Figure S2), was separated from the mother solution by centrifugation and then washed by DMSO, followed by drying in vacuum. The samples are denoted Sx and Sx-1, where x is the molar ratio of CsAc to PbBr2. Note that the difference between Sx and Sx-1 is the DMSO amount used for the purification. The detailed synthesis parameters and properties of the products are summarized in Table 1. The yielded product displays strong green PL under UV (365 nm) irradiation. We stress that this room-temperature synthetic technique is scalable for the large-scale preparation of such phosphors, as demonstrated in Figure 1b. The laboratory X-ray diffraction (lab-XRD) 8

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patterns of S1, S1-1, S2.5, S2.5-1, S4 and S4-1 match well with a rhombohedral Cs4PbBr6 phase in space group R-3c (Figure 1c), forming a Bergerhoff-Schmitz-Dumont-type structure. However, it is noted that impurity phases occur in S5.

Figure 1. (a) Schematic illustration of the room-temperature self-assembly process for the product. (b) Images of the product prepared on a large scale under daylight (left) and the illumination of 365 nm light in the dark (right); the ratio of CsAc to PbBr2 is 4, and the mass of the product is 9.3 g. (c) Lab-XRD patterns of the S1, S1-1, S2.5, S2.5-1, S4 , S4-1 and S5 samples.

Table 1. Summary of the synthesis parameters and properties of the yielded products.

PLQY (%)

Sizes of CsPbBr3 c (nm)

17.77

60.16

30

200

10.35

92.77

11

100

125

51.82

67.50

32

500

100

200

34.68

95.29

13

500

100

200

70.77

95.25

14

DMSOa HBr/H2O DMSOb Product (µl) (µl) (µl) yield (%)

Sample

Molar ratio of CsAc:PbBr2

S1

1:1

500

100

50

S1-1

1:1

500

100

S2.5

2.5:1

500

S2.5-1

2.5:1

S4

4:1

9

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S4-1

4:1

500

100

400

52.27

97.53

13

S5

5:1

500

100

250

/

/

/

a

DMSO amounts used for reaction.

b

DMSO amounts used for purification of the product.

c

The sizes of CsPbBr3 calculated by the Scherrer equation.

It is well recognized that inorganic perovskites with different dimensionalities possess quite different X-ray coherent scattering capabilities. For instance, CsPbBr3 perovskites show much weaker XRD patterns than Cs4PbBr6, as shown in Figure S3, thus making it hard to discriminate CsPbBr3 phase from Cs4PbBr6 when the relative amount of CsPbBr3 is low. To accurately identify the structure of the yielded products, we then took synchrotron XRD measurement. Astonishingly, different from the lab-XRD result, notable impurity peaks assigned to CsPbBr3 perovskites are observed. Crystal structure refinement by the Rietveld method has been carried out using the software package GSAS (general structure analysis system).37 Considering the coexistence of CsPbBr3 and Cs4PbBr6 phases, the following Rietveld refinement of the synchrotron data for S1 converged to Rwp=4.75% and Rp=6.73% (Figure 2a). The weight ratios of CsPbBr3 phase in S1 and S2.5, obtained by the Rietveld refinement, are roughly estimated to be 0.90% and 0.48%, respectively (Figure S4, Table S1 and S2). Unfortunately, the weight ratio of CsPbBr3 phase in S1-1, S2.5-1, S4 and S4-1 are difficult to be determined by the Rietveld refinement, as a consequence of a much lower concentration of CsPbBr3 phase (Figure S5). We emphasize that the use of synchrotron XRD helps us gain direct experimental evidence on the existence of CsPbBr3 phase in our products.

3.2 Photophysical Properties of the Cs4PbBr6/CsPbBr3 Composites, DFT Calculation 10

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and Luminescence Mechanism. Figure 2b displays the UV-Vis absorption, PL and PLE spectra of S4. The absorption spectrum of S4 shows an absorption onset at 550 nm and a strong absorption peak at 310 nm. Under UV or visible light excitation over a broad spectral range, S4 demonstrate strong green emission peaked at 519 nm and with an FWHM of 23.8 nm, of which the emission profile is similar to those of S1, S1-1, S2.5, S2.5-1, and S4-1 (Figure S6).

Figure 2. (a) Rietveld fit to the high-resolution synchrotron XRD pattern of the S1 sample. The solid lines and overlying crosses show the calculated and observed intensities, respectively. The pink lines (CsPbBr3 phase) and green lines (Cs4PbBr6 phase) indicate the positions of the calculated Bragg reflections. The difference between the observed and calculated profiles is shown in blue. Inset shows the enlarged image corresponding to the main peaks of CsPbBr3 phase. (b) Absorption, PL and PLE spectra of S4; the monitored wavelength for PLE is 519 nm, and the excitation wavelength for PL is 365 nm. 11

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To further know the emission characteristics of the yielded product, we took confocal emission microscopy measurement for S4 (Figure 3a-c, Figure S7). It is found that the particles show inhomogeneous green luminescence under 488 nm excitation, signifying that the emitting species are randomly distributed in the particles. Additionally, some regions look white in the bright-filed and overlaid images, suggesting the absence of absorption for the visible light. The origin of green luminescence from Cs4PbBr6-related systems is the subject of continuing debate.30-35,40-46 Previous investigations have been suggested that the PL originates from the excitonic emission from the Cs4PbBr6 phase or from the CsPbBr3 phase embedded in the PL-inactive Cs4PbBr6 crystalline matrix.30-35,40,41 Very recently, several authors proposed that the emission probably arises from bromine vacancy,42,43,44 given the fact that this defect generally leads to formation of trap states near the conduction band. To gain deep insight into the emission mechanism for our product, we carried out DFT calculation on the electronic structure and density of states (DOS) of the Cs4PbBr6 phase. As shown in Figure 3d, Cs4PbBr6 possesses a band gap of 3.84 eV, and the contributions to the upper valence band and the conduction band predominately originate from Br 4p and Pb 6p orbitals, respectively. We note that this is consistent with the results of the absorption spectrum and agrees well with the result of previous theoretical calculation.40 Coupled with structural and PL features as discussed, it is suggested that the observed white regions in the bright-field and overlaid confocal images correspond to the Cs4PbBr6 phase and that the green emission in our products is not from the Cs4PbBr6 phase.

12

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Figure 3. (a) Bright-field single-particle image of the S4 sample. (b) Dark-field single-particle image of the S4 sample under 488 nm excitation. (c) Overlaid image of the bright- and dark-field images as shown in (a) and (b), respectively. Scale bar: 5 µm. (d) DFT calculated band structure and partial DOS (PDOS) of the Cs4PbBr6 perovskites.

Considering that structural defects tend to be generated at the surfaces of the particles, the emission behaviors of the products might be well expected to be influenced when using more DMSO for the purification, if the emission stems from the structural defects. Nevertheless, as illustrated in Figure S6, the products washed by more DMSO show nearly identical emission profiles compared with the counterparts by less DMSO. We stress that changing the ratio of CsAc to PbBr2, while keeping the amount of HBr as a constant, can create reaction environments where the ratio of Cs:Pb:Br is in a broad range. In such a case, it is well expected that the emission center can be greatly altered if the emission is from structural 13

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defects like bromide vacancies. However, varying the precursor ratio of CsAc:PbBr2 from 1:1 to 4:1 merely leads to the shift of PL less than 4 nm (Figure S6), meaning that changing the reaction parameters almost cannot affect the emission center. All these observations leads us to rule out the possibility of the observed PL from bromide vacancies in our products. We stress that the excitation/emission profiles of the products are similar to those of CsPbBr3 NCs,16 and the CsPbBr3 NCs were unambiguously identified to exist in them. Taken all these experimental and theoretical results together, we conclude that the observed PL in our samples can be attributed to the CsPbBr3 NCs, although its concentration is low. Based on the above assignment, the absorption onset at 550 nm for S4 can be assigned to CsPbBr3 NCs, and the strong absorption peak at 310 nm is associated with the Cs4PbBr6 matrix.34 The sharp dip at ca. 310 nm in the PLE spectrum indicates that light is absorbed by the Cs4PbBr6 matrix and does not reach CsPbBr3. We note that the PL peak positions of our samples are longer than that of CsPbBr3 NCs. It has been revealed that CsPbBr3 NCs demonstrate size-dependent Stokes shift of PL. CsPbBr3 NCs with an edge length of 9 nm shows PL peaking at ca. 500 nm, whereas that with an edge length of 11.8 nm shifts to ca. 516 nm.16 The sizes of CsPbBr3 in S1 and S2.5 were calculated to be 30 and 32 nm, respectively, by the Scherrer equation (Table 1), which are much larger than NCs synthesized by the hot-injection strategy. However, close examination of the synchrotron XRD patterns of S1 and S2.5 shows that the diffraction peaks assigned to the CsPbBr3 phase consist of broad bands with sharp peaks (Figure S5a and S5c), and that the sharp peaks can be removed by using more DMSO for purification (Figure S5b and S5d). This can be well explained by the coexistence of Cs4PbBr6-passivated and unpassivated CsPbBr3 in S1 and S2.5 (Figure S8). It 14

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is noted that using more DMSO to wash away unpassivated CsPbBr3 in S1 and S2.5 results in PLQYs over 90% but much lower product yields (Table 1). We thus conclude that the low PLQYs of S1 and S2.5 is due to the existence of some PL-inefficient, large-sized CsPbBr3 powders that is not well passivated by Cs4PbBr6. The near-unity PLQYs for S4 and S4-1 suggest that almost all CsPbBr3 NCs are well passivated by Cs4PbBr6. Interestingly, it is found that the product yields and PLQYs are associated with the ratio of precursors, and the highest product yield of 71% and the near-unity PLQY (95%) can be achieved for S4 (Table 1, Figure S9). To the best of our knowledge, such a high product yield and PLQY are hard to be simultaneously achieved for lead halide perovskite phosphors. We note that the decay curves of all samples can be well fitted by a tri-exponential function (Figure 4a and b, Table S3). In general, luminescent systems with near-unity PLQYs show single-exponential decays. However, for one system with near-unity PLQY and containing more than one emitting species, it is possible to show multiexponential decays when all these emitters possess near-unity PLQYs but different radiative recombination rates. We thus surmise that the multi-exponential decay observed for S4 could be attributed to the radiative recombination of more than one luminescent CsPbBr3 species with different sizes.46 Although the average sizes of CsPbBr3 passivated by Cs4PbBr6 in S1-1, S2.5-1, S4 and S4-1 are hard to be accurately determined owing to the poor signal to noise ratio of the XRD pattern caused by its small amount and weak X-ray scattering capability, we presume that it should be in the nanometer scale based on the roughly calculated size (Table 1). Our work thus adds to the observations and resulting conclusions on the embedded NC origin of the efficient green luminescence. Under continuous illumination by 453 nm light, 15

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Cs4PbBr6/CsPbBr3 phosphors show much better stability than colloidal CsPbBr3 NCs (Figure 4c and d). The emission intensity of Cs4PbBr6/CsPbBr3 retains about 93% after illumination for 80 min, whereas that of CsPbBr3 NCs remains less than 45%.

Figure 4. Time-resolved PL of (a) the S1, S2.5, and S4 samples and (b) the S1-1, S2.5-1, and S4-1 samples monitored at 520 nm. Photostability of (c) Cs4PbBr6/CsPbBr3 composites and (d) CsPbBr3 NC film under continuous illumination by 453 nm light. The power density is 6.11 mW/mm2. The edge length of CsPbBr3 NC used here is around 9 nm. We note the PL peak wavelength of NC solution is 500 nm, and the redshift of emission in the NC film may be caused by the agglomeration of NCs.

Additionally, we find that the long-term stability of Cs4PbBr6/CsPbBr3 is superior to CsPbBr3 NCs (Figure 5a). It is noted that the PL intensity gradually decreases with an increase of temperature, which can almost restore to the original state when cooling the 16

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sample to room temperature. This behavior is similar to that observed in Cs4PbBr6-related system as reported previously.31 We also note that, under identical measurement conditions, our Cs4PbBr6/CsPbBr3 phosphors show comparable emission peak intensity with respect to commercially available green-emitting β-SiAlON:Eu2+ phosphors (Figure 5b).

Figure 5. (a) PLQYs of Cs4PbBr6/CsPbBr3 composites and CsPbBr3 NCs exposed to air for different times. (b) PL spectra of the S4 sample and commercially available green-emitting β-Si2AlON:Eu2+ phosphors when excited by 460 nm light. Both samples were measured under identical conditions.

3.3 Formation Mechanism of Cs4PbBr6/CsPbBr3 Composites. Considering the tantalizing PL features of the product, exploring its formation mechanism is therefore intriguing. One fact is that Cs4PbBr6/CsPbBr3 forms in the mixed solvents of water and DMSO. It is known that DMSO are a good solvent for PbBr2 and CsPbBr3. Additionally, the addition of HBr water solution could promote the dissolution of CsAc, thanks to excellent solubility of CsBr in water. We thus presume that the formation of Cs4PbBr6/CsPbBr3 composites could be based on a dissolution-crystallization process enabled by such a mixed solvent, and that highly luminescent CsPbBr3 NCs well protected by the Cs4PbBr6 framework can survive after the reaction (Figure 1a). As a consequence, the amount of solvents used is expected to affect 17

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the properties of the resulting products. Control experiment by adding more DMSO during the reaction produces powders with a comparable PLQY and decreased product yield, whereas decreasing DMSO amount results in a decreased PLQY and increased product yield. This may be caused by the dissolution of Cs4PbBr6 into DMSO (Table 2). Similarly, adding or decreasing water in the reaction system leads to products with relatively poor PLQYs (Table 2), although less water corresponds to a relatively larger product yield. These observations strongly signify that varying the amount of any solvent can affect the balance between precipitates and solutes in the reaction system, thus influencing the properties of Cs4PbBr6/CsPbBr3 composites. We stress that DMSO used for purification not only wash way PL-inefficient CsPbBr3 NCs unpassivated by Cs4PbBr6, but also can etch the Cs4PbBr6 powders that leads to a decreased product yield (Table 1); the almost identical PLQYs for S4 and S4-1 signify that most CsPbBr3 NCs in both cases are well passivated by Cs4PbBr6. Obviously, the key for the attainment of Cs4PbBr6/CsPbBr3 with a high PLQY and product yield lies in the judicious balance of the amounts of solutes and solvents used, which leads to the formation of high-quality CsPbBr3 NCs well passivated by the Cs4PbBr6 matrix. We also note that our method leads to phosphors with a much higher product yield than that by the method of Quan et al. where a large amount of Cs source was discarded.34

Table 2. Product yields and PLQYs of the products synthesized by changing the amounts of solvents used, while retaining the ratio of CsAc:PbBr2 as 4:1. DMSO (µl) 350

H2O (µl)

Product yield (%)

77

75.02 18

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PLQY (%) 76.75

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450

77

72.91

90.70

500

77

70.77

95.25

550

77

68.59

90.99

650

77

64.75

93.34

500

57

77.54

82.87

500

72

76.26

84.24

500

82

70.81

86.98

500

97

69.83

89.63

3.4 WLED Application. We emphasize that the strategy developed here can simultaneously fulfill the aforementioned criteria in terms of high PLQYs and good stability of the product, and environmentally friendly, low-cost, scalable, and high-yield synthesis, although increasing the ratio of highly luminescent CsPbBr3 phase in this composite remains an open question for further investigation to boost its brightness. We stress that the attainment of Cs4PbBr6/CsPbBr3 phosphors with a near-unity PLQY in solid state via such a facile route makes it promising for the solid-state lighting application. We constructed a pc-WLED using Cs4PbBr6/CsPbBr3 composites and K2SiF6:Mn4+ as green- and red-emitting phosphors, respectively, and a blue InGaN chip as the excitation source. The electroluminescence spectrum of the WLED operated at different currents is shown in Figure 6. The luminous efficacy is determined to be 73.8 lm/W operated at 5 mA, comparable to that constructed using CsPbBr3 NCs incorporated into a silica/alumina monolith.27

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Figure 6. Electroluminescence spectra of the constructed WLED operated at indicated currents. The inset shows the related photograph of the working LED.

4. Conclusions To summarize, we have presented a facile room-temperature self-assembly strategy for the synthesis of Cs4PbBr6/CsPbBr3 composite phosphors with a near-unity PLQY and having stability better than colloidal CsPbBr3 NCs. The method developed here is highly attractive in that it is easy-to-control, enviromentally friendly, cost-effective, scalable, and high product yield. The combined experimental and theoretical characterizations suggest that the PL originates from high-quality CsPbBr3 NCs passivated by the Cs4PbBr6 framework, which is formed based on a dissolution-crystallization process in the mixed solvent of water and DMSO. The ample evidences concerning the existence of CsPbBr3 NCs in 0-D Cs4PbBr6 highlight the importance in identifying the phase purity of such 0-D systems by synchrotron XRD rather than lab-XRD to gain deep insights into the structure-property relationship and to avoid any false positive. Our work provides a facile route for the synthesis of stable, highly 20

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luminescent perovskites based on the combination of 0-D and 3-D inorganic perovskites. We anticipate that our work could pave the way for the use of such composite perovskites as highly luminescent emitters in various applications such as lighting, displays, and other optoelectronic and photonic devices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Confocal emission microscopic images, SEM images, XRD, PL spectra, high-resolution synchrotron XRD patterns, supplementary tables.. AUTHOR INFORMATION Corresponding Author Email: [email protected] Author Contributions ||

Y.-M. Chen, Y. Zhou, Q. Zhao, and J.-Y. Zhang contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 11574225 and 51672106), Jiangsu Specially Appointed Professor program (Grant No.

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SR10900214), Natural Science Foundation of Jiangsu Province for Young Scholars (BK20140336), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The SPring-8 experiment was carried out with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal No. 2016B1205). H.-T. Sun greatly thanks the fruitful comments from Prof. Osman Bakr in King Abdullah University of Science and Technology (KAUST).

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Lett. 2017, 8, 3266-3271. (46) Xu, J.; Huang, W.; Li, P.; Onken, D. R.; Dun, C.; Guo, Y.; Ucer, K. B.; Lu, C.; Wang, H.; Geyer, S. M. Imbedded Nanocrystals of CsPbBr3 in Cs4PbBr6: Kinetics, Enhanced Oscillator Strength, and Application in Light-Emitting Diodes. Adv. Mater. 2017, 29, 1703703.

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