Continuous Synthesis of Highly Stable Cs4PbBr6 Perovskite

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Continuous Synthesis of Highly Stable Cs4PbBr6 Perovskite Microcrystals by a Microfluidic System and Their Application in White-Light-Emitting Diodes Zhen Bao,† Hung-Chia Wang,† Zhen-Feng Jiang,‡ Ren-Jei Chung,*,‡ and Ru-Shi Liu*,†,§,⊥ Department of Chemistry and §Advanced Research Center of Green Materials Science and Technology, National Taiwan University, Taipei 106, Taiwan ⊥ Department of Mechanical Engineering and Graduate, Institute of Manufacturing Technology, and ‡Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, 106, Taiwan Inorg. Chem. Downloaded from pubs.acs.org by NEWCASTLE UNIV on 10/17/18. For personal use only.

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S Supporting Information *

Recently, some reports demonstrated that reducing the dimensionality of the perovskite structure improved the PLQY because the exciton binding energy increased.12−14 Saidaminov et al.8 synthesized zero-dimensional perovskite solids through one-pot synthesis. The zero-dimensional Cs4PbBr6 perovskite solids demonstrated long-term stability in ambient conditions (relative humidity of 57%). Chen et al.15 demonstrated a cooling method to grow the centimeter-sized Cs4PbBr6 crystals. Their crystals can emit strong green light with absolute PLQY up to 97%, which is attributed to the embedded CsPbBr3 in the matrix of the Cs4PbBr6 crystals. However, Yang et al.16 demonstrated that green photoluminescence (PL) of Cs4PbBr6 materials may be caused by the bromine-vacancy-related defect. Up to now, the real mechanism for the green emission of Cs4PbBr6 is still underway. For practical applications, a microfluidic system is a considerable approach to mass production, which is another significant problem. Lignos et al.17 used a microfluidic reactor in the synthesis of CsPbX3 (X = Cl, Br, I) PQDs along with in situ PLQY and absorption measurement. The reaction parameters can be easily optimized through this microfluidic system. Ma et al.18 built the one-dimensional−two-dimensional microreactors by a microfluidic spinning technique to achieve continuous CsPbBr3 PQDs/poly(methyl methacrylate) nanocomposite production for various optoelectronic applications. In this work, we successfully used the microfluidic system (easy-Medchem; Figure S1) to continuous providely Cs4PbBr6 MC production. Different crystal growth temperatures and washing methods were used to optimize the Cs4PbBr6 MCs. Finally, the Cs4PbBr6 MCs were fabricated with K2SiF6:Mn4+ red phosphor into white-LEDs to improve the color gamut. The synthesis using a microfluidic system is illustrated in Scheme 1. The Cs4PbBr6 MCs were synthesized through a modified recrystallization procedure during heating. The PbBr2 and CsBr reagents were dissolved with oleic acid into dimethyl sulfoxide (DMSO) as the precursor solution, and oleylamine was dissolved in toluene as the precipitant solution. In the reaction procedure, the polar solvent DMSO was a good solvent for dissolving PbBr2 and CsBr, while the nonpolar solvent toluene thoroughly dissolved inorganic salts, which can promote their reprecipitation.19 The precursor and precipitant solutions

ABSTRACT: In this paper, we report a simple, rapid, and stable method for the continuous synthesis of highly stable Cs4PbBr6 perovskite microcrystals (MCs) using a microfluidic system. To demonstrate the potential application of Cs4PbBr6 MCs, the sample was fabricated with K2SiF6:Mn4+ phosphor onto InGaN blue chips as white-light-emitting diodes (LEDs). Our white-LED device achieved a high National Television Standards Committee value of 119% for backlight display, which indicated that the Cs4PbBr6 MC is a promising material for future applications.

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n recent years, perovskite nanocrystal has tremendous potential for many applications, such as light-emitting diodes (LEDs), solar cells, lasers, and photodetectors.1−4 It has a promising future in backlight display applications because of its excellent optical performance of high photoluminescence quantum yield (PLQY), narrow emission wavelength, and color-tunable property.5 Although it has outstanding optical properties, many challenges, such as poor thermal stability, photostability, water resistance, and anion exchange, still must be overcome. To improve the stability of perovskite quantum dots (PQDs), Wang et al.6 first encapsulated CsPbX3 PQDs into mesoporous (MP) silica matrix power, which enhanced the thermal stability and photostability. However, the decrease of the PLQY during the encapsulation of PQD nanocomposites was quite difficult to overcome. To obtain high and stable PLQY performance, an alternative is to reduce the structure dimensionality of the perovskite material. Perovskite materials can be classified into four kinds of dimensional crystal structures that depend on the connectivity property of the corner-sharing PbX64− octahedral.7−9 The general formula of a perovskite solid can be written as AnBX2+n, where A is a monovalent cation, such as Cs+, B is divalent metal, such as Pb2+, and X is a halogen anion, such as Cl−, Br−, and I−. The dimensionality of the perovskite material was calculated by 4 − n. Only a zero-dimensional Cs4PbX6 perovskite structure allows full isolation of each individual PbX64− octahedron. The isolated structure can promote the recombination of excitons, which makes the PLQY more stable.10,11 © XXXX American Chemical Society

Received: July 15, 2018

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DOI: 10.1021/acs.inorgchem.8b01985 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Scheme 1. Schematic of the Synthesis of Cs4PbBr6 Perovskite MCs through a Microfluidic System

Figure 1. (a) XRD patterns of CsPbBr3 powder and Cs4PbBr6 MCs washed with different solvents. SEM images of Cs4PbBr6 MCs (b) unwashed, (c) washed with TBA, and (d) washed with DMSO. EDS mapping images of (e) cesium, (f) lead, and (g) bromine that were acquired at the same position as part d.

were loaded into two reagent bottles, and then they were pumped into the microfluidic system. Before flowing into the reactor, the solutions were mixed with a Micro-T mixer. During mixing of the precursor and precipitant solutions, the Cs4PbBr6 perovskite tended to nucleate with the coprecipitation of PbBr2 and CsBr. Then, the Cs4PbBr6 perovskite crystal nuclear grew into MCs under heating in the reactor. Different flow rates, reagent ratios, and crystal growth temperatures were used to optimize the Cs4PbBr6 perovskite product (Tables S1 and S2). The flow rates of both the precursor and precipitant solvents were finally chosen as 1 mL/min, at which the reaction time was 5 min and the highest product yield was obtained. Then, a series of reagent ratios and reaction temperatures were used at this flow rate. As the reaction temperature increased, the Cs4PbBr6 MCs can emit stronger under UV light (Figure S2). To obtain pure Cs4PbBr6 MCs, the Cs4PbBr6 product was washed with tert-butyl alcohol (TBA) or DMSO. TBA could only dissolve the impurity in the product, while DMSO with higher polarity could also partly dissolve the Cs 4PbBr6 perovskite. The product could only be washed by DMSO at most twice because all of the product was dissolved after washing three times (Figure S3). As a consequence, the Cs4PbBr6 MC powder showed a clearer yellow-green color and emitted stronger green light under UV light after each time of washing (Figure S4). As a comparison, CsPbBr3 quantum dots were synthesized via the hot-injection method and precipitated with a hexane/TBA mixed solution (1:4 volume ratio). After washing twice, all of the Cs4PbBr6 MCs and CsPbBr3 powder samples were dried at 90 °C for 12 h. Figure 1a shows the structure characterization results and powder X-ray diffraction (XRD) patterns of the CsPbBr3 and Cs4PbBr6 powders washed with different solvents. The CsPbBr3 powder and both Cs4PbBr6 MC samples exhibited a highly crystalline structure and good agreement with the Crystallography Open Database (COD) reference nos. 1533063 and 1538416 respectively. This result proves that all of the samples are a pure single phase. Scanning electron microscope (SEM) images of the Cs4PbBr6 MCs unwashed sample and samples washed with TBA or DMSO are shown in Figure 1b−d. The surface morphology of Cs4PbBr6 MCs washed with TBA did not show an obvious difference compared with that of the unwashed sample. However, because of the higher solubility of DMSO, the surface morphology of the Cs4PbBr6 MCs washed with DMSO is much smoother. Parts e−g of Figure 1 show the energydispersive spectroscopy (EDS) mapping images of cesium, lead, and bromine that were acquired at the same position of Figure 1d. The elemental analysis data of the yellow square in Figure 1d

is shown in Figure S6. Moreover, the X-ray photoelectron spectroscopy (XPS) data (Figure S7) is consistent with the EDS data, which indicates uniform elemental distribution in our synthesized Cs4PbBr6 MCs not only in the bulk but also on the surface. The optical properties and thermal stability tests of Cs4PbBr6 MCs are shown in Figure 2. The PL intensity of unwashed Cs4PbBr6 MCs enhanced with an increase of the reaction temperature (Figure 2a). The Cs4PbBr6 MCs synthesized at 150 °C emitted highest PL intensity, which was purified for further

Figure 2. (a) PL spectra of unwashed Cs4PbBr6 MCs synthesized at different temperatures. (b) PL excitation spectra of Cs4PbBr6 MCs monitored under an emission wavelength of 520 nm and emission spectra excited by a wavelength of 460 nm. (c) PL spectra of the CsPbBr3 powder and Cs4PbBr6 MCs washed with different solvents. (d) PL spectra of Cs4PbBr6 MCs washed with DMSO after heating at different temperatures. Inset: Normalized PL intensity of Cs4PbBr6 MCs washed with DMSO and the CsPbBr3 powder after heating at different temperatures. (e) PLQYs of DMSO-washed Cs4PbBr6 MCs synthesized at different temperatures. (f) PL decay times of Cs4PbBr6 MCs and the CsPbBr3 powder. B

DOI: 10.1021/acs.inorgchem.8b01985 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry property tests. As shown in Figure 2b, the Cs4PbBr6 MCs washed with DMSO had board excitation spectra ranging from 340 to over 500 nm. While the emission spectra of the samples were quite narrow with a peak position at 521 nm, the full width at half-maximum (fwhm) was only 23 nm. As a comparison, the CsPbBr3 powder and Cs4PbBr6 MCs washed with TBA were emitted at similar positions of 519 and 521 nm with broader fwhm of 28 and 31 nm, respectively (Figure 2c). The Cs4PbBr6 MCs washed with DMSO were selected for the thermal stability test. The Cs4PbBr6 MCs had obviously higher thermal stability compared with the CsPbBr3 powder (Figure 2d). After heat treatment at 150 °C for 10 min, the Cs4PbBr6 MCs still exhibited 92% PL intensity. On the contrary, the PL intensity of the CsPbBr3 powder decreased to only 33% (Figure S8). The quantum yields of the CsPbBr3 powder and Cs4PbBr6 MCs were measured by using an absolute PLQY spectrometer (c11347, Hamamatsu). The excitation wavelength was set at 460 nm to simulated the working status on the InGaN blue chips. The absolute quantum yield of the CsPbBr3 powder was quite low (PLQY < 0.1%), while the absolute quantum yields of Cs4PbBr6 MCs washed with TBA or DMSO can reach 15% and 24%, respectively. Moreover, the PLQYs of Cs4PbBr6 MCs synthesized at different temperatures increased with an increase of the synthesis temperature, as shown in Figure 2e. The PL decay times of Cs4PbBr6 MCs and the CsPbBr3 powder are shown in Figure 2f. The origin data were fitted by the following formula:

Figure 3. Photographs of (a) LED devices fabricated with Cs4PbBr6 MCs and K2SiF6:Mn4+ phosphor and (b) their working status. (c) Color coordinate of the white-LEDs. (d) EL spectra of the fabricated white-LED. (e) Color gamut of the white-LEDs and the NTSC standard.

In summary, we presented a simple, rapid, and stable method to achieve the continuous production of high performance Cs4PbBr6 perovskite MCs.

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fit = A + B1e−t / T1 + B2 e−t / T2 + B3e−t / T3

The fitting results of Cs4PbBr6 MCs are T1 = 18.44 ns, T2 = 132.4 ns, and T3 = 1303 ns and those of the CsPbBr3 powder are T1 = 7.065 ns, T2 = 97.17 ns, and T3 = 561.0 ns. The Cs4PbBr6 MCs have obvious longer lifetimes than the CsPbBr3 powder. This may lead to the higher PLQYs and stronger luminescence from Cs4PbBr6 MCs. The Cs4PbBr6 MCs were fabricated with CsPb(Br/I)3 red PQDs onto InGaN blue chips to construct the white-LEDs. As shown in the electroluminescence (EL) spectra (Figure S9), the Cs4PbBr6 MCs did not exhibit any ion exchange with CsPb(Br/ I)3 red PQDs. These LEDs had stable white-light color coordinates (Figure S10). However, ion exchange occurred when CsPbBr3 green PQDs were fabricated with CsPb(Br/I)3 red PQDs (Figure S11), which caused the LED cannot emit white light stably. This result illustrated that Cs4PbBr6 MCs have better structure stability than CsPbBr3 quantum dots. The EL intensity of LEDs fabricated with Cs4PbBr6 or CsPbBr3 depends on the operating time shown in Figure S12, which indicates that the Cs4PbBr6 MCs are much more stable than the CsPbBr3 one. To achieve better device performance and stability, the Cs4PbBr6 MCs were fabricated with K2SiF6:Mn4+ red phosphor. The photographs of the fabricated LED devices and their working statuses are shown in Figure 3a,b. These LED devices mixed RGB color together to emit white light. The color coordinate was optimized at (0.2879, 0.2434) in CIE 1931 (Figure 3c), which is suitable for backlight display applications. As shown in Figure 3d, three peaks, corresponding to the InGaN chip, Cs4PbBr6 MCs, and K2SiF6:Mn4+ red phosphor, were observed in the EL spectra of the white-LED. This white-LED device exhibited a wide color gamut of 119% of National Television Standards Committee (NTSC) standard (Figure 3e) and a luminous efficiency of 13.91 lm/W. These results demonstrated the success of fabricating white-LEDs by Cs4PbBr6 MCs.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01985. Experimental section, photographs of the microfluidic system and different samples, lattice parameters, EDS elemental analyst result, XPS, PL, and EL spectra, and the color coordinate of LED, and structure of Cs4PbBr6 perovskite and its properties (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.J.C.). *E-mail: [email protected] (R.S.L.) ORCID

Ru-Shi Liu: 0000-0002-1291-9052 Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Advanced Research Center of Green Materials Science and Technology from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (Contracts MOST 107-2113-M-002008-MY3 and MOST 107-3017-F-002-001). C

DOI: 10.1021/acs.inorgchem.8b01985 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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(19) Zhang, Y.; Saidaminov, M. I.; Dursun, I.; Yang, H.; Murali, B.; Alarousu, E.; Yengel, E.; Alshankiti, B. A.; Bakr, O. M.; Mohammed, O. F. Zero-Dimensional Cs4PbBr6 Perovskite Nanocrystals. J. Phys. Chem. Lett. 2017, 8, 961−965.

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DOI: 10.1021/acs.inorgchem.8b01985 Inorg. Chem. XXXX, XXX, XXX−XXX