Innovatively Continuous Mass Production Couette-taylor Flow: Pure Inorganic Green-Emitting Cs4PbBr6 Perovskite Microcrystal for display technology

We report for the first time the mass production of Cs4PbBr6 perovskite microcrystal with a Couette-Taylor flow reactor in order to enhance the efficiency of the synthesis reaction. We obtained a pure Cs4PbBr6 perovskite solid within 3 hrs that then realized a high photoluminescence quantum yield (PLQY) of 46%. Furthermore, the Cs4PbBr6 perovskite microcrystal is applied with red emitting K2SiF6 phosphor on a blue-emitting InGaN chip, achieving a high-performance luminescence characteristics of 9.79 lm/W, external quantum efficiency (EQE) of 2.9%, and correlated color temperature (CCT) of 2976 K; therefore, this perovskite is expected to be a promising candidate material for applications in optoelectronic devices.

In the LED packaging process, solid state powder is generally more convenient to apply to optoelectronic applications as well as LED package relative to the film form. It is more useful if a fully inorganic Cs 4 PbBr 6 perovskite solid can be produced on a large scale. Although the synthesis and optical characteristics of Cs 4 PbBr 6 solid have been reported, to the best of our knowledge, there has been no report on the large-scale production of the Cs 4 PbBr 6 solid.
Herein, we report for the first time the large-scale synthesis of green-emitting Cs 4 PbBr 6 perovskite solid as a promising candidate for application in optoelectronic devices via the Couette-Taylor flow method. To date, only mass production of graphene oxide (GO) has been reported within the framework of the Couette-Taylor flow method. The Couette-Taylor flow reactor consists of two coaxial cylinders with inner cylinder rotating and generating toroidal vortices that are regularly spaced along the cylinder axis at the critical rotating speed. This toroidal motion of fluids leads to a highly efficient radial mixing of reactant in the system, thereby enhancing the synthesis reaction efficiency. Fully inorganic Cs 4 PbBr 6 perovskite powder were used as the luminescence source in the electroluminescence of LEDs. The optimized contents of Cs 4 PbBr 6 perovskite powder displayed an outstanding PLQY as well as 1931 CIE coordinates for device performance. This technique can be an important method for application in the fabrication of optoelectronic devices.
For synthesis of the Cs 4 PbBr 6 perovskite microcrystals with the Couette-Taylor flow method, 42.562 g of CsBr (0.2 mol) and 73.402 g of PbBr 2 (0.2 mol) precursor were dissolved in 5 L of DMF. Then, prepared 500 mL of OA and 500 mL OLA were added to 5 L of an n-hexane medium reactor solution. Precursor and medium reactor solutions were added in the Couette-Taylor flow reactor by a peristaltic pump at the same time and same volume (20 mL/min). The Couette-Taylor flow reactor (length: 500 mm) consists of two coaxial cylinders: the outer cylinder (radius: 68 mm) is fixed, and the inner cylinder (radius: 60 mm) rotates. After each solution was introduced into the gap between the two cylinders, the inner cylinder was rotated. The rotating speed of the inner cylinder was 1000 rpm for the reaction time. The mixture in a Couette-Taylor flow reactor produced a green-colored reactant within 15 min. To examine the dependence of the luminous characteristics on the synthetic technique, the stirring method is introduced. 0.085 g of CsBr (0.4 mmol) and 0.146 g of PbBr 2 (0.4 mmol) were dissolved in 10 mL of DMF. After dissolving the precursor solution, 10 mL was quickly injected into the medium reactor solution, consisting of OA (1 mL) and OAm (1 mL) in 10 mL of n-hexane to induce the reaction via various stirring conditions. The solution color was changed from milky-white to yellow-green. Then, after continuous stirring for 30 minutes, strong green emission was observed under a 365-nm wavelength lamp. The solutions were separated by centrifugation at 4000 rpm for 5 minutes. Then, the supernatant was discarded, and the collected precipitant was washed with 10 mL of cyclohexane. Finally, the precipitant was dried with a freeze-drier to obtain the Cs 4 PbBr 6 powder. All processes were carried out at room temperature.
Characterization. The crystalline phase of the Cs 4 PbBr 6 perovskite was identified using X-ray diffraction (XRD, D-MAX 2500, Rigaku, Tokyo, Japan) with Ni-filtered Cu Kα radiation. The XRD pattern was recorded in the 2θ range of 10°-50°. The photoluminescence (PL) were examined using a spectrometer (SCINCO, FS-2, Korea) in the range of 400-700 nm with a xenon lamp excitation source (150 W). The microstructure and morphology of the samples was investigated using field emission scanning electron microscopy (FE-SEM, JSM-7000F, JEOL). The absolute quantum yield (absQY) was measured using the absolute PL quantum yield measurement system of Hamamatsu C9920-2 and the PL intensity, luminous efficacy, CRI, CCT, CIE coordinate number of the LED operating at 3.2 V and 20, 40, 60, 70 mA were measured using the CSLMS LED 1060 of Labsphere.

Results
Figure 1(a) shows the procedure for the synthesis of green-emitting Cs 4 PbBr 6 perovskite materials. PXRD patterns of the green-emitting Cs 4 PbBr 6 prepared via the Couette-Taylor flow method were obtained. The prepared Cs 4 PbBr 6 sample shows good agreement with the JCPDS reference No. 73-2478, and no impurity phase is detected; the sample has a crystal structure with a rhombohedral crystal system and lattice parameters of a = b = 13.73 Å and c = 17.31 Å as shown in Fig. 1(b). By using the Couette-Taylor flow method, we confirm the possibility of synthesis of the various perovskite materials such as CsPbBr 3 , Cs 2 PbBr 4 , and Cs 4 PbBr 6 for innovative mass production. To examine the morphology of the synthesized materials, a field-emission scanning electron microscopy (FE-SEM) image of the synthesized microcrystal using the Couette-Tayler flow method was obtained and is shown in Fig. 1(c). Formation of rhombic prisms is displayed, which corresponds to Cs 4 PbBr 6 perovskite compounds, as well as previous reports.
The photoluminescence (PL) spectra of the Cs 4 PbBr 6 perovskite microcrystals are shown in Fig. 2(a) as a function of the synthetic route. The PLQY of approximately 46% in the Cs 4 PbBr 6 microcrystal prepared via the Couette-Taylor flow method displayed the higher luminous performance than the 35% for the sample obtained by the stirring method, and a slight variation of 6.57 nm in the full width at half-maximum (FWHM) was observed. This is related to the decrease in the crystalline defects and the improvement of the homogeneity in the Cs 4 PbBr 6 microcrystal because the Couette-Taylor flow can intensify the agitation and induce uniform fluidic motion, both of which enhance the uniformity of reactants as well as mass productivity via continuous production. Additionally, the luminous characteristics of the Cs 4 PbBr 6 perovskite microcrystal are shown ornation as a function of synthesis temperature in Figure S1 of the supporting Inf. Comprehensively consideration of the color purity for display technology shows that the Cs 4 PbBr 6 perovskite microcrystals synthesized using the Couette-Taylor flow method are suitable for use in display applications. The CIE chromaticity coordinates of the Cs 4 PbBr 6 perovskite microcrystal is indicated in Fig. 2(b). The CIE color space coordinate of the sample prepared with the Couette Taylor flow method is suited to the green region (x = 0.1254, y = 0.7377), with high color purity compared to the sample obtained using the stirring method (x = 0.1473, y = 0.7241). The overall luminous characteristics as a function of synthetic technique are displayed in Table 1.
Electroluminescence properties of a white LED package at the forward-bias current of 20 mA is shown in Fig. 3(a). It consists of a blue-emitting InGaN LED chip, a green-emitting Cs 4 PbBr 6 perovskite microcrystal, and a red-emitting K 2 SiF 6 :Mn 4+ phosphor for white light generation. To confirm the change of white target, the K 2 SiF 6 : Mn 4+ phosphor is composited with Cs 4 PbBr 6 perovskite microcrystal with the ratio of 0.2:1, 0.4:1, 0.6:1, and 0.8:1, respectively. The typical electroluminescence (EL) spectrum is obtained, corresponding to the emission wavelength of the Cs 4 PbBr 6 perovskite microcrystal and the K 2 SiF 6 : Mn 4+ phosphor. Furthermore, luminous efficacy increased with the increasing ratio of the composite. Under the forward-bias current of 20 mA, the highest power efficacies of 9.79 lm W −1 , CRI of 34.772, CCT of 2976 K, EQE of 2.9% are observed. With an increasing composite ratio, warm-white light is generated due to the addition of the red component. CIE color space coordinates for practical application in white LED are shown in Fig. 3(b). With an increasing composite ratio, the axis is moved to warm white based on the Flankian locus line, which is suitable for use in lighting technology through a perfect combination of green and red components. Additionally, the National Television System Committee (NTSC) has conducted investigations to identify candidates for next-generation display applications, which encompass 118%.  To the best of our knowledge, it is very impressive for Cs 4 PbBr 6 perovskite microcrystal to meet the requirements for use in the ultra-high definition (UHD) display. The overall characteristics of the white LED package are presented in Table 2. Based on the composite ratio of 0.6:1 (K 2 SiF 6 : Cs 4 PbBr 6 ), EL spectra and CIE color space coordinates are analyzed as shown in Fig. 4(a,b). We know that perovskite materials are unstable at higher temperatures. With increasing forward-bias current, the luminous efficacy is decreased from 9.79 to 4.48 lm W −1 , and CCT is changed from 2976 to 1657 K due to the unstable characteristics of the Cs 4 PbBr 6 perovskite microcrystal. The overall characteristics of the white LED package with the different forward-current bias values are presented in Table 3. Figures S2, S3, and S4 of the Supporting Information show the CIE color space coordinates as a function of the composite ratio under different forward-bias currents of 20, 40, 60 and 70 mA.

Conclusion
In summary, we have developed an innovative synthetic technique of Cs 4 PbBr 6 perovskite microcrystal using Couette taylor flow method. The prepared Cs 4 PbBr 6 perovskite microcrystal shows the behavior of excellent luminous properties with narrow FWHM and mass production possibility. Through these results, we believe that our innovative technique will represent a new strategy for applications in optoelectronic device, and findings are new family member as a next generation candidate in field of luminescence materials 29,30 .