Ultrasonication-assisted trace amount solvent synthesis of Cs₄PbBr₆ crystal with ultra-bright green light emission

Clean energy and efficient energy conversion are essential to achieve sustainable development. Halide perovskites are promising materials that can efficiently convert solar energy into electricity as well as electricity into light energy. Significant progress in halide perovskites ABX₃ (A = CH₃NH₃⁺, Cs⁺; X = I⁻, Br⁻, Cl⁻, SCN⁻) for solar cells makes them the most promising materials for the next-generation solar cells.^1–5 Meanwhile, halide perovskites have attracted lots of research interest due to the high-performance optoelectronic properties, such as light-emitting devices,^6–13 photodetectors (PDs),^14,15 and lasers.^16,17 ABX₃ perovskites have a perfect three-dimensional (3D) structure consisting of corner-sharing [BX₆]⁴⁺ octahedra, with the A⁺ ion filling the gap between adjacent [BX₆]⁴⁺ octahedra, achieving a superlow trap state density of 10¹⁰ cm⁻³.¹⁸ 

However, the joint angle of the three-dimensional perovskite structure prevents quantum confinement of charge carriers, resulting in low optical emissions. Furthermore, 3D perovskite ABX₃ has inherent vulnerability to moisture and heat, and long-term stability is still the biggest challenge. Therefore, it is an urgent goal to find other materials for solid halide perovskite illumination and to have high luminosity. In the commercialization of halide perovskite light emitting devices, the color gamut and brightness can be increased by adjusting the structural size of halide perovskites. The A₄BX₆ structure materials are promising candidates. They are more stable because the isolated [BX₆]⁴⁺ octahedron is inserted in all dimensions through the A⁺ ions in the crystal lattice and is separated from each other. However, the synthesis method of Saidaminov et al. requires PbBr₂ and CsBr to be dissolved in a large amount of dimethyl sulfoxide (DMSO) and stirred for 1 h.¹⁹ The solution was filtered and heated to 120 °C for extra 3 h. Then, the sample was washed with DMSO and dried at 100 °C. Bastiani et al. prepared the material with 1 ml of precursor solution (1 ml of DMSO, 0.25 mol of PbBr₂, and 0.25 mol of CsBr dissolved).²⁰ The precursor was inserted into a closed container containing 4 ml of diethyl ether (DE). After as long as 48 h, the immiscible interface between DE and DMSO formed crystals in the crystallization flask. Zhang’s method for preparing Cs₄PbBr₆ nanocrystals requires a complex procedure in which Cs–oleic acid precursor, n-hexane, and oleic acid (OA) were quickly injected into the flask with vigorous stirring with PbBr₂, HBr, OA, and oleylamine.²¹ These reported methods require either very long time, higher temperature, or more complicated steps in the preparation of Cs₄PbBr₆, and in some cases, there are certain restrictions for moisture and oxygen. These are disadvantages for the preparation of Cs₄PbBr₆ in large quantities. Meanwhile, a large amount of organic solvent has a huge impact on cost and environment.

Here, we discovered a one-step method for the rapid synthesis of Cs₄PbBr₆ high quality crystals at room temperature with a trace amount of solvent, which is feasible for large-scale industrial preparation with low costs and less environmental concerns. It has also been found that the Cs₄PbBr₆ crystals have higher PL brightness and can be used for ultra-bright light-emitting devices, achieving the maximum brightness of 3.8 × 10⁹ cd/m².

0.1 mol of CsBr (cesium bromide, 99.9%, Aladdin, China) and 0.025 mol of PbBr₂ [lead (II) bromide 99%, Aladdin, China] were placed in a 5 ml test tube. Then, 10 µl of DMSO (dimethyl sulfoxide 99.8%, Aladdin, China) was added dropwise. The resulting mixture was then sealed and placed in an ultrasonic cleaner (Fisher Scientific FS110D, 100 W) for sonication (sample green-DMSO). The duration of sonication can be found in Fig. 1(c). The sample was dried at 140 °C.

0.1 mol of CsBr and 0.025 mmol of PbBr₂ were placed in a 5 ml test tube. Then, 5 ml of ethanol was added dropwise. The resulting mixture was then sealed and placed in an ultrasonic generator for sonication (sample white-EtOH). The duration of sonication can be found in Fig. 1(d). The sample was dried at 140 °C.

CaAlSiN₃:Eu²⁺ (Shenzhen Looking Long Technology Co., China) and BaMgAl₁₀O₁₇:Eu (Shenzhen Looking Long Technology Co., China) were used as the blue light (450 nm) source and the red light (630 nm) source of the light emitting device, respectively. They were packaged with epoxy resin (Shenzhen Looking Long Technology Co., China). The white light emitting device was excited by a 365 nm optical core (Shenzhen Looking Long Technology Co., China). The green-DMSO sample was prepared in a matrix powder and excited using a 365 nm optical core for the green light emitting device.

Cs₄PbBr₆ was imaged by using a field emission scanning electron microscope (ZEISS Merlin Compact) at an accelerating voltage of 10 KV–15 KV. The x-ray diffraction pattern was taken by using an x-ray diffractometer (XRD-6000, Shimadzu, Japan) with Cu(Kα) radiation (λ = 1.54 Å). PL spectra were taken using Princeton Instruments SpectraPro HRS-750 (with a superconducting nanowire single photon detector). The light emitting devices were powered and tested by using a Keithley meter (2400 and 6487) with a photodetector (Digi-Key, USA).⁸ Emission spectra (devices) were collected on an F-380 fluorescence spectrometer (Tianjin Gangdong Sci. & Tech. Development Co., Ltd., China)

In Fig. 1(a), 10 µl of DMSO was added in the mixture of 0.1 mol CsBr and 0.025 mol PbBr₂ (4:1 ratio). The resulting mixture was then sealed and placed in an ultrasonic generator for sonication (sample green-DMSO). It is worth noting that with the help of ultrasonication, Cs₄PbBr₆ can be formed faster with a trace amount of DMSO. As can be seen in Fig. 1(c), only 20 min (second dot) is required to quickly prepare the Cs₄PbBr₆ crystals with the aid of ultrasonication. After drying in an oven, a yellow–green powder of Cs₄PbBr₆ was obtained, as shown in the upper left corner in Fig. 1(b). If the trace amount solvent was changed from DMSO to ethanol, no Cs₄PbBr₆ crystals could be synthesized. As the volume of ethanol was increased to 5 ml, a white bulk Cs₄PbBr₆ (sample white-EtOH) could be synthesized. As shown in Fig. 1(d), the color changed obviously at the time of 250 min (fifth dot), and it took ∼300 min (sixth dot) to finish the reaction, which is 15 times longer than the that for sample green-DMSO. Moreover, energy-dispersive x-ray spectroscopy (EDS) results indicate that there are more impurities in sample white-EtOH (Table S1), while the atomic ratio for sample green-DMSO is very close to the theoretic ratio (Table S2). With this method, we could not synthesize Cs₄PbBr₆ crystals with other solvents, such as water or isopropanol. The polarity of DMSO is very high, which makes it a better solvent for this reaction. With the additional assistance of ultrasonication, this reaction can be done quickly. In the case of less polar ethanol, it has lower solubility and takes longer time to react. Even though the polarity of water is higher than DMSO, however, water decomposes the final product (Cs₄PbBr₆ crystals) immediately, which makes water not a good solvent. Thus, the organic solvent DMSO with high polarity is the best choice for this reaction, which dissolves reactants quickly and barely dissolves the product.

Scanning electron microscopy (SEM) was used to image the morphology for the crystals of these two samples. As can been seen in Figs. 2(a) and 2(b), sample green-DMSO prepared with a trace amount of DMSO was found to be well-shaped crystals of 2 µm–8 µm. It has a typical Cs₄PbBr₆ crystal morphology.²² As a comparison, the sample white-EtOH prepared with ethanol is shown in Figs. 2(c) and 2(d). No good shape of Cs₄PbBr₆ crystals can be found even under different synthetic conditions, such as different precursor ratios, as shown in Fig. S1. The Cs₄PbBr₆ crystal is unstable under a transmission electron microscopy (TEM) beam (supplementary material).

Photoluminescence (PL) measurements were used to test the synthesized samples. In Fig. 3(a), a1 and a3 are optical images of sample green-DMSO and white-EtOH under visible light, respectively; a2 and a4 are the corresponding optical images under the UV light. Sample green-DMSO shows much stronger photoluminescence than sample white-EtOH. Solid state PL spectra for both samples were shown in Fig. 3(b). The emission peaks for both green-DMSO and white-EtOH are at 519 nm, with the full width at half maximum (FWHM) at about 19 nm and 23 nm, respectively. The narrower FWHM for sample green-DMSO indicates better color purity than sample white-EtOH. Notably, the FWHM of 19 nm for sample green-DMSO is narrower than the previously reported Cs₄PbBr₆, including powder (∼23 nm),¹⁹ bulk crystals (24 nm),²⁰ and uniform size nanocrystals (23 nm).²³ Steady state PL spectra for sample green-DMSO at different spots show almost the same peak profile, indicating the uniformity of the synthesized Cs₄PbBr₆ crystal. In Fig. 3(d), the fluorescence lifetime was found to be about 10 ns for sample green-DMSO, which is less than those in the literature, such as 17.7 ns for Cs₄PbBr₆ in Ref. 24. The PL measurement results indicate that the Cs₄PbBr₆ crystals prepared by the ultrasonication-assisted trace amount solvent method have a better color purity.

The green light emitting device was excited by a wick (Figs. S2–S4). As shown in Fig. 4(a), the detector reached the maximum current density of 2.2 × 10⁷ mA/cm² at 3.8 V. The current efficiency reached 14.8 cd/A at 3.5 V [Fig. 4(b)]. The inset of Fig. 4(c) shows the green light emitting device with an ultra-high brightness of 3.8 × 10⁹ cd/m², which is nearly 100 times higher than that of the wick (Figs. S2–S4). A related video can be found in the supplementary material. Figure 4(d) shows the schematics displaying a better penetration of green light the from Cs₄PbBr₆ microcrystal compared to an ultraviolet light. The performance of the light emitting device including PL is directly related to the quality of the synthesized material. As can be seen from the SEM images, the material synthesized by the ultrasonication-DMSO method used here shows very high quality Cs₄PbBr₆ crystals with smooth surfaces and sharp edges compared with those of the material prepared with ethanol. These well-shaped microcrystals play the role of resonance cave during the light emission with less dissipation.

In order to demonstrate the practicality of sample green-DMSO, the ultrasonication-assisted trace amount solvent synthesized Cs₄PbBr₆ microcrystals were mixed with a commercially available nitride (CaAlSiN₃:Eu²⁺) with red fluorescent emission at 630 nm and BaMgAl₁₀O₁₇:Eu material with blue fluorescent emission at 450 nm to assemble a white light emitting device, which was excited by a 365 nm optical core. Figure 5(a) demonstrates that the device with the ultrasonication synthesized Cs₄PbBr₆ crystals emits bright white light with a driven voltage of 3 V. A fluorescence detector was used to detect the emission spectrum. Figure 5(b) shows that the light emitting device can achieve full-band emission (400 nm–800 nm) in the visible range. The green light (wavelength around 520 nm) from Cs₄PbBr₆ microcrystals shows a strong and sharp peak, which is consistent with Fig. 3. According to the emission spectrum, the color coordinates are calculated by using the software CIE1931xy (V.1.6.0.2) according to the following formulas:

where R, G, and B denote red, green, and blue, respectively.

The calculated results in Fig. 6 indicate that the prepared light emitting device based on the Cs₄PbBr₆ crystal from sample green-DMSO can cover 32.3% of the color gamut and achieves 107.3% coverage for International Commission on Illumination (CIE) 1931 and 84.8% coverage for CIE 1976. The light emitting device with Cs₄PbBr₆ microcrystals as the green light source can cover 105% of National Television system committee (NTSC) gamut. These values are higher than those of light emitting devices made with the commonly used green phosphors, such as β-SiAlON:Eu²⁺ and CdS nanocrystals with NTSC gamut 86% and 104%, respectively.^25–27 Compared with these currently available commercial phosphors, the starting materials for Cs₄PbBr₆ microcrystals are cheaper, and the synthesis is easier. The resulting light emitting device can achieve wide color gamut coverage. These results reflect the great potential of the ultrasonication-assisted trace amount solvent method prepared Cs₄PbBr₆ microcrystals in the manufacture of wide color gamut light emitting devices while allowing for their simplicity, environmental friendliness, and scalable synthesis methods as potential materials for high device performance. This research expands the application of halide perovskite materials beyond solar cells.

In summary, Cs₄PbBr₆ crystals have been rapidly synthesized by ultrasonication with a trace amount of DMSO. It shows a strong fluorescence at 519 nm with a lifetime of 10 ns. The corresponding green light emitting device achieves a maximum brightness of 3.8 × 10⁹ cd/m². The white light emitting device can cover 32.3% of the color gamut, which covers 107.3% of CIE1931 and 84.8% of CIE1976. It is an excellent green light source material. This method offers a simple, fast, and environmentally friendly way to prepare the Cs₄PbBr₆ material.

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

See the supplementary material for a detailed description of the wick and the video of the device.

The authors gratefully acknowledge the funding for this project through the National Natural Science Foundation of China (Grant No. 51873083), the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University; Grant No. sklpme2018-4-27), the Key University Science Research Project of Jiangsu Province (Grant No. 18KJA130001), the Six Talent Peaks Project in Jiangsu Province (Grant No. 2015-XCL-028), the Postgraduate Research and Practice Innovation Program of Jiangsu Province (Grant Nos. SJCX19_0584 and SJCX18_0759), and the U.S. Department of Education, Office of Postsecondary Education, Institutional Services (Title III, Part B, HBCU Program). Q.J. and Z.X. acknowledge support by the U.S. Department of Energy (VFP), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.