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Abstract
Benefit from their near-unity photoluminescence quantum yield (PL QY), narrow emission band, and widely tunable bandgap, metal halide perovskites have shown promising in light-emitting applications. Despite such promise, how to facile, environmentally-friendly, and large-scale prepare solid metal halide perovskite with high emission and stability remains a challenging. Herein, we demonstrate a convenient and environmentally-friendly method for the mass synthesis of solid CsPbBr3/Cs4PbBr6 composites using high-power ultrasonication. Adjusting key experimental parameters, bright emitting CsPbBr3/Cs4PbBr6 solids with a maximum PL QY of 71% were obtained within 30 min. XRD, SEM, TEM, Abs/PL, XPS, and lifetime characterizations provide solid evidence for forming CsPbBr3/Cs4PbBr6 composites. Taking advantage of these composites, the photostability, thermostability, and polar solvent stability of CsPbBr3/Cs4PbBr6 are much improved compared to CsPbBr3. We further demonstrated CsPbBr3/Cs4PbBr6 use in flexible/stretchable film and high-power WLEDs. After being subjected to bending, folding, and twisting, the film retains its bright emission and exhibits good resistance to mechanical deformation. Additionally, our WLEDs display a superior, durable high-power-driving capability, operating currents up to 300 mA and maintaining high luminous intensity for 50 hours. Such highly emissive and stable metal halide perovskites make them promising for solid-state lighting, lasing, and flexible/stretchable display device applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the growing demand to develop bright, low-cost, green color converters for solid-state lighting, lasing, and display, it is imperative to explore highly stable light-emitting materials that can withstand the impact of heat, light, and polar solvents. Metal halide perovskites (i.e., CsPbX3, X = Cl, Br, and I), owing to their apparent advantages such as high photoluminescence quantum yield (PL QY) [1,2], narrow photoluminescence (PL) full width at half-maximum (FWHM) [3,4], widely tunable bandgap [5,6], and high charge-carrier mobility [7,8], have become appealing luminescent materials for optoelectronic devices. However, accompanied by excellent optoelectronic performance, the perovskites encounter some tough challenges in practical applications. Because of the low formation energy, labile surface, and metastable structure, perovskite nanoparticles are extremely sensitive to moisture, heat, and light, which leads to poor stability and durability [9–12]. In addition, the PL QY of perovskite nanoparticles is rapidly declining as they transform from liquid to solid [13]. At the same time, the most easily overlooked point is that energy consumption and environmental risks in the synthesis process should be taken seriously. For high-power and large-scale lighting and display applications, harsh environments and costs exacerbate these challenges.
Much effort has been devoted to solving the above issues. As for stabilizing perovskite particles and maintaining their high PL QY in the solid state, constructing different perovskite composites, such as CsPbBr3/Cs4PbBr6 and CsPbBr3/Cs2PbBr5, is a very effective route [14–17]. Recent studies have proved that the CsPbBr3/Cs4PbBr6 composites possess high PL QY and good stability [18–20]. For example, Wang et al. synthesized the CsPbBr3/Cs4PbBr6 solid nanocomposite with a high PL QY (72%). This composite has higher comprehensive stability than the CsPbBr3@SiO2 and CsPbBr3 quantum dots [18], which inspires us to design such a CsPbBr3/Cs4PbBr6 composites. CsPbBr3 nanoparticles with a wide-bandgap Cs4PbBr6 matrix can protect CsPbBr3 from direct contact with the environment, reduce the probability of electron leakage, and improve optoelectronic performance [21]. However, up to now, how to efficiently and environmentally-friendly obtain CsPbBr3/Cs4PbBr6 composites with high solid-state PL QY and stability is still a thorny problem.
In our previous work, we discovered that mechanochemical synthesis is an effective driving force for the environmentally friendly preparation of perovskite nanocomposites [7,22,23]. For example, our group developed a high-speed mechanical mixing method to prepare solid CsPbBr3/Cs4PbBr6 composites [23]. This method avoids using highly toxic anti-solvents (such as toluene and chloroform) in the synthesis process but also shuns multiple dissolution and reprecipitation and can significantly reduce the synthesis time and effort, thereby reducing production costs. However, due to the mechanical mixing of the preparation process, the preparation time is too long (more than one hour), and the production yield is relatively low, so it cannot meet the demands of practical applications.
With the motivation to resolve the above bottleneck, we propose a facile, environmental-friendly, and large-scale approach for preparing solid CsPbBr3/Cs4PbBr6 composites using high-power ultrasonication. Because no ligands or anti-solvents were used in the whole synthesis procedure, the gram-scale of the CsPbBr3/Cs4PbBr6 can be easily obtained by filtration within 30 min at room temperature. Interestingly, the filtered solvent can be recycled and reused to synthesize new CsPbBr3/Cs4PbBr6 solids, which reduces the cost and potential environmental risks resulting from organic solvents. By tuning the CsBr/PbBr2 molar ratios, ultrasound radiation time and power, a bright-emitting CsPbBr3/Cs4PbBr6 solid with a maximum PL QY of 71% and a narrow FWHM of 20 nm was achieved. In addition, compared with CsPbBr3 quantum dots, the photostability, thermostability, and polar solvent stability of CsPbBr3/Cs4PbBr6 were greatly improved, and the passivating mechanism of the Cs4PbBr6 matrix was investigated. To illustrate potential applications, the CsPbBr3/Cs4PbBr6 solid-powders were applied to fabricate flexible/stretchable film and white light-emitting diodes (WLEDs). The film maintains high green emission after bending, folding, and twisting, exhibiting good resistance to various mechanical deformations. In addition, compared to commercial WLEDs, our WLEDs show advantages in narrow emission, high color purity, and wider color gamut. Additionally, our WLEDs displayed a superior, durable high-power-driving capability with operating currents up to 300 mA and maintaining very high luminous intensity for 50 hours.
2. Experimental
2.1. Chemicals and Materials
Cesium bromide (CsBr, 99.9%), lead bromide (PbBr2, 99.99%), dimethyl sulfoxide (DMSO, 99.9%), oleic acid (OA, 90%), oleylamine (OAm, 90%), n-hexane (99.5%), and ethanol (75%) were purchased from Shanghai Aladdin Biochemical Technology Co.. Polydimethylsiloxane (PDMS) was achieved by Dow Corning Co., and the red phosphor (Ba, Ca, Sr)3SiO5:Eu was received from Nichia Co.. All materials used in the present work are without further purification.
2.2. Preparation of CsPbBr3/Cs4PbBr6 Microcrystals
The green luminescent CsPbBr3/Cs4PbBr6 microcrystals were conveniently prepared using a high-power ultrasonic processor (FS-300N, SXSONIC, China) at room temperature, and the preparation schematic is shown in Fig. 1 and Fig. S1. In a typical synthesis process, 3.28 mmol CsBr and 0.82 mmol PbBr2 (the molar ratio of CsBr/PbBr2 is 4:1) were added to a glass bottle, and then 2.0 mL of DMSO was injected. Subsequently, the glass was transferred to an ultrasonic processor and continually crushed for 30 min at 90 W of ultrasound power. After that, the yellow-green powders were finally collected by filtration. Our method is highly effective; the highest product yield was 91.3%. This method can also be readily scaled up, and over 2.74 g of the products (Fig. S2) can be prepared simultaneously without compromising PL QY. In addition, a series of experiments were conducted to study the effects of synthesis conditions on the synthesis of CsPbBr3/Cs4PbBr6.
Fig. 1. The preparation schematic of CsPbBr3/Cs4PbBr6 microcrystal solid-state powders using a high-power ultrasonic processor at room temperature.
2.3. Synthesis of CsPbBr3 quantum dots with room temperature supersaturated recrystallization strategy
CsPbBr3 quantum dots (QDs) were prepared by using the method reported by Li and co-workers with modifications [24]. In a typical synthesis of CsPbBr3 QDs, 0.4 mmol CsBr and 0.4 mmol PbBr2 were dissolved in 10 mL DMSO. Subsequently, 1.0 mL OA and 0.5 mL OAm were injected to stabilize the precursor solution. After that, the 1.0 mL mixture was quickly dropped in 10 mL n-hexane under vigorous stirring, and then bright green emission CsPbBr3 QDs were acquired.
2.4. Fabrication of green-emitting CsPbBr3/Cs4PbBr6 film, red phosphor film, and WLEDs devices
To obtain green-emitting CsPbBr3/Cs4PbBr6 film, the as-prepared CsPbBr3/Cs4PbBr6 solid-powders were dispersed into PDMS, and the mixture was stirred with a vacuum homogenizer for 12 min to degas bubbles. Subsequently, the mixture was injected into a Teflon mould and heated at 120 °C for one hour. Following the same steps above, the red phosphor film was prepared by adding red phosphors into PDMS and vigorously stirring with a vacuum homogenizer for 6 min to degas bubbles. After that, the mixture was poured into a Teflon mould and heated at 120 °C for one hour. To fabricate a high-powder WLEDs device, a green CsPbBr3/Cs4PbBr6 film, a red phosphor film, and six commercial blue LED chips were applied. The green CsPbBr3/Cs4PbBr6 and red phosphor film layers were coated on the surface of the blue LED chips (centered at 450 nm) to achieve white light.
2.5. Characterization
The crystal phase of the products was characterized by applying X-ray diffraction (XRD, D8-ADVANCE, Bruker, Germany) equipped with a Cu-Kαradiation source (λ = 0.15418 nm) under 35 kV and at a counting rate of 2°/min in the scanning angle range from 5° to 60°. The morphological features of the products were measured by a field-emission scanning electron microscope (FE-SEM, Merlin, Germany). The UV−Vis absorption spectra of the samples were obtained on a UV−Vis spectrometer (Tu-1901, Purkinje, China). The PL spectra of the products were recorded by a fluorescence spectrophotometer (RF-6000, Shimadzu, Japan) with a xenon lamp as an excitation source. The PL QY of the samples was measured on Hamamatsu Quantum Yield Measurement System (C9920-02G) under an excitation wavelength of 365 nm. X-ray photoelectron spectroscopy (XPS) determinations were carried out at a Thermo Scientific machine (Thermo K-Alpha) with a mono Al-Kα excitation source (1486.6 eV) as the X-ray source. PL lifetime was collected on Edinburgh Instruments (FLS 980). The obtained PL decay curves are fitted with the tri-exponential function as given in the following expression [25]:
3. Results and discussion
3.1. Effect of preparation conditions on the formation of CsPbBr3/Cs4PbBr6
Since the solubility of CsBr and PbBr2 in DMSO varies greatly, in order to determine whether different CsBr/PbBr2 molar ratios can produce CsPbBr3/Cs4PbBr6 crystals and which molar ratio produces the best optical properties, we first investigated the effects of different CsBr/PbBr2 molar ratios on the structural phases and optical properties of the prepared samples. The crystalline phase of the samples was analyzed by using XRD diffraction patterns. As shown in Figs. 2(a) and 2(b), the main diffraction peaks at 2θ = 12.6°, 20.1°, 22.4°, 25.4°, 28.6°, 30.3°, 30.9°, 34.4°, 38.9°, and 45.7°considerably correspond with the rhombohedral Cs4PbBr6 (PDF #73-2478) crystal planes (110), (113), (300), (024), (214), (223), (006), (134), (330), and (600), respectively [26]. Further, the characteristic peaks emerged at 2θ = 15.8°, 22.4°, 27.5°, 31.9°, 35.7°, 45.7°, and 48.7°, which is in good agreement with the crystal planes (100), (110), (111), (200), (210), (220) and (221) of the cubic CsPbBr3 phase (PDF #18-0364) [27]. Each sample was compared with standard CsPbBr3, Cs4PbBr6, CsBr, and PbBr2 XRD spectra to observe which phases were present. After careful comparison, it was found that the prominent diffraction peaks at 2θ = 12.6°, 25.4°, 28.6°, 30.3°, and 30.9° of the rhombohedral Cs4PbBr6 and 2θ = 29.3° of CsBr changed significantly. When the molar ratio of CsBr/PbBr2 is 1:1, the product contains three components of CsPbBr3, CsBr, and Cs4PbBr6; when the molar ratio of CsBr/PbBr2 is 2:1, the characteristic peak intensity of CsBr decreases (Fig. S3), while the characteristic peak intensity of Cs4PbBr6 increases, indicating that the content of CsBr in the product decreases, and the component of Cs4PbBr6 increases. When the CsBr/PbBr2 molar ratio continues to increase and exceeds 6:1, Cs4PbBr6 becomes the main crystalline component of the product, and a small amount of CsPbBr3 and almost no CsBr are observed. In addition, no other signals corresponding to other cesium lead bromide compounds, such as CsPb2Br5, are detected. Therefore, these results imply that these samples are mainly composed of CsPbBr3 and Cs4PbBr6.
Fig. 2. Structure phase and optical properties of the as-prepared samples. (a) XRD diffraction patterns of the samples formed with different molar ratios of CsBr/PbBr2 (1:1, 2:1, 4:1, 6:1, 8:1, 12:1, and 20:1). (b) Typical XRD patterns of the sample formed with a CsBr and PbBr2 molar ratio of 6:1. (c) Normalized PL intensity (d) PL QY of the samples resulting from different molar ratios of CsBr/PbBr2.
To explore the optical behavior, the PL spectra and PL QY of these samples were collected. As shown in Fig. 2(c), a PL blue shift of the composite material, from 530 to 520 nm, was found as the molar ratio of CsBr/PbBr2 was increased. This phenomenon probably relates to the crystals’ size-dependent bandgap, similar to previous reports on CsPbBr3/Cs4PbBr6 crystals [28,29]. In addition, the PL QY of the samples varied with molar ratios of CsBr/PbBr2. When the CsBr/PbBr2 molar ratio increased, the PL QY first increased rapidly and then decreased slowly and achieved the maximum of 71% at a CsBr/PbBr2 molar ratio of 6:1, as presented in Fig. 2(d). In short, the results of XRD and optical characterization indicated that as long as the molar ratio of precursors was kept above 1:1, the synthesis yielded a luminescent compound, and the optimal molar ratio is 6:1.
During the preparation process, we found that the other two very critical experimental parameters have a significant influence on the optical performance and morphology of perovskite solids, so we further investigated these parameters. The molar ratio of CsBr/PbBr2 is 6:1 and keeps other conditions unchanged. We first studied the effect of ultrasound radiation time on the optical properties and morphology of CsPbBr3/Cs4PbBr6 microcrystals. Figures 3(a) and 3(b) demonstrate the variation of UV–Vis absorption and PL spectra of the CsPbBr3/Cs4PbBr6 prepared at ultrasound radiation time of 10, 20, 30, 60, and 90 min. With the ultrasound radiation time increased, the first characteristic absorption peak changes to blue slowly, corresponding to the blue shift of the PL emission peak, which is similar to the previously reported findings [22]. However, too long ultrasound radiation time leads to the degradation of the product's performance. As displayed in Fig. 3(c), the PL QY first increases quickly and then decreases slowly with the rising of the radiation time and reaches the maximum of 70% at a radiation time of 30 min. Since Cs4PbBr6 will not be dissolved in DMSO, long-time ultrasonic treatment makes the CsPbBr3 embedded in the surface of Cs4PbBr6 dissolve, thereby reducing the PL QY. This can be confirmed by SEM characterization. As shown in Fig. 3(d), when the radiation time is between 10 and 30 min, the morphology of the samples gradually ranges from irregular to rhombohedral shape with small particles embedded in their surface. However, as the radiation time increased to 90 min, small particles disappeared, and some polygon shapes emerged. These results support the change in PL QY.
Fig. 3. Optical properties and morphology of the samples synthesized under different ultrasound radiation time. (a) UV-Vis spectra (solid line) and PL spectra (dash line). (b) Normalized PL emission spectra. (c) PL QY. (d) SEM images.
Furthermore, the effect of ultrasound power on the optical properties and morphology of CsPbBr3/Cs4PbBr6 microcrystals was investigated. As shown in Figs. 4(a) and 4(b), both the UV–vis absorption and PL spectra show a certain degree of blue shift. At the same time, the highest PL QY was obtained (90 W), which exhibited a pronounced bright green light emission. As expected, the corresponding maximum PL QY can reach up to ∼70% at an ultrasonic power of 90 W (Fig. 4(c)). However, higher ultrasound power supports faster dissolution, which also has a significant effect on the homogeneity of the products. This can be demonstrated by SEM images. As shown in Fig. 4(d), the morphologies of these samples gradually became amorphous and the surfaces of some samples were damaged when the ultrasound power is 150 W, leading to a decrease in PL QY. Therefore, the optimal ultrasound power is 90 W.
Fig. 4. Optical properties and morphology of the samples synthesized under different ultrasound power. (a) PL spectra (solid line) and UV-vis (dash line). (b) Normalized PL emission spectra. (c) PL QY. (d) SEM images.
3.2. Characterization of CsPbBr3/Cs4PbBr6 microcrystals
The morphological characterization of the optimized sample is shown in Figs. 5(a) and 5(b). The primary crystals reach a micrometer scale and present a rhombohedral shape with small particles embedded in their outer surfaces (Fig. 5(a)), agreeing with the Cs4PbBr6 compound as a matrix. To further confirm the CsPbBr3 nanoparticles embedded in Cs4PbBr6, TEM was applied. Since TEM cannot measure the microcrystal directly due to its large size, the resulting microcrystals were processed into small particles by mechanical milling and then re-dispersed in n-hexane with ultrasonic treatment. During the processing, no obvious PL emission change was observed (Fig. S4), indicating that this treatment could not destroy the structure of the resulting nanocrystals [14]. Figure 5(b) demonstrates a resulting TEM image and its corresponding high-resolution TEM (HRTEM) image, which shows a characteristic lattice spacing of 4.4 ± 0.1 Å corresponding to the (113) plane of Cs4PbBr6. Furthermore, a region with a small lattice spacing of only 2.9 ± 0.1 Å, corresponding to the (200) phase of CsPbBr3, was also observed, which directly evidences the coexistence of CsPbBr3 and Cs4PbBr6. Therefore, these characterizations proved the formation of the CsPbBr3/Cs4PbBr6 composite structure.
Fig. 5. Morphological characterization and optical properties of CsPbBr3/Cs4PbBr6 microcrystals. (a) SEM image. (b) TEM image. Inset is a high-resolution transmission electron microscopy (HRTEM) image. (c) Normalized photoluminescence (PL) (excited by 365 nm), photoluminescence excitation (PLE) (monitored at 524 nm) spectra, and ultraviolet-visible (UV-Vis) absorption spectra. (d) Contour plot of the colored PL intensity measured as a function of excitation wavelength (340 - 480 nm).
Furthermore, to better understand the fluorescence properties of the optimized CsPbBr3/Cs4PbBr6 microcrystals, their PL spectra, UV–Vis absorption spectra, and photoluminescence excitation (PLE) spectra were investigated. As shown in Fig. 5(c), the PL spectrum reveals that CsPbBr3/Cs4PbBr6 shows a narrow and intense emission band at 524 nm with an FWHM of 20 nm. The absorption spectrum and PLE spectrum are demarcated into two sections (termed sections I and II). As for the absorption spectrum, the CsPbBr3/Cs4PbBr6 microcrystals show a cliff absorption band with an absorption edge at about 510 nm (section II) and a sharp absorption peak in the ultraviolet (310 nm, section I). The green cliff absorption corresponds to the absorption by CsPbBr3 nanocrystals, and the absorption band well matches previous reports on bulk Cs4PbBr6 crystals that derive from the optical transitions between the localized states within the isolated PbBr64- octahedron [21,30]. It is noted that there is a slight Stokes shift (∼14 nm) between the first excitonic absorption peak and the PL emission peak, which coincides with a direct exciton recombination process [7]. When monitoring 524 nm emission, the PLE spectrum demonstrates that the CsPbBr3/Cs4PbBr6 possesses a high PLE fluorescence intensity in the extended spectral range (340-510 nm, section II), while a sharp dip in the short wavelength region (310-350 nm, section I). In the range of 340 nm to 510 nm, the PLE spectrum and absorption spectrum are consistent; the absorption enhancement leads to fluorescence enhancement, which is because the CsPbBr3 can independently absorb the excitation photons and generate fluorescence. However, from 310 nm to 350 nm, absorption enhancement leads to the weakened PL emission of the CsPbBr3/Cs4PbBr6. This phenomenon can be ascribed to the CsPbBr3 nanocrystals embedded in the Cs4PbBr6 matrix, where the light is fully absorbed by the Cs4PbBr6 matrix and does not reach the CsPbBr3 nanocrystals [28]. The contour plot (Fig. 5(d)) of the PL emission and PL excitation spectra of the resulting CsPbBr3/Cs4PbBr6 shows the most vigorous emission centered at about 524 nm when excited at different excitation wavelengths (340-480 nm). The observed contour demonstrates that the CsPbBr3/Cs4PbBr6 has an excitation-independent emission behavior.
XPS was further applied to characterize the resulting CsPbBr3/Cs4PbBr6 microcrystals and pure CsPbBr3 QDs powder (as a reference). Figures 6(a)-(d) demonstrated the XPS full-scan spectra and their corresponding high-resolution spectra of Cs-3d, Pb-4f, and Br-3d. The XPS full-scan spectra indicate that both samples consist of Cs-3d, Pb-4f, and Br-3d. As shown in Figs. 6(b) and 6(c), no significant changes were observed for the binding energy of Cs-3d and Pb-4f of the two samples. The Cs-3d spectra exhibit two peaks with two binding energy of 724.8 and 738.7 eV, assigned to Cs-3d5/2 and Cs-3d3/2, respectively. The Pb-4f spectra reveal two separated peaks at 139.4 and 143.4 eV, corresponding to the Pb-4f7/2 and Pb-4f5/2 levels. However, the binding energies for Br-3d are different for pure CsPbBr3 QDs powder and resulting CsPbBr3/Cs4PbBr6 microcrystals. The Br-3d peaks of pure CsPbBr3 QDs can be fitted to two peaks with the binding energies of 68.6 and 69.5 eV that correspond to the Br-3d5/2 and Br-3d3/2 levels. In comparison, the Br-3d peak of CsPbBr3/Cs4PbBr6 powder can be fitted by three peaks at a binding energy of 68.6 eV, 69.5 eV, and 67.8 eV, as shown in Fig. 6(d). The observed peak at 68.6 eV and 69.5 eV of CsPbBr3/Cs4PbBr6 matched well with the corresponding CsPbBr3 QDs, which can be ascribed to the Br atoms of corner-sharing [PbBr6]4− of CsPbBr3 [24]. The peak at 67.7 eV of CsPbBr3/Cs4PbBr6 can be devoted to the Br atoms in isolated [PbBr6]4− of Cs4PbBr6, which is consistent with previous reports [14]. Therefore, this result further supports forming the CsPbBr3/Cs4PbBr6 structure.
Fig. 6. XPS spectra of the CsPbBr3/Cs4PbBr6 microcrystals and pure CsPbBr3 QDs powders. (a) XPS full spectra. High resolution XPS analyses corresponding to (b) Cs-3d, (c) Pb-4f, and (d) Br-3d.
The time-resolved decay curve was also used to analyze PL dynamics for the CsPbBr3/Cs4PbBr6 microcrystals. To highlight the passivation effect of the Cs4PbBr6 matrix, we used pure CsPbBr3 QDs powder as a reference. Here, a 375 nm pulse laser was employed as an excitation source to measure the PL decay time of these two samples. Both PL decay curves can be accurately fitted by triple exponential functions according to Eqs. (1) and (2), as shown in Figs. 7(a)-(b) and Table 1. There are three characteristic time constants τ1, τ2, and τ3 in fitting curves, which indicate that these two samples include more than one emitting species with different recombination rates [15]. The detailed procedure is illustrated in Fig. 7(a) and Table 1 for CsPbBr3 QDs powder, for which time constants τ1, τ2, and τ3 are 1.568, 4.976, and 17.13 ns, and respective amplitudes A1, A2, and A3 are 0.487, 0.373, and 0.154. Based on this value, we obtain an average decay lifetime τavg. of 10.58 ns. A similar procedure has been applied to the CsPbBr3/Cs4PbBr6 microcrystals, and the results for time constants τ1, τ2, and τ3 are 3.611, 15.83, and 77.914 ns, and respective amplitudes A1, A2, and A3 are 0.437, 0.45, and 0.089. The average decay lifetime τavg. for the CsPbBr3/Cs4PbBr6 microcrystals is 42.13 ns (Fig. 7(b)), which is considerably longer than that for the CsPbBr3 QDs sample. This can be attributed to the surface passivation of CsPbBr3 nanoparticles by the Cs4PbBr6 matrix and the abundance of Br ions that gives them a higher radiative recombination ratio. In addition, according to Eq. (2), the weighted average decay lifetime τavg. is computed using all three time constants where the amplitude value is used as the weighing factor. Considering the corresponding weighing factor A1 and A2, we find that A1 for CsPbBr3 QDs is larger than that of CsPbBr3/Cs4PbBr6. At the same time, A2 for CsPbBr3 QDs is smaller than CsPbBr3/Cs4PbBr6, indicating that the surface defects of the CsPbBr3 QDs are passivated by the well-matched lattice Cs4PbBr6, thereby increasing the probability of radiative recombination [31]. In short, the CsPbBr3/Cs4PbBr6 composites prepared by our method can passivate the surface defects of CsPbBr3 nanocrystals and enable better solid-state photoluminescence performance.
Fig. 7. Time-dependent PL decay spectra of (a) CsPbBr3 QDs powders (as a reference) and (b) CsPbBr3/Cs4PbBr6 microcrystals.
Table 1. Fitted lifetimes of the CsPbBr3/Cs4PbBr6 and CsPbBr3 QDs powder.
3.3. Passivation mechanism of Cs4PbBr6 matrix
Figure 7 proves that the Cs4PbBr6 matrix can passivate CsPbBr3, but the specific passivation mechanism is unclear, so we further explore the passivation effect of Cs4PbBr6. As shown in Fig. 2(d), when the CsBr/PbBr3 molar ratio was increased from 1:1 to 6:1, a gradual increase in the PL QY of the product could be observed, benefiting from the unique crystalline composites of CsPbBr3/Cs4PbBr6 (Fig. 5(b)). With the onset of the sonication reaction, CsPbBr3 first grows with the CsBr seed layer as the carrier site and behaves hydrophilicity because there is no hydrophobic organic ligand or porous structure to protect the growth of CsPbBr3 crystals [32], which easily leads to the agglomeration of CsPbBr3 crystals by collision with each other (Fig. 3(d) and Fig. 8(a)) and the growth of large-sized CsPbBr3 with low fluorescence properties (brown squares in Fig. 8(a)). Therefore, when the CsBr content is relatively low (CsBr/PbBr3 molar ratio of 1:1), the main components of the products are bulk CsPbBr3 nanocrystals with low PL QY, and a small amount of Cs4PbBr6 and CsBr without photoluminescence properties (Fig. 2(a)). To make the product with better fluorescence properties, the crystal size of CsPbBr3 must be restricted within the exciton Bohr radius. When the CsBr content increases, more non-fluorescent Cs4PbBr6 matrix was generated, and more CsPbBr3/Cs4PbBr6 composites (Fig. 8(b)) were formed with CsPbBr3 crystals. The Cs4PbBr6 crystals encapsulated in these composites can passivate and inhibit the growth of CsPbBr3 crystals and limit the size of CsPbBr3 crystals to the exciton Bohr radius so that the in situ prepared perovskite phosphors have strong photoluminescence properties despite the absence of organic ligand passivation protection. In addition, the Cs4PbBr6 component forms an encapsulated protective layer both to passivate the CsPbBr3 crystals and to avoid the regrowth of CsPbBr3 crystals after mutual collisional agglomeration (Fig. 8(b)). However, after the precursor CsBr/PbBr3 molar ratio continues to increase and becomes larger than 6:1, Cs4PbBr6 becomes the main crystalline component of the product. At this point, a small amount of CsPbBr3 and almost no CsBr are observed. It is known from previous studies that the pure Cs4PbBr6 has no photoluminescence properties [33,34]. However, the product in this work has obvious fluorescence properties, which indicates that the luminescence properties of the product still come from the small amount of CsPbBr3 crystals with crystal radii in the exciton Bohr radius range in its components. In summary, Cs4PbBr6 plays a role in passivating CsPbBr3 crystals and inhibiting their growth, and preventing further agglomeration and regrowth of CsPbBr3 crystals, which is necessary for the strong fluorescence properties of the ligand-free CsPbBr3/Cs4PbBr6 composites. The revealing of this mechanism provides a theoretical basis for in-situ encapsulation of ligand-free perovskite luminescent material.
Fig. 8. (a) Lack of protection by organic ligands or porous structures causes CsPbBr3 crystals to collide with each other to aggregate (yellow bidirectional arrows) and grow into large-sized CsPbBr3 (brown squares) with low fluorescence properties. (b) Cs4PbBr6 (the semi-transparent squares) forms an encapsulated protective layer that serves to passivate CsPbBr3 crystals (red fork) and avoid regrowth of CsPbBr3 crystals after agglomerating between them.
3.4. Thermotolerance, storage stability, UV photostability, and polar solvent stability of CsPbBr3/Cs4PbBr6 microcrystals
It is well known that the performance of perovskite crystals is significantly affected by ambient environments, such as heat, UV light, water, and other polar solvents. Excellent thermotolerance, UV light resistance, and solvent stability are crucial to meeting the requirements of practical lighting and display applications. Thus, the thermotolerance, storage stability, UV photostability, and ethanol stability of CsPbBr3/Cs4PbBr6 powders were systematically investigated under different conditions. As shown in Fig. 9(a), temperature-dependent relative PL intensity and fluorescence photographs were obtained at ambient temperature after heating at different temperatures. When heated at 200 °C for one hour, CsPbBr3/Cs4PbBr6 powders still possessed bright green emissions. The Bing Dwen Dwen images made by our optimized CsPbBr3/Cs4PbBr6 exhibited high PL intensity of their initial values of 100%, 97%, 95%, and 87% after heating at 25, 100, 150, and 200 °C, respectively. Moreover, compared to the CsPbBr3 QDs powders synthesized by the room-temperature supersaturated recrystallization strategy, our CsPbBr3/Cs4PbBr6 microcrystals demonstrated outstanding performance. Figure 9(c) shows the relative PL intensity of CsPbBr3/Cs4PbBr6 and CsPbBr3 QDs powders recorded at 150 °C. The results demonstrated that the CsPbBr3/Cs4PbBr6 powders decay only 23.5% after 48 hours, while the CsPbBr3 QDs powders are nearly fluorescence quenched after heating for ten hours, which further verified the satisfactory thermotolerance. In addition, Our CsPbBr3/Cs4PbBr6 showed stable green emission, with minimal degradation after 90 days of storage under ambient conditions (HH 80% and HT 25 °C), as demonstrated in Figs. 9(b) and 9(d). In Fig. 9(b), the green-emitting in a hand-writing luminescent STU logo made by our CsPbBr3/Cs4PbBr6 powders under excitation at 365 nm remained nearly unchanged even after 60 days of storage. The time-dependent PL intensity of CsPbBr3/Cs4PbBr6 and CsPbBr3 QDs powders are obtained in Fig. 9(d). It can be seen that the CsPbBr3/Cs4PbBr6 can still maintain about 92% of its original intensity, while CsPbBr3 QDs are nearly quenched.
Fig. 9. Thermotolerance, storage stability, UV photostability, and polar solvent stability. (a) Fluorescence photographs and PL intensity of CsPbBr3/Cs4PbBr6 powders at ambient temperature after heat treatment (25–200 °C) for one hour. (b) A hand-writing luminescent STU logo made by CsPbBr3/Cs4PbBr6 powders stored at ambient temperature after 60 days. (c) Time-dependent PL intensity stability after heating at 150 °C for various times ranging from 0 to 48 hours. (d) PL intensity of the CsPbBr3/Cs4PbBr6 powders after 90 days of storage under ambient conditions (HH 80% and HT 25 °C). (e) PL intensity of the CsPbBr3/Cs4PbBr6 powders under continuous illumination by 365 nm UV light, the power density is 16 mW/cm2. (g) PL intensity of the CsPbBr3/Cs4PbBr6 powders and CsPbBr3 QDs powders dissolved in ethanol (20 mg/mL) after ten days.
Resistance to UV light and stability to polar solvents are other key parameters determining the long-term operation of perovskite-based lighting devices. To evaluate the UV photostability of the prepared samples, the CsPbBr3/Cs4PbBr6 and CsPbBr3 QDs powders were irradiated under continuous UV light with an optical power density of 16 mW/cm2 for 15 hours. In Fig. 9(e), the PL intensity of CsPbBr3/Cs4PbBr6 showed only a 12.9% decrease after the irradiation of 15 hours, while CsPbBr3 QDs dropped 65.4% at the same measurement conditions. The excellent UV photostability of CsPbBr3/Cs4PbBr6 is probably linked to stable Cs4PbBr6 encapsulation since this framework is known to be resistant to UV deterioration [30]. Furthermore, the polar solvent (ethanol) stability of the obtained samples is evaluated. Figure 9(f) exhibits the PL intensity change of the CsPbBr3/Cs4PbBr6 powders and CsPbBr3 QDs powders dissolved in ethanol (20 mg/mL) after ten days. It can be found that the CsPbBr3/Cs4PbBr6 maintains 85% of its PL intensity after immerging in ethanol for ten days, while CsPbBr3 QDs decay is swift and lose its luminescence. The green luminescence of the bare CsPbBr3 QDs only can last less than two hours under immerging in ethanol because its structure is destroyed by ethanol. Therefore, all these results indicate that our CsPbBr3/Cs4PbBr6 has excellent stability and great potential applied as optical converters in harsh conditions.
3.5. Application of CsPbBr3/Cs4PbBr6 microcrystals in lighting and displays
The easy large-scale preparation and excellent photothermal properties of CsPbBr3/Cs4PbBr6 make it an excellent candidate for solid-state light and display applications, such as backlighting, flexible/stretchable information displays, and high-power optoelectronic devices. The above prospect has been primarily demonstrated as shown in Fig. 10. First, a highly green emissive CsPbBr3/Cs4PbBr6 film with a dimension of 100 × 100 × 1 mm was successfully and conveniently prepared. Interestingly, it maintained a bright green emission after bending and folding dozens of times, showing good flexibility (Fig. 10(a)). Furthermore, we cut this large-scale film into rectangular strips with large aspect ratios, and their optical properties under different mechanical deformations were investigated. Amazingly, these strips possessed a bright green emission after rolling and twisting (Fig. 10(b)). Further bending and twisting still could not affect its emission performance, indicating that the optical property was robust against various mechanical deformations.
Fig. 10. (a) A large dimensional CsPbBr3/Cs4PbBr6 film was prepared that could emit bright green light under different deformations: bending and folding. The bar scale is 2 cm. (b) Optical images of the light-emitting performance of CsPbBr3/Cs4PbBr6 under various deformations: bending, twisting, and rolling. The bar scale is 2 cm. (c) Diagram of the white light-emitting diodes (WLED) combing blue LED chips, green CsPbBr3/Cs4PbBr6 film, and red phosphor film. (d) EL spectrum of WLED. Inset is a photograph of the WLED device operated at 200 mA. (e) CIE chromaticity coordinates correspond to the above WLED device (yellow line) and are compared to NTSC standard (white dash line). (f) Luminous flux (purple curve) and luminous efficacy (green curve) of the above WLED device.
Based on the above merits, we fabricated WLEDs using as-prepared CsPbBr3/Cs4PbBr6 film as the green conversion hub. Figure 10(c) shows a diagram of the remote package WLEDs device combing blue LED chips, green CsPbBr3/Cs4PbBr6 film, and red phosphor film. Figure 10(d) presents the electroluminescence (EL) spectra obtained at a current density of 20 mA, which is the typical operating current for WLEDs as to their use in ambient conditions. The working WLED exhibits bright white light with a color rendering index (CRI) of 86 and a correlated color temperature (CCT) of 5632 K. Moreover, the color gamut of the WLEDs covers 115.4% of the National Television Systems Committee (NTSC) (white dashed triangle) gamut, as demonstrated in Fig. 10(e) (solid yellow triangle). In addition, the forward-bias current-dependent luminous flux and luminous efficacy of CsPbBr3/Cs4PbBr6 WLEDs are presented in Fig. 10(f). The luminous flux of WLEDs gradually increased with increasing device current. In contrast, luminous efficacy showed the opposite trend. The high luminous efficacy of 50.1 lm/W is achieved at a forward-bias current of 20 mA. Remarkably, our WLEDs demonstrate a superior high-power-driving capability, with operating currents up to 300 mA and maintaining very high luminous intensity (over 90% of the initial value) for 50 hours (Figs. 11(a) and 11(b)), which is much higher than most previously reported perovskite materials [14,30]. Surprisingly, our WLED devices can continue to operate at such high power even when their electrode temperature has exceeded 80°C (Fig. S5). This shows that our WLEDs can overcome the structural damage caused by higher operating temperatures and high drive currents, which is critical for their applications in phosphor converted WLEDs, especially in high power conditions. We hope this superior luminescent material can set the stage for broad applications of solid-state lighting, lasing, and display devices.
Fig. 11. (a) EL spectra of WLEDs measured at different working time (0 -50 h). (b) Long-term operation of the CsPbBr3/Cs4PbBr6-based WLEDs devices at 300 mA for 50 hours.
4. Conclusions
In summary, we have demonstrated a facile and environmental-friendly approach for the large-scale synthesis of solid CsPbBr3/Cs4PbBr6 composites using high-power ultrasonication. Changing the CsBr/PbBr2 molar ratios, ultrasound radiation time, and ultrasound power, the bright emission CsPbBr3/Cs4PbBr6 solid-powders with a maximum PL QY of 71% and a narrow FWHM of 20 nm were achieved. By applying XRD, SEM, TEM, Abs/PL/PLE, XPS, and PL decay lifetime characterizations, all these results provide conclusive evidence supporting the formation of CsPbBr3/Cs4PbBr6 composites. In addition, we further investigated the passivation mechanism of the Cs4PbBr6 matrix, revealing that Cs4PbBr6 plays a role in passivating CsPbBr3 crystals and inhibiting their growth, and preventing further agglomeration and regrowth of CsPbBr3 crystals, which is the key to obtaining ligand-free, highly emissive CsPbBr3/Cs4PbBr6 solids. Taking advantage of these composites, the photostability, thermostability, and polar solvent stability of CsPbBr3/Cs4PbBr6 are greatly improved compared to CsPbBr3 QDs. The relative PL intensity of CsPbBr3/Cs4PbBr6 can still be maintained at even 76.5% when heated at 150 °C for 48 hours and showed superior stability with minimal degradation after 90 days of storage under ambient conditions. Even after immersing in a polar solvent (ethanol) for ten days, its PL intensity remained at about 85% of the original value. We further demonstrated CsPbBr3/Cs4PbBr6 solids use in flexible/stretchable film and high-input-power durable solid-state WLEDs. After being subjected to bending, folding, and twisting, the film retains its bright green emission, indicating its optical property was robust against various mechanical deformations. In addition, our WLEDs displayed a superior durable high-power-driving capability, operating currents up to 300 mA and maintaining very high luminous intensity (over 90% of the initial value) for 50 hours. Such highly emissive and stable metal halide perovskite provides a robust candidate for solid-state lighting, lasing, and display device applications.
Funding
Open Fund of Hubei Key Laboratory of Mechanical Transmission and Manufacturing Engineering at Wuhan University of Science and Technology (MTMEOF2020B04); Characteristic Innovation Projects of Ordinary Colleges and Universities in Guangdong Province (2020KTSCX038); General Program of Natural Science Foundation of Guangdong Province (2021A1515010662, 2022A1515011280); National Natural Science Foundation of China (52005314); STU Scientific Research Foundation for Talents (NTF19045, NTF20010, NTF22029).
Acknowledgments
The authors would like to thank Prof. Zongtao Li, Binhai Yu, and Xingrui Ding of South China University of Technology for giving us recommendations for LED device design and fabrication, and thank Shudong Yu, Guanwei Liang, and Jiasheng Li of South China University of Technology for experimental help.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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