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Abstract
All-inorganic perovskites are promising alternatives to organic-inorganic hybrid halide perovskites in the field of optoelectronic materials. Optimizing the luminescent property of CsPbBr3 is very important to promote its application in optoelectronic devices, and thus a facilitative method to prepare a large number of efficient luminous samples is desirable. In this study, we effectively improve the luminous performance of CsPbBr3 powder by a simple mechanosynthesis strategy. The emission intensity of CsPbBr3 powder is improved by ca. 40 times and the PLQY of the powder sample exceeds 40%. It is disclosed that the defects of CsPbBr3 are passivated and the ratio of the non-radiation recombination is reduced. The formed Cs4PbBr6 during grinding has a zero-dimensional perovskite structure that can limit the migration of the carriers between the [PbBr6] octahedrons. Now CsPbBr3 can be used in powder form other than film or quantum dots. A UV-chip LED is fabricated by using the prepared phosphor, and the narrow-band emission property (FWHM is 22 nm) makes it more suitable for application in back-lighting systems.
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1. Introduction
Organic-inorganic hybrid halide perovskites have been widely studied in the field of photovoltaic devices due to their excellent photoelectric properties, such as extremely high light absorption coefficient, ultra-long carrier diffusion length, and adjustable band gap [1–4]. However, the intrinsic instability of organic groups in hybrid perovskite makes them extremely sensitive to water, oxygen, light, and thermal stress, which limits their commercialization [2,4–6]. Therefore, all-inorganic halide perovskites CsPbX3 (X = Cl, Br, I) are widely studied due to their better stability [1,4,5].
Similar to the rapid development of solar cells, perovskites have also gained increasing attention and become a rising star in the field of luminescence [1,3,7–9]. The ultra-long carrier diffusion length is ideal for their use in solar cells. However, it is not conductive to the radiative recombination required by phosphors [5,6,9,10]. To solve this problem and improve the luminescent efficiency of halide perovskite, a series of methods have been designed to limit the size of perovskite crystals [5,6,10–12]. The quantum confinement effect is used to tailor the transport behaviour of the carriers in nanoscale [5,10,13]. For example, Qin et al. prepared perovskite materials with quasi-2D/3D structure, in which carriers mainly accumulate in the low band gap region of the luminescent material [14]. At present, colloidal perovskite quantum dots, perovskite nanocrystals (NCs) and thin film with very good luminescent efficiency have been prepared successfully [15–17]. However, when the quantum dots or NCs are purified, serious luminescence quenching will occur due to the aggregation of crystal particles [1,18], and the microscale size composites are hard to meet the needs of applications in other photoelectric devices, such as micro-light-emitting-diodes (micro-LEDs) [19,20]. The controllable synthesis of the nanoscale composites with a high emission efficiency also remains a challenge [21]. There are very few reports on the perovskite powder materials with effective luminescence properties [12,18,22]. Therefore, it is of great significance to find a simple and repeatable method to improve the luminescence performance of halide perovskites, especially the form that is convenient for practical application.
Mechanosynthesis is an attractive synthesis method with advantages such as rapidity, simplicity, reproducibility. It has been widely employed in the design and synthesis of nanoparticles, metal-organic frameworks (MOFs) and carbon hybrid materials [23,24]. Recently, mechanosynthesis has been successfully applied to the synthesis of halide perovskite. However, surface treatment with appropriate ligands is still necessary to obtain high bright perovskite [25,26].
Herein, we synthesized CsPbBr3 embedded in the Cs4PbBr6 composite with bright emission in powder form via mechanosynthesis strategy, which can be directly applied to LEDs as phosphors. By adding a proper amount of CsBr into the pre-synthesized CsPbBr3 and then successive grinding, the best luminous performance of the powder material is improved by ca. 40 times. The crystal structure of the samples is characterized by XRD and TEM, and a series of optical performance tests including PL, PLE, UV-vis DRS, time-resolved PL and PLQY are performed to analysis and characterize the hybrid system. Moreover, a green LED with 22 nm FWTH was fabricated by using the powder sample for back-lighting system to prove its practical application potential. The method does not require any organic ligand. It is simple and easy to be realized, with high repeatability and low cost. In addition, this method greatly reduced the content of Pb element in the material and minimize the problem of Pb toxicity to a certain extent.
2. Methods
Perovskite crystals were synthesized by the following methods. CsBr and PbBr2 (1:1 molar ratio) were dissolved in DMSO in ambient air under continuous stirring, until no powder was observed. The precursor solution was coated on a clean glass substrate. Then, the samples were annealed at 100°C for about 12 h. As the solvent evaporated, CsPbBr3 crystallized. After cooling to room temperature, the perovskite crystals were collected from the substrate. Finally, CsBr powder was added into the prepared CsPbBr3 crystals with the ratios of 0:1, 1:1, 3:1, 5:1, and 7:1 respectively and ground carefully. The prepared samples were named as CsPbBr3, Mix-1.0, Mix-3.0, Mix-5.0, Mix-7.0, respectively.
3. Results and discussions
The luminescent photos and photoluminescence emission (PL) spectra of the samples are shown in Fig. 1. All samples were prepared and tested with the same conditions. Under the excitation of 365 nm, they emit green light. The brightest one is the sample Mix-5.0. PL spectra show a broadband emission peak centered at about 512 nm (Fig. 1(b)). The emission intensity increases with the increasing doping amount of CsBr until reaching the maximum for the sample Mix-5.0, which is ca. 40 times of that of CsPbBr3. The emission spectrum of Mix-5.0 show a small blue shift compared with the peaks of other samples. The self-absorption mainly appeared in the short wavelength due to overlap between absorption and emission spectra, so the emission in the short wavelength region is enhanced comparatively compared with that in the long wavelength region when the self-absorption is decreased by doping CsBr and the samples are effectively dispersed. The results indicate that the adding amount of CsBr powder and grinding affects the luminescence properties of the perovskite powder samples. To make clear the mechanism behind this phenomenon, a series of characterizations are carried out furtherly.
Fig. 1. Luminescent photos (a) and PL emission spectra (b) of CsPbBr3 with different amounts of CsBr in CsPbBr3 powder. The powders are excited at 365 nm. It can be seen that with the increase of the proportion of CsBr, the luminescent intensity of the sample first increases and then decreases, and reaches maximum intensity when the ratio of CsBr : CsPbBr3 is 5:1
To study the phase composition of the powder samples, XRD patterns of the samples are collected and shown in Fig. 2(a). The standard XRD patterns of CsPbBr3 (PDF-#54-0752) and Cs4PbBr6 (PDF-#73-2478) are both added as the references[27]. With adding CsBr powder and grinding, obvious diffraction peaks belonging to Cs4PbBr6 begin to appear in the XRD patterns. The samples change into mixtures of CsPbBr3 and Cs4PbBr6. Figures 2(b) and (c) are the high resolution TEM images of sample CsPbBr3 and sample Mix-5.0, respectively. Only the lattice of CsPbBr3 phase is observed in the sample CsPbBr3. However, two different lattices belonging to CsPbBr3 and Cs4PbBr6 respectively are observed in the sample of Mix-5.0 (Fig. 2(c)), which further confirm the formation of Cs4PbBr6 during the grinding process.
Fig. 2. (a) XRD patterns of CsPbBr3 with different amounts of CsBr. (b) High resolution TEM image of the sample CsPbBr3. (c) High resolution TEM image of the sample Mix-5.0.
To further study the properties of the sample Mix-5.0, PL excitation (PLE) and UV–vis diffuse reflectance spectra were collected in ambient condition and are presented in Figs. 3(a) and (b), which are in good agreement each other. Figure 3(c) is a comparison of the absorption spectra of the sample CsPbBr3 and Mix-5.0. Obviously, the absorption intensity in the range of 350-550 nm for the sample Mix-5.0 is significantly reduced compared with that of the sample CsPbBr3, which is because the content of CsPbBr3 in the sample Mix-5.0 is lower than that in pure CsPbBr3. As a consequence, the exciton absorption of CsPbBr3 in the sample Mix-5.0 is reduced greatly in this region. At the same time, a characteristic absorption peak of Cs4PbBr6 at 320 nm is observed [22,28]. However, only the emission peak of CsPbBr3 can be measured in PL spectra, no additional emission peak appears and no obvious shift of PL peak position for all the samples is observed, as shown in Fig. 1(b). The emission mechanism of Cs4PbBr6-related systems remains controversial [1,22,28]. Some researchers attribute it to the intrinsic emission of Cs4PbBr6, while others attribute it to the presence of green luminescent species in the Cs4PbBr6 matrix, such as CsPbBr3 NCs or defects (Br vacancy). Combined with previous reports [22,28], the band gap of Cs4PbBr6 is about 3.8 eV. Thus, Cs4PbBr6 cannot emit green light itself. In our experiment, changing the mixing amount of CsBr can change the ratio of Cs/Pb/Br in a large range. In this case, the emission center will change greatly if the light emission comes from structural defects, such as bromine vacancy. However, as the ratio of CsBr vs CsPbBr3 changed from 0 to 5.0, the offset of PL is less than 5 nm. Therefore, it seems that the green emission at about 510 nm should originates from CsPbBr3. Such a phenomenon is similar with the case of CsPbBr3/Cs4PbBr6 composite NCs, in which cubic-phase CsPbBr3 can align with all three dimensions of the Cs4PbBr6 lattice computationally. Cs4PbBr6 can provide endotaxy to the CsPbBr3 NCs and passivate their surfaces defects. Simultaneously, the CsPbBr3 emitters are dispersed in the Cs4PbBr6 matrix avoiding agglomeration fluorescence quenching, which results in a high PLQY of the composites [21,29].
Fig. 3. PL excitation spectra (a), UV-Vis diffuse reflectance spectra (b), and UV-vis absorption spectra (c) of CsPbBr3 with different adding amounts of CsBr powder. With the content increase of CsBr in the mixing sample, the intensity of the first exciton absorption peak of CsPbBr3 at 510 nm gradually decreases because of the content decrease of CsPbBr3 in the mixed samples. A characteristic and sharp absorption peak of Cs4PbBr6 at 325 nm appears. These results further confirm that the gradual evolution process from CsPbBr3 to Cs4PbBr6.
More insight into the luminescent mechanism can be obtained from the time-resolved PL spectra and the photoluminescence quantum yield (PLQY), as shown in Fig. 4. All the time-dependent PL spectra can be fitted well by a double-exponential function as follows:
It is reported that the rapid decay component of the decay curve is associated with the trap-assisted non-radiation recombination at the grain boundary or surface, while the slow decay component is attributed to the radiation recombination of excitons in perovskite crystals [8,11,12]. As shown in Table 1, the average lifetime ${\tau _{a\textrm{v}e}}$ of Mix-5.0 is one order of magnitude higher than CsPbBr3. Both the rapid and the slow decay components of sample Mix-5.0 are longer than those of sample CsPbBr3, which indicates that the defects of the samples are greatly reduced after being modified by our method. Thus, the non-radiative recombination in the sample Mix-5.0 is effectively inhibited.
Fig. 4. Fluorescence lifetime decay curves (a) and PLQY (b) of CsPbBr3 with different adding amounts of CsBr powder.
Table 1. PLQY, time constants ${\tau _1}$ and ${{\tau}_2}$, components ${{A}_1}$ and ${{A}_2}$ of ${{\tau}_1}$ and ${{\tau}_2}$, average lifetime ${{\tau}_{{a}\textrm{v}{e}}}$, the radiative decay rate ${\varGamma_{rad}}$ and the nonradiative decay rate ${\varGamma _{{non} - {rad}}}$ of the CsPbBr3 with different adding amounts of CsBr powder.
Then the PLQY of the sample CsPbBr3, Mix-3.0, Mix-5.0, and Mix-7.0 are tested and shown in Fig. 4(b). The PLQY of the powder sample CsPbBr3 is less than 1%, while that of the samples Mix-3.0, Mix-5.0, and Mix-7.0 is 17.11%, 40.73% and 20.87%, respectively. Especially, the PLQY of the sample Mix-5.0 is 40.73%, which is about 40 times of that of the powder CsPbBr3. It is an extremely high result for the powder perovskite sample due to fluorescent light reabsorption and fluorescence resonance energy transfer [18]. When the excessive CsBr (more than 5 times of CsPbBr3) is added, a large amount of CsBr cannot react with CsPbBr3 and remains as residues, which has a negative effect on the luminescence of the material. PLQY is the ratio of the number of emitted photons to the number of absorbed photons. Both radiative recombination and non-radiative recombination can reduce the excited states. Therefore, PLQY can also be defined as the ratio of the radiative recombination rate to the total recombination rate, as shown in Eq. (3), in which ${\Gamma _{rad}}$ is radiative recombination rate, and ${\Gamma _{non - rad}}$ is non-radiative recombination rate. The average lifetime is the reciprocal of the total recombination rate and can be expressed by Eq. (4). ${\Gamma _{rad}}$ and ${\Gamma _{non - rad}}$ of the perovskite phosphor can be obtained by combining the Eq. (1) and Eq. (4).
The relevant data of PLQY, time constants ${\tau _1}$ and ${\tau _2}$, components ${A_1}$ and ${A_2}$ of ${\tau _1}$ and ${\tau _2}$, average lifetime ${\tau _{a\textrm{v}e}}$, the radiative decay rate ${\Gamma _{rad}}$ and the non-radiative decay rate ${\Gamma _{non - rad}}$ of CsPbBr3 with different adding amounts of CsBr powder are obtained by the Eq. (1) to Eq. (6) and listed in Table 1. The calculated results show that the non-radiative recombination rate decreases from 1966.28 μs−1 (sample CsPbBr3) to 77.75 μs−1 (sample Mix-5.0), and the radiation recombination rate increases from a very low level to 53.43μs−1 (sample Mix-5.0). The results of ${\Gamma _{rad}}$ and ${\Gamma _{non - rad}}$ confirm that the sample Mix-5.0 is with the best luminescent properties.
The above discussions indicate that mixing CsBr into CsPbBr3 by grinding can induce the formation of Cs4PbBr6. During the grinding process, the particle size of CsPbBr3 crystals is decreased and the surface defect density is increased, which will results in the non-radiation energy loss [24,26,27]. Interestingly, adding an appropriate amount of CsBr during the grinding process can combine the unstable CsPbBr3 with poor quality and transform it into Cs4PbBr6 [28]. On the one hand, the defects of CsPbBr3 are passivated and the non-radiation recombination is reduced. On the other hand, Cs4PbBr6 crystallizes in a zero-dimensional perovskite structure that can limit the migration of the carriers between the [PbBr6] octahedrons, thereby increasing the radiation recombination rate [22,28]. Therefore, even if the light absorption of CsPbBr3 is weakened by mixing with CsBr powder, the light utilization efficiency is elevated, then the luminous efficiency is improved and the emission is enhanced greatly.
To evaluate the potential of the Cs4PbBr6/CsPbBr3 composite material for practical applications, we constructed a green LED using sample Mix-5.0 coated on a commercially available 365 nm LED chip. Figure 5 is the electroluminescence spectra of the constructed LED operated at indicated currents. The inset shows the related photographs of the working LED in daylight and darkness, respectively. When the excitation current is 50 mA, the green light intensity reaches its maximum. The photoelectric transformation efficiency from the input electronic power to green emissions (ηgreen/input) can reach 15.0%. The full width at half-maximum (FWHM) is 22 nm. The narrow-band emission property makes it more suitable for application in back-lighting systems.
Fig. 5. Electroluminescence spectra of the constructed LED operated at indicated currents. The inset shows the related photograph of the working LED.
4. Conclusion
In conclusion, we have proposed a simple method to greatly improve the luminous intensity of CsPbBr3 powder by adding an appropriate amount of CsBr into the pre-synthesized CsPbBr3 powder and then grinding. The highest PLQY of the powder sample exceeds 40% when CsBr vs CsPbBr3 is 5:1. In this process, the defects of CsPbBr3 are passivated and the non-radiation recombination is reduced. Meanwhile, partially unstable CsPbBr3 is combined with CsBr additive to ripen into Cs4PbBr6, which has a zero-dimensional perovskite structure that can limit the migration of the carriers between the [PbBr6] octahedrons. Thus the radiation recombination rate is increased. The samples are obtained directly in powder form, which are convenient for solid state lighting applications with broad market prospect. This study not only offers a promising candidate for high-level back-lighting devices, but also paves a new way to design and synthesize other new perovskite functional materials or improve the performance of them.
Funding
National Natural Science Foundation of China (11774187, U1902218); Natural Science Foundation of Tianjin City (19JCYBJC17600); 111 Project (B07013).
Acknowledgments
This work was supported by the National Natural Science Foundation of China (11774187, U1902218), the Natural Science Foundation of Tianjin City (19JCYBJC17600), and the 111 project (B07013).
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.
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