Low Temperature Solution-Processable Cesium Lead Bromide

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Low Temperature Solution-Processable Cesium Lead Bromide Microcrystals for Light Conversion Quyet Van Le, Jong Won Lee, Woonbae Sohn, Ho Won Jang, Jong Kyu Kim, and Soo Young Kim Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00264 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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Crystal Growth & Design

Low Temperature Solution-Processable Cesium Lead Bromide Microcrystals for Light Conversion Quyet Van Le†, Jong Won Lee††, Woonbae Sohn†††, Ho Won Jang†††*, Jong Kyu Kim††*, and Soo Young Kim†* †

School of Chemical Engineering and Materials Science, Integrative research center for two-

dimensional functional materials, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul, 06974, Republic of Korea ††

Department of Materials Science and Engineering, Pohang University of Science and

Technology (POSTECH), Pohang, Gyeongbuk, 790-784, Republic of Korea †††

Department of Materials Science and Engineering, Research Institute of Advanced Materials,

Seoul National University, Seoul 08826, Republic of Korea

Key word: Perovskite, CsPbBr3, Cs4PbBr6, UV-LEDs

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ABSTRACT

In this report, we present a new approach for the fabrication and application of Cs4PbBr6 microcrystal (Cs4PbBr6 MCs). The Cs4PbBr6 MCs are synthesized via an anti-solvent induced crystallization of PbBr2:CsBr directly in dimethylsulfoxide (DMSO) by introducing HBr (HBr, 48% aqueous solution). The ratio of HBr and DMSO plays a vital role in the formation of Cs4PbBr6. By controlling the HBr/DMSO ratio, pure Cs4PbBr6 or the CsPbBr3 phase can be obtained. The Cs4PbBr6 MCs were initially obtained by adding HBr to CsBr:PbBr2/DMSO. However, on increasing the amount of the added HBr, Cs4PbBr6 MCs were converted to CsPbBr3 MCs and the photoluminescence (PL) disappeared. It was also found that CsPbBr3 MCs can be transformed to Cs4PbBr6 MCs by simply adding DMSO to the dried CsPbBr3 MCs. The Cs4PbBr6 MCs exhibit a strong PL at 516 nm with a full width at half maximum of 25 nm regardless of the crystal size (5-10 µm). On using Cs4PbBr6 MCs as a light converter in ultraviolet-light emitting diodes, a PL intensity that is three times higher than that of CsPbBr3 quantum dots based devices could be achieved, unraveling the potential of this material for optoelectronic applications.


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INTRODUCTION In recent years, cesium lead halide materials (CsPbX3, X: I, Br, and Cl) with a perovskite structure have drawn significant attraction as novel materials for optoelectronic devices such as solar cells, light emitting diodes (LEDs), resistive switching memories and photodetectors.1-16 CsPbX3 has been successfully used as absorption layers in solar cells, for achieving efficiency as high as 9.5% by employing a low cost and solution processable method.1, 17 CsPbX3 has also been used as emitting layers in LEDs and the efficiency strongly depends on the crystal size and uniformity of the perovskite film.3 Perovskite thin films are usually fabricated using spin-casted precursors composed of cesium halide (CsX, X: I, Br, and Cl) and lead halide (PbX2). The perovskite thin films fabricated with the spin-casting method are generally non-uniform owing to the formation of extremely large perovskite crystals, resulting in a weak PL intensity and very low external quantum efficiency when used in LEDs.3 To overcome these obstacles, efforts have been made for developing new methods of producing inorganic/organic perovskite lead halide quantum dots (QDs) exhibiting quantum yields of up to 70–90 % for use as an emitting layer in LEDs, as similar to metal chalcogenide QDs.6, 18-20 However, perovskite QDs are surrounded by organic ligands with long chains, which limit the electrical and optical performances in optoelectronic applications.5-6 More recently, a new family of materials derived from cesium lead halide, namely Cs4PbBr6 perovskite, has been discovered.21-25 Cs4PbBr6 with a size of a few micrometers shows a photoluminescence quantum yield (PLQY) of >45% regardless of the size distribution of the particles (up to a few micrometers).22 Interestingly, the PLQY of Cs4PbBr6 MCs did not degrade after keeping the sample for 3 weeks at a relative humidity of 58%, making it a promising candidate for fabricating optoelectronic devices with a long term stability.22 In addition, a


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prototype of LEDs using Cs4PbBr6 MCs as a green emissive layer was demonstrated, suggesting the huge potential of Cs4PbBr6 MCs in optoelectronic devices. In this study, a simple method for the large-scale production of highly pure Cs4PbBr6 MCs for use as a green light converter in ultra violet LEDs (UV-LEDs) is presented. Cs4PbBr6 MCs were produced in PbBr2:CsBr/DMSO solution by adding HBr as an anti-solvent. The HBr/DMSO ratio plays a crucial role in the formation of Cs4PbBr6 MCs. According to the HBr/DMSO ratio, the pure Cs4PbBr6 phase or CsPbBr3 phase was obtained. The crystal structure and composition of the synthesized cesium lead halide were investigated using x-ray diffraction (XRD), selected area electron diffraction (SEAD), energy dispersive x-ray spectroscopy (EDS), and field emission scanning electron spectroscopy (FE-SEM). UV-LEDs using Cs4PbBr6 as the green converter from UV light was fabricated and discussed in detail.

EXPERIMENTAL SECTION The synthesis of Cs4PbBr6 MCs The precursor composed of CsBr and PbBr2 with a molar ratio of 1:1 was prepared with different concentrations of 10–40 wt% in DMSO and heated for 24 h in air at the temperature of 70 °C. After cooling down to ambient temperature, 100 µL bromic acid (HBr, 48% aqueous solution)) was swiftly injected to the as-prepared precursor. Monoclinic Cs4PbBr6 MCs were formed immediately after introducing HBr. In order to complete the crystallization process, the mixture was stirred for 10 minutes. The precipitate was then washed with DMSO and collected by centrifugation. The synthesis of CsPbBr3 MCs


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CsBr and PbBr2 solutions were separately prepared in HBr with a concentration of 0.5 M. CsPbBr3 MCs were immediately formed on adding 1 mL CsBr/HBr to 1 mL PbBr2/HBr at ambient temperature. The synthesis of CsPbBr3 QDs CsPbBr3 were synthesized according to previous reports.18 Preparation of Cs-oleate: 0.814 g Cs2CO3 was loaded into a 3-neck flask along with 20 mL octadecene (ODE) and 2.5 mL oleic acid (OA). The mixture was then dried at 120 °C under N2 atmosphere for 1 h before heating to 150 °C for 2 h. Preparation of CsPbBr3 QDs: ODE (5 mL) and 0.188 mmol PbBr2 were loaded into a 25 mL 3-neck flask and dried under vacuum at 120 °C for 1 h. Dried OA (0.5 mL) and dried oleylamine (0.5 mL) were injected at 120 °C for 30 min. Subsequently, the mixture was heated to 140–200 °C. The as-prepared Cs-oleate was swiftly injected to the mixture. The reaction mixture was then cooled down with an ice-water bath to terminate the reaction. The perovskite QDs were washed 3 times with hexane/methyl acetate and dispersed in toluene for further characterizations and device fabrication. Preparation of CsPbBr3 QDs, CsPbBr3 MCs, and Cs4PbBr6 MCs in poly(methyl methacrylate) (PMMA) Sapphire (1.5 × 1.5 cm2) was used as the substrate. PMMA solution was prepared by dissolving PMMA powder in chlorobenzene with a concentration of 40 mg/mL. CsPbBr3 QDs, CsPbBr3 MCs, and Cs4PbBr6 MCs were then dispersed in PMMA to obtain a concentration of 100 mg/mL. CsPbBr3 QDs/PMMA, CsPbBr3 MCs/PMMA, and Cs4PbBr6 MCs/PMMA films were fabricated by drop-coating. The thickness of the film was controlled by modulating the dropping volume as shown in Figure S8. The thicknesses of the CsPbBr3 QDs/PMMA films


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were 5.86, 8.54, 8.63, and 10.63 µm for the dropping volume of 25, 50, 100, and 200 µL. The thicknesses of the Cs4PbBr6 MCs/PMMA films are 12.54, 13.92, 71.36, and 212.2 µm with the dropping volume of 25, 50, 100, and 200 µL. The Cs4PbBr6 MCs/PMMA film is thicker than the CsPbBr3 QDs/PMMA films with the same dropping volume because the size of the Cs4PbBr6 MCs is a few micrometers. Application of CsPbBr3 QDs/PMMA and Cs4PbBr6 MCs/PMMA films as a UV-LED down conversion layer A commercial UV-LED (UVC LED 6868) was prepared. The size of the UV LED was 6.8 × 6.8 × 1.35 mm3. The current-voltage curve and electroluminescence spectrum of the UV-LED are shown in Figure S9. The maximum electroluminescence was achieved at the wavelength of 265.5 nm. After placing the UV-LED on the chuck of the probe station, CsPbBr3 QDs/PMMA or Cs4PbBr6 MCs/PMMA film is located on top of the UV-LED as shown in Figure S10. The PL spectra of CsPbBr3 QD/PMMA and Cs4PbBr6 MCs/PMMA films were measured at 8V and a 20 mA. In order to check the stability, PL spectra were recorded under the same conditions after keeping the films in air for 4 and 7 days. Characterization UV-vis absorption spectra (V670 UV-vis spectrophotometer) were recorded to check the wavelength range for the highest absorption region of Cs4PbBr6 MCs and CsPbBr3 QDs. The shape and size of materials were measured using FE-SEM. An arc xenon lamp and detector (PMT 1527 photomultiplier) were used to detect emissions from the samples for performing the PL measurements. XRD pattern (D8-Advance/Bruker-AXS) was utilized to confirm the crystal structure of the Cs4PbBr6-MCs. SEAD patterns were recorded using TEM (JEOL-2100F, Japan).


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RESULTS AND DISCUSSION Figure 1 shows the photographs of the Cs4PbBr6 MCs produced in DMSO solvent on introducing HBr as the anti-solvent. The precursors were prepared by mixing PbBr2 and CsBr in DMSO at different concentrations of 5, 10, 20, 30, and 40 wt%, which is denoted as the samples 1, 2, 3, 4, and 5 in Figure 1. The volume of DMSO is fixed as 1 mL. Before introducing HBr acid in the precursors, the precursor solutions are transparent except for the sample 5 (saturated concentration) as shown in Figure S1. The color of the precursor solution with >40 wt% of PbBr2 and CsBr changes to yellow. After injecting 100 µL HBr into the precursor solution in DMSO, the color of the sample 4 and 5 immediately changes to yellow-green, suggesting the formation of Cs-based crystals (Figure 1(a)). The samples 4 and 5 exhibit a green luminescence emission under 365 nm UV-light. On adding 200 µL HBr, the color of the sample 3 changes to yellowgreen while the color of the samples 4 and 5 is unchanged (Figure 1(b)). Sample 3, 4, and 5 exhibit a green luminescence emission under 365 nm UV-light. On adding 100 µL HBr again to the solution containing 200 µL HBr (total 300 µL), aggregates are formed in the sample 2. In this case, the color of the sample 5 changes to brown, suggesting the change in the crystal composition or structure. The transition of color from yellow-green to brown is assumed to indicate the formation of the perovskite structure, which was further confirmed by XRD. After adding 300 µL HBr into the solution, the samples 2, 3, 4, and 5 exhibit a green luminescence emission under 365 nm UV-light. However, the luminescent intensity of the sample 5 containing 300 µL HBr decreased as compared with the sample 5 containing 200 µL HBr. Therefore, the color change of the precursor solution to yellow-green with the addition of HBr suggests the formation of crystals exhibiting a luminescence under UV irradiation. The color change of the sample 1 is not observed in all cases, suggesting that no crystal was formed with a precursor concentration of

Low Temperature Solution-Processable Cesium Lead Bromide

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