Article pubs.acs.org/IC
Homogeneous Synthesis and Electroluminescence Device of Highly Luminescent CsPbBr3 Perovskite Nanocrystals Song Wei,†,‡ Yanchun Yang,†,‡ Xiaojiao Kang,† Lan Wang,†,‡ Lijian Huang,† and Daocheng Pan*,† †
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China ‡ University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *
ABSTRACT: Highly luminescent CsPbBr3 perovskite nanocrystals (PNCs) are homogeneously synthesized by mixing toluene solutions of PbBr2 and cesium oleate at room temperature in open air. We found that PbBr2 can be easily dissolved in nonpolar toluene in the presence of tetraoctylammonium bromide, which allows us to homogeneously prepare CsPbBr3 perovskite quantum dots and prevents the use of harmful polar organic solvents, such as N,Ndimethylformamide, dimethyl sulfoxide, and N-methyl-2-pyrrolidone. Additionally, this method can be extended to synthesize highly luminescent CH3NH3PbBr3 perovskite quantum dots. An electroluminescence device with a maximal luminance of 110 cd/m2 has been fabricated by using high-quality CsPbBr3 PNCs as the emitting layer.
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INTRODUCTION Highly luminescent lead-based perovskite nanocrystals, including CsPbX3 and CH3NH3PbX3 (X = Cl, Br, or I), have recently received a tremendous amount of attention because of their facile synthesis and excellent optical properties.1−23 Compared with group II−VI and III−V quantum dots (QDs), perovskite nanocrystals can be synthesized at lower reaction temperatures in only several seconds.1−23 More importantly, the photoluminescence quantum yield, the tunable emission range, and the emission line width of perovskite nanocrystals (PNCs) are completely comparable to or even better than those of highly luminescent group II−VI and III−V QDs.24−28 These outstanding optical properties of PNCs make them very attractive as luminescent materials in light-emitting diodes (LEDs).18−23 Therefore, the synthesis of highly luminescent CsPbX3 and CH3NH3PbX3 PNCs is critically important for their device applications. Highly luminescent CsPbX3 and CH3NH3PbX3 PNCs have been extensively synthesized in the literature.1−23 In most cases, PbX2 was used as the starting material and was dissolved in a polar organic solvent, such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO).1−8 CsPbX3 and CH3NH3PbX3 PNCs were formed by mixing a small amount of a PbX2 precursor solution with a large amount of a nonpolar organic solvent. It was reported that these polar organic solvents can severely degrade CsPbX 3 and CH3NH3PbX3 perovskite semiconductors.29 As a result, the production yield of PNCs was strongly influenced by polar organic solvents.5 Recently, Kovalenko and co-workers developed a homogeneous synthesis route for preparing highly luminescent CH3NH3PbX3 (X = Br or I) nanocrystals; however, tetrahydrofuran as a polar solvent was still required.16 © XXXX American Chemical Society
Therefore, developing a polar organic solvent-free synthesis route is quite important for the high-yield preparation of CsPbX3 and CH3NH3PbX3 PNCs. It is well-known that inorganic PbBr2 is insoluble in nonpolar organic solvents. To overcome this problem, we added tetraoctylammonium bromide (TOAB) to toluene in an attempt to improve the ability of PbBr2 to dissolve in toluene. Surprisingly, PbBr2 can easily dissolve in toluene upon addition of TOAB, which allows us to homogeneously prepare CsPbBr3 perovskite nanocrystals and prevents the use of harmful polar organic solvents. In addition, CH3NH3PbBr3 perovskite QDs can be synthesized by this facile method.
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EXPERIMENTAL SECTION
Chemicals. PbBr2 (99%), Cs2CO3 (99.9%), an aqueous solution of methylamine (AR, 40 wt %), tetraoctylammonium bromide (TOAB, 98%), zinc acetate [Zn(OAc)2·2H2O, 99%], and MoO3 (99.9%) were purchased from Aladdin Inc. Oleic acid (90%) was obtained from Sigma-Aldrich. N,N-Dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), toluene, 2-methoxyethanol, and monoethanolamine were of analytical grade and were used as received without further purification. 4,4-N,N-Dicarbazole-biphenyl (CBP, 99.5%) was purchased from LumTec Inc. Homogeneous Synthesis of CsPbBr3 and CH3NH3PbBr3 Nanocrystals. First, cesium oleate was prepared by loading 0.5 mmol of Cs2CO3 and 5.0 mL of oleic acid into a 20 mL vial, and the vial was placed in a 120 °C oven for 60 min. Then, 5 mL of toluene was added to the vial at room temperature, forming a 0.1 M cesium oleate precursor solution. The PbBr2 precursor solution was prepared Received: November 18, 2016
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DOI: 10.1021/acs.inorgchem.6b02763 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. Digital photographs of DMF, NMP, and toluene solutions of PbBr2 that were injected into toluene at (a) a high concentration and (c) a low concentration. Digital photographs of DMF, NMP, and toluene solutions of PbBr2 that were injected into a toluene solution of cesium oleate at (b) a high concentration and (d) a low concentration. (e) UV−vis absorption and PL spectra of CsPbBr3 PNCs achieved by using PbBr2 as the precursor that was dissolved in different organic solvents.
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by dissolving 0.2 mmol of PbBr2 in 2.0 mL of toluene, DMF, or NMP. Note that 0.4 mmol of TOAB was used to improve the ability of PbBr2 to dissolve in toluene. For the synthesis of CsPbBr3 nanocrystals, 1.0 mL of a cesium oleate precursor solution, 2.0 mL of oleic acid, and 20 mL of toluene were mixed in a 50 mL conical flask while being magnetically stirred at room temperature in open air. Subsequently, 0.5 mL of a PbBr2 toluene, DMF, or NMP solution was swiftly injected into the conical flask. After 120 s, the CsPbBr3 nanocrystals were precipitated by adding γ-butyrolactone. Finally, the CsPbBr3 nanocrystals were centrifuged and redispersed in toluene for various characterizations. The same procedure that is described above was employed to synthesize CH3NH3PbBr3 nanocrystals, except Cs2CO3 was replaced with a methylamine aqueous solution. Fabrication of CsPbBr3 Nanocrystal LEDs. CsPbBr3 nanocrystal-based LEDs were fabricated with an ITO/ZnO (40 nm)/ CsPbBr3 (30 nm)/CBP (45 nm)/MoO3 (10 nm)/Al inverted structure. First, ITO was patterned by a fiber laser marker. Next, a ZnO thin film was achieved by spin-casting a sol−gel precursor solution at 3000 rpm, followed by a sintering process at 320 °C for 30 min on a preheated hot plate. A ZnO precursor solution was prepared by dissolving 2.0 mmol of Zn(OAc)2 in 1.0 mL of monoethanolamine and 9.0 mL of 2-methoxyethanol at 100 °C for 2 h. Then, a CsPbBr3 nanocrystal toluene solution (∼20 mg/mL) was spin-casted onto a ZnO thin film at 2500 rpm. Finally, CBP, MoO3, and an Al electrode were subsequently deposited by thermal evaporation. Note that both the synthesis of CsPbBr3 nanocrystals and the fabrication of a CsPbBr3 nanocrystal thin film were conducted in open air, and the LED with an active area of ∼9 mm2 was tested in an ambient environment without any encapsulation. Characterizations. The X-ray diffraction (XRD) patterns were recorded using a Bruker D8 FOCUS X-ray diffractometer. UV−vis absorption and photoluminescence (PL) spectra were recorded with Metash 5200 and Shimadzu RF 5301PC spectrophotometers, respectively. The PL quantum yields (QYs) of the NCs at room temperature were determined by comparing the integrated emission of the NC samples in an aqueous solution with that of a fluorescence dye (coumarin 6 in ethanol, 78% QY). Transmission electron microscopy (TEM) images were taken on a FEI Tecnai G2 F20 instrument with an accelerating voltage of 200 kV. Energy-dispersive spectroscopy (EDS) was performed using a scanning electron microscope (Hitachi S-4800) equipped with a model 4010 Bruker AXS XFlash detector. The I−V curve was measured with a LabTracer-controlled Keithley 2400 source meter, and the luminance was synchronously recorded with a Minolta luminance meter (LS-160). The electroluminescence spectra were recorded with an Ocean Optics spectrometer (Maya2000 Pro) and a Keithley 2400 source meter.
RESULTS AND DISCUSSION In literature reports, TOAB has been widely used as the phasetransfer agent for the two-phase synthesis of metallic Au, Pt, and Pd nanocrystals.30−32 Inorganic HAuCl4, H2PtCl6, and PdCl2 can be transferred into nonpolar toluene by TOAB, which motivates us to dissolve PbBr2 in toluene in the presence of TOAB by the formation of (C8H17)4N+Pb[Br3]−. As shown in Figure 1a, a clear toluene solution of PbBr2 was obtained by dissolving PbBr2 and TOAB in toluene, and this solution can be randomly diluted with toluene without aggregation. However, a large amount of white precipitate will occur when a transparent DMF or NMP solution of PbBr2 is injected into nonpolar toluene. When these DMF and NMP solutions of PbBr2 are mixed with a toluene solution of cesium oleate at room temperature, large CsPbBr3 particles and small CsPbBr3 nanocrystals will form at the same time, as shown in Figure 1b. A similar phenomenon was observed in the synthesis of CH3NH3PbX3 perovskite nanocrystals.5 Although the large CsPbBr3 particles can be removed by direct centrifugation, these large CsPbBr3 particles will severely decrease the production yield of CsPbBr3 nanocrystals. In contrast, a toluene solution containing PbBr2 and TOAB was added to a toluene solution of cesium oleate, forming a clear CsPbBr3 nanocrystal solution without aggregation (see Figure 1b). Obviously, homogeneously synthesized CsPbBr3 nanocrystals have a higher production yield and a higher photoluminescence quantum yield (PLQY). To better elucidate the advantages of our “homogeneous” synthesis, another control reaction was conducted at a 10-fold lower concentration, as shown in panels c and d of Figure 1. No aggregation and large CsPbBr3 particles were formed when polar DMF and NMP solutions of PbBr2 were injected into a nonpolar toluene solution of cesium oleate at a low concentration (see Figure 1d). This phenomenon is the same as those of previously reported synthesis using DMF and DMSO.1−8 Thus, the advantage of our polar solvent-free approach is that it allows us to synthesize high-quality CsPbBr3 nanocrystals at a high concentration without a significant decrease in production yield or PLQY. Figure 1e shows UV−vis absorption and photoluminescence spectra of CsPbBr3 PNCs achieved via injection of a DMF, NMP, or toluene solution of PbBr2 into a toluene solution of cesium oleate. It was noticed that large CsPbBr3 particles were formed when polar DMF and NMP were used as the solvent, and they were removed by centrifugation prior to various B
DOI: 10.1021/acs.inorgchem.6b02763 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry characterizations. These oleic acid-capped CsPbBr3 PNCs have a similar PL peak at 511−515 nm with a full width at halfmaximum (fwhm) of 19−20 nm. However, the homogeneously synthesized CsPbBr3 nanocrystals have a photoluminescence quantum yield of 80−85%, which is significantly higher than those of polar−solvent synthesized nanocrystals (usually 20− 30%). Note that the synthesis of CsPbBr3 nanocrystals was conducted in open air at room temperature; thus, this facile synthesis is very easy to scale up to the order of grams. This reaction was successfully scaled up 40-fold in a 1.0 L beaker, and gram-scale CsPbBr3 nanocrystals with a PLQY approaching 80% were achieved, as shown in Figure S1. The optical quality of our CsPbBr3 nanocrystals is comparable to those of CsPbBr3 nanocrystals that were synthesized by a high-temperature hotinjection approach.9−12 More importantly, for small-scale and large-scale CsPbBr3 nanocrystals, their UV−vis absorption and PL spectra, PLQYs, sizes, and size distributions (see Figure S2) are quite similar, confirming that our method is highly reproducible. The crystal structure and chemical composition of CsPbBr3 PNCs were characterized by powder X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS), respectively. It was reported that CsPbBr3 usually crystallizes in an orthorhombic, tetragonal, or cubic structure.12,33 According to the literature reports, at low temperatures, CsPbBr3 crystallizes in a stable orthorhombic phase and will transform into a tetragonal or cubic structure at 88−91 or 130−132 °C, respectively.12,33 However, our XRD patterns of CsPbBr3 PNCs that were synthesized at room temperature cannot be indexed to the three structures mentioned above. Figure 2a presents the XRD
Figure 3. (a and c) Low-resolution TEM and (b and d) highresolution TEM images of (a and b) small-scale and (c and d) largescale CsPbBr3 PNCs by a homogeneous method.
CsPbBr3 PNCs that were synthesized on a small scale and a large scale by a homogeneous approach. The sizes of small- and large-scale CsPbBr3 PNCs are 12.7 and 12.9 nm, respectively, and both of them are beyond the calculated Bohr exciton diameter of CsPbBr3 (∼7 nm),9 suggesting a weak quantum confinement effect for our CsPbBr3 PNCs. Additionally, the calculated optical band gap of CsPbBr3 PNCs is around 2.4 eV, according to the onset of UV−vis absorption spectra, which is quite close to the previously reported value of 2.3 eV for bulk CsPbBr3 single crystals,33 revealing that our CsPbBr3 PNCs have a weak quantum size effect. Both the small-scale and the large-scale CsPbBr3 PNCs exhibit a decent size distribution, as shown in Figure S2. It should be mentioned that oleic acid serves as the capping agent and plays a decisive role in controlling the size and size distribution of CsPbBr3 PNCs. The carboxyl group of oleic acid has a strong bonding strength with Pb2+ ions; thus, oleic acid has been extensively used in the synthesis of high-quality PbS quantum dots.34−36 It should be mentioned that TOAB is used as the secondary ligand by passivating the Br− anions on the nanocrystal surface. For oleic acid-capped CsPbBr3 PNCs, they can be highly dispersed in many types of nonpolar organic solvents, such as hexane, chloroform, toluene, and chlorobenzene. Consequently, a highly luminescent CsPbBr3 nanocrystal thin film can be fabricated by spin-casting a CsPbBr3 PNC solution. When DMF and NMP were used as the solvent to synthesize CsPbBr3 PNCs, small CsPbBr3 PNCs with a narrow size distribution can also be achieved by the removal of large CsPbBr3 particles. Panels a and c of Figure S3 show their TEM images. Their HRTEM analysis revealed that CsPbBr3 nanocrystals are highly crystalline (see Figure S3c,d). The polar organic solvent-free approach can also be extended to synthesize high-quality CH3NH3PbBr3 nanocrystals by substituting cesium oleate with methylammonium oleate. Figure 4a shows UV−vis absorption and PL spectra of
Figure 2. (a) XRD patterns of oleic acid-capped CsPbBr3 PNCs using PbBr2 as the precursor that was dissolved in different organic solvents (inset, partially enlarged XRD pattern of CsPbBr3 PNCs). (b) EDS spectrum and chemical composition of homogeneously synthesized CsPbBr3 PNCs.
patterns of room-temperature-synthesized CsPbBr3 PNCs. As shown in the inset of Figure 2a, two diffraction peaks were observed in the range of 2θ from 15° to 16°, and these two peaks should be assigned to monoclinic CsPbBr3 (see JCPDS Card No. 18-0364), because orthorhombic, tetragonal, or cubic CsPbBr3 has only one diffraction peak in the same region. Therefore, our CsPbBr3 PNCs should possess a monoclinic structure, instead of the commonly observed orthorhombic, tetragonal, or cubic structure. Figure 2b displays the EDS spectrum of homogeneously synthesized CsPbBr3 PNCs. It was found that as-prepared CsPbBr3 PNCs have a Cs:Pb:Br ratio of 1.00:1.15:3.26, and the actual chemical composition of each element is in good agreement with the stoichiometric value. The morphology, size, and size distribution of CsPbBr3 PNCs were characterized by TEM. Panels a−d of Figure 3 show the TEM and high-resolution TEM (HR-TEM) images of C
DOI: 10.1021/acs.inorgchem.6b02763 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 5a shows current density−voltage−luminance (J−V−L) curves for a typical CsPbBr3 nanocrystal-based LED. The
Figure 4. (a) UV−vis absorption and PL spectra of homogeneously synthesized CH3NH3PbBr3 PNCs. (b) XRD pattern of CH3NH3PbBr3 PNCs. (c) HR-TEM image of CH3NH3PbBr3 PNCs. Photographs of CH3NH3PbBr3 PNCs under (d) normal indoor light and (e) UV light illumination. Figure 5. (a) Current density−voltage−luminance (J−V−L) curves of a CsPbBr3 PNC LED. The inset is a photograph of the LED at an operating voltage of 6.5 V. (b) EL spectra of a CsPbBr3 nanocrystalbased LED at different voltages.
homogeneously synthesized CH3NH3PbBr3 PNCs. The PLQY of CH3NH3PbBr3 PNCs can be as high as 90% at room temperature, which is even higher than that of CsPbBr3 PNCs and is comparable to those of the best CH3NH3PbBr3 PNCs in the literature.1−23 More importantly, no large particles were observed during the synthesis of CH3NH3PbBr3 PNCs. The XRD results confirmed that as-prepared CH3NH3PbBr3 PNCs are highly crystalline, as shown in Figure 4b. The roomtemperature-synthesized CH3NH3PbBr3 PNCs crystallize in a typical perovskite structure (space group Pm3̅m). The crystalline nature of CH3NH3PbBr3 PNCs was further confirmed by their HR-TEM image (see Figure 4c). The luminescence of a CH3NH3PbBr3 nanocrystal solution can be clearly observed under the illumination of normal indoor light and UV light (see Figure 4d,e). These low-cost and highly luminescent CH3NH3PbBr3 PNCs are particularly beneficial for perovskite nanocrystal-based LED application. To determine the PL mechanism of CsPbBr 3 and CH3NH3PbBr3 PNCs, their PL decay curves were recorded and are shown in Figure S4. For group II−VI QDs, a band edge emission with a short PL lifetime of tens of nanoseconds and a defect-related emission with a long PL lifetime of hundreds of nanoseconds are commonly observed.37−39 However, all of PL decay curves of PNCs can be well fitted by a single-exponential decay. When NMP, DMF, and toluene were used as the solvent to synthesize CsPbBr3 PNCs, their PL lifetimes are 16.1, 13.5, and 11.4 ns, respectively. For homogeneously synthesized CH3NH3PbBr3 PNCs, a shorter PL lifetime of 9.4 ns was observed. These results suggest that the photoluminescence of CsPbBr3 and CH3NH3PbBr3 PNCs is dominated by band edge emission. No defect-related emission was observed on PL and PL decay curves. Recently, perovskite-based LEDs have been extensively reported.18−23 Herein, the EL devices with an ITO/ZnO/ CsPbBr3/CBP/MoO3/Al inverted structure have been successfully fabricated. ZnO and CBP were chosen as the electrontransporting layer and hole-transporting layer, respectively.
rectification property was clearly observed for the CsPbBr3based LED, and the maximal luminance can reach 110 cd/m2. As shown in the inset of Figure 5a, very bright green emission was observed for the CsPbBr3-based LED at an operating voltage of 6.5 V. The highest EQE and current efficiency are 0.015% and 0.042 cd/A, respectively, at a driving voltage of 6.8 V (see Figure S5). Moreover, the LED exhibits a low turn-on voltage of 3.2 V. EL spectra of a CsPbBr3 nanocrystal-based LED at different voltages are shown in Figure 5b. The EL peak is centered at 515 nm with a fwhm of 20 nm, which is almost the same as that of PL spectrum of CsPbBr3 nanocrystals. Note that the PNC synthesis, the deposition of a PNC thin film, and the LED test were conducted in open air.
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CONCLUSIONS
In summary, a polar organic solvent-free approach via the dissolution of PbBr2 in toluene has been developed for the synthesis of highly luminescent CsPbBr3 and CH3NH3PbBr3 perovskite nanocrystals, which can prevent the use of harmful DMF, DMSO, NMP, or tetrahydrofuran. Their PLQYs can be as high as 80−90%, which are comparable to those of the best CsPbBr3 and CH3NH3PbBr3 PNCs in the literature. The morphology, structure, and chemical compositions of PNCs were investigated in detail. Through this facile approach, gramscale perovskite nanocrystals can be prepared in open air at room temperature. CsPbBr3 PNCs have been successfully applied in LEDs as an emitting layer. The results suggest that room-temperature-synthesized CsPbBr3 PNCs have great potential for use in LEDs. D
DOI: 10.1021/acs.inorgchem.6b02763 Inorg. Chem. XXXX, XXX, XXX−XXX
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02763. UV−vis absorption and PL spectra of large-scale CsPbBr3 PNCs, size distributions of CsPbBr3 PNCs synthesized on small and large scales, TEM images of CsPbBr3 PNCs that were synthesized by injecting DMF and NMP PbBr2 precursor solutions, PL decay curves of oleic acid-capped CsPbBr3 and CH3NH3PbBr3 PNCs, and current efficiency and external quantum efficiency (EQE) versus driving voltage curves (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Telephone and fax: +86-431-85262941. E-mail:
[email protected]. cn. ORCID
Daocheng Pan: 0000-0002-8273-6331 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51672267 and 51402285). REFERENCES
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DOI: 10.1021/acs.inorgchem.6b02763 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b02763 Inorg. Chem. XXXX, XXX, XXX−XXX
