Halide-Rich Synthesized Cesium Lead Bromide Perovskite

Jun 1, 2017 - The fabricated light-emitting diode (LED) devices with NCs made in a halide-rich circumstance demonstrated performance better than that ...
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Halide-Rich Synthesized Cesium Lead Bromide Perovskite Nanocrystals for Light-Emitting Diodes with Improved Performance Peizhao Liu,†,‡ Wei Chen,‡,§ Weigao Wang,‡ Bing Xu,‡ Dan Wu,∥ Junjie Hao,‡ Wanyu Cao,‡ Fan Fang,† Yang Li,† Yuanyuan Zeng,† Ruikun Pan,† Shuming Chen,‡ Wanqiang Cao,*,† Xiao Wei Sun,*,‡ and Kai Wang*,‡ †

School of Materials Science and Engineering, Hubei University, Wuhan 430062, China Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen 518055, China § Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, 85748 Garching, Germany ∥ School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 ‡

S Supporting Information *

ABSTRACT: To improve the quality and durability of inorganic halide perovskite nanocrystals (NCs), ammonium halide and lead oxide (PbO) were separately employed for the synthesis of NCs with a tunable ratio of metal to halide. The halide-rich circumstance was therefore successfully set up and found to be beneficial for obtaining good quality NCs with high photoluminescence quantum yields and remarkable stability against purification compared to those qualities of previous regular methods with lead halide. The fabricated light-emitting diode (LED) devices with NCs made in a halide-rich circumstance demonstrated performance better than that of devices with NCs made in a halide-poor circumstance. A LED with CsPbBr3 NCs with a 1:4 Pb:Br ratio showed an obvious improved maximal luminance of 12090 cd m−2, a current efficiency of 3.1 cd A−1, and an external quantum efficiency of 1.194%, which were much higher than those of devices with NCs synthesized by the regular method.

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“polymer-assisted method”,10 and the device performances were consequently improved with maximal luminances of 7627 and 53525 cd m−2, respectively. Unlike Cd-based QDs, the perovskite NCs are more intrinsically ionic and therefore very sensitive to polar solvents and surfactants.5 The ligands binding to the NCs surface are also highly dynamic and labile, which makes them debond easily during the isolation and purification process.11 Thus, the QY value of CsPbBr3 NCs will be decreased by this poor colloidal stability,11 which would consequently have an detrimental effect on the performance of the device. Recently, Li et al. had proposed that the high quality of CsPbBr3 was related to the self-passivating effect of the halide element of Br,12 and a similar phenomenon was also found in hybrid perovskites.13 Moreover, Zeger et al. found that the surface of CsPbBr3 NCs could be passivated by oleylammonium bromide.11 The halide elements are supposed to play an important role in maintaining the high quality of CsPbBr3 NCs for the device.

INTRODUCTION All inorganic perovskite nanocrystals (NCs) have attracted a great deal of attention in terms of optoelectronic devices, including photovoltaics as well as light-emitting diodes (LEDs),1−6 because of their outstanding optoelectronic performance. Inorganic perovskite NCs have become promising materials in advanced display technology because of advantages such as their high quantum yield (QY), narrow emission with a narrow full width at half-maximum (fwhm), and the wide range of their tunable emission wavelength. In 2015, CsPbBr3 perovskite NC LEDs were first reported by the Zeng group and exhibited a promising external quantum efficiency (EQE) of 0.12% and a luminance of 946 cd m−2.1 The luminance of the CsPbBr3 LED had been improved to 1377 cd m−2 through structural modification by the Rogach group.3 Meanwhile, in our previous work, we further optimized the device performance, reaching a higher luminance of 3853 cd m−2 and an EQE of 2.21%, by introducing the dual-phase perovskite composites.7 Moreover, a perovskite thin film (TF) had also become a promising material in LED applications. Yantara et al. first reported a TF LED consisting of CsPbBr3 perovskite materials, which exhibited a maximal luminance of 407 cd m−2.8 The maximal luminance of LEDs was further improved by the “one-step solution method”9 and the © 2017 American Chemical Society

Received: February 20, 2017 Revised: May 31, 2017 Published: June 1, 2017 5168

DOI: 10.1021/acs.chemmater.7b00692 Chem. Mater. 2017, 29, 5168−5173

Article

Chemistry of Materials

Figure 1. (a) PL spectra of as-prepared perovskite NCs in a halide-rich circumstance. Insets show photographs of NCs under UV light with a wavelength of 365 nm. (b) X-ray diffraction patterns of NCs. (c) TEM and (d) HRTEM images of CsPbBr3 NCs.

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To the best of our knowledge, there are three main routes for the synthesis of high-quality inorganic perovskite NCs: “hot injection method”,14 “anion-exchange method”,15 and “poor solvent method”.12,16,17 As previously described in the literature for the typical synthesis of CsPbBr3, the methods relied strongly on Cs2CO3, CsBr, PbBr2, etc., as raw materials for metal and halide precursors, and the element ratios of halides to metals were somehow fixed to be low, resulting in a fragile halide-passivated effect on NCs. Thus, as-prepared perovskite NCs in this synthesis system revealed poor durability against variable circumstances, e.g., purification and device fabrication. Moreover, as previously described in the literature, lead oleate would be generated as a byproduct during the NC synthesis process, and excess Pb components would act as trap states, producing an obvious quenching effect after purification. Cho et al. had used an increased level of methylammonium bromide (MABr) to prevent the formation of Pb components.18 It has been proven that a halide-rich circumstance would be beneficial for obtaining higher-quality perovskite NCs. In this work, we utilized NH4X (X = Cl, Br, or I) and PbO to substitute for conventional PbX2 (X = Cl, Br, or I) as sources of halides and lead separately, so that the Pb:X ratio (X = Cl, Br, or I) could be adjusted at will for the sake of investigation. Moreover, perovskite NCs were synthesized in this halide-rich circumstance with a much higher quality and stronger durability against variation in circumstances, like purification treatment for further device fabrication. It was also found that when the raw materials had a 4:1 NH4Br:PbO ratio for the synthesis of CsPbBr3, the absolute photoluminescence quantum yield (abs QY) of purified CsPbBr3 NCs could still remain at a considerably high level (∼75%); a related LED demonstrated a high level of luminance of 12090 cd m−2, and the related CE and EQE were 3.1 cd A−1 and 1.194%, respectively.

EXPERIMENTAL SECTION

Chemicals. Cs2CO3 (99.99%), PbO (99.90%), NH4Cl (99.99%), NH4Br (99.99%), NH4I (99.999%), octadecene (ODE, 90%), oleic acid (OA, AR), and oleylamine (OLA) were purchased from Aladdin. n-Hexane (>97.0%) and acetone (>99.5%) were purchased from Ling feng Reagent Co. and toluene (99.8%) and acetonitrile (99.8%) from Sigma. Preparation of Cesium Oleate. The synthesis method is similar to that described in a previous report,15 using PbO and NH4X instead of PbX2. First, OA and OLA were dried under vacuum for 1 h at 120 °C. Cs2CO3 (0.0815 g) was loaded into a 25 mL three-neck flask along with octadecene (4 mL) and dried OA (0.25 mL), dried under vacuum for 1 h at 120 °C, and then heated under Ar to 150 °C to obtain a clear solution. Synthesis of CsPbX3 NCs. For CsPbX3 (PbO and 3NH4X), ODE (5 mL), PbO (0.188 mmol, 0.04195 g), and a NH4X species such as NH4I (0.564 mmol, 0.08178 g), NH4Br (0.564 mmol, 0.05525 g), NH4I (0.564 mmol, 0.03016 g), or a mixture of the three were loaded into a 25 mL three-neck flask and degassed for 0.5 h at 120 °C. Dried OLA (0.5 mL) and OA (0.5 mL) were injected at 120 °C under vacuum and then degassed for 1 h at 120 °C until PbO and NH4X had been completely solubilized. The temperature was then increased to 180 °C under Ar, and a cesium oleate solution (0.4 mL, maintained at 80 °C before injection) was quickly injected. Five seconds later, the reaction mixture was cooled to room temperature with an ice/water bath. Isolation and Purification of CsPbBr3 NCs. The 4.5 mL crude solution was added to an equal volume of acetone and the mixture centrifuged for 3 min at 10000 rpm; after centrifugation, the precipitate was redispersed in 4.5 mL of toluene, and an equal volume of acetonitrile was added. The mixture was centrifuged again for 1 min at 5000 rpm, and then the precipitate was dispersed in 1 mL of dehydrate n-hexane and centrifuged for 4 min at 8000 rpm to obtain a clear solution; this solution was kept in a refrigerator at 4 °C. Characterization of Materials. As-purified NCs were dispersed in extra dry n-hexane, and their ultraviolet−visible (UV−vis) 5169

DOI: 10.1021/acs.chemmater.7b00692 Chem. Mater. 2017, 29, 5168−5173

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Chemistry of Materials absorption spectra and photoluminescence (PL) spectra were recorded by the PERSEE TU-1901 spectrophotometer and the Gilden Photonics fluoroSENS spectrofluorometer, respectively. The QY and lifetime τ of the purified NCs in solution were also measured by Quantaurus-QY C11347-11 and Quantaurus-Tau C11367-11 instruments, respectively. All the QY values were absolute quantum yields that were all obtained by counting the photons and described by the equation QY = Pemi/Pabs, where Pabs stands for the number of absorbed photons and Pemi stands for the number of emitted photons. Three drops of as-purified NCs in the last section were continuously diluted in 2 mL of dehydrate n-hexane in a glass tube and then placed in the chamber integrated with an integrating sphere. The final absolute QY values were directly read from the terminal monitor, and the PL decay curves were fitted with a triexponential decay model. Moreover, X-ray diffraction (XRD) was performed with a Smartlab 9KW instrument to analyze the NC powder on a glass substrate. Transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM) images were recorded with a Tecnai F30 S-TWIN instrument with an operating voltage of 300 kV to observe the NCs doped on Cu mesh substrates. X-ray photoelectron spectroscopy (XPS) was measured by ESCALAB 250Xi. Device Fabrication and Characterization. Patterned ITOcoated glass was successively cleaned with soap, deionized water, ethanol, chloroform, acetone, and isopropanol and treated with UV and ozone for 30 min. A 40 nm PEDOT:PSS film was spin-coated onto ITO glass at 3000 rpm for 45 s and annealed in air at 120 °C for 30 min. Then the substrate was transferred into a glovebox, and 40 nm poly-TPD (dissolved in chlorobenzene at a concentration of 10 mg mL−1) was spin-coated onto the PEDOT:PSS film at a speed of 4000 rpm for 40 s and annealed at 110 °C for 30 min. The perovskite NCs (15 mg mL−1) active layer was spin-cast from their colloidal solution at 2000 rpm for 45 s. TPBI (40 nm), LiF (1 nm), and Al (150 nm) layers were sequentially deposited by thermal evaporation in a vacuum deposition clamber (1 × 10−7 Torr). The Al cathode was deposited through a shadow mask defining device area of 2 mm × 2 mm. All devices were characterized in ambient air. The luminance−current− voltage characteristics were measured with an Agilent 2400 luminance meter. The EL spectra were recorded using a USB 2000 spectrometer.

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direction of CsPbBr3. The TEM and HRTEM images of the remaining NCs with blue emission and red emission are given in Figure S1. To investigate the relationship between the variable halide circumstance and the quality of the NCs, four samples of CsPbBr3 NCs were synthesized in different halide circumstances, and the NCs synthesized with the regular method by utilizing PbBr2 as precursor, at a 1:2 Pb:Br ratio, were set as the reference sample. The three remaining samples were synthesized with lead oxide and ammonium bromide as metal and halide sources separately with Pb:Br ratios of 1:2, 1:3, and 1:4, as described in Table 1. The absolute QY values of NCs Table 1. Spectral Peaks, fwhm Values, QY Values, and Average PL Lifetimes of NCs Synthesized with Different Ratios of Raw Materialsa sample (precursors) sample sample sample sample a

1 2 3 4

(PbBr2), reference (PbO and 2NH4Br) (PbO and 3NH4Br) (PbO and 4NH4Br)

PL λ (nm)/fwhm (nm)

QY (%)

τ (ns)

512/25 512/25 514/19 513/19

48 55 69 75

15.9 10.6 8.1 6.3

All data were recorded for purified NCs.

were all tested after the purification process and determined to be the average values. All the test results are listed in the Table 1 and illustrated in Figure 2. Figure 2a depicts the spectra for the PL and absorption of different NCs. From the spectra data, the emission peaks for all samples are all maintained at 513 ± 1 nm. Meanwhile, with the increase in the level of Br in raw materials, the fwhm was becoming narrower and the QY value was increasing, which could be seen from Table 1 and Figure 2c. These could be explained well by the halide self-passivation effect mentioned in the work of Li et al.12 Additionally, the time-resolved PL decay curves of as-prepared NCs were fitted to triexponential decay functions as shown in Figure 2b. The short-, intermediate-, and long-lived PL lifetimes and the values of the distribution coefficient (A) and the percentages (P) are also listed in Table S1. All these equations also were relevant to the fitting and average lifetime determinations and the percentages. The long-lived PL lifetime, caused by electron trap states,16 was observed to decrease with an increase in the level of Br in raw materials, which indicated the Br-rich circumstance was beneficial for obtaining NCs with fewer electron trap states and devices with better performance.19 Figure 3 illustrates the proposed schematics for NCs synthesized in halide-poor or -rich (typically for Br) circumstances. NCs before purification are surrounded by oleylammonium bromide. However, during the purification process, the QY of NCs was normally decreased obviously due to the “eroding” effect to the passivating layer and “peeling off” of consequent ligands.5 Thus, with the increase in the amount of oleylammonium bromide around NCs, the halide is supposed to remain to a large extent on the surface of NCs, which is consequently beneficial for maintaining the self-passivation conditions. In addition, as described in Figure 3, the Br-rich element could not only passivate the surface electron traps of NCs, ensuring a high QY value, but also improve the durability of NCs during the purification process and device fabrication. The XPS data in Figure S4 confirmed the increase in the level of Br in the NCs with the addition of Br components to the raw materials. The shoulder peaks for Br also indicated the valence information for Br on the inner and outer surface of NCs.

RESULTS AND DISCUSSION

We had synthesized perovskite NCs with different emissions, with all PbO:NH4X (X = Cl, Br, I or their mixtures) ratios being 1:4 as a halide-rich circumstance and obtained NCs with saturated blue, green, and red emission for wide color gamut display. The emission of NCs was determined by the ratio of halides: 7:3 NH4Cl:NH4Br (CsPbBr0.9Cl2.1) for blue, pure CsPbBr3 for green, and 7:3 NH4I:NH4Br (CsPbBr0.9I2.1) for red emissions as illustrated in the insets of Figure 1a. The PL spectra of the as-prepared perovskite NCs, CsPbBr0.9Cl2.1, CsPbBr3, and CsPbBr0.9I2.1, are also shown in Figure 1a. Their PL peaks (and fwhm) were found at 441 nm (16 nm), 514 nm (24 nm), and 662 nm (45 nm), respectively. The XRD patterns in Figure 1b indicate the cubic perovskite phase for as-designed NCs with blue, green, and red emission. From CsPbBr0.9Cl2.1 to CsPbBr3 to CsPbBr0.9I2.1, all main diffraction peaks, including (100), (110), (200), (211), and (202) peaks, had been observed shifted toward the small angle direction, because of the different constitutions of NCs with different halide ion radii [R(Cl−) < R(Br−) < R(I−)], which could be well explained by Bragg’s theory.14 Panels c and d of Figure 1 show the size distribution and morphology of CsPbBr3 NCs as determined by TEM and HRTEM, respectively. All the NCs reveal a narrow size distribution and square block in morphology because of the intrinsic cubic phase of perovskites. From the high-resolution image, the width space between the lattice fringes was recorded as 0.30 nm for the (200) phase 5170

DOI: 10.1021/acs.chemmater.7b00692 Chem. Mater. 2017, 29, 5168−5173

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Figure 2. (a) PL and UV spectra of NCs synthesized in different precursors in different halide circumstances. (b) Time-resolved PL decay and fitting curves of purified CsPbBr3 NCs. (c) Average QY values of NCs.

Figure 3. Schematics for halide-poor and halide-rich circumstances for synthesis of NCs.

transporting layer, and LiF/Al as the cathode. The structures and fabrication processes from device sample (DS) 1 to DS 4 corresponding to samples 1−4 of NCs, respectively, were all the same except for as-used NCs, which were made in different halide circumstances. The EL peak and fwhm of DS 4 are 515 and 18 nm, respectively, which indicate saturated green corresponding to Commission Internationale de L’Eclairage

Panel a of Figure 4 demonstrates the LED device structure and panel b its energy band diagram for all functional layers. The LED device consists of multiple layers in the following order: indium tin oxide (ITO) as the anode, poly(ethylenedioxythiophene):polystyrene sulfonate film, polyTPD film as the hole-transporting layer, CsPbBr3 perovskite NCs as the emitting layer, TPBi (40 nm) as the electron5171

DOI: 10.1021/acs.chemmater.7b00692 Chem. Mater. 2017, 29, 5168−5173

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Figure 4. Illustration of the CsPbBr3 perovskite NC LED device. (a) LED device structure and (b) flat band energy level diagram. (c) PL spectrum of CsPbBr3 in solution and EL spectrum of a related EL device (inset, device with an operation voltage of 9.5 V with an active area of 2 mm × 2 mm; NCs synthesized in a 1:4 Pb/Br solution). (d) Comparison of device performance with different NCs, including current density (CD) and luminance (L) vs driving voltage characteristics. (e) Current efficiency (CE) and external quantum efficiency (EQE) as a function of luminance.

and recombination at the lighting layer that were higher than those of halide-poor cases. This could also be verified from panels d and e of Figure 4. Meanwhile, the maximal luminescence of DS 4 reached a considerable level of 12090 cd m−2 at 9.8 V, which is not only much higher than that of DS 1 but also higher than most data of CsPbBr3 perovskite NC LEDs previously published in the literature to the best of our knowledge (Table S2).

(CIE) color coordinates of (0.073, 0.6813). The inset shows the performance of the DS 4 LED operated under a bias of 8.5 V indicating vivid green. Table 2 summarizes the device Table 2. Device Performance Parameters of LEDs Based on CsPbBr3 Perovskite NCs Synthesized with Different Ratio of Raw Materials device sample DS DS DS DS

1 2 3 4

EL λ (nm)/ fwhm (nm)

Von (V)

L (cd m−2)

CE (cd A−1)

EQE (%)

PE (lm W−1)

516/22 515/19 515/19 515/18

5.8 5.4 4.5 4.6

3583 4778 9565 12090

0.935 1.167 3.036 3.106

0.336 0.445 1.132 1.194

0.349 0.439 1.478 1.38

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CONCLUSIONS

We have set up a halide-rich and halide-tunable circumstance by separately using low-toxicity ammonium halide and lead oxide (PbO) as halide and metal sources to obtain perovskite NCs with higher quality and stronger durability because of the enhanced surface passivation effect. With an increase in the level of the halide elements for the synthesis circumstance, QYs of as-prepared NCs increased and PL lifetimes consequently decreased, which was supposed to be an efficient method for obtaining better quality NCs. Moreover, LED devices with CsPbBr3 NCs synthesized in a halide-rich circumstance demonstrated an operational performance much better than that of a device with NCs made in a halide-poor circumstance, in which the maximal luminescence of LED has reached a considerable level of 12090 cd m−2 at 9.8 V, which is not only higher than those of the remaining devices made in halide-poor cases but also higher than those of most devices described previously in the literature. NCs made in a halide-rich circumstance have improved and optimized the charge transportation and exciton recombination of related LED devices.

operation performance for DS 1−DS 4. The NCs optimized by the halide circumstance greatly improved the LED device performance. With the increase in the QY value of NCs, the charge mobility was supposed to be optimized and the defect midstates of NCs were also somehow eliminated in the NCs layer in the device. The decreased average lifetime of NCs with the increase in the level of Br in the reaction system was supposed to optimize the exciton recombination in luminescent layers. The EQE and PE values were therefore improved. Device performances from DS 1 to DS 4, the maximal luminance (L), the current efficiency (CE), and the external quantum efficiency (EQE) are all improved >2-fold from 3583 cd m−2, 0.935 cd A−1, and 0.336% to 12090 cd m−2, 3.106 cd A−1, and 1.194%, respectively. Detailed device performances of DS 1−DS 4 are all shown in Figures S5−S8. Moreover, the turn-on voltage, defined as the voltage necessary to detect a luminescence of 1 cd m−2, of DS 4 is 4.6 V, lower than the value of 5.8 V of DS 1, which indicates the NCs from the halide-rich circumstance possessed efficiencies for charge transportation 5172

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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.chemmater.7b00692. TEM and HRTEM images of CsPbBr0.9Cl2.1 (blue), CsPbBr3 (green), and CsPbBr0.9I2.1 (red) and the related size distribution, PL spectra for NCs synthesized via different methods, detailed information about timeresolved PL decay and related equations, XPS data for NCs and the elements, and device performances and comparisons (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Peizhao Liu: 0000-0002-6367-6847 Author Contributions

P. Liu and W. Chen contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the financial support of the National Key Research and Development Program of China administered by the Ministry of Science and Technology of China (2016YFB0401702), the National Natural Science Foundation of China (61674074, 51402148, and 61405089), the Shenzhen Peacock Team Project, and the Shenzhen Innovation Project (JCYJ20160301113356947, KC2014JSQN0011A, JCYJ20150630145302223, and JCYJ20160301113537474).

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REFERENCES

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DOI: 10.1021/acs.chemmater.7b00692 Chem. Mater. 2017, 29, 5168−5173