Non-blinking Quantum-Dot-Based Blue Light-Emitting Diodes with

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Non-blinking Quantum-Dot-Based Blue Light-Emitting Diodes with High Efficiency and Balanced Charge-Injection Process Qingli Lin, Lei Wang, Zhaohan Li, Huaibin Shen, Lijun Guo, Yanmin Kuang, Hongzhe Wang, and Lin Song Li ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01195 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Non-blinking Quantum-Dot-Based Blue LightEmitting Diodes with High Efficiency and Balanced Charge-Injection Process Qingli Lin,† Lei Wang,† Zhaohan Li,† Huaibin Shen,†,* Lijun Guo,‡ Yanmin Kuang,‡ Hongzhe Wang,† and Lin Song Li†,*

†

Key Laboratory for Special Functional Materials of Ministry of Education, Henan University,

Kaifeng 475004, China

‡

Institute of Photo-biophysics, School of Physics and Electronics, Henan University, Kaifeng

475004, China

ABSTRACT: Blue non-blinking (>98% ‘on’ time) ZnCdSe/ZnS//ZnS quantum dots (QDs) with absolute fluorescence quantum yield (QY) of 92% (λpeak =472 nm) were synthesized via a low temperature nucleation and high temperature shell growth method. Such bright non-blinking ZnCdSe/ZnS//ZnS core/shell QDs exhibit not only good emission tunability in the blue-cyan range with corresponding wavelength from 450 to 495 nm, but also high absolute photoluminescence (PL) QY and superior chemical and photochemical stability. Highly efficient blue quantum dot-based light-emitting diodes (QLEDs) have been demonstrated by using nonblinking ZnCdSe/ZnS//ZnS QDs as emissive layer and the charge-injection balance within QD active layer were improved by introducing an non-conductive layer of poly(methyl methacrylate)

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(PMMA) between electron transport layer (ETL) and QD layer, where PMMA layer takes the role of coordinator to impede excessive electron flux. The best device exhibits outstanding features such as the maximum luminance of 14,100 cd/m2, current efficiency of 11.8 cd/A, and external quantum efficiency (EQE) of 16.2%. Importantly, the peak efficiency of the QLEDs with PMMA is achieved at ~ 1,000 cd/m2 and high EQE>12% can be sustained in the range of 100 to 3,000 cd/m2.

KEYWORDS: blue quantum dot, non-blinking, electroluminescence, poly(methyl methacrylate), charge-injection balance

Inorganic semiconductive quantum dots (QDs) offer many unique properties, such as saturated photoluminescence (PL), high quantum yield (QY), high photochemical stability, and sizecontrolled tunable emission spectra; as a consequence, QD-based devices with various emission wavelengths may be used as next generation light sources in many fields including solid-state lighting and flat panel displays.1-9 Especially, quantum dot-based light-emitting diodes (QLEDs) do show strong potential in the realization of devices with unprecedented color and brightness with reduced energy consumption, it can be projected to consume only 10 to 20 % of the power of liquid crystal displays (LCDs) and manufacture for less than half of the cost of LCDs.10 Recent progress on QLEDs begins to demonstrate these significant advantages in good color purity of narrow full width at half-maximum (FWHM) less than 35 nm, high brightness and efficiency, long operation lifetime, low power consumption, and low-cost manufacturing


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process.11-14 The most excellent device performance of red, green, and blue QLEDs have been reported by QD Vision, Lee, and our group, respectively.12,15,16 The maximum luminance for red (λ = 620 nm), green (λ = 520 nm), and deep-blue (λ = 445 nm) QLEDs have reached up to 165,000, 218,800, and 10,400 cd/m2.12,15,16 Moreover, the peak values for external quantum efficiency (EQE) have been recorded to 20.5% (λ = 640 nm) by Jin, 23.68% (λ = 538 nm) by Chen, and 15.6% (λ = 445 nm) by us for the red, green, and deep-blue devices.16-18

To date, the performance of red and green QLEDs achieve similar levels of the state-of-the-art OLEDs. However, the performance of blue QLEDs in the range of 450 ~ 495 nm, which is the most important color for practical applications, is still significantly below those of red and green QLEDs. This is mainly attributable to the following two reasons: (1) Low PL QY ( 10%, many efforts have been used to improve blue QLED device performance.13,23 As alloy QD, ZnCdSe does show high PL QY in a wide range of 440 to 550 nm, high photostability, as well as a smooth interfacial potential barrier for the alloy core/shell QDs, which is beneficial to suppress the nonradiative recombination.19,24 Therefore, ZnCdSe alloy QDs were chosen as “good” emitting layer for blue QLEDs. By adopting our recent established “low temperature nucleation and high temperature shell growth” method,25,26 ZnCdSe/ZnS//ZnS alloy QDs were synthesized first, and the detailed experimental procedure was presented in Supporting Information. The UV-vis and normalized PL spectra of the ZnCdSe core, ZnCdSe/ZnS, and ZnCdSe/ZnS//ZnS alloy core/shell QDs are presented in Figure 1a. In fact, two layers of ZnS shells have been chosen to growth over the ZnCdSe cores, respectively. Along with the growth of the first ZnS shell, up to 13 nm blue shift of PL spectra is observed, and the decrease of FWHM is about 3 nm (from 42 nm to 39 nm). The PL QY of ZnCdSe/ZnS QDs is significantly increased from 64% to 87%. After the growth of a second ZnS shell layer, the PL QY of


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ZnCdSe/ZnS//ZnS QDs boosted to 92% eventually. Such superior quantum efficiency is in no small part due to the successful surface passivation realized by the wider bandgap ZnS shells. It is worth mentioning that the growth of shell material at high temperature is a crucial step, which can facilitate the diffusion of Zn and S atoms into the ZnCdSe cores and form a ZnCdSeS graded interlayer, as evidenced by the blue shift of PL emission.25,26

Such interfacial alloy layers are normally existed in both ZnCdSe/ZnS and ZnCdSe/ZnS//ZnS QDs, which consist of intermediate ZnCdSe-ZnS alloy layers with a radial composition gradient to the outer ZnS shells. Consequently, the ZnCdSe/ZnS//ZnS QDs are more beneficial to suppress the Auger recombination than the ZnCdSe/ZnS, due to the additional overcoating of ZnS shells can pin the alloy layer in a more inner part compared to the ZnCdSe/ZnS QDs, forming a stable structure with continuous composition transition of core/alloy shell//out shell structure.13 Even though the PL is shifted only 9 nm to 472 nm with the growth of second ZnS shell, a significant improvement of FWHM is observed and it is narrowed to 31 nm by the end of reaction. The decrease of PL FWHM is most likely the result of the improved matching between the reactivity of octanethiol and the infusion rate of shell precursors, which guarantees the steady epitaxial growth of ZnS shell and is similar to those recent reports.27,28

To characterize the structure evolution of as-prepared QDs, their crystallographic parameters were investigated using XRD (Figure 1b). According to the XRD patterns, the peak positions of ZnCdSe core QDs lie between the bulk zinc blende CdSe and zinc blende ZnSe. Such a


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diffraction pattern indicates that the peaks of the sample are derived not from a mixture of CdSe and ZnSe, but ZnCdSe alloy QDs with compositional homogeneity. An obvious trend of peak shift to that of zinc blende phase ZnS has been noticed after the coating of ZnS shells onto ZnCdSe cores and the cubic phase structure can still be kept for all samples. TEM images shown in Figure 1c reveals the uniform shape and narrow particle size distribution of as-prepared QDs, as well as average particle diameters of 3.5±0.7, 6.3±0.5, and 7.8±0.6 nm for ZnCdSe, ZnCdSe/ZnS, and ZnCdSe/ZnS//ZnS QDs, respectively. Typical HRTEM images proved high crystallinity for all three kinds of QDs with continuous lattice fringes throughout the whole particles. The size and optical properties for QDs used in this paper are summarized in Table 1.

Ever since the phenomenon of random fluctuations in PL emission from QDs has been discovered, it has been recognized as a detrimental effect that limits the stability of QD-based devices.27,29 Applications for QDs in fields like diodes, lasers, and bioimaging are all adversely affected by their tendency to blink.27,30 In recent years, various QD materials with wellcontrolled compositions and high PL QY have abounded, but only several special red emission QDs have been found to be non-blinking.27,30-32 So far, there is still no reports of non-blinking blue-cyan QDs. The QD blinking behavior were further studied at single-emitter level to characterize as-synthesized blue QDs. Diluted blue ZnCdSe core, ZnCdSe/ZnS, and ZnCdSe/ZnS//ZnS core/shell QDs were immobilized in a PMMA matrix, respectively. As shown in Figure 2, their fluorescence traces were recorded under continuous wave (CW).


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Figure 2a shows the corresponding blinking trace and Figure 2b shows photon counting histograms (PCHs) of each whole trace. Compared to the other samples, it is evident that the ZnCdSe/ZnS//ZnS core/shell//shell QD sample with ~ 2.2 nm ZnS outer-shell thickness shows a significantly increased on-state and the average ‘on’ time is >98%. This reveals that zinc-blende ZnCdSe/ZnS//ZnS QDs core/shell//shell QDs are indeed nearly non-blinking within targeted single-exciton regime. We speculate that the graded thick shell with highly crystalline structure, which can effectively suppress the interception mechanism, should account for the significantly reduced blinking phenomenon observed here.33 By using high QY non-blinking ZnCdSe/ZnS//ZnS QDs, blue QLEDs was first fabricated as conventional structure. Its energy band diagram and corresponding cross-sectional SEM images are shown in Figure S1a-d (Supporting Information). Compared to commonly used HTL material of PVK, the mobility of TFB (µh ~ 1.0 × 10-2 cm2 V-1 s-1)21,34 is adopted due to more favorable hole transport in the devices. The as-prepared device exhibits the maximum luminance of 12,500 cd/m2, current efficiency of 7.2 cd/A, and EQE of 9.8%, respectively (Supporting Information, Figure S2). Relative to Lee and co-workers’ report about blue QLEDs,19 EQE of this device increases by 38%, which is attributed to adopting the high QY non-blinking QDs as emissive layer that suppressing nonradiative Auger recombination and increasing the exciton recombination probability in QD layer. However, the imbalance of charge injection is still a drawback and may decrease the overall device efficiency because much more electrons is still


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injected than holes due to the great disparity between HOMO level of TFB and the valance band of QDs.

To obtain balanced electron and hole injections, an insulating layer may be introduced between ETL and QD layer.17 By the insertion of a PMMA interlayer between ZnO ETL and blue QD emissive layer, a multilayer structure has been demonstrated in Figure 3a. It exhibits the schematic structure of multilayered QLEDs, consisting of ITO (~150 nm), PEDOT:PSS/TFB (~70 nm), QDs (~20 nm), PMMA, and ZnO (~40 nm). In Figure S3 (Supporting Information), the scanning electron microscopic (SEM) images display relatively uniform and compact QD and ZnO layers. We expect this PMMA interlayer can prevent direct contact of QDs with ZnO nanoparticles, and therefore it effectively blocks the overflow of electrons and promotes the hole-electron injection balance. Here TFB is adopted as HTL, which differs from Jin’s report that using poly(N,N’-bis(4-butylphenyl)-N,N’-bis(phenyl)-benzidine) (Poly-TPD) and PVK as bilayer-structured HTL.17 Compared to the HOMO energy level and hole mobility of both PolyTPD (-5.2 eV and µh~ 1.0 × 10-4 cm2 V-1 s-1)34,35 and PVK (-5.8 eV and µh~ 2.5×10-6 cm2 V-1 s1 21,22

)

, while, as expected from the energy band diagram of Figure 3b, HOMO energy level of

TFB is -5.3 eV36 and hole mobility is as high as µh~ 1.0 × 10-2 cm2 V-1 s-1,21,34 which conduces to the hole injection and transport. While, ZnO nanoparticles have been proved to be an effective electron injection/transport layer and hole blocking layer, owing to an electron affinity of ca.


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~4.3 eV and an ionization potential of ca. ~7.6 eV, which brings great advantages in the recombination of electron-hole within QD layer.20

To compare the balance of carrier injection with and without PMMA, single carrier (hole-only and electron-only) devices have been fabricated. Figure S5 (Supporting Information) provides the energy level diagrams of single carrier devices. Therein, PEDOT:PSS/TFB promoted the hole injection/transport and blocked the electrons in the hole-only device (Figure S5a, Supporting Information). While, the two separate layers of ZnO blocked the hole injection from the electrodes in the electron-only device (Figure S5b, Supporting Information). Figure 3d shows the current density-voltage characteristics of single carrier devices. For the devices without PMMA, the electron current is more than 7 times higher than the hole current at the driving voltage of 98%. The significantly suppressed blinking phenomenon observed here is mainly caused by the graded thick shell with highly crystalline structure, which effectively suppressed the interception mechanism. The second aspect is the improved charge balance. Herein, an insulating layer of PMMA was inserted between QD and ZnO layers in the architecture of QLEDs. PMMA layer plays the role of moderately impeding the excess electron injecting from the cathode. In addition, the single carrier data (Figure 3d) suggests that the balance of hole and electron injection increased by 50% after introducing the PMMA layer. In conclusion, both the advanced QD materials and optimized device structure suppress the efficiency roll-off in QLEDs, which mainly originates from reduced charge-injection imbalance and all kinds of non-radiative exciton quenching processes, such as non-radiative Auger recombination, dissociation of excitons under high fields, and electric-field-induced decrease in PL efficiency of QD. Importantly, the average value of peak EQE is reached as high as 14.3% (Figure 4c) with a low standard deviation of 4.41%, which is obtained from the statistic of 55 different batches of devices and suggested high reproducibility of device performance.

Further insight into the influence of PMMA on the improvement of QLEDs performance, which is expressed by the PL lifetime of QD films contacting different layers, is presented in Figure 4d. When the ZnO nanoparticles are directly spin-coated onto the QD layers, the PL lifetime of QD film decreases from 9.03 to 8.75 ns, due to the negative charging of QDs resulted from the spontaneous charge-transfer process. While, the spontaneous charging can be


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suppressed by inserting a thin PMMA layer between QDs and ZnO, increasing the exciton lifetime from 8.75 to 9.08 ns, which is on a par with that of layers without ZnO. In addition, it is worth noting that this PL lifetime shortening on QD films is less than that of previously reported results based on QDs with blinking characteristics,13,17 and this can be explained by the suppression of the Auger recombination in non-blinking QDs.

The QLEDs were simply sealed with glass cover using UV resin and their operational lifetime was further detected in air by using a constant current, corresponding to the initial luminance (L0) of 460 cd/m2. Figure S6 shows the time dependence of the luminance and applied biases for the device. The time at which the luminance is reduced to 50% of its initial value, or T50, is 36 h. The operating voltage increases from 2.94 V to 4.27 V during the same period. Using the relation L0n × T50 = constant (1.5 ≤ n ≤ 2)37, and assuming the acceleration factor of n = 1.5, we obtain a T50 of 355 h for QLEDs at L0=100 cd/m2.

Figure 5a depicts the PL spectra of QD solution as well as EL spectra of QLEDs. The EL spectra shows a 7 nm red-shift compared to PL spectra (PL peak at 472 nm), which is attributed to both the combination of inter-dot interactions within the close-packed QD solids and the strong electric field. For the first aspect, the PL spectrum of QD film redshifts about ~2 nm and is slightly broadened relative to PL spectrum of QD solution, as shown in the inset of Figure 3c. While, for the second aspect, the Stark effect arising from the electric field reduces the energy of exciton recombination.38,39,40 Note that no any parasitical emission could be observed either in


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the low or high energy region for QLEDs, suggesting the good confine of excitons in the QD layers. The FWHM of EL spectra increases to 34 nm. The uniform emission photograph taken at 5.0 V can be seen in the inset of Figure 5a. No obvious broadening of EL emission can be detected with the driving voltage spanning from 3 to 6 V. As a result of quantum confined Stark effect (QCSE), which usually leads to the red-shift of EL emission of QLEDs under ascending external electric field, the EL peak redshifted for about 2 nm with increasing bias voltage.

Based on the above results, QDs with PL peaks at 454 and 495 nm have been used to fabricate QLEDs with optimized device structure (as shown in Figure 3a) and the relevant electrical characteristics are provided in Figure S7 (Supporting Information). QLEDs with turn-on voltage of 2.75 and 2.27 V, the maximum luminance of 20,900 and 33,800 cd/m2 and peak EQE of 13.2% and 15.8% have been achieved, respectively based on QDs with PL peaks at 454 and 495 nm as emissive layers (Figure 6a). Both EL spectra show a red shift of 7 ~ 8 nm, relative to corresponding PL spectra (Figure 6b). The Commission Internationale de l'Eclairage (CIE1931) color coordinates of (0.151, 0.042), (0.119, 0.154), and (0.105, 0.540), corresponding to the emission peaks of 462, 479, and 502 nm, respectively, are marked in Figure 6c. The important parameters of three devices with different QDs are summarized and presented in Table 2.

CONCLUSIONS

In summary, ZnCdSe/ZnS//ZnS QDs with non-blinking characteristic and high PL QY of 92% as well as high stability were synthesized via a low temperature nucleation and high temperature


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growth method. Highly efficient QLEDs based on this non-blinking ZnCdSe/ZnS//ZnS QDs have been demonstrated by adopting a modified device structure, where an insulator of PMMA layer is introduced between ZnO and QD layers. Such PMMA layer plays a crucial role that moderately impedes the electron injection, and thus improves charge-injection balance within QD active layers. The best device shows superior performance indexes like the maximum luminance of 14,100 cd/m2 and current efficiency of 11.8 cd/A, respectively. Such blue QLEDs show a 65.3% increase in EQE relative to QLEDs without PMMA and it is recorded as high as 16.2%. More importantly, the peak efficiency of the QLEDs with PMMA is achieved at ~ 1,000 cd/m2, and high efficiency (EQE > 12%) can be maintained in the luminance range of 100 to 3,000 cd/m2. Histogram of peak EQE of devices shows average peak EQE of 14.3% and a low standard deviation of 4.41%, suggesting high reproducibility of device performance. Additionally, the QLEDs with PL peaks of 454 and 495 nm display the maximum values of 13.2% and 15.8% in EQE by adopting the optimized device structure. These results suggest that such outstanding performance of blue QLEDs would be promising in the application of the solid-state lighting and next generation flat panel displays.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.


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Detailed experimental procedure and characterization, cross-sectional SEM images of QLEDs without PMMA and correspongding the diagram of energy level, characteristics of QLEDs without PMMA, SEM images of QD and ZnO layers, PL decay of ZnCdSe/ZnS//ZnS films at different emission wavelengths, schematic illustration of energy levels for the single-carrier devices, and characteristics of LEDs based on QDs with different PL peak. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]; [email protected].

Author Contributions

Qingli Lin and Lei Wang contributed equally to this paper. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the research project of the National Natural Science Foundation of China (61474037, 21671058, and 61504040), Key Project of National Natural Science Foundation of China (Grant No.U1604261). REFERENCES


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(21) Choulis, S. A.; Choong, V. E.; Mathai, M. K.; So, F. The Effect of Interfacial Layer on the Performance of Organic Light-Emitting Diodes. Appl. Phys. Lett. 2005, 87, 113503. (22) Lee, D.-H.; Liu, Y.-P.; Lee, K.-H.; Chae, H.; Cho, S. M. Effect of Hole Transporting Materials in Phosphorescent White Polymer Light-Emitting Diodes. Org. Electron. 2010, 11, 427-433. (23) Wang, L.; Chen, T.; Lin, Q.; Shen, H.; Wang, A.; Wang, H.; Li, C.; Li, L. Song. HighPerformance Azure Blue Quantum Dot Light-Emitting Diodes via Doping PVK in Emitting Layer. Org. Electron. 2016, 37, 280-286. (24) Zhong, X.; Zhang, Z.; Liu, S.; Han, M.; Knoll, W. Embryonic Nuclei-Induced Alloying Process for the Reproducible Synthesis of Blue-Emitting ZnxCd1-xSe Nanocrystals with Long-Time Thermal Stability in Size Distribution and Emission Wavelength. J. Phys. Chem. B 2004, 108, 15552-15559. (25) Shen, H.; Bai, X.; Wang, A.; Wang, H.; Qian, L.; Yang, Y.; Titov, A.; Hyvonen, J.; Zheng, Y.; Li, L. S. High-Efficient Deep-Blue Light-Emitting Diodes by Using High Quality ZnxCd1-xS/ZnS Core/Shell Quantum Dots. Adv. Funct. Mater. 2014, 24, 2367-2373. (26) Wang, A.; Shen, H.; Zang, S.; Lin, Q.; Wang, H.; Qian, L.; Niu, J.; Li, L. S. Bright, Efficient, and Color-Stable Violet ZnSe-Based Quantum Dot Light-Emitting Diodes. Nanoscale 2015, 7, 2951-2959. (27) Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H. -S.; Fukumura, D.; Jain, R. K.; Bawendi, M. G. Compact High-Quality CdSe-CdS Core-Shell


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Nanocrystals with Narrow Emission Linewidths and Suppressed Blinking. Nat. Mater. 2013, 12, 445-451. (28) Clark, M. D.; Kumar, S. K.; Owen, J. S.; Chan, E. M. Focusing Nanocrystal Size Distributions via Production Control. Nano Lett. 2011, 11, 1976-1980. (29) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Fluorescence Intermittency in Single Cadmium Selenide Nanocrystals. Nature 1996, 383, 802-804. (30) Qin, H.; Niu, Y.; Meng, R.; Lin, X.; Lai, R.; Fang, W.; Peng, X. Single-dot Spectroscopy of Zinc-Blende CdSe/CdS Core/Shell Nanocrystals: Nonblinking and Correlation with Ensemble Measurements. J. Am. Chem. Soc. 2014, 136, 179-187. (31) Meng, R.; Qin, H.; Niu, Y.; Fang, W.; Yang, S.; Lin, X.; Cao, H.; Ma, J.; Lin, W.; Tong, L.; Peng, X. Charging and Discharging Channels in Photoluminescence Intermittency of Single Colloidal CdSe/CdS Core/Shell Quantum Dot. J. Phys. Chem. Lett. 2016, 7, 5176-5182. (32) Park, Y.-S.; Bae, W. K.; Padilha, L. A.; Pietryga, J. M.; Klimov. V. I. Effect of the Core/Shell Interface on Auger Recombination Evaluated by Single-Quantum-Dot Spectroscopy. Nano Lett. 2014, 14, 396-402. (33) Galland, C.; Ghosh, Y.; Steinbrück, A.; Sykora, M.; Hollingsworth, J. A.; Klimov, V. I.; Htoon, H. Two Types of Luminescence Blinking Revealed by Spectroelectrochemistry of Single Quantum Dots. Nature 2011, 479, 203-207.


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(34) Thesen, M. W.; Höfer, B.; Debeaux, M.; Janietz, S.; Wedel, A.; Köhler, A.; Johannes, H.H.; Krueger, H. Hole-Transporting Host-Polymer Series Consisting of Triphenylamine Basic Structures for Phosphorescent Polymer Light-emitting Diodes. J. Polym. Sci. A: Polym. Chem. 2010, 48, 3417-3430. (35) Sun, Q.; Wang, Y. A.; Li, L. S.; Wang, D.; Zhu, T.; Xu, J.; Yang, C.; Li, Y. Bright, Multicoloured Light-Emitting Diodes Based on Quantum Dots. Nat. Photonics 2007, 1, 717-722. (36) Ma, H.; Liu, M. S.; Jen, A. K.-Y. Interface-Tailored and Nanoengineered Polymeric Materials for (Opto)electronic Devices. Polym. Int. 2009, 58, 594-619. (37) Wellmann, P.; Hofmann, M.; Zeika, O.; Werner, A.; Birnstock, J.; Meerheim, R.; He, G.; Walzer, K.; Pfeiffer, M.; Leo, K. High Efficiency p-i-n Organic Light-Emitting Diodes with Long Lifetime. J. Soc. Inf. Disp. 2005, 13, 393-397. (38) Mashford, B. S.; Stevenson, M.; Popovic, Z.; Hamilton, C.; Zhou, Z.; Breen, C.; Steckel, J.; Bulovic, V.; Bawendi, M.; Coe-Sullivan, S.; Kazlas, P. T. High-Efficiency Quantum-Dot Light-Emitting Devices with Enhanced Charge Injection. Nat. Photonics 2013, 7, 407-412. (39) Mashford, B. S.; Nquyen, T.-L.; Wilson, G. J.; Mulvaney, P. All-Inorganic Quantum-Dot Light-Emitting Devices Formed via Low-Cost, Wet-Chemical Processing. J. Mater. Chem. 2010, 20, 167-172.


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(40) Bae, W. K.; Park, Y.-S.; Lim, J.; Lee, D.; Padilha, L. A.; McDaniel, H.; Robel, I.; Lee, C.; Pietryga, J. M.; Klimov, V. I. Controlling the Influence of Auger Recombination on the Performance of Quantum-Dot Light-Emitting Diodes. Nat. Commun. 2013, 4, 2661.


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Figure 1. (a) Evolution of UV-vis and PL spectra of ZnCdSe cores, ZnCdSe/ZnS, and ZnCdSe/ZnS//ZnS alloy core/shell QDs. (b) XRD pattern of as-prepared QDs. Blue and red lines indicate the peak positions of standard references of bulk zinc-blende CdSe and ZnS, respectively. (c) TEM images of ZnCdSe core, ZnCdSe/ZnS, and ZnCdSe/ZnS//ZnS alloy core/shell QDs. Correspondingly, HRTEM images of each kind of QDs are shown as the upperright insets in the images of Figure 1c where the bottom-left insets show the schematic structures of QD cores and core-shell QDs.


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Table 1. Summary of size, thickness of ZnS shells and optical properties of ZnCdSe core, ZnCdSe/ZnS, and ZnCdSe/ZnS//ZnS alloy core/shell QDs. QD structures

d [nm] by TEM

ZnS Thickness [nm]

PL peak [nm]

FWHM [nm]

PL QY (%)

ZnCdSe

3.5±0.7

N/A

494

42

64

ZnCdSe/ZnS

6.3±0.5

1.4

481

39

87

ZnCdSe/ZnS//ZnS

7.8±0.6

2.2

472

31

92


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Figure 2. (a) Representative PL intensity time traces (1,800 s) of the as-synthesized single ZnCdSe core, ZnCdSe/ZnS, and ZnCdSe/ZnS//ZnS core/shell QDs under continuous wave (CW) laser excitation. The binning time is 15 ms. The gray traces are the corrected background noise intensities. (b) Photon counting histograms for the fluorescence traces shown in (a).


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Figure 3. (a) Schematic illustration of the multilayered QLEDs in this study. (b) Energy level diagram of materials used in QLEDs. (c) Variations of PL lifetime in solution versus solid film states of ZnCdSe/ZnS//ZnS QDs with PL peak at 472 nm. Insets show the normalized corresponding PL spectra and PL QY of QDs in solution and film states. (d) Current densityvoltage curves of single-carrier devices with and without PMMA layer.


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Figure 4. (a) Current density-voltage-luminance (J-V-L) of QLEDs with and without PMMA layer. (b) Current efficiency and EQE as a function of luminance of QLEDs (ηA-L-ηEQE) with and without PMMA layer. Inset shows the corresponding efficiency in the luminance range of 100~3,000 cd/m2. (c) Histogram of peak EQEs from statistical data of 55 devices. (d) PL lifetime of the QD films contacting different layers. The thicknesses of the layers are identical to those in QLEDs after structure optimization.


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Figure 5. (a) Normalized PL (solid line) and EL (dashed line) spectra. Inset shows the photograph taken at 5 V. (b) Normalized EL spectra of QLEDs with increasing voltage.


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Figure 6. (a) The maximum of EQE and luminance of QLEDs with QDs of PL peaks locating at 454 and 495 nm, respectively. (b) Normalized PL spectra (solid lines) and EL spectra (dashed lines) of two QLEDs with different emission wavelengths. (c) Commission Internationale de l’Eclairage (CIE1931) chromaticity coordinates of QLEDs based on different QDs.


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Table 2. Summary of performance parameters of QLEDs based on QDs with different emissive wavelengths. PL λmax (nm)

EL λmax (nm)

CIE (x,y)

FWHM (nm)

VT (V)

Lmax (cd/m2)

max.ηEQE (%)

454

462

(0.151, 0.042)

30

2.75

20900

13.2

472

479

(0.119, 0.154)

34

2.40

14100

16.2

495

502

(0.105, 0.540)

45

2.27

33800

15.8


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Table of Contents Graphic and Synopsis

Non-blinking Quantum-Dot-Based Blue Light-Emitting Diodes with High Efficiency and Balanced Charge-Injection Process

Qingli Lin,† Lei Wang,† Zhaohan Li,† Huaibin Shen,†,* Lijun Guo,‡ Yanmin Kuang,‡ Hongzhe Wang,† and Lin Song Li†,*

Highly efficient blue light-emitting diodes (LEDs) based on non-blinking ZnxCd1-xSe/ZnS//ZnS QDs (quantum yield >90%) have been demonstrated via enhanced charge-injection balance using a modified device structure, where an insulator layer of poly(methyl methacrylate) (PMMA) was introduced between ZnO and QD layers. Maximum current efficiencies of 11.8 cd/A and peak external quantum efficiencies (EQE) of 16.2%, were achieved for blue QLEDs (λPL=472 nm).


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Non-blinking Quantum-Dot-Based Blue Light-Emitting Diodes with

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