Nonblinking Quantum-Dot-Based Blue Light-Emitting Diodes with

Article Cite This: ACS Photonics 2018, 5, 939−946

Nonblinking Quantum-Dot-Based Blue Light-Emitting Diodes with High Efficiency and a 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 and ‡Institute of Photo-biophysics, School of Physics and Electronics, Henan University, Kaifeng 475004, China S Supporting Information *

ABSTRACT: Blue nonblinking (>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 nonblinking 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 dotbased light-emitting diodes (QLEDs) have been demonstrated by using nonblinking ZnCdSe/ZnS//ZnS QDs as emissive layer, and the charge−injection balance within the QD active layer was improved by introducing a nonconductive layer of poly(methyl methacrylate) (PMMA) between the electron transport layer (ETL) and the QD layer, where the PMMA layer takes the role of coordinator to impede excessive electron flux. The best device exhibits outstanding features such as 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, nonblinking, electroluminescence, poly(methyl methacrylate), charge-injection balance

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by Jin, 23.68% (λ = 538 nm) by Chen, and 15.6% (λ = 445 nm) by us for red, green, and deep-blue devices.16−18 To date, the performance of red and green QLEDs has achieved similar levels of 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 recently 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 is presented in the 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 grow over the ZnCdSe cores, respectively. Along with the growth of the first ZnS shell, an up to 13 nm blue shift of PL spectra is observed, and the decrease of fwhm is about 3 nm (from 42 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 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 the PL emission.25,26 Such interfacial alloy layers normally exist 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 compared to the ZnCdSe/ZnS, due to the additional overcoating of ZnS shells being able to 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//outer shell structure.13 Even though the PL is shifted only 9 to 472 nm with the growth of the second ZnS shell, a significant improvement of fwhm is observed and it is narrowed to 31 nm by the end of the 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 the ZnS shell and is similar to those recent reports.27,28 940

DOI: 10.1021/acsphotonics.7b01195 ACS Photonics 2018, 5, 939−946

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ACS Photonics 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 diffraction pattern indicates that the peaks of the sample are derived not from a mixture of CdSe and ZnSe but from 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. The TEM images shown in Figure 1c reveal 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. 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 ZnCdSe/ZnS ZnCdSe/ZnS//ZnS

3.5 ± 0.7 6.3 ± 0.5 7.8 ± 0.6

N/A 1.4 2.2

494 481 472

42 39 31

64 87 92

Figure 2. (a) Representative PL intensity time traces (1800 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).

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 the EQE of this device increases by 38%, which is attributed to adopting the high QY nonblinking QDs as emissive layer that suppresses nonradiative Auger recombination and increases the exciton recombination probability in the QD layer. However, the imbalance of charge injection is still a drawback and may decrease the overall device efficiency because many more electrons are still injected than holes due to the great disparity between the HOMO level of TFB and the valence band of QDs. To obtain balanced electron and hole injections, an insulating layer may be introduced between the ETL and the QD layer.17 By the insertion of a PMMA interlayer between the ZnO ETL and the 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 the HTL, which differs from Jin’s report 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 Poly-TPD (−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 s−1),21,22 as expected from the energy band diagram of Figure

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 such as diodes, lasers, and bioimaging are all adversely affected by their tendency to blink.27,30 In recent years, various QD materials with well-controlled compositions and high PL QY have abounded, but only several special red emission QDs have been found to be nonblinking.27,30−32 So far, there is still no report of nonblinking blue-cyan QDs. The QD blinking behavior was further studied at single-emitter level to characterize assynthesized 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). 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 nonblinking within the 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 nonblinking ZnCdSe/ZnS//ZnS QDs, blue QLEDs were first fabricated using conventional structures. Their energy band diagrams and corresponding cross-sectional SEM images are shown in Figure S1a−d (Supporting 941

DOI: 10.1021/acsphotonics.7b01195 ACS Photonics 2018, 5, 939−946

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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. The 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 nonradiative exciton quenching processes, such as nonradiative Auger recombination, dissociation of excitons under high fields, and electric-field-induced decrease in PL efficiency of QD. Importantly, the average value of the peak EQE reaches 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 suggests 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 resulting from the spontaneous charge-transfer process. While the spontaneous charging can be suppressed by inserting a thin PMMA layer between QDs and ZnO, increasing the exciton lifetime from 8.75 to 9.08 ns is on 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 nonblinking QDs. The QLEDs were simply sealed with a 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 (Supporting Information) 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 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 show a 7 nm red-shift compared to PL spectra (PL peak at 472 nm), which is attributed to both the combination of interdot interactions within the close-packed QD solids and the strong electric field. For the first aspect, the PL spectrum of the QD film red-shifts about ∼2 nm and is slightly broadened relative to the PL 943

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

spectrum of the 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.38−40 Note that no parasitical emission could be observed in either the low or high energy region for QLEDs, suggesting the good confinement 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 the quantum confined Stark effect (QCSE), which usually leads to the redshift of the EL emission of QLEDs under ascending external electric field, the EL peak red-shifted 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, 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.

Figure 6. (a) 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

CONCLUSIONS In summary, ZnCdSe/ZnS//ZnS QDs with nonblinking characteristic and high PL QY of 92% as well as high stability were synthesized via a low temperature nucleation and high temperature growth method. Highly efficient QLEDs based on this nonblinking 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 a PMMA layer plays a crucial role that moderately impedes the electron injection and thus improves chargeinjection balance within QD active layers. The best device shows superior performance indexes, such as 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

PL λmax (nm)

EL λmax (nm)

CIE (x,y)

fwhm (nm)

VT (V)

Lmax (cd/m2)

max.ηEQE (%)

454 472 495

462 479 502

(0.151, 0.042) (0.119, 0.154) (0.105, 0.540)

30 34 45

2.75 2.40 2.27

20900 14100 33800

13.2 16.2 15.8

relative to QLEDs without PMMA, which is recorded as high as 16.2%. More importantly, the peak efficiency of the QLEDs with PMMA is achieved at ∼1000 cd/m2, and high efficiency (EQE > 12%) can be maintained in the luminance range of 100 to 3000 cd/m2. A histogram of the peak EQE of devices shows average peak EQE of 14.3% and a low standard deviation of 944

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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 solid-state lighting and next generation flat panel displays.

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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/acsphotonics.7b01195. Detailed experimental procedure and characterization, cross-sectional SEM images of QLEDs without PMMA and corresponding 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 peaks (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Lin Song Li: 0000-0001-7015-3211 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.

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

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REFERENCES

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DOI: 10.1021/acsphotonics.7b01195 ACS Photonics 2018, 5, 939−946

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DOI: 10.1021/acsphotonics.7b01195 ACS Photonics 2018, 5, 939−946