Enhancing the Performance of Quantum-Dot Light-Emitting Diodes by

Jun 19, 2018 - The external quantum efficiency (EQE) of blue quantum-dot light-emitting ... Postmetallization annealing is widely adopted in the semic...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 23218−23224

Enhancing the Performance of Quantum-Dot Light-Emitting Diodes by Postmetallization Annealing Qiang Su, Heng Zhang, Yizhe Sun, Xiao Wei Sun, and Shuming Chen* Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen 518055, P. R. China

ACS Appl. Mater. Interfaces 2018.10:23218-23224. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/15/18. For personal use only.

S Supporting Information *

ABSTRACT: The effect of postannealing on the device characteristics is systematically investigated. The external quantum efficiency (EQE) of blue quantum-dot light-emitting diodes (QLEDs) is significantly improved from 5.22 to 9.81% after postannealing. Similar results are obtained in green and red QLEDs, whose EQEs are enhanced from 11.47 and 13.60 to 15.57 and 16.59%, respectively. The annealed devices also exhibit a larger current density. The origin of efficiency improvement is thoroughly investigated. Our finding indicates that postannealing promotes the interfacial reaction of Al and ZnMgO and consequently leads to the metallization of the AlZnMgO contact and the formation of the AlOx interlayer. Because of the metallization of AlZnMgO, the contact resistance is effectively reduced, and thus the electron injection is enhanced. On the other hand, the formation of the AlOx interlayer can effectively suppress the quenching of excitons by the metal electrode. Because of the enhancement of electron injection and suppression of exciton quenching, the annealed blue, green, and red QLEDs exhibit a 1.9-, a 1.3-, and a 1.2-fold efficiency improvement, respectively. We envision the results offer a simple yet effective method to enhance the charge injection and the efficiency of QLED devices, which would promote the practical application of QLEDs. KEYWORDS: quantum-dots, light-emitting diodes, post-annealing, interlayer, exciton quenching

1. INTRODUCTION Considerable research efforts have been devoted to colloidal quantum-dot (QD) light-emitting diodes (QLEDs) recently because of their attractive features such as tunable emission color, high color purity, excellent flexibility, and simple and low-cost fabrication, which make them promising candidates for the next-generation display.1−5 With the rapid development of QD synthesis and device engineering, the external quantum efficiency (EQE) and the stability of QLEDs are substantially improved in the past few years. For example, the EQEs and the lifetime of red and green QLEDs are over 20.5%, 100 000 h (predicted half-lifetime, T50) @ 100 cd/m2 and 21%, 76 000 h (T50) @ 100 cd/m2,6−8 respectively, which are very close to those of organic (O) LEDs and approach the entry level of the industrial requirement. However, the performance, especially the stability of the blue QLEDs,9,10 is significantly lower than those of the red and green QLEDs and is still far below the industrial requirement, which limits the application of QLEDs in full color display. The poor performance of blue QLEDs is in part due to the inefficient charge injection induced by the wide band gap of blue QDs.1,11 In addition, the blue excitonic energy, which is relatively large, can efficiently dissipate through various nonradiative channels such as inter-QD resonant energy transfer, Auger recombination, and/or quenched by interfacial defects.12 To improve the performance of blue QLEDs, the charge injection and transport have been enhanced by replacing the insulated oleic acid ligands with shorter 1-octanethiol ligands,13 or modifying the surface of © 2018 American Chemical Society

ZnO using PEIE, which lowers the work function of ZnO and thus decreases the electron injection barrier.14 To suppress the quenching of excitons, QDs with a thick shell and gradientalloyed structure have been adopted, which effectively suppress the nonradiative Auger recombination and inter-QD resonant energy transfer.9,15 The quenching of excitons can further be suppressed by inserting a thin insulated layer (e.g., poly(methyl methacrylate), poly(ethyleneimine), etc.) between the QDs and the ZnO electron-transport layer.16,17 All these device engineering efforts are aimed at improving the charge injection and/or reducing the exciton quenching in blue QLEDs. In this work, we develop a simple processing method, that is, postmetallization annealing, to simultaneously enhance the charge injection and reduce the exciton quenching. The resultant annealed blue QLEDs exhibit a significantly improved EQE of 9.81%, which is 1.9-fold higher than 5.22% of the unannealed devices. Postmetallization annealing is widely adopted in the semiconductor industry to reduce the contact resistance of metals/semiconductors.18−20 For example, in the standard complementary metal oxide semiconductor (CMOS) process, metal deposition onto silicon is followed by annealing to form silicide (i.e., a metal silicon compound), which yields an alloyed AlSi interface with Ohmic contact characteristics.18 Inspired by the benefits brought by postmetallization Received: May 22, 2018 Accepted: June 19, 2018 Published: June 19, 2018 23218

DOI: 10.1021/acsami.8b08470 ACS Appl. Mater. Interfaces 2018, 10, 23218−23224

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic device structure and (b) energy-level alignment of the QLEDs. (c) Schematic illustration of the postannealing process.

Figure 2. J−V−L and EQE−J characteristics of (a,b) blue QLEDs, (c,d) green QLEDs, and (e,f) red QLEDs postannealed at different temperatures. (g) Quantity statistics of maximum EQE of blue QLEDs without and with postannealing. (h) EL spectra and (i) photographs of the red/green/blue monochromatic QLEDs.

annealing, we tend to reduce the contact resistance and thus enhance the charge injection in QLEDs using the same method. We find that the alloyed contact can similarly be obtained by postannealing the QLED devices. Our results indicate that postannealing promotes the interfacial reaction of Al and ZnMgO and consequently leads to the metallization of the AlZnMgO contact and the formation of the AlO x interlayer. Because of the metallization of AlZnMgO, the contact resistance is effectively reduced and thus the electron injection is enhanced. On the other hand, the formation of the AlOx interlayer can effectively suppress the quenching of excitons by the metal electrode. The proposed method is also applicable to red and green QLEDs, whose EQEs are enhanced from 11.47 and 13.60 to 15.57 and 16.59%, respectively. Our results offer a simple yet effective method to enhance the

charge injection and the efficiency of QLED devices, which would promote the practical application of the QLEDs.

2. RESULTS AND DISCUSSION QLEDs with glass/ITO (indium-tin-oxide)/PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate)/ PVK (poly(9-vinlycarbazole))/QDs/ZnMgO/Al structure were fabricated, followed by postannealing in a hot plate at 100−160 °C for 20 min. The schematic device structure, energy-level alignment, and postannealing process are shown in Figure 1. The effect of postannealing on the device performance is systematically investigated. Figure 2a−f shows the current density−voltage−luminance (J−V−L) and the EQE−J characteristics of the devices, which were postannealed at different temperatures. It is clear that after postannealing, 23219

DOI: 10.1021/acsami.8b08470 ACS Appl. Mater. Interfaces 2018, 10, 23218−23224

Research Article

ACS Applied Materials & Interfaces Table 1. Performance Data of Blue, Green, and Red QLEDs with and without Postannealing 2

luminance (cd/m2)

CEa (cd/A)

EQEb (%)

PEc (lm/W)

devices

postannealing

Von (V) (1 cd/cm )

@ 12 (V)

max

max

max

EQE enhancement factor (%)

blue QLEDs

without 120 °C without 140 °C without 100 °C

4.7 4.7 3.7 3.7 2.8 2.5

19 250 29 750 447 066 576 211 185 500 237 100

4.30 7.80 50.43 68.55 22.12 27.04

5.22 9.81 11.47 15.57 13.60 16.59

1.63 3.28 21.43 26.39 8.81 12.21

∼85

green QLEDs red QLEDs

∼30 ∼20

a

Current efficiency. bExternal quantum efficiency. cPower efficiency.

Figure 3. EQE−J characteristics of blue QLEDs postannealed (a) before or after Al deposition and (b) in air or in nitrogen ambience.

or after Al cathode deposition. As shown in Figure 3a, if annealing is performed prior to the deposition of Al cathode, the EQE of the device is similar with that of the unannealed device, indicating that postannealing does not alter the thinfilm morphologies as well as the contacts of PVK/QD and QD/ZnMgO. However, if annealing is performed after the Al deposition, the EQE is significantly improved from 4.8 to 8.3%. This implies that the Al/ZnMgO contact is effectively modified by postannealing, and such modification accounts for the performance improvement. To investigate whether annealing ambience affects the performance, the devices were annealed in air or in N2-filled glovebox. As shown in Figure 3b, the devices exhibit similar EQE, indicating that the annealing ambience does not play a major role in modifying the Al/ZnMgO contact. How does postannealing modify the Al/ZnMgO contact? Previous studies disclosed that postannealing induces interdiffusion and promotes the interaction of metals and semiconductors, and, as a result, leads to the metallization of the metal/semiconductor contact and thereby reduces the contact resistance.18−20 To characterize the Al/ZnMgO contact, X-ray photoelectron spectroscopy (XPS) was performed. To probe the interface, Al was carefully removed by sputter etching, and the etching depth was monitored in situ by analyzing the amount of elements, as schematically shown in Figure 4a. Figure 4b shows the O 1s spectra of ZnMgO, which are Gaussian-fitted by three curves centered at 529.7 ± 0.1 eV (O-I), 531.1 ± 0.1 eV (O-II), and 531.7 ± 0.1 eV (O-III), corresponding to the oxygen in the oxide lattices (O-I), oxygen vacancies (O-II), and oxygen bonds in the hydroxide (O-III), respectively.21 It is clear that after postannealing, the O-I state is effectively reduced from 70.14 to 44.84%, whereas the O-II and the O-III states are significantly increased. This implies that the oxygen is outdiffused from ZnMgO, leading to the accumulation of oxygen vacancies at the surface of ZnMgO. As the oxygen vacancies are regarded as donors in ZnO, this generates a heavily doped

the devices exhibit significantly larger current density, higher luminance, and more importantly, higher efficiency. The improvement is strongly related with the annealing temperature. For example, at an optimal annealing temperature of 120 °C, the EQE of blue QLEDs is significantly improved from 5.22 to 9.81%, whereas it is decreased to 8.34% when the temperature is increased to 140 °C. It is possible that at high temperature, the organic ligands of QDs are damaged, leading to the reduction of efficiency, whereas at low temperature, the alloying process is not fully activated, thus resulting in a low current density and a low EQE, as shown in Figure 2a,b. Similar results are also obtained in red and green QLEDs. For example, the red and green devices exhibit high EQEs of 13.60 and 16.59% when annealed at optimal temperatures of 140 and 100 °C, respectively, which are 1.3- and 1.2-fold higher than 11.47 and 15.57% of the unannealed devices. It is noted that the EQE improvement of the red and green devices is not as significant as that of the blue devices, which might imply that the improvement mechanisms are rather different, as will be discussed later. The detailed device performance data are summarized in Table 1. To investigate whether such improvement is repeatable, the EQEs of over 48 blue QLEDs from different fabrication batches were measured. As statisticed in Figure 2g, most annealed devices exhibit a 1.8- to 2.0-fold EQE improvement, indicating the high reproducibility of our method. Figure 2h,i shows the electroluminescence (EL) spectra and the photos of the annealed devices, which are identical to those of the unannealed devices, indicating the QDs remain intact after postannealing. The performance of QLEDs is effectively improved by simply postannealing the devices, which is rather useful because it neither increases the fabrication complexity nor alters the structure of devices. During annealing, the thin-film morphologies and the interfacial contacts such as PVK/QD, QD/ZnMgO, or Al/ZnMgO could be altered, which might be responsible for the efficiency improvement. To identify which contact plays a major role, postannealing was performed before 23220

DOI: 10.1021/acsami.8b08470 ACS Appl. Mater. Interfaces 2018, 10, 23218−23224

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Figure 4. (a) Schematic sample structure used for XPS characterization. XPS spectra of (b) O 1s scan and (c) Al 2p scan at the interface of Al/ ZnMgO. (d) Schematic front view and cross-sectional view of the test structure for transmission line measurement (TLM) characterization. (e) TLM data of samples with and without postannealing. (f) J−V characteristics of electron-only devices with and without postannealing.

region near the ZnMgO surface18−20,22 and thus enhances the conductivity of ZnMgO. The out-diffused oxygen subsequently reacts with Al and thereby leads to the formation of interfacial AlOx, as can be verified from the Al 2p spectra shown in Figure 4c. The Al 2p spectra consist of two peaks, which are centered at 72.6 ± 0.1 and 75.3 ± 0.1 eV, corresponding to Al and amorphous AlOx, respectively.23 It is noted that there already exists a thin AlOx layer for the sample without postannealing, which is reasonable because of the strong reaction between Al and O in the ZnMgO layer. Such an interfacial reaction can further be effectively promoted by postannealing, as can be verified by a higher AlOx intensity. This behavior is consistent with the fact that the enthalpy of formation for Al2O3 (ΔG°298 = −1492 kJ/mol) is much smaller than that for ZnO (ΔG°298 = −324 kJ/mol).24 The interdiffusion and interfacial reactions of Al and oxygen yield an AlZnMgO-alloyed contact that would significantly reduce the contact resistance. To verify this assertion, the contact resistance was measured by using the TLM method, which is commonly used to characterize the metal/semiconductor contact resistance of transistors and other electronic devices.25 The configuration of the test structure is schematically shown in Figure 4d. To enhance the current laterally spreading between two Al contacts, ZnMgO was deposited onto ITO. The width of the semiconductor tested is W = 0.5 mm, and the length L is varied from 4 to 16 mm. The total resistance between the two Al contacts is RT = 2R m + 2R C + R s

RT =

L W

W

L + 2R C

(3)

We can see that RT is a function of L. By measuring several RT of resistors with different lengths L, we can plot the curve of RT ≈ L, as displayed in Figure 4e. In the limit of L = 0, the RT value would be just twice the contact resistance and can be obtained from the graph by extrapolating back to L = 0. The curves shown in Figure 4e are fitted by the linear functions of RT = 0.99L + 0.026 kΩ and RT = 0.99L + 0.05 kΩ, corresponding to the samples with and without postannealing, respectively. By setting L = 0, the contact resistance can be obtained, which is effectively decreased from 0.05 to 0.026 kΩ after postannealing. In addition, according to eq 3, the sheet resistance of ZnMgO/ITO can also be obtained from the slope of the curves. Both samples exhibit the same reasonable sheet resistance of 0.99 kΩ/mm × 0.5 mm ≈ 0.5 kΩ/□, indicating that postannealing does not affect the resistance of ZnMgO/ ITO. The reduction of the contact resistance and the generation of the heavily doped ZnMgO surface contribute to the improvement of electron injection, and thus the postannealed devices exhibit significantly larger current density, as shown in Figure 2a−e. This can further be verified by measuring the electron current in electron-only devices. As shown in Figure 4f, the postannealed devices indeed exhibit larger electron current, whereas the hole current (shown in the Supporting Information, Figure S1) remains almost the same. It should be noted that, in blue QLEDs, the carrier injection is relatively inefficient because of the wide band gap of the blue QDs,11,26 and therefore, by enhancing the electron injection, the efficiency of the blue QLEDs can be significantly improved. However, in other color QLEDs, especially the red QLEDs, the electron injection is relatively efficient,6,27 and thus the enhancement of electron injection might not improve the efficiency of the red and green QLEDs. The observed EQE improvement in the red and green QLEDs therefore implies that there are other mechanisms responsible for the improve-

(1)

where Rm is the resistance of Al, RC is the contact resistance of Al/ZnMgO, and Rs is the resistance of the bilayer ZnMgO/ ITO. The resistivity of the metal is so low that RC ≫ Rm, and thus Rm can be ignored. In addition, R S = R S□

R S□

(2)

where RS□ is the sheet resistance of the bilayer ZnMgO/ITO. The total resistance can then be rewritten as 23221

DOI: 10.1021/acsami.8b08470 ACS Appl. Mater. Interfaces 2018, 10, 23218−23224

Research Article

ACS Applied Materials & Interfaces

Figure 5. PL spectra of (a) quartz/QD/ZnMgO and (b) quartz/QD/ZnMgO/Al with and without postannealing, (c) TRPL decay curves of quartz/QD/ZnMgO/Al with and without postannealing, and (d) schematic illustration of the exciton quenching process.

of electron trapping and consequently suppress exciton quenching, as briefly illustrated in Figure 5d. To conclude, the efficiency improvement of the red and green QLEDs is mainly due to the suppression of exciton quenching, whereas both the enhancement of electron injection and suppression of exciton quenching contribute to the improvement of the blue QLEDs. This also explains why blue QLEDs exhibit the highest efficiency improvement of 1.9-fold, as compared to 1.3and 1.2-fold of the green and the red devices, respectively. It is worth to mention that Al could react with oxygen from ZnMgO even without postannealing. However, this native reaction is very slow and usually takes several days to form a thin AlOx. Because of the gradual formation of AlOx and the progressive reduction of contact resistance, the performances of the QLED devices are increased with time, as shown in the Supporting Information, Figure S3. This phenomenon is termed as positive aging.28 More details about positive aging and the exact origin will be discussed in another report. Fortunately, positive aging can be alleviated by postannealing the devices, as shown in the Supporting Information, Figure S3. This is because the formation of the alloyed contact and the AlOx interlayer can rapidly be obtained after postannealing, and thus the devices can achieve the highest efficiency soon after annealing.

ment. Besides carrier injection, the performance of devices is largely affected by the photoluminescence (PL) of the QDs, and this prompts us to further probe the exciton recombination in QDs. The exciton recombination is probed by using PL and timeresolved (TR) PL spectroscopy. To ensure the accuracy of the measurement, the PL intensity was collected using an integrating sphere, and the excitation/detection conditions were kept the same for different measurements. As shown in Figure 5a and the Supporting Information, Figure S2, for the sample without Al, the PL intensity and TRPL decay of the postannealed sample are identical to those of the unannealed sample, indicating that postannealing does not affect the exciton recombination in QD/ZnMgO. However, if the Al cathode is introduced, the postannealed sample exhibits a significantly higher PL intensity and slower TRPL decay, as shown in Figure 5b,c, indicating that Al plays a major role in enhancing the PL efficiency of QDs. The TRPL decay curves were fitted with a triexponential function, and the average exciton lifetime is estimated by the relationship τav = A1τ1 + A2τ2 + A3τ3, with the parameters of time components (τi) and corresponding weights (Ai). The fitting results are summarized in the Supporting Information, Table S1. After postannealing, the average exciton lifetime of the blue, green, and red samples is increased from 3.67, 4.87, and 9.13 ns to 4.10, 5.23, and 10.74 ns, respectively. The lengthened exciton lifetime and the enhancement of the PL intensity indicate that the exciton quenching is effectively suppressed, which is likely due to the formation of interfacial AlOx. As schematically illustrated in Figure 5d, the defect states that originated from the oxygen vacancies in ZnMgO could effectively trap holes from the QDs and electrons from the metal electrode; these defect states are regarded as effective charge recombination centers,21 which quench the excitons. In other words, the electrons from the Al cathode can be effectively captured by the defect states of ZnMgO. Because of the well-aligned energy levels, the trapped electrons can further transfer to the valence band of QDs and thus quench the excitons. The formation of interfacial AlOx, as promoted by postannealing, can effectively block the pathway

3. CONCLUSIONS In conclusion, postmetallization annealing has been proposed as a simple and effective method to enhance the performance of QLED devices. The effect of postannealing on device characteristics is systematically investigated. The results indicated that postannealing promotes the interfacial reaction of Al and ZnMgO and consequently leads to the metallization of the AlZnMgO contact and the formation of the AlOx interlayer. Because of the metallization of AlZnMgO, the contact resistance is effectively reduced, and thus the electron injection is enhanced. On the other hand, the formation of the AlOx interlayer can effectively suppress the quenching of excitons. Because of the enhancement of electron injection and suppression of exciton quenching, the annealed blue, green, 23222

DOI: 10.1021/acsami.8b08470 ACS Appl. Mater. Interfaces 2018, 10, 23218−23224

Research Article

ACS Applied Materials & Interfaces

the Basic Research Program of Science, Technology and Innovation Commission of Shenzhen Municipality (grant no. JCYJ20170307105259290), the Shenzhen Peacock Plan (grant no. KQTD2015071710313656), and the National Key R&D Program of China (grant no. 2016YFB0401702).

and red QLEDs exhibit a 1.9-, a 1.3-, and a 1.2-fold efficiency improvement, respectively. The positive-aging effect can also be alleviated by postannealing. The proposed method, which neither increases the fabrication complexity nor alters the structure of devices, can effectively enhance the charge injection and the efficiency of QLED devices, which would promote the practical application of QLEDs.

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4. EXPERIMENTAL SECTION 4.1. Device Fabrication. ITO with sheet resistance = 25 Ω sq−1 was used as an anode. The ITO-coated glass substrates were thoroughly cleaned by scrubbing, followed by ultrasonic cleaning in detergent for 30 min, soaking in deionized water for 10 min, and baking for 30 min in an oven. Prior to the deposition of functional layers, the ITO-coated glass substrates were treated by a UV−ozone cleaner for 30 min. The spincoating processes were carried out in an ambient atmosphere. Then, the hole injection layer PEDOT:PSS was spin-casted at 3000 rpm and baked at 130 °C for 20 min. Then, the hole transport layer, PVK, 10 mg/mL in chlorobenzene, was spin-casted at 4000 rpm for 45 s and baked at 120 °C for 20 min. After this, the red QD (15 mg/mL in octane solution, CdxZn1−xSe@ZnS/OT), green QD (10 mg/mL in octane solution, CdxZn1−xSeyS1−y@ZnS/oleic acid), and blue QD (10 mg/mL in octane solution, CdxZn1−xSeyS1−y@ZnS/OT) layers were spin-casted at 3000, 3000, and 2500 rpm for 45 s, respectively. (All QDs are commercially available from Mesolight Inc.) After this, the commercially available Zn0.85Mg0.15O nanoparticles were spin-casted on QDs at 3000 rpm from a 20 mg/mL ethanol solution and baked at 100 °C for 10 min. Finally, the Al cathode with a thickness of 100 nm was evaporated in a high-vacuum thermal evaporator under a pressure of less than 5 × 10−4 Pa. 4.2. Device Characterization. The thicknesses of the films were measured by a Stylus Profiler (Bruker DektakXT). The PL and TRPL spectra were measured with Edinburgh instruments FS5 Fluorescence Spectrometer. The J−V−L (current density−voltage−luminance) characteristics were characterized by a programmable source meter (Keithley 2614B) and a PR670 spectrometer.

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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/acsami.8b08470. The hole current of the hole-only devices with and without postannealing; TRPL and fitting results of sample: quartz/QDs/ZnMgO with and without postannealing; J−V−L and CE−J characteristics tested at different time periods of greed QLEDs with and without postannealing (PDF)

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REFERENCES

(1) Yang, Y.; Zheng, Y.; Cao, W.; Titov, A.; Hyvonen, J.; Manders, J. R.; Xue, J.; Holloway, P. H.; Qian, L. High-Efficiency Light-Emitting Devices Based on Quantum Dots with Tailored Nanostructures. Nat. Photonics 2015, 9, 259−266. (2) Dai, X.; Deng, Y.; Peng, X.; Jin, Y. Quantum-Dot Light-Emitting Diodes for Large-Area Displays: Towards the Dawn of Commercialization. Adv. Mater. 2017, 29, 1607022. (3) Bozyigit, D.; Wood, V. Challenges and Solutions for HighEfficiency Quantum Dot-Based LEDs. MRS Bull. 2013, 38, 731−736. (4) Zhang, H.; Sun, X.; Chen, S. Over 100 cd A−1 Efficient Quantum Dot Light-Emitting Diodes with Inverted Tandem Structure. Adv. Funct. Mater. 2017, 27, 1700610. (5) Zhang, H.; Chen, S.; Sun, X. W. Efficient Red/Green/Blue Tandem Quantum-Dot Light-Emitting Diodes with External Quantum Efficiency Exceeding 21%. ACS Nano 2018, 12, 697−704. (6) Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X. Solution-Processed, High-performance LightEmitting Diodes Based on Quantum Dots. Nature 2014, 515, 96−99. (7) Manders, J. R.; Qian, L.; Titov, A.; Hyvonen, J.; Tokarz-Scott, J.; Acharya, K. P.; Yang, Y.; Cao, W.; Zheng, Y.; Xue, J.; Holloway, P. H. High Efficiency and Ultra-Wide Color Gamut Quantum Dot LEDs for Next Generation Displays. J. Soc. Inf. Disp. 2015, 23, 523−528. (8) Li, Z.; Hu, Y.; Shen, H.; Lin, Q.; Wang, L.; Wang, H.; Zhao, W.; Li, L. S. Efficient and Long-Life Green Light-Emitting Diodes Comprising Tridentate Thiol Capped Quantum Dots. Laser Photonics Rev. 2017, 11, 1600227. (9) Shen, H.; Lin, Q.; Cao, W.; Yang, C.; Shewmon, N. T.; Wang, H.; Niu, J.; Li, L.; Xue, J. Efficient and Long-Lifetime Full-Color Light Emitting Diodes Using High Luminescence Yield Thick-Shell Quantum Dots. Nanoscale 2017, 9, 13583−13591. (10) Lee, K.-H.; Lee, J.-H.; Song, W.-S.; Ko, H.; Lee, C.; Lee, J.-H.; Yang, H. Highly Efficient, Color-Pure, Color-Stable Blue Quantum Dot Light-Emitting Devices. ACS Nano 2013, 7, 7295−7302. (11) Wang, O.; Wang, L.; Li, Z.; Xu, Q.; Lin, Q.; Wang, H.; Du, Z.; Shen, H.; Li, L. S. High-Efficiency, Deep Blue ZnCdS/CdxZn1-xS/ ZnS Quantum-Dot-Light-Emitting Devices with an EQE Exceeding 18%. Nanoscale 2018, 10, 5650−5657. (12) 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. (13) Shen, H.; Cao, W.; Shewmon, N. T.; Yang, C.; Li, L.; Xue, J. High-Efficiency, Low Turn-on Voltage Blue-Violet Quantum-DotBased Light-Emitting Diodes. Nano Lett. 2015, 15, 1211−1216. (14) Shi, Y.-L.; Liang, F.; Hu, Y.; Zhuo, M.-P.; Wang, X.-D.; Liao, L.S. High Performance Blue Quantum Dot Light-Emitting Diodes Employing Polyethylenimine Ethoxylated as the Interfacial Modifier. Nanoscale 2017, 9, 14792−14797. (15) Bae, W. K.; Padilha, L. A.; Park, Y.-S.; McDaniel, H.; Robel, I.; Pietryga, J. M.; Klimov, V. I. Controlled Alloying of the Core-Shell Interface in CdSe/CdS Quantum Dots for Suppression of Auger Recombination. ACS Nano 2013, 7, 3411−3419. (16) Lin, Q.; Wang, L.; Li, Z.; Shen, H.; Guo, L.; Kuang, Y.; Wang, H.; Li, L. S. Nonblinking Quantum-Dot-Based Blue Light-Emitting Diodes with High Efficiency and a Balanced Charge-Injection Process. ACS Photonics 2018, 5, 939−946. (17) Zhong, Z.; Zou, J.; Jiang, C.; Lan, L.; Song, C.; He, Z.; Mu, L.; Wang, L.; Wang, J.; Peng, J.; Cao, Y. Improved Color Purity and Efficiency of Blue Quantum Dot Light-Emitting Diodes. Org. Electron. 2018, 58, 245−249.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shuming Chen: 0000-0001-5218-3257 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Q.S. and H.Z. contributed equally to this work. This work was supported by the National Natural Science Foundation of China (grant no. 61775090), the Guangdong Natural Science Funds for Distinguished Young Scholars (grant no. 2016A030306017), the Guangdong Special Funds for Science and Technology Development (grant no. 2017A050506001), 23223

DOI: 10.1021/acsami.8b08470 ACS Appl. Mater. Interfaces 2018, 10, 23218−23224

Research Article

ACS Applied Materials & Interfaces (18) Xu, Y.; Liu, C.; Amegadze, P. S. K.; Park, W.-T.; Long, D. X.; Minari, T.; Balestra, F.; Ghibaudo, G.; Noh, Y.-Y. Significant Roles of Low-Temperature Post-Metallization Annealing in Solution-Processed Oxide Thin-Film Transistors. Appl. Phys. Lett. 2014, 105, 133505. (19) Kim, H.; Kim, K.-K.; Lee, S.-N.; Ryou, J.-H.; Dupuis, R. D. Low Resistance Ti/Au Contacts to Amorphous Gallium Indium Zinc Oxides. Appl. Phys. Lett. 2011, 98, 112107. (20) Kim, H.-K.; Seong, T.-Y.; Kim, K.-K.; Park, S.-J.; Yoon, Y. S.; Adesida, I. Mechanism of Nonalloyed Al Ohmic Contacts to N-type ZnO : Al Epitaxial Layer. Jpn. J. Appl. Phys. 2004, 43, 976−979. (21) Sun, Y.; Jiang, Y.; Peng, H.; Wei, J.; Zhang, S.; Chen, S. Efficient Quantum Dot Light-Emitting Diodes with a Zn0.85Mg0.15O Interfacial Modification Layer. Nanoscale 2017, 9, 8962−8969. (22) Zeng, H.; Duan, G.; Li, Y.; Yang, S.; Xu, X.; Cai, W. Blue Luminescence of ZnO Nanoparticles Based on Non-Equilibrium Processes: Defect Origins and Emission Controls. Adv. Funct. Mater. 2010, 20, 561−572. (23) Wagner, C. D.; Muilenberg, G. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, USA, 1979; p 42. (24) Kim, H.-K.; Kim, K.-K.; Park, S.-J.; Seong, T.-Y.; Adesida, I. Formation of Low Resistance Nonalloyed Al/Pt Ohmic Contacts on N-type ZnO Epitaxial Layer. J. Appl. Phys. 2003, 94, 4225−4227. (25) Reeves, G. K.; Harrison, H. B. Obtaining the Specific Contact Resistance From Transmission-Line Model Measurements. IEEE Electron Device Lett. 1982, 3, 111−113. (26) Cheng, T.; Wang, Z.; Jin, S.; Wang, F.; Bai, Y.; Feng, H.; You, B.; Li, Y.; Hayat, T.; Tan, Z. Pure Blue and Highly Luminescent Quantum-Dot Light-Emitting Diodes with Enhanced Electron Injection and Exciton Confinement via Partially Oxidized Aluminum Cathode. Adv. Opt. Mater. 2017, 5, 1700035. (27) Peng, H.; Jiang, Y.; Chen, S. Efficient vacuum-free-processed quantum dot light emitting diodes with printable liquid metal cathodes. Nanoscale 2016, 8, 17765−17773. (28) Acharya, K. P.; Titov, A.; Hyvonen, J.; Wang, C.; Tokarz, J.; Holloway, P. H. High Efficiency Quantum Dot Light Emitting Diodes From Positive Aging. Nanoscale 2017, 9, 14451−14457.

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DOI: 10.1021/acsami.8b08470 ACS Appl. Mater. Interfaces 2018, 10, 23218−23224