Ultrasonic Spray Processed, Highly Efficient All-Inorganic Quantum

Article pubs.acs.org/journal/apchd5

Ultrasonic Spray Processed, Highly Efficient All-Inorganic QuantumDot Light-Emitting Diodes Wenyu Ji,†,§ Shihao Liu,‡ Han Zhang,† Rong Wang,† Wenfa Xie,*,‡ and Hanzhuang Zhang*,†,§ †

College of Physics, ‡State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, and §Key Lab of Physics and Technology for Advanced Batteries (Ministry of Education), Jilin University, Qianjin Street 2699, Changchun 130012, China S Supporting Information *

ABSTRACT: All-inorganic and low-cost quantum-dot light-emitting diodes (QLEDs) are always desired considering the easy processing and outstanding physical and chemical stability of inorganic oxides. Herein, efficient all-inorganic QLEDs are demonstrated by using NiO and ZnO as the charge transport layers fabricated via ultrasonic spray processes. Excellent device performance is achieved thanks to the introduction of an Al2O3 interlayer between quantum dots (QDs) and an amorphous NiO layer. Transient photoluminescence and electricity measurements indicate that the Al2O3 layer can suppress the exciton quenching induced by the NiO layer and reduce the electron leakage from QDs to NiO. In consequence, relative to that of a device without an Al2O3 layer, the efficiency of an Al2O3-containing device is enhanced by a factor of 539%, increasing from 3.8 cd/A to 20.5 cd/A, and it exhibits colorsaturated green emission (peak at 530 nm) and high luminescence (>20 000 cd/m2). These are the best performances for all-inorganic QLEDs reported to date. Meanwhile, it is demonstrated that ultrasonic spray is a feasible and cost-effective technology to construct efficient all-inorganic QLEDs. We anticipate that these results will spur the progress toward realization of high performance and mass production of all-inorganic QLEDs as a platform for QD-based full-color displays. KEYWORDS: quantum-dot light-emitting diodes (QLEDs), inorganic metal oxides, insulating layer, passivation, exciton quenching

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HTLs and/or ETLs renders the device vulnerable to oxygen and humidity in the air. Hence, an additional strict encapsulation technology has been required and is very important. Accordingly, the use of inorganic materials as CTLs instead of organic counterparts to construct all-inorganic QLEDs has attracted increasing interest in recent years due to their excellent photoelectric and chemical stability. Inorganic metal oxide semiconductor films fabricated through a solution process have also facilitated major progress in the emerging field of flexible, stable, and low-cost electronic devices. Allinorganic QLEDs with metal oxides as the CTLs are considered as prospective candidates to substitute the hybrid ones due to low material/technique cost and the robust stability of inorganic materials. To date, ZnO is confirmed to be the optimal choice as the electron transport material and is competent for highly efficient QLEDs.9−11,13,14 The biggest obstacle to achieving efficient all-inorganic QLEDs is the limited hole transport materials and strong interactions between QDs and the metal oxide HTL. It has been reported that an alternative HTL material is nickel oxide (NiO),15−19 which is one of the most widely studied oxide semiconductors due to its optical transparency and excellent stability and can be used for a number of optical and electrical applications.

olloidal inorganic semiconductor quantum dots (QDs) have attracted much attention due to their ultrahigh color purity, wide color tunability, and low-cost solution processability.1−8 Especially, light-emitting diodes based on QDs as the emitting layer (QLEDs) have been considered as nextgeneration display technologies.3−8 Great progress has been achieved since the first QLED was reported by A. P. Alivisatos,3 including device efficiency and stability.9−14 Recently, organic− inorganic hybrid QLEDs, in which the charge transport layers (CTLs) consist of the inorganic metal oxide ZnO as the electron transport layer (ETL) and organic polymer as the hole transport layer (HTL), exhibit excellent performance for both conventional and inverted QLEDs.9−11,13,14 For instance, QLEDs with a maximum luminance of more than 200 000 cd/m2 have been reported with an inverted device structure.10 Moreover, QLEDs with record efficiency (external quantum efficiency, EQE > 20%) and device lifetime (over 100 000 h) have been successfully built by employing a ZnO ETL combined with a poly(methyl methacrylate) (PMMA) insulating interlayer.13 However, some of the key parameters in QLEDs still cannot meet the commercial requirements. So it is necessary to further improve the device performance and lower the fabrication cost. At present, the most efficient QLEDs are strongly dependent on the organic charge transport layer for both conventional and inverted structures. As we know, the instable nature of organic © 2017 American Chemical Society

Received: March 7, 2017 Published: April 4, 2017 1271

DOI: 10.1021/acsphotonics.7b00216 ACS Photonics 2017, 4, 1271−1278

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cost of materials for device fabrication. In addition, it has been demonstrated that a mist-deposition process can achieve successive deposition of multicolor QDs through a set of registered shallow masks and thus is recognized as a low-cost solution for large-area displays.22 In response to these concerns, in this work we construct a set of efficient all-inorganic QLEDs with Al2O3-passivated NiO as the HTL and ZnO nanoparticles as the ETL, which were deposited through all-solution techniques (ultrasonic spray processes) except for the Al cathodes. A schematic diagram of an ultrasonic spray system is shown in Figure 1a. The devices

Moreover, air-stable all-inorganic QLEDs have also been demonstrated using NiO as the HTL,16,18 but the device efficiency is at a rather low level. Up to now, it is still an open challenge to construct efficient metal oxide HTLs for allinorganic QLEDs. The key issue faced in the present allinorganic QLEDs is the rather low efficiency and short device lifetime, which is due primary to the quenching of QD emission induced by the NiO layer and large leakage current across the oxide HTL. To date, all of the all-inorganic QLEDs using inorganic metal oxides as the CTLs rely on a similar p−i−n type architecture with the QD emitters directly contacting the CTLs,15−18 which leads to exciton quenching due to QD charging and/or nonradiative energy transfer between the QDs and metal oxides.13,17 The QD charging process usually results from charge transfer between QDs and metal oxides,13 while energy transfer between QDs and metal oxides is often due to the relatively high carrier concentration of metal oxides at the interface adjacent to the QD layer.17 These interactions activate QD luminescence quenching processes and are detrimental to efficient device fabrication. Moreover, a thin QD emission layer ( Va > 0 V, and (c) Va > 3 V.

the Al2O3 layer, the longer the exciton lifetimes. We know that the exciton lifetime is defined via the following equation: 1 τ= k r + k nr (1) where kr and knr are the radiative and nonradiative rates of excitons in QDs, respectively. The knr consists of two terms: one originates from the intrinsic nonradiative decay (ki‑nr) in QD particles and the other (referred to as kt‑nr) originates from the interactions between QDs and NiO. It is certain that there are no chemical changes to the QDs after inserting the Al2O3 interlayer since the underlying Al2O3 layer has been thermally treated at 150 °C and the subsequent QDs undergo a very low annealing process at 80 °C. So the increased exciton lifetimes of 1276

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technology is used to build these devices. We believe that this ultrasonic spray technology is a very feasible technology for industrial production at a high throughput rate. Although the lifetime (∼8000 h) of the present all-inorganic QLEDs is lower than that of the hybrid ones, considerable progress has been achieved compared to the previously reported results, and we think there is still enormous room to further improve the allinorganic device performance with the advances of inorganic hole transporting materials and QDs. Our work offers a reliable, large-scale, and cost-effective approach to fabricate all-inorganic QLEDs. These results provide a rational guideline of device design and also offer a practicable platform for display and lighting applications based on organic-free QLEDs.

improved device stability, ease of preparation, and potential for mass production. Moreover, all the active layers are fabricated in air conditions except for the Al cathode, and this is the first report on device lifetime of all-inorganic QLEDs to date. Most importantly, our unpackaged all-inorganic QLEDs exhibit long shelf lives and still enable stable EL emission after 20 days placed in air, as shown in Figure S5 in the Supporting Information. This result further demonstrates that our allinorganic QLEDs possess better resistant to oxygen and humidity than the hybrid ones.

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MATERIALS AND METHODS The Ni(CH3COO)2 was purchased from Sigma-Aldrich. The synthesis procedures of ZnO nanoparticles and QDs were described elsewhere.10,23−27 The low-cadmium QDs possess a quaternary-alloy ZnCdSSe/ZnS core/shell structure. The Al2O3 precursor solution was prepared according to the method reported in ref 28. Before use, the ITO-coated glass substrates were cleaned by sonication for 10 min each in acetone, ethanol, and deionized water in sequence, after which they were treated by UV-Ozone for 7 min to enhance the wettability of the substrates. Then all of the layers except the Al cathode were deposited by ultrasonic spray processes under atmospheric conditions. During the deposition processes, the solution precursor flowed into the atomizer from a reservoir by nitrogen pressure, and the N2 carrier gas passing through the atomizer was introduced to control the spray rate at 0.3 mL/min, which ensures a fine atomization and high film quality. The distance between the nozzle tip and substrate was fixed at 10 cm by the holder. In order to ensure uniformity, the substrate was slowly rotated (500 rpm) during the deposition process to overcome the surface tension, hence obtaining uniform and smooth oxide and QD films. The solution concentrations of QDs (in toluene), Ni(CH3COO)2 (in ethanol), Al2O3 (in ethanol), and ZnO (in ethanol) were 2, 4, 0.2, and 3 mg/mL, respectively. The thicknesses of the NiO, QDs, and ZnO layers were 40, 30, and 45 nm, respectively, accessed through atomic force microscope (AFM) measurements. For the calibration of the Al2O3 layer thickness, a thick Al2O3 layer of over 30 nm was deposited on a silicon substrate, and the thickness was determined by means of ellipsometry. Given the solution concentration, the Al2O3 layer thickness is believed to vary linearly with the spray time. Then, we can adjust the thicknesses (from 2 to 8 nm) of the Al2O3 layers in different devices with various spray times. Each layer was thermally cured in a glovebox (MBraun) for 40 min after deposition to create homogeneous and high-conductive films, and the annealing temperature was 280, 150, 80, and 70 °C for NiO, Al2O3, QD, and ZnO layers, respectively. Detailed measurement processes for devices have been described in our previous report (ref 25) and in the Supporting Information.

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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.7b00216. Photoluminescence emission and absorption spectra; TEM image; XPS spectra; J−V for the ZnO electron-only device; shelf stability images; PL lifetime and quantum yield (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (W.-F. Xie): [email protected]. *E-mail (H.-Z. Zhang): [email protected]. ORCID

Wenyu Ji: 0000-0003-2932-5119 Author Contributions

W.J. and W.X. designed the experiments and analyzed the data. W.J. wrote the manuscript. S.L. implemented the measurements of impedance spectroscopy of the devices. H.Z. carried out the steady-state and time-resolved experiments. H.Z.Z. gave some significant comments on the experiments and data analysis. All authors reviewed the manuscript and discussed the results. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the program of the National Natural Science Foundation of China (Nos. 11674315, 11474131, and 61474054).

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REFERENCES

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CONCLUSIONS In conclusion, using giant QDs and an Al2O3 passivating layer, we have fabricated a highly efficient all-inorganic QLED with NiO and ZnO as the HTL and ETL, respectively. The current efficiency of 20.5 cd/A is achieved, and the maximum luminescence is more than 20 000 cd/m2 for the green inorganic QLED with a 5 nm Al2O3 interlayer. These are the best performances for the all-inorganic QLEDs to date, and all the results are 2 orders of magnitude higher than that previously reported. Moreover, the low-cost ultrasonic spray 1277

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