Effect and mechanism of encapsulation on aging characteristics of quantum-dot light-emitting diodes
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Guangdong University Key Lab for Advanced Quantum Dot Displays and Lighting, Shenzhen Key Lab for Advanced Quantum Dot Displays and Lighting, and Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen 518055, China 2 Key Laboratory of Energy Conversion and Storage Technologies (Southern University of Science and Technology), Ministry of Education, Shenzhen 518055, China © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 13 July 2020 / Revised: 4 September 2020 / Accepted: 5 September 2020
ABSTRACT The aging characteristics, e.g., the evolution of efficiency and luminance of quantum-dot light-emitting diodes (QLEDs) are greatly affected by the encapsulation. When encapsulated with ultraviolet curable resin, the efficiency is increased over time, a known phenomenon termed as positive aging which remains one of the unsolved mysteries. By developing a physical model and an analytical model, we identify that the efficiency improvement is mainly attributed to the suppression of hole leakage current that is resulted from the passivation of ZnMgO defects. When further encapsulated with desiccant, the positive aging effect vanishes. To fully take the advantage of positive aging, the desiccant is incorporated after the positive aging process is completed. With the new encapsulation method, the QLED exhibits a high external quantum efficiency of 20.19% and a half lifetime of 1,267 h at an initial luminance of 2,800 cd·m−2, which are improved by 1.4 and 6.0 folds, respectively, making it one of the best performing devices. Our work provides an in-depth and systematic understanding of the mechanism of positive aging and offers a practical encapsulation way for realizing efficient and stable QLEDs.
KEYWORDS quantum-dot, light-emitting diodes, positive aging, encapsulation, ZnO defects
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Introduction
After decades of development, the external quantum efficiency (EQE) of colloidal CdSe-based quantum-dot light-emitting diodes (QLEDs) has been improved from initial 0.1% to over 20% [1–5], which has already reached their theoretical limit, assuming that the out-coupling efficiency of planar devices is around 20% [6]. Coupled with their advantages of high color saturation, high brightness and simple solution processability, the QLEDs could find a wide variety of applications, such as display [7, 8], lighting and phototherapy [9–12]. Although the performance especially the efficiency and the brightness of QLEDs have been rapidly improved, the lifetime of QLEDs is still far behind that of their rivals, e.g., organic light-emitting diodes (OLEDs) [13–16]. Besides, the underlying physical mechanisms, such as charge injection, exciton quenching and device degradation of QLEDs still remain unclear. Therefore, to improve the stability and promote the application of QLEDs, more efforts should focus on understanding the in-depth mechanisms of QLEDs. To prolong the lifetime, the QLEDs should be well packaged to protect them from ambient mo
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