Effects of double heat treatment of NiO hole transport layer on the performance of QLEDs
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Effects of double heat treatment of NiO hole transport layer on the performance of QLEDs Sun-Kyo Kim1, Heesun Yang1, and Yong-Seog Kim1,* 1
Department of Materials Science and Engineering, Hongik University, Seoul, Korea
Received: 7 May 2020
ABSTRACT
Accepted: 30 August 2020
Parameters affecting the performance of QLED devices using NiO hole transport layer (HTL) thin film via the sol–gel process route were investigated. In the preparation of the NiO HTL thin film, a double heat treatment scheme, crystallization heat treatment at elevated temperatures followed by vacuum annealing treatment at lower temperatures, was introduced to form a crystalline NiO single phase as well as surface phases with reduced defect concentration. The decoupling of crystallization and vacuum annealing treatment of NiO HTL thin film was effective in enhancing the performance of the QLEDs. Also, the insertion of an interlayer between QD EML and NiO HTL was very effective in enhancing the performance of the device. With the modification of processing conditions, the luminance of QLED with the NiO HTL was improved from 30 to 11500 cd/m2 and the current efficiency from 0.25 to 23 cd/A, respectively.
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Springer Science+Business
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Introduction Recently, there have been significant advancements in luminous efficiencies of electroluminescence hybrid quantum-dot light-emitting devices (QLEDs) which use an organic hole transporting layer (HTL), a QD emissive layer (EML), an inorganic ZnO electron transporting layer (ETL) with ITO anode, and aluminum cathode. Their efficiencies were enhanced to the levels comparable to those of the OLEDs. Redemitting QLEDs reached a maximum EQE 20.5% with a lifetime longer than 100000 h at 100 cd/m2 [1]. Green-emitting QLEDs using a tandem structure
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https://doi.org/10.1007/s10853-020-05223-z
achieved an EQE over 23.68% [2]. For the blue QLEDs, EQE has been also improved to 18.0% [3–5]. The lifetime of the hybrid QLEDs, especially the blue-emitting QLEDs, is rather short, mainly attributed to the use of organic HTLs. For example, poly (ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS), which is acidic and hygroscopic, has been used in many studies as an HTL of the hybrid QLEDs, but its charge injection and transport characteristics degraded over the time due to the reactions with oxygen and moisture in the atmosphere [6]. To overcome such environmental instability of the organic HTLs, numerous studies have explored the possibilities of using metal oxide semiconductors
J Mater Sci
as the HTL. The metal oxides are expected to have better stabilities against oxygen and humidity of the environments. Besides, the metal oxide HTLs have been projected to have higher charge carrier mobility compared to that of the organic counterparts. This should enhance the charge balance in the QD EML, which would eventually lead to higher efficiency and longer lifetime of the QLED devices. For
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