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Title: Colloidal quantum dot light-emitting diode architectures for high performance
Authors: Leck, Kheng Swee
Keywords: DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
DRNTU::Engineering::Electrical and electronic engineering::Nanoelectronics
DRNTU::Engineering::Electrical and electronic engineering::Semiconductors
DRNTU::Engineering::Materials::Photonics and optoelectronics materials
Issue Date: 2016
Source: Leck, K. S. (2016). Colloidal quantum dot light-emitting diode architectures for high performance. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: Recently scientific research and development on colloidal quantum dot light-emitting diodes (QD-LEDs) have attracted considerable interest thanks to their advantages over conventional epitaxial-based light-emitting diodes, which require expensive deposition tools and high temperature growth on a substrate template, limiting the choices of materials for epitaxial growth. Colloidal quantum dots (QDs) are chemically synthesized tiny nanocrystals that can be easily dispersed in organic/aqueous solvents and integrated into organic/polymer light-emitting diode architectures using cost-effective solution-based fabrication techniques. In addition, the emission spectra of QD-LEDs can be conveniently adjusted (e.g., tuned from blue to red) by simply changing the size of their QDs. This shows a huge potential for future display and lighting applications. Therefore, there is a strong motivation for understanding the underlying device physics and improving the device performance of the QD-LEDs. This thesis work systematically studies the influence of device architecture on the performance of QD-LEDs using various means including optimizing device charge injection and balance and using efficient indium-free electrodes. In this thesis, white QD-LEDs were demonstrated using a specially designed loosely-packed QD layer and device architecture. In the device, 2,2’,2”-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) electron transport layer (ETL) and poly(N,N’-bis(4-butylphenyl)-N,N’-bis(phenyl)-benzidine) (poly-TPD) hole transport layer (HTL) were made partially in contact to each other. As a result, a certain amount of the injected electrons and holes recombined at the interface, giving rise to red exciplex emission. Combining the red exciplex emission, the green emission from the QDs, and the blue emission from poly-TPD layer, a set of white QD-LEDs with high colour rendering index (CRI) of 89 and correlated color temperature (CCT) of 4098 K were successfully demonstrated. Second, an additional HTL was employed between the QD layer and the TPBi ETL to improve the device performance. Adding HTL at the electron side limits excess electrons to be injected into the QD layer, resulting in better charge balance. Here an improvement in the device performance of over five-folds was obtained with the proposed device architecture in comparison to the reference device in which the QD layer is deposited between the HTL and the ETL. The significant improvement results from the balanced hole and electron injection into the QDs emissive layer. Exciton distribution analysis of the studied device showed that nonradiative energy transfer from the organic transport layer to the QDs is minimum and the improvement mainly stems from the optimized charge balance. Third, the effects of different electron injection materials on the operating voltage were investigated. Notable performance improvement was observed from the devices using Cs2CO3 electron injection material. A 35% increase in the EQE, a 19% reduction in the operating voltage, and a 24% improvement in the power conversion efficiency were achieved compared with the reference device using LiF as the electron injection layer (EIL). Device exciton distribution study showed that Cs2CO3 promoted exciton formation in the QD layer and reduced exciton leakage to the organic layer. The increase of exciton recombination in the QD layer thus enabled better device performance. In addition, in the thesis, we used gallium-doped zinc oxide (GZO) to replace tin-doped indium oxide (ITO) used as the electrode in QD-LEDs due to the increasingly expensive raw materials of ITO. The lowest sheet resistance and resistivity of the radio-frequency (RF) sputtered GZO films are 12.28 Ω/□ and 9.48×10-4 Ω·cm, respectively, comparable to those of commercial grade ITO. These resulting QD-LEDs using the GZO electrodes exhibit a similar level of performance as the devices with ITO as the electrode, indicating that the GZO electrode prepared by the RF sputtering process can be a promising ITO replacement for low-cost QD-LEDs. QD-LEDs have made great progress in terms of device performance over the years and there is room for further improvement. With all these findings and demonstrations in this thesis, we conclude that QD-LEDs hold great promise for potential applications in lighting and displays
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