Light-emitting diodes (LEDs) are widely used in lighting and display applications. Their use has rapidly grown over the past few years due to a lot of efforts and research that have been put in place to realize more efficient and durable LEDs. Today, the focus has been shifted to the use of colloidal quantum dots in LEDs rather than organic as it was widely practiced before. Quantum dots have the advantage of its chemical and optical properties such as tunable emissions color and high material stability that makes them suitable for LED applications.
Although the discovery and use of quantum dots in LEDs have significantly improved the design and architecture of the device, there is still more to be done concerning the general performance and durability of such devices to achieve the competitive advantage over the organic LEDs and others.
With the demand of all kinds of LEDs ranging from red, green, blue among others expected to increase in the future, the need to improve the performance efficiency and lifetime durability of QLEDs much higher than their counterparts have increased. Although there are numerous ways of realizing this, the solution, however, is seen to be the use of tandem structures. Such structures constitute of electroluminescent units that are connected in series with the transparent interconnecting layers (ICL).
A group of researchers from Southern University of Science and Technology, Department of Electrical and Electronic Engineering in China led by Professor Shuming Chen developed efficient tandem QLEDs from enhanced ICL. This was in a bid to improve the performance of QLEDs. Their research work is currently published in the journal, ACS Nano.
In their experiments, the authors used evaporated small molecule HATCN together with metal oxide to serve as the p-type for the development of the ICL. On the other hand, electrical properties of the ICL were improved by using ultrathin aluminum layer sandwiched between the p-type and n-type materials.
The authors successfully demonstrated full-color tandem QLEDs. They comprised of red, blue and green QLEDs with very high external quantum efficiency. For example, the external quantum efficiencies obtained stood at 21.4% for B-QLEDs, 23.1% for R-QLEDs and 27.6% for G-QLEDs, which are the world-records currently.
This is the first research to report such high efficiencies observed in the full-color tandem QLEDs. This milestone achievement would further promote the practical application of QLED. They also exhibited increased operation lifetime, high color purity and minimal roll-off efficiency. The ICL developed, in this case, is, therefore, a suitable method that is highly expected to increase the use of QLEDs in the future especially for an application that requires full-color displays.
The authors also pointed out that the performance of the tandem devices highly depends on the optical and the electrical properties of the interconnected layers. Besides, the ICL is capable of preventing the solvents used in the treatments from damaging the bottom quantum dot layer especially during the deposition of the upper layer. This shows the robustness and effectiveness of the ICL as far as charge injection is concerned.
Highly efficient full-color tandem quantum-dot light-emitting diodes (QLED) with EQE exceding 23%, 27% and 21% for red-, green and blue-QLEDs are achieved by using a novel inter-connecting layer (ICL) of ZnMgO/Al/HATCN/MoO3. The demonstrated QLEDs, with extremely high efficiency, long lifetime, low efficiency roll-off and high color purity, would be ideal candidates to bring QLEDs into the next generation full-color displays and the solid-state lighting market.
Zhang, H., Chen, S., & Sun, X. (2017). Efficient Red/Green/Blue Tandem Quantum-Dot Light-Emitting Diodes with External Quantum Efficiency Exceeding 21%. ACS Nano, 12(1), 697-704.
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