While quantum dot-based light-emitting diodes (QLEDs) are not made of organic materials, they share many of the same advantages as organic LEDs (OLEDs). For instance, both QLEDs and OLEDs outshine semiconductor-based LEDs in terms of their greater flexibility, better color quality, and potential for lower cost since they can be fabricated using a simple process on a large-area substrate. But ever since the first QLEDs were demonstrated in the mid-'90s, about a decade after OLEDs, their performance has lagged behind OLEDs despite ongoing improvements. Now in a new study, a team of researchers from South Korea has designed and demonstrated QLEDs with an improved efficiency and unprecedented brightness that matches the brightness of today's best fluorescent OLEDs.
The key to improving the brightness and efficiency of the QLEDs is improving the injection of current-carrying electrons and holes into the quantum dots. The more efficiently the electrodes can inject electrons and holes into the quantum dots, the more efficiently the device can emit light. Usually, the anode is made of indium tin oxide, whose transparency allows light to escape. But here, the researchers inverted the device by making the indium tin oxide the cathode with the help of zinc oxide nanoparticles as an electron transport layer, which performed charge carrier injection much more efficiently than before.
“The most important cause of the low performance of QLEDs is the poor injection of holes into the quantum dots (QDs) from the anode and neighboring hole transport layer due to a huge potential energy barrier,” Changhee Lee told Phys.org. “Because of that, the electron-hole balance is not achieved, resulting in low quantum efficiency and low maximum brightness. Furthermore, the excess electrons or holes, which do not recombine in the QD layer and enter the neighboring organic hole-transport orelectron-transport layers (HTL or ETL), can cause leakage current and device degradation, resulting in poor efficiency and stability. Therefore, good carrier injection is a key factor for realizing high-performance QLEDs.”
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