Charge Accumulation Behavior in Quantum Dot Light-Emitting Diodes

Quantum dot light-emitting diodes (QLEDs) constitute the next-generation display technology because of their wide color gamut, narrow emission spectrum, adjustable emission wavelength, and ease of solution processability. With the development of novel material and device preparation techniques, the QLEDs not only show an external quantum efficiency (EQE) of more than 20% in red, green, and blue (primary color) devices, but also achieve 100% Rec.2020 (recommendation standard for ultrahigh-resolution display) color gamut coverage. However, the future commercialization of QLEDs is still a challenge. The T95 lifetime (defined as 95% time for the luminance to decay to the initial value L-0 = 1000 cd.m(-2)) of red, green, and blue QLED devices is significantly lower than that of commercially available organic light-emitting diodes (OLEDs). This is ascribed to the lacking of understanding and argument to hypothesis of degradation mechanisms. A QLED is a sandwich structure composed of a quantum dot (QD) emitter layer, carrier transport layer, and electrode layer. The QLED works on the principle of electroluminescence: electrons and holes injected from the electrodes on both sides of the device cross multiple interfaces and reach the QD emitter layer to undergo radiation recombination. Generally, the QD emitter layer adopts the structure of a wide-band gap shell wrapped around a narrow band-gap core. Because of the deep valence band maximum, the hole injection barrier is higher, and the hole injection efficiency is reduced. This not only disturbs the injection balance but also leads to the accumulation of interfacial holes, which is one of the important factors affecting the efficiency and life of the device. Past studies have attempted to understand charge accumulation behavior in QLEDs by predicting the interfacial energy band structure, and there are very few reports on the direct measurement of charge accumulation. In this work, we built a charge extraction circuit to investigate the charge accumulation behavior before and after aging in a prototype red QLED. In the fresh red QLEDs, the number of accumulated charges gradually increased with the driving current density and tended to saturate above turn-on current density. In the aged red QLEDs, the accumulated charges increased with a decrease in luminance. Our method to investigate the charge accumulation behavior developed can be extended to various kinds of LEDs, such as OLEDs and perovskite LEDs, thus providing insight into their working mechanism.

Automated summary

QLEDs are considered the next-generation display technology due to their wide color gamut and narrow emission spectrum, but the future commercialization faces challenges, mainly due to the lower T95 lifetime. The working principle of QLED is based on electroluminescence, but its hole injection efficiency is low, leading to the accumulation of interfacial holes, thus affecting the device's lifetime and efficiency.