Influence of Encapsulation on the Efficiency and Positive Aging Behavior in Blue Quantum Dot Light-Emitting Devices

Abstract

Encapsulating blue quantum dot light-emitting devices (QLEDs) using an ultraviolet curable resin is known to lead to a significant increase in their efficiency. Some of this efficiency increase occurs immediately, whereas some of it proceeds over a period of time, typically over several tens of hours following the encapsulation, a behavior commonly referred to as positive aging. The root causes of this positive aging, especially in blue QLEDs, remain not well understood. Here, it is revealed that contrary to the expectation, the significant improvement in device efficiency during positive aging arises primarily from an improvement in electron injection across the QD/ZnMgO interface and not due to the inhibition of interface exciton quenching as is widely believed. The underlying changes are investigated by XPS measurements. Results show that the enhancement in device performance arises primarily from the reduction in O-related defects in both the QDs and ZnMgO at the QD/ZnMgO interface. After 51.5 h, the blue QLEDs reach the optimal performance, exhibiting an EQEmax of 12.58%, which is more than sevenfold higher than that in the control device without encapsulation. This work provides design principles for realizing high efficiency in blue QLEDs with oxide electron-transporting layers (ETLs) and provides a new understanding of the mechanisms underlying positive aging in these devices and thus offers a new starting point for both fundamental investigations and practical applications.

28‐4: Positive Aging Resulted in Highly Efficient Blue Quantum Rod Light Emitting Diodes

“…For instance, the fresh device had a current density of 121 mA/cm^2 at 6V, while the aged devices showed 280, 480, and 520 mA/cm^2 at 6V for day 1, day 3, and day 4, respectively. Previous reports from Qiang et al, Acharya et al, and Chen et al have shown that the aging process significantly improves the current density of QLEDs by forming a thin AlOx interlayer among the ZnMgO/Al interface through the aging process [20][21][22] . Which suppresses the electron injection barrier and increases the conductivity of ZnMgO by creating oxidation deficiency in ZnMgO.…”

Efficient and stable InP quantum-dot light-emitting diodes formed by premixing 2-hydroxyethyl methacrylate into ZnMgO

“…17–19 Specifically, significant advances in the stability of QLEDs have been reported through a remarkable phenomenon called “positive aging,” which accompanies post-treatments such as thermal annealing or acidic resin treatment. 20–28 In general, QLEDs exhibit negative aging behavior owing to the rapid deterioration of the charge transporting layers, where the luminance continuously decreases as a function of time. 29–32 In contrast, QLEDs exhibiting positive aging overcome the rapid luminance decline in the initial stage of device operation and offer drastically extended lifetimes; 20,25 however, the precise principle of positive aging remains unclear.…”