Mechanisms and Perspectives of Positive Ageing Effect in Quantum-Dot Light-Emitting Diodes

The widespread integration of ZnO nanoparticles (NPs) as the electron transport layer has yielded significant advancements in the performance of hybrid quantum dot light-emitting diodes (QLEDs). It has been widely demonstrated that conventional hybrid QLEDs exhibit a positive ageing effect, which refers to the markedly increased whole performance of QLEDs with prolonged annealing, voltage stressing, or storage time. This phenomenon is believed to be closely associated with the ZnO (or its derivative such as ZnMgO) electron transport layer. This review aims to summarize the role of ZnO in positive ageing and to discuss the existing challenges in the hybrid QLEDs. Currently, the origins of the positive ageing can be categorized into two types: i) interface effect between ZnO and metal cathode and ii) passivation effect of ZnO NPs. Then the underlying mechanisms for positive ageing are discussed in detail and their limitations are pointed out. Finally, a perspective and outlook for the positive ageing effect are shown, as well as a suggestion for the future research directions and opportunities of hybrid QLEDs.