Quantum-dot light-emitting diodes (QLEDs) have shown great potential in display and lighting applications. In particular, the incorporation of zinc oxide nanoparticles (ZnO NPs) as the electron transport layer (ETL) has significantly enhanced QLED performance, including efficiency, luminance, and operational lifetime. However, several challenges remain for the commercialization of ZnO-based QLEDs. Among these, a critical issue is the so-called “positive ageing” effect, where device efficiency gradually increases and eventually stabilizes after fabrication during storage or low-voltage operation. While this positive ageing effect can lead to optimized device performance, its uncontrollable nature poses significant difficulties. It results in inconsistent device performance across QLED arrays and carries a risk of complete device failure during the ageing process. To date, the underlying physical mechanisms of the positive ageing effect in QLEDs remain unclear.
In a new paper published in Light: Science & Applications , a team of scientists, led by Professor Wenyu Ji from Key Lab of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, China, and co-workers have systematically investigated the effects of acid and metal ions on the morphology and optoelectronic properties of ZnO nanoparticle (NP) films during the positive ageing process. Through a series of in-situ characterizations, they unveiled the mechanism underlying the positive ageing effect in hybrid QLEDs employing ZnO NP-based electron transport layers.
The study reveals that acid treatment induces an in-situ recrystallization of ZnO NPs. Specifically, under acidic conditions, ZnO NPs undergo oriented fusion along the c -axis of the wurtzite crystal structure, leading to the formation of long-range ordered ZnO crystals and passivation of surface defects. Meanwhile, the diffusion of active metal ions (such as Al and Ag) from the cathode into the ZnO layer further promotes the recrystallization process and enhances defect passivation.
As a result, the recrystallized ZnO NP films exhibit significantly reduced carrier concentration and markedly enhanced electron mobility. The high mobility ensures efficient electron injection, while the lower carrier concentration suppresses exciton quenching within the quantum dot (QD) layer. Additionally, the passivation of trap states in ZnO further suppresses exciton quenching. Collectively, these improvements lead to a substantial enhancement in device performance, with the external quantum efficiency (EQE) of red QLEDs increasing from 17.2% to 33.7%.
Light Science & Applications
Operando ZnO recrystallization for efficient quantum-dot light-emitting diodes