Mechanisms for Efficiency Improvement in ZnO-Based Hybrid QLEDs: Beyond Suppression of Emission Quenching

The presence of surface defects on ZnO nanoparticles provides additional pathways for exciton quenching, which consequently diminishes the efficiency of quantum-dot-light-emitting diodes (QLEDs). Insulating layers are commonly inserted between the ZnO electron-transport layer (ETL) and quantum dot (QD) emission layer to enhance device performance, often with the improvement being attributed, sometimes without sufficient justification, to the suppression of emission quenching, as evidenced by photoluminescence characterization. Here, we revisit the ZnO/QDs bilayer interface and carry out in situ temperature-dependent transient electroluminescence spectroscopy measurements to unravel the influence of the Al2O3 insulating layer on the charge dynamics in the device. The results indicate that, in contrast to the control device without an insulating layer, QLED incorporating an Al2O3 insulating layer exhibits a preferential distribution of holes toward QDs farther from the QD/ZnO interface at lower driving voltages. Besides the reduced emission quenching of QDs (if any), this spatial redistribution of holes should primarily elevate the efficiency of the exciton formation, hence resulting in a substantial enhancement of device efficiency. Our research highlights the crucial role of charge distribution in determining the performance of QLEDs.