Effect of Excess Carriers on the Degradation of InP-Based Quantum-Dot Light-Emitting Diodes

Improving operational stability is one of the most crucial issues for the practical application of quantum-dot (QD)-based light-emitting diodes (QLEDs), particularly in devices with heavy-metal-free QDs. Although a few potential reasons for their operational instability have been suggested, such as poor charge balance and surface defects of QDs, the origin of degradation needs to be disclosed more clearly to achieve higher device stability. Here, we systematically investigate the effect of excess charges on the degradation of InP-based QLEDs. For this, we intentionally designed charge-imbalanced QLEDs, i.e., hole-excess and electron-excess devices, by inserting an insulating layer adjacent to QDs, and measured their lifetime discharging the devices at regular intervals to observe the effect of each excess carrier separately. Based on multilateral analysis, we found that excess holes cause rapid deterioration of QDs and resultant permanent degradation of QLEDs at an early stage, whereas excess electrons temporarily charge the QDs. To minimize the permanent degradation of QDs, we adopted an electrochemically robust ligand for QDs by ligand exchange, enabling QLEDs to exhibit improved lifetime (by a factor of ∼109) with no notable permanent degradation. We believe that our device design, analysis methods, and strategies for better stability would not only expand the fundamental understanding of QLEDs but also contribute to the development of highly stable InP-based QLEDs.