Understanding Electrical Excitation Dynamics in Quantum-Dot Light-Emitting Diodes: From Steady-State to Temporal Behaviors

Light-emitting diodes based on colloidal quantum dots (QDs) promise a new generation of color-pure, cost-effective, and flexible light sources. Despite of the tremendous advances in the synthesis of QDs, the electroluminescent (EL) properties of QDs generally lag behind their photoluminescent (PL) properties. These EL-PL gaps originate from the essential difference in the charge dynamics of electrical excitation with respect to optical excitation. Here, we report our recent advances in understanding the electrical-excitation dynamics of QDs in active quantum-dot light-emitting diodes (QLEDs) in both steady-state operations, long-term evolutions, and transient EL dynamics under pulsed excitations.<br/>First, we unraveled how the dynamic charge balance can be achieved in the steady-state operation of red, green, and blue QLEDs, leading to high-performance devices with near-limiting efficiencies [1,2]. Key to this is the sequential charge injection at single-nanocrystal level and effective blocking of electron leakage at the polymer/QD interface. Then, we address an anomalous efficiency-elevation phenomenon induced by operational degradation. Comprehensive in-situ characterizations reveal that this is caused by the ligand migration between the electron-transport layer and the cathode, which in turn improves the charge balance in long-term operation. Finally, we report the excitation-memory effects in the transient EL dynamics of QLEDs modulated by deep-level trap states. By utilizing this unique process, we demomstrate 100 MHz modulation and data transmission based on micro-QLEDs.<br/><br/>[1] Deng, Y., et al., Deciphering exciton-generation processes in quantum-dot electroluminescence. <i>Nat. Commun.</i> 2020 11.<br/>[2] Deng, Y., et al. Solution-processed green and blue quantum-dot light-emitting diodes with eliminated charge leakage. <i>Nat. Photon. </i>2022 16.