Charging and Charged Species in Quantum Dot Light-Emitting Diodes

Despite recent rapid advances in improving quantum dot light-emitting diodes, many fundamental aspects of the device operating mechanism remain unresolved. Through transient electroluminescence and time-resolved photoluminescence measurements, the effects of offset voltage on charging and charge transport are examined. First, capacitive charging occurs with a time constant of ∼500 ns, followed by electron transport through quantum dots with a mobility of ∼10^-5 cm² V^-1 s^-1. Hole injection then initiates an electroluminescence rise that is independent of offset voltage. The photoluminescence lifetime is also unaffected by the offset voltage, indicating no injection of charges into the quantum dots or on their surfaces prior to the voltage pulse. A slower equilibration to steady-state electroluminescence is dependent on the offset voltage, indicative of another charging process. Elemental mapping shows that ZnO deposition from solution can lead to the diffusion of charged species into the quantum dot layer, which may cause the slower process.