Positive charging effect in quantum-dot light-emitting diodes

It is widely accepted that there are significantly more electrons than holes in ZnO-based quantum-dot light-emitting diodes (QLEDs) due to the outstanding electrical properties of the ZnO film. However, here we found that holes but not electrons are stored in the quantum dots (QDs) after turning off the driving voltage. This unexpected hole storage (positive charging) behavior is attributed to the strong confinement effect caused by the wide bandgap shell outside the QDs. Temperature-dependent transient electroluminescence (TrEL) measurements were carried out to confirm the sign of the residual charges and their distribution in the QD emission layer. As temperature decreases, the holes tend to be immobile in the QDs, elevating the concentration of holes within the QD layer away from the ZnO. Consequently, increased EL spike intensity is observed for the QLED during the off period. This positive charging effect reminds us to reconsider the operational mechanisms of QLEDs, especially for the display applications driven by an alternative current mode.