Charge-storage behaviors in quantum-dot light-emitting diodes

Understanding the charge dynamics in the quantum-dot light-emitting diodes (QLEDs) is essential to further improve their performance. Here we demonstrate that the holes can be stored for over 30 ms in QLEDs based Cd-based quantum dot (QD) emission layer. This ultralong-term hole storage is examined by inserting an electron-capturing unit (ECU) in the hole-transport layer. Through superimposing a negative offset voltage during transient electroluminescence measurements, the electrons captured by the ECU were transported back to the QDs, where holes are stored during the typical operation for the QLED. Then, excitons are formed and electroluminescence signal is detected. We found that the electroluminescence signal can be detected until 30 ms after turning off the driving voltage for the QLED. Given the limited electron capturing time in the ECU, this should be the lower limit for hole storage time. It is abnormal and unanticipated for the QLEDs based on the ZnO electron-transport layer, for which electrons are widely considered as the majority charge carriers. We believe our results can offer significant insights into the working mechanism and degradation of the QLEDs.