Revisiting Hole Injection in Quantum Dot Light-Emitting Diodes

Injecting holes from the hole transport layer (HTL) into the quantum dot (QD) emitting layer in quantum dot light-emitting diodes (QLEDs) is considered challenging due to the presence of a relatively high hole injection barrier at the HTL/QD interface. However, QLEDs with exceptional brightness and efficiency are achieved, prompting a reevaluation of the traditional hole injection mechanisms. This study examines the hole injection mechanism in QLEDs using a combination of experiments and simulations. The results demonstrate that the applied bias significantly reduces the barrier height between the highest occupied molecular orbital level of the HTL and the valence band (VB) of the QDs, facilitating hole injection. The bending of the lowest unoccupied molecular orbital energy level of the HTL at the HTL/QD interface confines electrons within the QD, effectively minimizing leakage current. Additionally, the triangle-shaped potential barrier arising from the bending of the VB energy level of the QDs creates favorable conditions for hole–tunneling injection. Moreover, both simulations and experiments consistently demonstrate that the predominant pathway for hole injection from the HTL to the QDs in the QLED device involved thermally assisted tunneling. This study is important to understand the hole injection mechanism in QLEDs.