Promoted Hole Transport Capability by Improving Lateral Current Spreading for High-Efficiency Quantum Dot Light-Emitting Diodes.

Carrier imbalance resulting from stronger electron injection from ZnO into quantum-dot (QD) emissive layer than hole injection is one critical issue that constrains the performance of QDs-based light-emitting diodes (QLEDs). This study reports highly efficient inverted QLEDs enabled by periodic insertion of MoO into (4,4'-bis(-carbazolyl)-1,1'-biphenyl) (CBP) hole transport layer (HTL). The periodic ultrathin MoO/CBP-stacked HTL results in improved lateral current spreading for the QLEDs, which significantly relieves the crowding of holes and thus enhances hole transport capability across the CBP in QLEDs. Comprehensive analysis on the photoelectric properties of devices shows that the optimal thickness for MoO interlayer inserted in CBP is only ≈1 nm. The resulting devices with periodic two insertion layers of MoO into CBP exhibit better performance compared with the CBP-only ones, such that the peak current efficiency is 88.7 cd A corresponding to the external quantum efficiency of 20.6%. Furthermore, the resulting QLEDs show an operational lifetime almost 2.5 times longer compared to CBP-only devices.