Stepwise Bi-Layer Hole-Transport Interlayers With Deep Highest Occupied Molecular Orbital Level for Efficient Green Quantum Dot Light-Emitting Diodes

The imbalanced charge transport/injection in quantum dot (QD) light-emitting diodes (QLEDs) due to the existing excess electrons has greatly restricted the device performance. Here, we designed a stepwise bi-layer structured hole-transport layer (HTL) containing 4,4'-bis-(carbazole-9-yl)biphenyl (CBP)/1,3-bis(9H-pyrido[2,3-b]indol-9-yl) benzene (mCaP) with deep highest occupied molecular orbital (HOMO) level to realize more efficient hole injection into QD emissive layer for balancing charge in the QLED devices. The resulting green QLEDs show a maximum current efficiency (CE) of 41.2 cd A-1corresponding to an external quantum efficiency (EQE) of 12.6%, which is a significant efficiency enhancement compared with the devices with only single-layer HTL. In addition, the device turn-on voltage is also reduced from 5 to 3 V, and meanwhile the operational lifetime is increased by more than twofold.