Enhanced hole injection by introducing an electron-withdrawing layer in inverted quantum dot light-emitting diodes

Abstract The effects of different hole transport layers (HTLs) have been investigated in the inverted quantum dot light-emitting diodes (QD-LEDs). Surprisingly, the HTL with shallower highest occupied molecular orbital (HOMO) level leads to higher current. The unique energy levels of di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) HTL cause Auger-assisted hole injection and better carrier confinement within the QD emitting layer, resulting in lower driving voltage and higher efficiency. In addition, the hole injection is further effectively enhanced by inserting a thin 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) layer between TAPC and MoO 3. And the optimized device with 4-nm-thick HATCN obtains a maximum power efficiency of 26.2 lm/W, meaning a 50% improvement compared to the device employing individual MoO 3 hole injection layer. Owing to the deep lowest unoccupied molecular orbital (LUMO) level and strong electron withdrawing ability of HATCN, electrons can easily transport from the HOMO of TAPC to the LUMO of HATCN, and the holes left on TAPC can greatly enhance the hole transport ability. Besides, the reduced barrier from the high work function of MoO 3 /Al as well as the smoother surface morphology of HATCN/MoO 3 film also assists hole injection. It is believed that the combination of HTL and insertion of HATCN between HTL and MoO 3 can be applied to many other opto-electronic devices to improve hole injection.