High-Efficiency Blue Phosphorescence Organic Light-Emitting Diode with Ambipolar Carbazole-Triazole Host

An ambipolar material with a high triplet energy of 3.1 eV, 9-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole (CbzTAZ), was synthesized by conjugating an electron-transporting 1,2,4-triazole and a hole-transporting carbazole and employed as the host doping with blue emitter iridium(III)bis[4,6-(difluorophenyl)-pyridinato-N,C2']picolinate (FIrpic) to be the emitting layer (EML) of an efficient blue phosphorescent organic light-emitting diode (PhOLED). By adjusting the ratio between CbzTAZ and FIrpic, an efficient OLED performance with a current efficiency (?C) of 50 cd/A and an external quantum efficiency (?EQE) of 23.8% can be obtained. The position of the recombination zone located inside the EML close to the electron-transporting layer (ETL) can be detected by partially doping FIrpic in one of the three EML regions. By fine-tuning the thickness of the ETL to balance electrons and holes in the EML, a highly efficient blue PhOLED with a maximum ?C of 52.1 cd/A, a maximum power efficiency of 46.1 lm/W, and a maximum ?EQE of 24.4% was obtained. In addition, the device demonstrated a favorable efficiency roll-off (?C of 44.6 cd/A and ?EQE of 21%) at a high brightness of 1000 cd/m2.