Carrier transport manipulation for efficiency enhancement in blue phosphorescent organic light-emitting devices with a 4,4′-bis(N-carbazolyl)-2,2′-biphenyl host

4,4 '-N,N '-Dicarbazol-biphenyl (CBP) is a common host in phosphorescent organic light emitting devices (PhOLEDs). However, it is rarely used to fabricate blue PhOLEDs due to its lower triplet energy level. Here, a 53.8% enhancement of the maximum external quantum efficiency (EQE(max)) from CBP-based blue PhOLEDs by introducing a carrier adjustment layer (CAL) is reported. A EQE(max) of 16.0% is achieved in blue PhOLEDs with a CAL using CBP:bis[(4,6-difluorophenyl)pyridinato-N,C-2 '](picolinato)iridium(iii) (FIrpic) as an emitting layer (EML). This efficiency is the highest record of CBP-based blue PhOLEDs, and it is comparable to most blue PhOLEDs which use a single host with a high triplet energy level. The efficiency enhancement is attributed to the balanced transport and distribution of electrons and holes in the EML. The results suggest that the effect of a more balanced distribution of carriers in the EML on the efficiency of blue PhOLEDs is comparable to the triplet energy level of the host materials.