Origin of efficiency roll-off for FIrpic based blue organic light-emitting diodes and implications on phosphorescent molecule design

This work reports our effort on understanding the efficiency roll-off in blue phosphorescent organic light-emitting diodes (OLEDs), based on bis[3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)]-iridium (FIrpic) doped in 4,4-N,N-dicarbazole-biphenyl (CBP) host. The performance of a set of blue phosphorescent OLEDs with different FIrpic dopant concentrations was analyzed. Unusually, the device having a 30-nm-thick pure FIrpic emissive layer with a high luminous efficiency of 7.76 cd/A at 100mA/cm2 was obtained. Theoretical calculation, based on the density functional theory, reveals that the exciton self-quenching process is suppressed due to the strong Coulomb repulsion between FIrpic molecules. It is found that the efficiency roll-off in FIrpic-based OLEDs is closely related to the exciton quenching induced by the CBP+ radical cations that are present in the CBP host.