Reduced efficiency roll-off in phosphorescent OLEDs with a stack emitting layer facilitating triplet exciton diffusion

A high efficiency and low efficiency roll-off phosphorescent organic light-emitting diode (PHOLED) is demonstrated based on a stacked emissive layer by alternating [CBP : 4 wt% Ir(ppy)(3) (5 nm)] and [CBP : 8 wt% Ir(ppy)(3) (5 nm)] ultrathin films. The results show that when the number of the stack cell is equal to three, the peak current efficiency of ca. 48.8 cd A(-1) and the peak external quantum efficiency (EQE) of ca. 19.6% are obtained and kept until 1000 cd m(-2). As the luminance further increases from 1000 to 100 000 cd m(-2), there only exists a less than 25.1% drop in current efficiency and 25.5% drop in EQE. The improved properties for the proposed PHOLED are attributed to greatly weakened triplet exciton quenching due to the very effective triplet exciton diffusion by the abovementioned stacked emissive structure.