Improved electrophosphorescence efficiency for organic light-emitting diodes using the cohost with stepwise blending profile

A tris(2-phenylpyridine) iridium doped phosphorescent organic light-emitting diode using a mixed host with stepwise blending profile was demonstrated. We adopted the mixture of 4,4′,4″-tris(N-carbazolyl) triphenylamine and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene as the host in the emitting layer. By using a stepwise blending ratio profile of the two host materials, a peak external quantum efficiency of 17.7% and a peak power efficiency of 45.7 lm/W were attained, which increased by 59.5% and 60.9% relative to the optimal uniform blending ratio profile based device, respectively. The efficiency improvement was attributed to the multiple exciton formation interfaces, which could expand the recombination regions.