Enhanced efficiency and reduced roll-off in blue and white phosphorescent organic light-emitting diodes with a mixed host structure

We report a significant enhancement and a reduced efficiency roll-off in blue and white phosphorescent organic light-emitting diodes (PHOLEDs) based on a mixed host structure. The combination of a hole-transport-type host and an electron-transport-type host as a mixed host emissive layer resulted in an effective charge carrier injection and distribution of the recombination zone, and therefore highly efficient blue PHOLEDs with peak external quantum and power efficiencies of 21.6% and 44.9 lm/W were achieved. Moreover, white PHOLEDs with a mixed host structure showed a power efficiency of 37 lm/W, which is over four times higher than 8.7 lm/W in a single host structure device at a luminance of $1000\text{cd}/{\text{m}}^2$ .