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Highly efficient blue phosphorescent organic light-emitting diodes (PHOLEDs) were achieved by blending the same hole- and electron-transporting layer materials in device as the host in the emissive layer. The design of this kind of device structure not only decreases the amount of used organic materials but also greatly reduces the structural heterogeneities and effectively facilitates the charge injection into the emissive layer. The resulting blue PHOLEDs exhibit higher electroluminescent efficiency. The maximum external quantum efficiency and power efficiency reach 20.4% and 55.4 lm W-1, respectively, yet keep 19.5% and 49.9 lm W-1, 18.3% and 40.9 lm W-1, respectively, at a luminance of 100 and 1000 cd m-2, showing a low efficiency roll-off property. The mechanism studies fully demonstrate that the bipolar co-host system extends the lifetime of excitons, broadens the recombination zone, and improves the charge carrier injection and transport balance, which are of greatly critical to get the improved efficiency and efficiency roll-off at high luminance in the blue PHOLEDs.