Two ambipolar host materials, 9,9-bis(9-methylcarbazol-3-yl)-4,5-diazafluorene (MCAF) and 9,9-bis(9-phenylcarbazaol-3-yl)-4,5-diazafluorene (PCAF), comprising two electron-donating carbazole units and an electron-accepting 4,5-diazafluorene group, have been designed, synthesized, and characterized. Given the nonplanar structure of the sp3-hybridized C9 atom of the 4,5-diazafluorene unit, MCAF and PCAF exhibit high triplet energy levels of 2.82 and 2.83 eV, as well as high glass-transition temperatures of 187 and 188 °C, respectively. Equipped with ambipolar transport properties as well as suitable highest occupied and lowest unoccupied molecular orbital energy levels, the two compounds excellently perform in blue phosphorescent organic light-emitting devices (PHOLEDs). The MCAF-based blue PHOLED has a very low turn-on voltage of 2.6 V, a high current efficiency of 32.2 cd A–1, a high external quantum efficiency of 17.9%, a high power efficiency of 31.3 lm W–1, and a low efficiency roll-off with a high eff...