Engineering the Substitution Position of Diphenylphosphine Oxide at Carbazole for Thermal Stability and High External Quantum Efficiency Above 30% in Blue Phosphorescent Organic Light‐Emitting Diodes

A carbazole derivative substituted with two diphenylphosphine oxide groups at asymmetric positions of carbazole is synthesized and the substitution position is correlated with the photophysical properties and device performances of blue phosphorescent organic light‐emitting diodes. The carbazole type host with substituents at 2‐ and 5‐ positions of carbazole shows the merits of low driving voltage of 2‐position substitution, and high thermal stability and high quantum efficiency of 5‐ position substitution. Therefore, the carbazole type host exhibits excellent thermal and morphological stability up to 140 °C and record high quantum efficiency of 31.4% and power efficiency of 53.1 lm W^‐1 without any outcoupling enhancement and p‐ or n‐doped charge transport layer in blue phosphorescent organic light‐emitting diodes.