A rational design of carbazole-based host materials with suitable spacer group towards highly-efficient blue phosphorescence

Two novel carbazole derivatives, 3,3'-(2,6-dimethyl-1,4-phenylene)bis(9-phenyl-9H-carbazole) (DMCz) and 3,3'-(2,3,5,6-tetramethyl-1,4-phenylene)bis(9-phenyl-9H-carbazole) (TMCz), are designed and synthesized as host materials for blue phosphorescent organic light emitting diodes (PHOLEDs). Both the materials possess sufficiently high glass transition temperatures and triplet energies for the blue dopant Ir-complex iridium(m) bis(4,6-(difluorophenyl)-pyridinato-N,C)picolinate (Firpic, 2.65 eV). The device using DMCz as the host material showed a better performance than that of TMCz with a maximum power efficiency of 36.0 +/- 0.7 lm per W, a maximum current efficiency of 43.7 +/- 0.6 cd per A and a maximum external quantum efficiency (EQE) of 18.5 +/- 0.1%.