Isomer dependent molecular packing and carrier mobility of N-phenylcarbazole–phenanthro[9,10-d]imidazole based materials as hosts for efficient electrophosphorescence devices
Two isomers of N-phenylcarbazole substituted phenanthro[9,10-d]imidazole, PI22PPCz and PI33PPCz, have been synthesized in this study. The target molecules provide perfect models for studying the effects of different molecular structures and molecular packing on the performance of host materials for organic light-emitting diodes (OLEDs). The ortho-substituted PI22PPCz forms extremely twisted molecular structure with intramolecular π…π interactions and intramolecular charge transfer characteristic. For meta-substituted PI33PPCz, the molecules aggregate into solid state based on intermolecular π…π interactions and its electron donor and acceptor moieties separately assemble into different columns as in the crystal, which is beneficial to enhance hole and electron mobility. Therefore, PI33PPCz is a better host material for OLEDs compared with PI22PPCz. The green phosphorescent OLED using PI33PPCz as host achieves very high brightness and extremely low efficiency roll-off, and its external quantum efficiency (EQE) can still be maintained above 22% when the brightness is 5000 cd m-2. Moreover, the red phosphorescent OLED using PI33PPCz as host also shows the maximum EQE of over 20% with enhanced performance.