Structural effect of phenylcarbazole-based molecules on the exciplex-forming co-host system to achieve highly efficient phosphorescent OLEDs with low efficiency roll-off†
In this work, five phenylcarbazole-based molecules (PhCzs), i.e., PhCzp-Me, PhCzm-Me, PhCNCzp-Me, PhCNCzp-MeCzPh, and PhCNCzm-MeCzPh, were synthesized by linking two phenylcarbazole moieties with m-xylene or p-xylene as the π-linker. Density functional theory calculations showed that the dihedral angles of the molecules using p-xylene as the π-linker were in the range of 54.8°–55.7°, which are much smaller than those of the molecules using m-xylene as the π-linker (86.9°–91.4°). These molecules can form an exciplex with B3PyMPM, which is suitable for developing efficient OLEDs based on the co-host system. In this study, the OLED fabricated with the optimal exciplex host of PhCNCzm-MeCzPh/B3PyMPM doped with a phosphorescent emitter of Ir(ppy)2(acac) exhibited an external quantum efficiency (EQE) of 31.5% with the main emission peak at 524 nm corresponding to the CIE coordinates of (0.316, 0.641). This high EQE could be attributed to the higher triplet energy of the exciplex caused by the increase in the intermolecular distance between the donor and the acceptor in the exciplex when sterically hindered m-xylene was applied as the π-linker of the PhCzs. Efficient charge transfer from the co-host to the dopant in the emitting layer was identified via the analysis of the photophysical and electrical properties of the OLED devices, which explains the high performance of the devices.