Efficiency phosphorescent OLEDs with a low roll-off based on a hetero-triplet iridium complex

In order to explore the triplet-triplet annihilation in cyclometalated iridium complexes, a hetero-triplet iridium complex [Ir(ppy)2pbi] employing two 2-phenylpyridine (Hppy) ligands and one 1,2-diphenyl-1H-benzo[d]-imidazole (Hpbi) ligand with similar triplet energies was synthesized. A device based on Ir(ppy)2pbi exhibited high current efficiency 26.7 cd/A at high doping concentration 12 wt%. More interestingly, the current efficiency was still maintained at 25.8 cd/A when the current density was above 253.2 mA/cm2. The efficiency roll-off was only about 3.4%, much lower than that of traditional cyclometalated iridium complexes. It is proposed that the complicated energy level and unsymmetical ligand in Ir(ppy)2pbi may help to suppress the triplet-triplet annihilation.