Rational Ligand Design of Heteroleptic Iridium (III) Complexes toward Nearly Perfect Horizontal Dipole Orientation for Highly Efficient Red‐Emitting Phosphorescent Organic Light‐Emitting Diodes

Abstract The horizontal orientation of the transition dipole moment of the phosphorescent emitters is understood to be an important factor to enhance the external quantum efficiency (EQE) of organic light‐emitting diodes by improving light out‐coupling in optical microcavity structures. Here, red‐emitting heteroleptic iridium (III) complexes exhibiting an extremely high horizontal ratio of emitting dipole orientation (EDO) and photoluminescence quantum yield (PLQY), as well as longer device operational lifetime, without scarifying any other photophysical properties are reported. The systematic molecular design of main and ancillary ligands in heteroleptic iridium complexes leads to the achievement of both a horizontal EDO of 92% and a PLQY of 98% in the red‐emitting phosphorescent devices along with a shorter exciton decay time of 0.71 µs. Accordingly, the red‐emitting devices show excellent performances of maximum EQE of 32% and low‐efficiency roll‐off with the 1931 Commission Internationale de L′Eclariage coordinates of (0.66, 0.34). Therefore, this approach opens the way for further development of new red‐emitting iridium complexes pushing the device efficiency toward the theoretical limits.