Samsung Advanced Institute of Technology

The light-emitting dipole orientation (EDO) of a phosphorescent emitter is a key to improving the external quantum efficiency (EQE) of organic light-emitting diodes (OLEDs) without structural modification of the device since the optical loss due to the microcavity structure is reduced. In this work, four homoleptic Ir complexes as a phosphorescent emitter were systematically designed based on the molecular structure of tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) to control the EDO. Trimethylsilane, methyl, 2-methylpropyl, and cyclopentylmethyl group substituted to pyridine ring of the ligand contributed to the improvement of the EDO from 76.5％ for Ir(ppy)3 to 87.5％. A linear relationship between the EDO and the aspect ratio (geometric anisotropy factor) was founded, implying the importance of the effective area for the non-bonding force between host and dopant molecules. Also, it was investigated that the EDO enhancement mainly originated from the vertical alignment of the C3 axis of molecule in the substrate axis rather than the change in the direction of the transition dipole alignment in the molecular axis. The optical simulation revealed that the outcoupling efficiency of phosphorescent OLEDs adopting new dopants reaches 38.4％. The green OLEDs exhibiting 28.3％ of EQE, 103.2 cd A-1 of current efficiency, and 98.2 lm W-1 were demonstrated, which was understood to have little electrical loss.