Design of Heteroleptic Ir Complexes with Horizontal Emitting Dipoles for Highly Efficient Organic Light-Emitting Diodes with an External Quantum Efficiency of 38%

Orientation of emitting dipoles is one of the most important material properties influencing the efficiency of organic light-emitting diodes (OLEDs). Recently, even globular-shaped Ir complexes, especially heteroleptic Ir complexes, have been reported to have horizontal emitting dipole orientation (EDO) to improve the external quantum efficiency (EQE) over 30%. Still the relationship between molecular structure of Ir complexes and their EDO has not been fully understood yet. Here, we report that substituents at the para-position of the pyridine in the main ligands of Ir complexes play a pivotal role inducing the orientation of heteroleptic Ir complexes. Substitution of aliphatic and aromatic functional moieties at the position leads to high horizontal emitting dipole orientation with the horizontal dipole ratio up to 86.5% to realize unprecedentedly high-efficiency yellow and green OLEDs, with EQEs of 38% and 36%, respectively. Elongated and planar substituents with high electrostatic potential enlarge the interacting surface region between Ir complex and host molecules, resulting in stacking Ir complexes parallel to the film surface.