Fundamental guidelines for the active control of the molecular orientation of heteroleptic iridium complexes enabled by carbazole-based host materials

Abstract

Realizing a perfectly horizontal orientation of the transition dipole moment (TDM) in an emitter molecule is one of the most promising approaches used to boost the efficiency of organic light-emitting devices (OLEDs). However, fundamental guidelines for horizontal orientation are still under discussion, and active control of the molecular orientation by a host material is rarely realized. In this study, we systematically investigated the structure–molecule orientation relationships of representative heteroleptic iridium complexes (HICs) in carbazole-based host materials to formulate guidelines for active control of the TDM. The horizontal orientation ratio (Θ) of the TDM is strongly correlated with (i) the glass transition temperature (T_g) of the host material and (ii) the surface area (S_ML) of the aromatic main ligand in the HICs. Furthermore, compared with emitters with the same S_ML values, the Θ values tended to increase when the contact area with the underlying layer increased. These results suggest that (i) the strength of the intermolecular interaction between the emitter and host material and (ii) the contact area of the main ligand in the emitter are crucial factors in the control of TDM in HICs. Finally, we successfully fabricated a yellow phosphorescent OLED with an external quantum efficiency of 29.0%.