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

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Materials for optical and electronic devices, 2024-02, Vol.12 (8), p.2772-2779
• Main Authors: Arai, Hiroki, Yoshida, Naoto, Sasabe, Hisahiro, Sagae, Yuki, Hoshi, Keigo, Yokoyama, Daisuke, Kido, Junji
• Format: Article
• Language: English
• Subjects: Active control; Carbazoles; Devices; Dipole moments; Emitters; Glass transition temperature; Guidelines; Horizontal orientation; Iridium compounds; Ligands; Molecular structure; Organic light emitting diodes; Orientation relationships; Phosphorescence; Quantum efficiency
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/d3tc04542k

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%. The transition dipole moments of a heteroleptic iridium complex are actively controlled by using cyano-containing carbazole-based hosts. The orientation ratio ( Θ ) is improved from 65% to 78%. The Θ value is correlated with the T g value of host material.