Counterion-engineered ionic iridium(iii) complexes for solution-processed red OLEDs

Abstract

Ir(III) complexes have garnered considerable interest for use in optoelectronic applications due to their remarkable photophysical characteristics, including emission wavelength-tunable characteristics, superior phosphorescence quantum yields, and excellent photostability. The photoluminescence and electroluminescence performances of these complexes can be finely modulated by modifying their ligands and counterions. In particular, the choice of counterions plays a crucial role in influencing solubility, film-forming properties, and charge transport characteristics, which are indispensable for the implementation in OLEDs. In the present study, five ionic Ir(III) complexes (Ir–PF₆, Ir–Cl, Ir–Br, Ir–I, and Ir–C₂₄H₂₀B) were rationally designed and synthesized, using 1-(6-methoxy-2-naphthyl)isoquinoline as the primary ligand and 2,2′-bipyridine-4,4′-diformaldehyde as the auxiliary ligand with different counterions (PF₆⁻, Cl⁻, Br⁻, I⁻, and C₂₄H₂₀B⁻). Five complexes exhibited red and deep-red emission with the maximum emission wavelengths of 663 (Ir–PF₆), 623 (Ir–Cl), 646 (Ir–Br), 664 (Ir–I), and 625 nm (Ir–C₂₄H₂₀B), respectively. In addition, deep-red electroluminescent devices based on Ir(III) complexes (Ir–Cl, Ir–Br and Ir–C₂₄H₂₀B) were prepared via solution-based spin-coating. Among them, the OLED prepared with Ir–Br exhibited the best performance, achieving a maximum brightness of 720 cd m⁻² and an external quantum efficiency (EQE) of 1.01%.