New Mixed-C^N Ligand Tris-Cyclometalated IrIII Complexes for Highly-Efficient Green Organic Light-Emitting Diodes with Low Efficiency Roll-Off

Abstract

This article reports new tris-cyclometalated IrIII complexes containing mixed-C^N ligands, (C^N1)2Ir(C^N2), which are a rarely explored class of complexes. Two mixed 2-phenylpyridine (ppy)/4-methyl-2,5-diphenylpyridine (mdp) complexes (ppy)2Ir(mdp) 3 and (ppy)Ir(mdp)2 4 have been synthesized and their optoelectronic properties have been systematically characterized. Photoluminescence quantum yield values are 0.76–0.85; excited state lifetimes are 0.07–0.11 µs. Phosphorescent organic light emitting devices (PhOLEDs) based on these emissive complexes in a very simple device architecture reproducibly showed slightly higher external quantum efficiency (EQE) and power efficiency (PE) values compared with devices using the analogous conventional homoleptic Ir complexes Ir(ppy)3 and Ir(mdp)3, and the heteroleptic complexes (ppy)2Ir(acac) and (mdp)2Ir(acac). The new complexes (ppy)2Ir(mdp) 3 and (ppy)Ir(mdp)2 4 give PhOLEDs with EQEmax and PEmax values of 26.7 and 26.0 %/94.7 and 94.1 lm W–1, respectively, and very high values are retained at the practical luminances of 100 and 1000 cd m–2. The values for (ppy)2Ir(mdp) are 26.3 % and 73.6 lm W–1 (at 100 cd m–2) and 24.6 % and 53.5 lm W–1 (at 1000 cd m–2). These efficiencies are comparable with the highest reported values of PhOLEDs using conventional IrIII emitters in more complicated device structures. Mixed C^N ligand systems are, therefore, a viable strategy for developing new efficient phosphorescent emitters.