Solution-Processible Multi-component Cyclometalated Iridium Phosphors for High-Efficiency Orange-Emitting OLEDs and Their Potential Use as White Light Sources

• Published in: Advanced functional materials Vol. 17; no. 15; pp. 2925 - 2936
• Main Authors: Ho, C.-L., Wong, W.-Y., Zhou, G.-J., Yao, B., Xie, Z., Wang, L.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 15.10.2007; WILEY‐VCH Verlag
• Subjects: Carbazoles; Iridium complexes; Light-emitting diodes; Light‐emitting diodes, organic (OLEDs); organic (OLEDs); Phosphorescence
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200601205

The synthesis and photophysical studies of several multifunctional phosphorescent iridium(III) cyclometalated complexes consisting of the hole‐transporting carbazole and fluorene‐based 2‐phenylpyridine moieties are reported. All of them are isolated as thermally and morphological stable amorphous solids. Extension of the π‐conjugation through incorporation of electron‐pushing carbazole units to the fluorene fragment leads to bathochromic shifts in the emission profile, increases the highest occupied molecular orbital levels and improves the charge balance in the resulting complexes because of the propensity of the carbazole unit to facilitate hole transport. These iridium‐based triplet emitters give a strong orange phosphorescence light at room temperature with relatively short lifetimes in the solution phase. The photo‐ and electroluminescence properties of these phosphorescent carbazolylfluorene‐functionalized metalated complexes have been studied in terms of the coordinating position of carbazole to the fluorene unit. Organic light‐emitting diodes (OLEDs) using these complexes as the solution‐processed emissive layers have been fabricated which show very high efficiencies even without the need for the typical hole‐transporting layer. These orange‐emitting devices can produce a maximum current efficiency of ∼ 30 cd A–1 corresponding to an external quantum efficiency of ∼ 10 % ph/el (photons per electron) and a power efficiency of ∼ 14 lm W–1. The homoleptic iridium phosphors generally outperform the heteroleptic counterparts in device performance. The potential of exploiting these orange phosphor dyes in the realization of white OLEDs is also discussed. New homoleptic and heteroleptic cyclometalated iridium(III) complexes of (9,9‐diethyl‐7‐pyridinylfluoren‐2‐yl)‐9‐carbazole and ‐3‐(9‐phenylcarbazole) (see Figure) are shown to be highly efficient orange‐emitting electrophosphors. The combined features of improved hole transport and short triplet state lifetimes render the resulting organic light‐emitting diodes to achieve efficiencies as high as 10 % photons/electron and 30 cd A–1. Utilization of these orange phosphors in the realization of white light sources is also proposed.