High-efficiency solution processable electrophosphorescent iridium complexes bearing polyphenylphenyl dendron ligands

We report the synthesis and electrophosphorescent behavior of a series of novel iridium complex materials (Complexes A-F), which are composed of ligands bearing polyphenylphenyl dendron groups and acetylacetonate. Yellow to saturated red organic light-emitting diodes (OLEDs) based on these newly developed Ir complexes were fabricated through solution process by doping the complex materials into polyvinyl carbazole (PVK)/2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) matrices. The emission wavelengths of the materials could be effectively tuned from 549 nm to 640 nm by changing the conjugation of the ligands either through incorporating additional aromatic segment (e. g. phenyl or fluorenyl group) onto the basic dendron ligand or fusing two of the phenyl rings on the polyphenylphenyl dendron group. High performance devices with the configuration of ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) (50 nm)/PVK:PBD (40%):Ir complex (6%) (70 nm)/2, 9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (12 nm)/Alq(3) (20 nm)/Mg:Ag (150 nm) have been demonstrated. For example, when Complex B was used as the emissive layer, maximum current efficiency of 34.0 cd/A and external quantum efficiency of 10.3% have been achieved. When 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBI) was used as the block layer, the efficiencies can be further improved to 46.3 cd/A and 13.9%, respectively. These solution processed OLED devices demonstrated quite stable EL efficiencies over a large range of current density, which indicated that triplet-triplet annihilation in electrophosphorescence could be effectively suppressed by incorporation of the polyphenylphenyl dendron structure into iridium complexes. (C) 2008 Elsevier B.V. All rights reserved.