Highly branched phosphorescent dendrimers for efficient solution-processed organic light-emitting diodes

Intermolecular interactions play a crucial role in the performance of organic light-emitting diodes (OLEDs). Here we report the photophysical and electroluminescence properties of a fac-tris(2-phenylpyridyl)iridium(III) cored dendrimer in which highly branched biphenyl dendrons are used to control the intermolecular interactions. The presence of fluorene surface groups improves the solubility and enhances the efficiency of photoluminescence, especially in the solid state. The emission peak of the dendrimer is around 530 nm with a PL quantum yield of 76 % in solution and 25 % in a film. The photophysical properties of this dendrimer are compared with a similar dendrimer with the same structure but without the fluorene surface groups. Dendrimer LEDs (DLEDs) are prepared using each dendrimer as a phosphorescent emitter blended in a 4,4'-bis(N-carbazolyl)biphenyl host. Device performance is improved significantly by the incorporation of an electron-transporting layer of 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene. A peak external quantum efficiency of 10 % (38 Cd A(-1)) for the dendrimer without surface groups and 13 % (49.8 Cd A(-1)) for the dendrimer with fluorene surface groups is achieved in the bilayer LEDs.