Highly Branched Phosphorescent Dendrimers for Efficient Solution‐Processed Organic Light‐Emitting Diodes

Abstract

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.

A Dendrimer‐Based Electropolymerized Microporous Film: Multifunctional, Reversible, and Highly Sensitive Fluorescent Probe

“…Thermal evaporation requires materials to have highly thermal stability and causes much waste of materials. Solution‐process not only wastes materials but also difficultly controls the morphology of films …”

Structural Mimics of Phenyl Pyridine (ppy) – Substituted, Phosphorescent Cyclometalated Homo and Heteroleptic Iridium(III) Complexes for Organic Light Emitting Diodes – An Overview

“…To control the intermolecular interactions Raghu N. Bera et al ., substituted the fluorene groups on the complex‐83 and explained the electroluminescence properties of the complex‐84 . The solution absorption spectrum of complex‐84 has relatively weak long‐wavelength (380–425 nm) absorptions due to MLCT states, and another stronger shorter wavelength absorption (280–370 nm) due to the ligand π‐π* transition and dendron absorption in a similar manner observed for complex‐83.…”

Novel green-light-emitting hyperbranched polymers with iridium complex as core and 3,6-carbazole-co-2,6-pyridine unit as branch

“…In addition, the branched structure can effectively mask the facial isomer of Ir(ppy) 3 that is, the face-to-face structure of the facial iridium core could be well restrained by the highly branched structures, thus interactions among the cores in the solid state could be avoided. 15 The observed high efficiency above correlated with the employed carbazole-containing ligand framework for the Ir complex, which had been proved to be able to enhance hole transport properties, thus facilitating the electrical excitation in the electrophosphorescent process. [37][38][39][40] Moreover, because of the extremely distorted molecular structure of 3,6-carbazole or 3,6-carbazole-co-2,6-pyridine, the delocalizated p-electron cloud is interrupted in the meta-position linkage.…”