Solution-processed dendrimer-based bis-tridentate iridium(iii) complexes with red, green, and blue phosphorescence for white OLEDs

Abstract

We have synthesised blue (D-Blue), green (D-Green), and red (D-Red) emissive solution-processable dendrimers featuring a bis-tridentate iridium(III) complex core for incorporation into monochrome and white organic light-emitting diodes (WOLEDs). The iridium(III) complex cores have a common dendronised bis[imidazolyl]phenyl ligand and different ancillary ligands to control the emission colour. The dendronised bis[imidazolyl]phenyl ligand has three first-generation biphenyl dendrons with t-butyl surface groups attached, with two of the dendrons projecting over the core of the dendrimer. The three dendrimers, D-Blue, D-Green, and D-Red had similar solution photoluminescence quantum yields (PLQYs) of around 75%. The neat film PLQYs were decreased relative to those in solution due to intermolecular interchromophore interactions. Blending the dendrimers at a concentration of 1 mol% with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) increased the PLQYs but not to the level measured in solution (at best around 50% of the value), which was ascribed to the dendrimers and TCTA not forming a blend film in which the guest was evenly distributed in the host. Neat film monochrome three-layer devices had modest performance with maximum external quantum efficiencies (EQEs) of between 1.6% (red emission) and 5.3% (green emission). When the dendrimers were blended into TCTA at a concentration of 1 mol% the maximum EQEs of the monochrome OLEDs increased to 5.0%, 12%, and 8.3% for blue, green, and red emission, respectively. Host-free WOLEDs composed of a blend of D-Blue (95%), D-Green (2.5%), and D-Red (2.5%) in a single emissive layer had an EQE of 5.4%, which was higher than expected based on the monochrome device performance and was ascribed to D-Blue acting as a host for the emissive D-Green and D-Red. The optimised WOLED with the three dendrimers at a combined concentration of 1.0 mol% in TCTA had an EQE of 9.3%. Both the host-free and host-containing WOLED devices emitted a warm white.