Effect of emissive ligand number on the optoelectronic properties of dendronised heteroleptic green emitting iridium(III) complexes

Abstract

We compare first- and second-generation dendrimers with biphenyl-based dendrons and green emissive homoleptic or heteroleptic fac-iridium(III) complex cores. The core complexes had one, two or three 2-phenylpyridyl ligands, which were responsible for the emission of light. The dendronised co-ligand, 5-phenyl-1-methyl-3-n-propyl-1H-1,2,4-triazole, was found not to contribute to the colour of the emission. The first- and second-generation dendrimers that had two emissive 2-phenylpyridyl ligands were found to have high solution photoluminescence quantum yields (PLQYs) of nearly 90 % whereas the homoleptic dendrimers and the one with a single emissive ligand had PLQYs of around 70 %. The PLQY values decreased in the solid-state, with the second-generation dendrimer with a single emissive ligand found to have the highest neat film PLQY of 55 %. This was attributed to the dendronised co-ligand acting as a self-host to reduce the intermolecular interactions that lead to the quenching of the luminescence. Organic light-emitting diodes (OLEDs) composed of neat films of the dendrimers had relatively poor performance, with unbalanced charge transport and a maximum external quantum efficiency (EQE) of 6.5 % for the second-generation dendrimer with a single emissive ligand. OLEDs with an emissive layer composed of a blend of the dendrimers in tris(4-carbazoyl-9-ylphenyl)amine all had similar current density-voltage characteristics, with the EQEs following the observed trend in the solution PLQY values. The OLED with the blend emissive layer containing the first-generation dendrimer with two emissive ligands was found to have an EQE of up to 14.1 %. Thus, both dendrimer generation and the number of emissive ligands can be used to control OLED performance.