Effect of dendrimer generation and surface groups on the optoelectronic properties of green emitting bis-tridentate iridium(iii) complexes designed for OLEDs

Abstract

A series of solution-processable green emitting dendronised bis-tridentate iridium(III) complexes composed of [2,6-diimidazolylidene]benzene and the dianionic ligand, 2-[3-(trifluoromethyl)-1H-pyrazol-5-yl]-6-[4-(trifluoromethyl)phenyl]pyridine, have been synthesised to investigate the role of surface groups (t-butyl versus 2-ethylhexyloxy) and dendron generation (first versus second) on the photoluminescent (PL) and electroluminescent (EL) properties of the dendrimers. The dendrimers exhibited similar solution photoluminescent quantum yields (PLQYs) in toluene (60–70%) independent of the surface groups and dendrimer generation. In contrast, the PLQY values of the second-generation dendrimers were significantly higher than those with the first-generation dendrons in the neat film due to the higher generation dendrons providing more effective encapsulation of the emissive cores. When the dendrimers were blended at a concentration of 2 mol% in tris[4-carbazoyl-9-ylphenyl]amine (TCTA) the PLQYs increased but were still below the values measured in solution suggesting that the dendrimers were not evenly distributed throughout the host. Emission from the guest and host in the 2 mol% blends provided evidence for uneven distribution of the guest and inefficient energy transfer from the host to the dendrimers. The difference in encapsulation of the cores by the dendrons also affected charge transport in the neat films. In the case of the second-generation dendrimer with 2-ethylhexyloxy groups, the hole mobility was <10⁻⁸ cm² V⁻¹ s⁻¹, which was an order of magnitude lower than for the less sterically encumbered dendrimers. Simple two-layer organic light-emitting diodes (OLEDs) with a solution-processed light-emitting layer were prepared. The best of the host-free devices exhibited a maximum external quantum efficiency (EQE) of 13.4%. The devices with the dendrimers blended with TCTA had more uniform EQEs of 12.2% to 13.5%, which are amongst the highest reported for solution-processed OLEDs containing a bis-tridentate iridium(III) complex emitter. Finally, it was found that devices containing the dendrimer with the t-butyl surface groups had greater stability.