Highly efficient and concentration-insensitive OLEDs based on alkyl sterically modified red homoleptic phenylphthalazine iridium complexes

Four red homoleptic iridium (III) complexes (Ir1–Ir4) with steric phenylphthalazine ligands have been synthesized. Single X-ray structural analysis shows that the steric hindrance groups can prevent molecular aggregation. All complexes exhibited red emission with peaks between 614 and 629 nm in CH2Cl2 and photoluminescence quantum yields (QYs) of 44%–86% in doped films. They were concentration-insensitive, their QYs in neat powder and air atmosphere can reach a maximum of 15%. These complexes exhibited excellent thermal stability with Td higher than 416 °C. The OLEDs based on these complexes showed good performances at 10 wt% high doping concentration. Especially, the device based on Ir1 gave a peak current efficiency and external quantum efficiency (EQE) of 25.0 cd A−1 and 24.2% with mild efficiency roll-off, which is the most efficient OLED based on phthalazine iridium complexes. The maximum EQEs of non-doped devices are between 4.0% and 5.6%, which are considerably high, compared with the reported efficiency of iridium complexes based red-emitting non-doped devices. These studies show that increasing the density of the rigid steric groups of iridium complexes is very effective to achieve high efficiency and low concentration sensitive red phosphorescent devices.