Improved Turn-on Times of Iridium Electroluminescent Devices by Use of Ionic Liquids

We demonstrate an improvement in the turn-on time of electroluminescent devices based on the iridium complex [Ir(ppy) 2 (dtb-bpy)] + (PF 6 - ), where ppy is 2-phenylpyridine and dtb-bpy is 4,4‘-di- tert -butyl-2,2‘-dipyridine, by introduction of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate BMIM + (PF 6 - ). Addition of 0.46 mol of the ionic liquid per mole of Ir complex reduces the turn-on time from 5 h to 40 min. However, the device lifetime is also reduced by a factor of 3 over this range, suggesting a tradeoff between device speed and stability. These results are discussed within the framework of the electrodynamic model of device operation and are found to be consistent with an increase in the ionic conductivity of the [Ir(ppy) 2 (dtb-bpy)] + (PF 6 - ) films upon the addition of ionic liquid.