Effect of an assistant dopant on the vibrational satellites of a phosphorescent emitter: Application to solution–processed single–layer white organic light–emitting diodes

Vibrational effects are crucial for optimizing the performances of phosphorescent Ir compounds as emitters in OLEDs. But, usually purely electronic spin–orbit coupling, where the coupling remains unchanged by vibrational motion, is taken into account as the sole mechanism for mixing singlets and triplets states, and mechanisms that depend on molecular vibrations are largely overlooked. Here I investigate a doubly doped system with iridium(III)tris(2-(4-tolyl)phenylpyridine [Ir(mppy)3] as assistant dopant, and bis(2–(9,9–dihexylfluorenyl)–1–pyridine)–(acetylacetonate) iridium(III) [Ir((dhfpy)2(acac)] as phosphorescent emitter dopant blended into a mixed phase polyfluorene matrix. The electroluminescence of the fully solution–processed single–layer devices show a well–resolved structured emission of the triplet emitter that can be interpreted in terms of Franck–Condon activity in the presence of the assistant dopant. Changes in the local environment of the emitter brought about by modifying the doping levels lead to different ratios of these vibronic bands, whose balanced combination along with blue fluorescent emission from the polymer host enable white electroluminescence with CIE coordinates (0.37, 0.33) in a simple device structure. Furthermore, the introduction of the assistant dopant boots significantly device performance.