Highly efficient phosphorescent OLEDs with host-independent and concentration-insensitive properties based on a bipolar iridium complex

A bipolar iridium complex, (ppy)(2)Ir(dipig), based on the ancillary ligand N,N'-diisopropyl-diisopropylguanidinate (dipig) with well-known cyclometalated ((CN)-N-boolean AND) ligand ortho-(2-pyridyl)phenyl (ppy), is applicable in phosphorescent organic light-emitting diodes (PHOLEDs) as an efficient emitter, using easily available host materials and a simple device fabrication process. The corresponding PHOLEDs are dominated by an efficient direct-exciton-formation mechanism and show very high EL efficiency together with gratifying host-and doping-concentration-independent features. EL efficiency values of more than 93 lm W-1 for power efficiency (eta(p)) and 24% for external quantum efficiency (eta(ext)) accompanied by little efficiency roll-off at high luminance are achieved in the (ppy)(2)Ir(dipig)-based devices by adopting the common materials 4,4'-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB) and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) as the host, with rather random concentration ranges of 8-15 wt% and 15-30 wt%, respectively. To the best of our knowledge, these values are the highest efficiencies ever reported for yellow PHOLEDs, and are even comparable with the highest levels for PHOLEDs in the scientific literature. Moreover, the eta(p) and eta(ext) values of the non-doped device can reach 70 lm W-1 and 18% respectively. They are almost two times higher than those of the most efficient reported PHOLEDs based on a neat emitting layer (EML).