Photoemission and X-ray absorption spectroscopies of phosphorescent organic iridium complexes

Phosphorescent materials represent an attractive route to high-efficiency organic LEDs, since their maximum theoretical quantum efficiency is 100%, compared to the 25% of fluorescent materials. We have studied the electronic structure of three phosphorescent iridium complexes, the red-emitter Iridium bis (2-(2′-benzothienyl) pyridinato-N, C3′) (acetylacetonate), the blue emitter, Iridium bis(2-(4,6- difluorophenyl) pyridinato-N,C2′) picolinate, and the green emitter Iridium tris (2-(4-totyl)pyridinato N,C2) using electron spectroscopies. The materials were evaporated in-situ onto a clean gold substrate in ulta-high vacuum. The valence band electronic structures were investigated using resonant photoemission, with the unoccupied states determined using X-ray absorption. The data suggests a hybridisation of the Ir 5d states with the π orbitals of the ligand. NEXAFS spectra are shown to be highly sensitive to the local environment.