Characterisation of thin films of organic phosphorescent materials using synchrotron radiation

Synchrotron radiation photoemission and X-ray absorption spectroscopy (NEXAFS) have been used to investigate the electronic structure of evaporated films of the phosphorescent organic iridium complexes iridium tris-(2-(4-totyl)pyridinato-N,C2), iridium bis(2-(4,6-difluorophenyl)pyridinato-N,C2)picolinate, and iridium bis(2-(2′-benzothienyl)pyridinato-N,C3)-(acetylacetonate) and spin coated films of these materials in a polymer host. Resonant photoemission at the Ir N6,7 edge indicates that the Ir 5d states are hybridised with the π orbitals of the organic ligands, in agreement with recent calculations. The nitrogen K-edge NEXAFS shows the difference in the unoccupied orbitals induced by the acetylacetonate group compared to those of the pyridinate ligands. Although the valence bands of the ex situ prepared films are not accessible to photoemission, the Ir 4f core levels remain visible, and demonstrate that the polymer host serves to lower the electron injection barrier in the iridium complexes in comparison to the pure films.