Photoemission study of interfaces in organic light-emitting diodes

The importance of the interfacial properties in organic light-emitting devi ces (OLEDs) is well recognized. We have investigated the interface formatio n between a metal, namely Al or Ca, and tris-(8-hydroxyquinoline) aluminum (Alq(3)) using X-ray and ultraviolet photoemission spectroscopy (XPS and UP S). In the case of Al/Alq(3), the metal was found to react preferentially w ith the quinolate oxygen as soon as it was deposited onto Alq(3). UPS spect ra show a quick disappearance of the Alq(3) features as early as 1 Angstrom of Al deposition, and also suggest the formation of a rather poorly define d gap state induced by AI. On the other hand, in the case of Ca/ Alq(3), th e interface is characterized by a staged interface reaction: for low Ca cov erages (< 4 Angstrom of Ca), electron transfer from the Ca to the nitrogen of the pyridine side of the quinolate ligand occurs. At higher coverages, t he Ca reacts with the phenoxide oxygen resulting in the decomposition of th e Alq(3) molecule.