Photoemission study of interfaces in organic light-emitting diodes

The importance of the interfacial properties in organic light-emitting devices (OLEDs) is well recognized. We have investigated the interface formation between a metal, namely Al or Ca and tris-(8-hydroxyquinoline) aluminum (Alq3) using X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). In the case of Al/Alq3, the metal was found to react preferentially with the quinolate oxygen as soon as it was deposited onto Alq3. UPS spectra show a quick disappearance of the Alq3 features as early as 1 إ of Al deposition, and also suggest the formation of a rather poorly defined gap state induced by Al. On the other hand, in the case of Ca/ Alq3, the interface is characterized by a staged interface reaction: for low Ca coverages (< 4 إ of Ca), electron transfer from the Ca to the nitrogen of the pyridine side of the quinolate ligand occurs. At higher coverages, the Ca reacts with the phenoxide oxygen resulting in the decomposition of the Alq3 molecule.