We have studied the interface formation of a vinylene phenylene oligomer with a Ca substrate using photoemission spectroscopy. The evolution of core and valence spectra during the deposition of the oligomer on Ca has indicated the molecular energy level bending. The total energy level bending was 0.5 eV and the thickness of the level bending region was about 100 Å. We propose an energy level diagram of the oligomer-Ca interface based on the information obtained from the photoemission spectra.

Metal/polymer interfaces play an important role in polymeric light-emitting diodes. Recent transport measurement studies by Parker promote a rigid band model with triangular barrier tunneling at the metal/polymer interface. This finding, however, stands in contradiction to the band bending and Schottky barrier formation observed using surface analytical techniques. We found that this apparent contradiction can be nicely reconciled if a modification of the tunneling model by band bending is included in the interpretation of the transport data. Band bending modified tunneling gives a clear physical picture of the tunneling process across the metal/polymer interface and is important for low field tunneling processes.

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We report on our recent x-ray photoemission spectroscopy investigations of the interface formation of Al with poly(p-phenylene vinylene) (PPV) prepared under various conditions. We have found that during deposition Al reacts with residual hydroxyl groups in the polymer. In addition, we have found that Schottky barrier formation and the associated band bending depend strongly on surface preparation. Samples converted in situ, containing 5% surface oxygen, show band bending that depends on the thickness of the Al overlayer, with effects arising after as little as 1 Å Al. By contrast, a sample converted ex situ, with 10% surface oxygen, is insensitive to aluminum deposition. In view of the results obtained, we feel that surface impurities and adsorbed species may delay Schottky barrier formation by acting as a buffer layer which prevents the PPV substrate from interacting with the growing layer of Al. In the in situ samples where band bending occurs, we find that it takes place after the formation of metallic Al.

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