The surface species of poly (p-phenylene vinylene) and their effects on metal interface formation

We used X-ray photoemission spectroscopy (XPS) to investigate the surface species of poly (p-phenylene vinylene) (PPV) and its interface formation with Ca and Al. PPV surface compositions varied with sample preparation. For relatively ‘clean’ surfaces with 4–5% O, analysis of the O( Is) peak revealed four types of oxygen species, namely carbonyl (C=0), hydroxyl (C-OH), ether (C-O-C) and the carboxylic groups (HO-C=O). The oxygen groups, excluding ether, reacted with Al or Ca to form the corresponding metal oxides. Chemical interactions between the metals and the phenylene and vinylene units to yield new species were not detected. For sulfur-free surfaces, a C(Is) peak shift of +0.5 eV followed the deposition of 15-30 Å of Ca on PPV. For sulfur-containing surfaces, the C(Is) peak shift was -0.5 eV. We attribute this difference to the interaction of metal atoms with the sulfur impurities. For Al/PPV, a C(Is) peak shift occurred at < 2 Å of Al deposition and reached a constant value of about + 0.4 eV after ∼ 8 Å of Al. Again, the direction of the peak shift depended on the presence of sulfur impurities. We attribute the C(Is) peak shifts to surface band bending and to Schottky-barrier formation. Since surface oxidation of PPV can inhibit band bending, our overall results suggest that the barrier height at the metal/PPV interface is highly sensitive to the surface preparation and relatively insensitive to the work function of the metals. A qualitative band picture is presented to account for the different directions of band bending.