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 surf ace species of poly (p-phenylene vinylene) (PPV) and its interface for mation with Ca and Al. PPV surface compositions varied with sample pre paration. For relatively 'clean' surfaces with 4.5% O, analysis of the O(1s) peak revealed four types of oxygen species, namely carbonyl (C= O), 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 det ected. For sulfur-free surfaces, a C(1s) peak shift of +0.5 eV followe d the deposition of 15-30 Angstrom of Ca on PPV. For sulfur-containing surfaces, the C(1s) peak shift was -0.5 eV, We attribute this differe nce to the interaction of metal atoms with the sulfur impurities, For Al/PPV, a C(1s) peak shift occurred at < 2 Angstrom of Al deposition a nd reached a constant value of about + 0.4 eV after similar to 8 A of Al. Again, the direction of the peak shift depended on the presence of sulfur impurities. We attribute the C(1s) 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 ba rrier height at the metal/PPV interface is highly sensitive to the sur face preparation and relatively insensitive to the work function of th e metals. A qualitative band picture is presented to account for the d ifferent directions of band bending.