INTERFACE FORMATION OF METALS AND POLY(P-PHENYLENE VINYLENE) - SURFACE SPECIES AND BAND BENDING

We used X-ray photoemission spectroscopy (XPS) to investigate the surf ace species of poly(p-phenylene vinylene) (PPV) and ifs interface form ation with Ca and Al. PPV surfaces compositions varied with sample pre paration. For relatively ''clean'' surfaces with 4-5% O, analysis of t he O Is peak revealed four types of oxygen species, namely carbonyl (C =O), hydroxyl (C-OH) ether (C-O-C) anti the carboxylic groups (HO-C=O) . The oxygen groups, excluding ether, reacted with Al or Ca to form th e corresponding metal oxides. Chemical interactions between the metals and the phenylene and vinylene units to yield new species were not de tected. For sulfur-free surfaces, a C Is peak shift of +0.5 eV followe d the deposition of 15-30 Angstrom of Cir on PPV. For sulfur-containin g surfaces, the C Is 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 Is peak shift occurred at <2 Angstrom of Al deposition and reached it constant value of about +0.4 eV after similar to 8 Angstro m of Al. Again, the direction of the peak shift depended on the presen ce of sulfur impurities. We attribute the C 1s peak shifts to surface band bending and to Schottky barrier formation. Since surface oxidatio n of PPV can inhibit band-bending, our overall results suggest that th e barrier height at the metal/PPV interface is highly sensitive to the surface preparation and relatively insensitive to the work function o f the metals. The shift seen by XPS in the C Is cove level spectra of: PPV points clearly to charge transfer and Schottky barrier formation a t the interface as a result of met-al deposition. These results imply that the metal/polymer interface is not rigid and that triangular barr ier tunneling fails to take into account the effect of barrier formati on. We propose a band-bending modified tunneling (BBMT) model to expla in charge transfer at the Ca/polymer interface. (C) 1997 by John Wiley & Sons, Ltd.