Interface formation between Ca and poly(p-phenylene vinylene) derivatives having electron-withdrawing 1,3,4-oxadiazole or electron-donating alkoxy pendants

We have studied the interface formation between Ca and poly(p-phenylene vinylene) derivatives having electron deficient 1,3,4-oxadiazole group (poly[2-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenyl}-5-(2′- ethylhexyloxy)-1,4-phenylenevinylene]) (P-1) or electron rich alkoxy groups (MEh-PPV) (P-2) on the polymer backbone during metal deposition. In situ X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) technique under ultra high vacuum condition were employed in this investigation. The changes of core-level XPS spectra imply that band bending and charge accumulation occur both for P-1 and P-2 during Ca deposition due to the strong charge transfer from Ca metal to the polymers. The NEXAFS spectral study suggests that charge transfer at the Ca/P-2 interface induces the formation of intragap state at the interface, whereas such a gap state is not formed between Ca and P-1. The difference is interpreted based on the contrasting electronic effect by the electron-withdrawing oxadiazole group and electron-donating alkoxy pendant: In P-1 the electrons injected from the Ca electrode tend to reside on the oxadiazole rings rather than in the π-conjugated backbone, whereas in P-2 they tend to remain in the conjugated π-electron system and form new intragap state.