THE CHEMICAL AND ELECTRONIC-STRUCTURE OF THE INTERFACE BETWEEN ALUMINUM AND POLYTHIOPHENE SEMICONDUCTORS

We have investigated the chemical nature and the electronic structure of the interface between a low work function metal, aluminum, and a co njugated polymer semiconductor, polythiophene. We have studied the ini tial stages of the interface formation by depositing the metal onto th e surface of a polymer film. Charge transfer processes between the met al and the polymer are analyzed using core-level x-ray photoelectron s pectroscopy (XPS); the evolution upon metallization of the valence ele ctronic levels directly related to the polymer electronic structure is followed with ultraviolet photoelectron spectroscopy (UPS). With thes e techniques, we investigate the deposition of aluminum on two polythi ophene systems (i) the alkyl-substituted poly-3-octylthiophene and (ii ) the alpha-sexithiophene oligomer. The experimental data are compared to the results of a recent quantum chemical study on model systems co nsisting of thiophene oligomers (up to sexithiophene) interacting with a few Al atoms. The interaction of polythiophene with Al atoms is fou nd to modify dramatically the structure of the conjugated backbone, as strong carbon-aluminum bonds are formed in the a positions of the thi ophene rings. A large charge transfer takes place from the Al atoms to the polymer chain, and the upper pi levels of the polymer are strongl y affected. The metallization is contrasted to the doping of conjugate d polymers with alkali metals.