Theoretical investigations of the aluminum/polythiophene interface J. Chem. Phys. 97, 9144 (1992); 10.1063/1.463340Chemical and electronic structure of compound semiconductor-metal interfaces Abstract: Electronic structure of semiconductor interfacesWe have investigated the chemical nature and the electronic structure of the interface between a low work function metal, aluminum, and a conjugated polymer semiconductor, polythiophene. We have studied the initial stages of the interface formation by depositing the metal onto the surface of a polymer film. Charge transfer processes between the metal and the polymer are analyzed using core-level x-ray photoelectron spectroscopy (XPS); the evolution upon metallization of the valence electronic levels directly related to the polymer electronic structure is followed with ultraviolet photoelectron spectroscopy (UPS). With these techniques, we investigate the deposition of aluminum on two poly thiophene systems (i) the alkyl-substituted poly-3-octylthiophene and (ii) the a-sexithiophene oligomer. The experimental data are compared to the results of a recent quantum chemical study on model systems consisting of thiophene oligomers (up to sexithiophene) interacting with a few AI atoms. The interaction of polythiophene with AI atoms is found to modify dramatically the structure of the conjugated backbone, as strong carbon-aluminum bonds are formed in the a positions of the thiophene rings. A large charge transfer takes place from the AI atoms to the polymer chain, and the upper rr levels of the polymer are strongly affected. The metallization is contrasted to the doping of conjugated polymers with alkali metals.
The interactions between different low work function metals aluminium, calcium and sodium, and α,ω-diphenyltetradecaheptaene, a model molecule for certain conjugated polymers, have been investigated using both x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum chemical calculations performed within the local spin density (LSD) approximation methodology. The metals are found to interact with the conjugated system in very different ways. Aluminium forms a covalent bond, which strongly modifies the π-electronic structure of the conjugated molecule, while both the sodium and the calcium atoms act as doping agents, inducing new states in the otherwise forbidden bandgap. These new gap states can be viewed as a soliton–antisoliton pair for the Na/DP7 and a bipolaronic-like defect for Ca/DP7.
The ^-electronic structural changes in a polyene molecule containing seven double bonds, a,codiphenyltetradecaheptaene (DP7), have been studied upon gradually doping with sodium, using x-ray and ultraviolet photoelectron spectroscopies. The spectra are interpreted with the help of detailed quantum chemical calculations. Analysis of the evolution of the XPS and UPS spectra as a function of doping with sodium indicates that the extra charges are stored in the form of two charged solitons on the polyene part of the molecules, which results in two new energy levels in the originally forbidden energy gap.
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