Electronic structure and mechanism for conductivity in thiophene oligomers and regioregular polymer

The electronic structure of polythiophene (PTP) and oligothiophenes are investigated using localized electron models combined with quantum chemical calculations, including ab initio and DFT methods and the Su-Schrieffer-Heeger (SSH) model. In PTP an electron hole is found to localize over about five thiophene units, with reorganization energy for ionization, lambda approximate to 0.1 eV. Coupling between thiophene oligomers and polymer chains, referring to the electronic hole state, is calculated using ab initio methods. The quite small coupling to reorganization energy ratio (delta/lambda) explains activated behaviour of the conductivity in some cases. Nuclear tunnelling occurs in the limit of low temperature.