ELECTRONIC TRANSPORT AND OPTICAL-PROPERTIES IN CONDUCTING POLYMERS - POLYALKYLTHIOPHENES AND POLYPHENYLENEVINYLENE - PREFACE

Electronic transport and optical absorption properties in poly(3-alkyl thiophenes) (PAT) and polyphenylenevinylene (PPV) have been investigat ed, in both pristine (semiconducting) and fully doped (metallic) polym ers, PATs with different side chain lengths were used, In PATs optical absorption at room temperature and the thermochromic effect, in which the side chains twist the planar main chain causing a blue shift in t he absorption, were studied in Langmuir-Blodgett thin films. The trans ition temperature in these films was found to be lower than in ordinar y spin coated films, As the side chain length increases the transition temperature decreases. Varying the concentration of the side chains r esults in differing magnitude of the thermochromic blue shift. This la tter effect was modeled using semi-empirical band structure calculatio ns, The pressure dependence of the conductivity in PATs, with doping l evel varying from the pristine state to the fully doped state tvas stu died at hydrostatic pressures up to 5 kbar. The relative magnitude of the pressure dependence depends strongly on the doping level and also on the side chain length. These effects are described with common hopp ing models for disordered materials. Using pristine PAT as the active semiconductor in a field effect transistor the mobility was measured u nder pressure, showing that the increase in conductivity when pressure is applied can be attributed to an increase in mobility, in accord wi th the hopping character of the conduction. Conductivity was measured in H2SO4-doped stretch-oriented PPV down to 1.3 K. The high conductivi ty (10(4) S/cm), weak temperature dependence and the appearance of a p ositive temperature coefficient of resistivity below 20 K show this ma terial to be the most metallic conducting polymer so far demonstrated. Magnetoconductance (MC) measurements at low temperatures exhibit both positive and negative MC. These observations are described as an inte rplay between weak localization and electron-electron interaction in a disordered metal. The very strong anisotropy in the MC, depending on the direction of the magnetic field with respect to the polymer chain orientation, is demonstrated to be due to weak localization effects.