Anisotropy of charge transport and thermoelectric properties in oriented conducting polymer films prepared by high temperature rubbing*

A general method is proposed to produce oriented and highly crystalline conducting polymer layers by combining the controlled orientation/crystallization of polymer films by high-temperature rubbing with a soft-doping method. Doping highly aligned films of regioregular poly(3-alkylthiophene)s with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) yields highly oriented conducting polymer films. These doped layers display polarized UV-Visible-NIR absorption, anisotropy in charge transport and thermoelectric properties. Grazing Incidence X-Ray diffraction, Transmission Electron Microscopy and polarized UV-Vis-NIR spectroscopy help understand and clarify the structure of the films and the doping mechanism. The ordering of dopant molecules in the layers of alkyl side chains depends closely on the length and packing (interdigitated versus non-interdigitated) of the alkyl side chains. The high orientation results in anisotropic charge conductivity σ and thermoelectric properties that are both enhanced in the direction of the polymer chains (σ=22±5 S/cm and S=-60±2 μV/K). The method of fabrication of such highly oriented conducting polymer films is versatile and is applicable to a large palette of semi-conducting polymers.