Structure and thermoelectric properties of electrochemically doped polythiophene thin films: Effect of side chain density

Intermolecular interaction between a conducting polymer and a dopant molecule is a fundamental factor to determine the structure and electronic properties of doped films, whereas clear guidelines of the molecular design to control such interactions are absent. We compared the structure and transport properties of two typical electrochemically doped polythiophenes with different side chain densities: poly(3-hexylthiophene) and poly[bis(3-dodecyl-2-thienyl)-2,2-dithiophene-5,5-diyl] (PQT). X-ray diffraction patterns show that more conspicuous lattice expansion and crystallinity improvement take place after doping PQT, the low side chain density polymer, although the crystallinity of the pristine film is rather low. The crystallinity improvement results in a large increase in the electrical conductivity, reaching 140 S/cm after doping and indicating the importance of side chain engineering to optimize the electronic properties of the doped state. We further suggest the dynamic motion of the dopant molecules with a temperature gradient in the doped PQT film based on thermoelectromotive force measurements, indicating the weakly bound state of the dopant molecules.