Anisotropic conductivity–temperature characteristic of solution-cast poly(3-hexylthiophene) films

For the purpose of developing a positive temperature coefficient (PTC) device, thin films of poly(3-hexylthiophene) (P3HT) were prepared by solution-cast from chloroform. In the present work, the P3HTs of high molecular weight (Mw) with various regioregularities were employed, and the structure anisotropy and the temperature dependence of conductivities in directions parallel (σ∥) and perpendicular (σ⊥) to the film surface were investigated. For highly orientated P3HT films, the temperature dependence of σ⊥ was found to reflect an amorphous character of charge hopping along this direction. However, the σ∥ decreased greatly above 50 °C, which was attributed to a decrease in the in-plane π-stacking caused both by the melting of crystallized side chains and the enhanced side-chain disturbance with increasing temperature. Different charge transport mechanisms were proposed to explain the anisotropic conductivity–temperature characteristics observed in the two directions. The high conductivity in the parallel direction was deteriorated significantly by thermal recycles, probably due to a reduction of the orientation degree of P3HT crystallites in the bulk film, rather than a change of conjugated length of polymer backbone. A reversible PTC effect was observed for the P3HT film with a high Mw and medium regioregularity in the perpendicular direction, which suggested that the high Mw was more important than the high regioregularity to design the thin film PTC devices based on the soluble P3HT films.