Highly anisotropic P3HT films with enhanced thermoelectric performance via organic small molecule epitaxy

Conducting polymers are potential candidates for thermoelectric (TE) applications owing to their low thermal conductivity, non-toxicity and low cost. However, the coil conformation and random aggregation of polymer chains usually degrade electrical transport properties, thus deteriorating TE performance. In this work, we fabricated poly(3-hexylthiophene) (P3HT) films with highly oriented morphology using 1,3,5-trichlorobenzene (TCB), an organic small-molecule, as a template for polymer epitaxy under a temperature gradient crystallization process. The resulting P3HT molecules, which were confirmed to be highly anisotropic by a combination of scanning electron microscopy, atomic force microscopy, polarizing microscope, polarized Raman spectroscopy, and two-dimensional-grazing incidence X-ray diffraction (GIXRD) analysis, not only markedly reduced the conjugated defects along the polymer backbone, but also effectively increased the degree of electron delocalization. These combined phenomena produced an efficient, 1D path for carrier movement and therefore resulted in enhanced carrier mobility in the TCB-treated P3HT films. The maximum values of the electrical conductivity and Seebeck coefficient were 320 S cm 1 and 269 μV K 1, respectively. Consequently, the maximum TE power factor and ZT value at 365 K reached 62.4 μWmK 2 and 0.1, respectively, in the parallel direction of the TCB-treated P3HT film. To the best of our knowledge, these are the highest values reported for pure P3HT TE materials. The method of using organic small-molecule epitaxy to generate highly anisotropic polymer films is expected to be valid for many conducting polymers. NPG Asia Materials (2016) 8, e292; doi:10.1038/am.2016.97; published online 22 July 2016