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

Abstract

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 μW mK−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. A process for stretching twisted polymer chains into ordered fibers can benefit production of low-cost materials that harvest heat losses. Most thermoelectric energy converters are made from inorganic semiconductors. Conductive polymers, however, are an attractive alternative because of their simple manufacturing requirements and intrinsic thermal insulating properties. Lidong Chen from the Shanghai Institute of Ceramics and co-workers have developed a way to reduce the defects that often plague plastic thermoelectrics with a dissolvable organic template. The team co-crystallized the conductive polymer poly(3-hexylthiophene) with small, trichlorobenzene-based molecules that solidify into needle-like structures under a temperature gradient. The aromatic stacking forces within the trichlorobenzene crystals lock the polymer chains into elongated positions that persist after template removal. Experiments on the resulting poly(3-hexylthiophene) filaments revealed near-record thermoelectric conversion efficiencies for this material. Herein, we fabricated pure poly(3-hexylthiophene) (P3HT) film with highly oriented morphology using a small-molecule 1,3,5-trichlorobenzene (TCB) as the template for polymer epitaxy with a temperature gradient crystallization process. The strong π–π conjugated interactions as well as the very close matching between the repeat distance of the thiophene units in P3HT and the repeat distance of TCB molecules have successfully induced the epitaxy process, allowing the P3HT polymer chains to lock onto the lattice of TCB, forming highly ordered P3HT chains in the direction parallel to the fiber axis. As a result, the electrical conductivity in the direction parallel to the fiber axis was significantly improved. The maximum thermoelectric power factor and ZT value at 365 K reached 62.4 μW mK-2 and 0.1 in the parallel direction of the TCB-treated P3HT film.