Effects of structural order in the pristine state on the thermoelectric power-factor of doped PBTTT films

The thermoelectric property of doped films of a thermotropic liquid-crystalline polythiophene poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) PBTTT-C14 was studied for the first time. Drop-casted films of PBTTT-C14, annealed or not, were doped by nitrosonium phosphate NOPF6 and their thermoelectric performance was compared. Power factor of the doped films was increased by a factor of three at room temperature when pristine films were thermally annealed. Both the electrical conductivity and the Seebeck coefficient of doped films benefited from the annealing process. Films with and without thermal annealing were characterized by ultraviolet–visible near-infrared (UV–vis NIR) spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) both before and after being doped. Improvement in structure order caused by annealing was verified in pristine films. Furthermore, the ordered structure in annealed films was largely retained after the doping treatment. The higher structure order in pristine films with annealing and the preservation of structure feature in the annealed films throughout the doping process were suggested to lead to higher bulk mobility which accounted for the enhanced power factor.