Increasing the Thermoelectric Power Factor of a Semiconducting Polymer by Doping from the Vapor Phase

We demonstrate how processing methods affect the thermoelectric properties of thin films of a high mobility semiconducting polymer, PBTTT. Two doping methods were compared: vapor deposition of (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTS) or immersion in a solvent containing 4-ethylbenzenesulfonic acid (EBSA). Thermally annealed, thin films doped by FTS deposited from vapor yield a high Seebeck coefficient (α) at high electronic conductivity (σ) and, in turn, a large power factor (PF = α²σ) of ∼100 μW m^-1 K^-2. The FTS-doped films yield α values that are a factor of 2 higher than the EBSA-doped films at comparable high value of σ. A detailed analysis of X-ray scattering experiments indicates that perturbations in the local structure from either dopant are not significant enough to account for the difference in α. Therefore, we postulate that an increase in α arises from the entropic vibrational component of α or changes in scattering of carriers in disordered regions in the film.