Doped Semiconducting Polymers As Solution-Processable Thermoelectric Materials

Due to inherently low thermal conductivity and tunable electronic properties, semiconducting polymers are very promising solution-processable-materials for thermoelectric applications. Polymeric semiconductors are insulators in its pristine state, but are rendered conductive through chemical doping. Here, we studied the thermoelectric properties of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT-C₁₄) doped either from solution or from the vapor phase. Overall, vapor doped films yield large Seebeck coefficients at high electrical conductivity (Power Factors ~ 100 μWm^-1K^-2). It important to note that the introduction of the chemical dopants can perturb the molecular packing and the morphology relative to the neat film. By leveraging various synchrotron X-ray scattering techniques that probe both the local and long range order, we discovered key structural features that can be correlated to the enhancement in the Seebeck coefficient and electrical conductivity for vapor doped films. Specifically, the introduction of the chemical dopant must limit the structural perturbation at small length scales while maintaining long-range correlation of polymer chains essential to efficient macroscopic charge transport. Lastly, to further elucidate the charge transport mechanism driving the thermoelectric properties, we report on the temperature-dependent measurements of both the Seebeck coefficient and electrical conductivity.