Thermoelectric properties of conducting polymers: The case of poly(3-hexylthiophene)

Conducting polymers have recently been suggested as thermoelectric materials for use in large-area thermogenerators. To help assessing the feasibility of this the electrical conductivity and Seebeck coefficient of a series of heavily doped regioregular poly (3-hexylthiophene) films are measured between 220 and 370 K. p-type chemical doping of up to 34% is accompanied by the introduction of negatively charged counterions, PF6-. The counterions produce a disordered environment within which the p-type electronic carriers move. This disorder diminishes with increasing doping as the effect of the counterions is smoothed out. Concomitantly the thermally activated electrical conductivity rises strongly while its activation energy decreases. On the other hand, the Seebeck coefficient is found to be weakly dependent on temperature and it decreases with increasing doping. When combined, these results indicate that the thermoelectric power factor reaches a broad maximum between 20% and 31% doping. These results are discussed in terms of the thermally activated hopping-type mobility of bipolarons, deduced from the absence of electron spin resonance signal in the heavily doped materials.