Thermoelectric Properties of Semiconducting Polymers

An emerging application of semiconducting polymers is as thermoelectric materials. The performance of thermoelectrics is related to their electrical conductivity, thermopower and thermal conductivity. These properties all rely on careful control of the carrier concentration by electrical doping. In many polymers, an empirical power-law relationship is found between the thermopower and electrical conductivity. We will discuss our efforts to model the connection between the electrical conductivity and thermopower of semiconducting polymers using poly(3-hexylthiophene) and a thienothiophene-based polymer, PBTTT. We find that electrical conductivity of doped semiconducting polymers is a strong function of morphology, whereas the thermopower is more weakly dependent on structural order. Temperature-dependent conductivity and thermopower measurements are in reasonable agreement with a recently proposed model that uses a power-law conductivity function to describe their behavior. This model also suggests a power law form for the electronic density of states of doped semiconducting polymers near the Fermi level. The implications of this behavior for improvement of the thermoelectric properties of polymers will be discussed.