Polaron delocalization dependence of the conductivity and the seebeck coefficient in doped conjugated polymers

Abstract

Conjugated polymers are promising materials for thermoelectrics as they offer good performance at near ambient temperatures. Current focus in polymer thermoelectric research mainly targets higher power factor (a product of conductivity and square of Seebeck coefficient) through improving charge mobility. This is accomplished via structural modification in conjugated polymers, different processing techniques and dopants. As a result, structure-charge transport relationship in conjugated polymers is generally well-established. In contrast, the relationship between structure and Seebeck coefficient is poorly understood due to its complex nature. A theoretical framework by David Emin (Phys. Rev. B, 1999, 59, 6205) suggests that Seebeck coefficient can be enhanced via carrier-induced vibrational softening, whose magnitude is governed by the size of the polaron. In this work, we seek to unravel this relationship in conjugated polymers using a series of highly identical pro-quinoid polymers. These polymers are ideal to test this framework experimentally, as the quinoid character and polaron delocalization in these polymers can be well controlled even by small atomic differences (<10 atm. % per repeating unit). By increasing polaron delocalization, i.e. polaron size, we demonstrate that both conductivity and the Seebeck coefficient (and hence power factor) can be increased simultaneously, and the latter is due to the increase in the polaron’s vibrational entropy.