Validity of the Mott formula and the origin of thermopower in π -conjugated semicrystalline polymers

Charge and thermoelectric transport should be closely correlated with each other; however, little is known regarding the origin of thermopower in semicrystalline π-conjugated polymers, particularly those doped with molecular dopants. It is controversial whether the well-established Mott formula is valid for such conducting polymers, which inevitably have a finite structural disorder. We show that a truly metallic regime that can be realized in a highly crystalline domain gives rise to thermopower, which is demonstrated unambiguously by the observation of a linear temperature dependence of the Seebeck coefficient in semicrystalline polythiophene-based conducting polymers. The presence of onset metallicity is also verified comprehensively by the Hall effect and Drude optical response, which indicates that the Mott formula, which is frequently used for degenerated semiconductors and metals, can be applicable to highly crystalline conjugated polymers. This provides insight into the structure-thermoelectric property relationships in semicrystalline conducting polymers.