Boosting the Seebeck Coefficient for Organic Coordination Polymers: Role of Doping-Induced Polaron Band Formation.

Organic polymers are becoming emerging thermoelectric materials. Tremendous progress has been achieved for p-type doping, but efficient n-type organic materials are still rare. By investigating potassium-doped n-type poly(nickel-ethylenetetrathiolate) using density functional theory coupled with Boltzmann transport equation, we find that (i) formation of the electron polaron band (EPB) split from the conduction band (CB) dominates electron transport; (ii) at low doping concentration, the upper CB gets involved in transport in addition to the EPB as the temperature rises, leading to a highly elevated Seebeck coefficient and power factor; and (iii) at even higher temperature, because the CB starts to dominate, the Seebeck coefficient levels off and then decreases with temperature. Such an "exotic" nonmonotonic temperature effect has been found in experiment but has never been explained. We find that such behavior is primarily due to a polaron effect. A doping-induced polaron band can be employed to boost the Seebeck coefficient, making the organic coordination polymer a peculiar n-type thermoelectric material.