Significant enhancement in the Seebeck coefficient and power factor of thermoelectric polymers by the Soret effect of polyelectrolytes

Abstract

Intrinsically conductive polymers are promising as next-generation thermoelectric (TE) materials. They have high electrical conductivity but a low Seebeck coefficient. On the other hand, polyelectrolytes have a high Seebeck coefficient but very low electrical conductivity. Mixtures of conductive polymers with polyelectrolytes exhibit a low power factor. In this work, a polyelectrolyte, poly(4-styrenesulfonic acid) (PSSH) or poly(sodium 4-styrenesulfonate) (PSSNa), is coated on a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film instead of forming a mixture. The polyelectrolyte can enhance the Seebeck coefficient while not remarkably affecting the electrical conductivity of PEDOT:PSS, and thus significantly enhance the TE properties. At 100% relative humidity (RH), the PEDOT:PSS film coated with PSSH exhibits a power factor of 401 μW m⁻¹ K⁻² with a Seebeck coefficient of 43.5 μV K⁻¹ and an electrical conductivity of 2120 S cm⁻¹. This power factor is higher than that of PEDOT:PSS mixed with PSSH by several orders of magnitude. In addition, these samples provide a stable and continuous output voltage under a temperature gradient. The enhancement is ascribed to the energy filtering of the charge carriers of PEDOT:PSS arising from the Soret effect of the ions in the polyelectrolyte layer.