(Invited) Thermoelectric Parameters in Blends of Polymers with Slightly Offset Carrier Energies

Abstract

The mechanical flexibility, low temperature processing, potential printability, capability of blending to form composites, and use of common elements are attractive features for the use of polymers in thermoelectrics. The subject of this presentation is the use of polymers blended with dopants and polymer additives to tune the charge carrier density and Fermi energy-transport energy difference while maintaining charge carrier mobility. Two thiophene polymers, poly(bisdodecylquaterthiophene) and poly(bisdodecyl thioquaterthiophene) (PQT12 and PQTS12, respectively), were the principal polymers used. Cyclic voltammetry and current-voltage measurements indicated that the introduction of sulfurs into the side chains induces traps in films containing PQTS12. Doping the polymer with sulfur in side chains (PQTS12) with the strong oxidant nitrosonium tetrafluroborate (NOBF4), we obtained an especially high conductivity up to 350 S cm-1. Furthermore, the high conductivity is stable in air without extrinsic ion contributions that are associated with the commonly used conductive polymer poly(ethylenedioxythiophene) (PEDOT:PSS). The thermoelectric power factor compared favorably with prior reports for p-type polymers that were made by the alternative process of immersion of polymer films into dopant solutions, and fit the established models for thermoelectric polymers. Additional data obtained from thiophene copolymers containing the ethylenedioxithiophene and thieno[3,2-b]thiophene subunits supported these conclusions. Blend microstructure, assessed using grazing incidence X-ray scattering, and mobility evaluated in field-effect transistors, was not adversely affected by the blending. Experiments, calculations and simulations are consistent with the idea that blending and doping polymers with closely-spaced energy levels and compatible morphologies to promote carrier mobility favor increased power factors relative to the individual polymers.