Thermoelectric Transport Signatures of Carrier Interactions in Polymer Electrochemical Transistors

Electronic phase transitions of polymers in the high carrier density regimes are still not well understood. Early works in the field have elegantly demonstrated that several polymers could be turned from a band-insulator to a metal upon chemical doping. However, little is known about carrier interaction effects that would be expected to arise, for instance, as the metallic state gets doped even more. The closest transport signature to that of a correlated system so far is the Efros-Skhlovskii variable-range hopping temperature dependence of electrical conductivity, reflecting the opening of a Coulomb gap around the Fermi level because of interactions between localised carriers in the insulating regime.



In this study, changes in the electronic phases of polymers are analysed by means of thermoelectric transport signatures. A wide range of carrier densities could be explored in our ionic liquid gated electrochemical transistors. We demonstrate the generality of a band-insulator to metal transition across a range of polymers with significantly different microstructures. At even higher doping levels, possible transport signatures of carrier interactions will be presented and discussed. X-ray photoemission spectra justify that the carrier densities of the samples are in the regime where carrier interactions shall be expected, and complementary ultraviolet photoemission spectroscopy analyses allow changes in the electronic structure to be followed.