Controlling the thermoelectric properties of polymers: application to PEDOT and polypyrrole

Poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole (PPy) films have been prepared by an electrochemical method in a three electrode cell. The films have been obtained at different oxidation levels regarded as bipolaron, polaron and neutral states by varying the voltage, as is usually done in conjugated heterocyclic polymers. The voltage (-0.2 < V < 1.0 V) has been applied versus a Ag/AgCl reference electrode, producing a variation of one order of magnitude in the electrical conductivity and the Seebeck coefficient of the films. In the voltage range explored, the electrical conductivity increases from 80 to 766 S cm(-1) in PEDOT and from 15 to 160 S cm(-1) in PPy, while the Seebeck coefficient decreases from 37.0 to 9.6 mu V K-1 for PEDOT and from 51.0 to 6.7 mV K-1 for PPy. The thermal conductivity remains unchanged with the oxidation state of the film, k approximate to 0.35 +/- 0.02 W m(-1) K-1 for PEDOT and 0.17 +/- 0.02 W m(-1) K-1 for PPy. A maximum thermoelectric efficiency of 1.4 x 10(-2) for PEDOT and 6.8 x 10(-3) for PPy has been achieved. These changes are related to the doping level of the polymer films and they can be accurately controlled by the applied voltage. In this work, we provide a very simple method to control and optimize the power factor or the figure of merit of conducting polymers.