This work reports on the synthesis of the intrinsically conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with several counter-ions, ClO₄, PF₆ and bis(trifluoromethylsulfonyl)imide (BTFMSI), by electro-polymerization and its thermoelectric properties. We show that, depending on the counter-ion size, the thermoelectric efficiency of PEDOT can be increased up to two orders of magnitude. A further chemical reduction with hydrazine optimizes the power factor (PF). By changing the counter-ions, we were able to increase the electrical conductivity (σ) of PEDOT by a factor of three, while the Seebeck coefficient remains at the same order of magnitude in the three polymers. The best thermoelectric efficiency has been observed in PEDOT:BTFMSI. From the measurement of the Seebeck coefficient and σ, a PF of 147 μW m⁻¹ K⁻² has been deduced, while the measured thermal conductivity is κ = 0.19 W m⁻¹ K⁻¹, resulting in a ZT ∼ 0.22 at room temperature, one of the highest values reported in the literature for polymers. The increase in σ with the change of the counter-ion is mainly due to the stretching of the polymer chains. In this work, we provide a chemical route to further improve ZT in polymers and demonstrate a method of synthesis based on the electro-polymerization on gold. After removing the gold layer, a very thin semiconducting polymer film can be isolated.

In this review, we report the state-of-the-art of polymers in thermoelectricity. Classically, a number of inorganic compounds have been considered as the best thermoelectric materials. Since the prediction of the improvement of the figure of merit by means of electronic confinement in 1993, it has been improved by a factor of 3–4. In the mean time, organic materials, in particular intrinsically conducting polymers, had been considered as competitors of classical thermoelectrics, since their figure of merit has been improved several orders of magnitude in the last few years. We review here the evolution of the figure of merit or the power factor during the last years, and the best candidates to compete with inorganic materials. We also outline the best polymers to substitute classical thermoelectric materials and the advantages they present in comparison with inorganic systems.

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 μV K⁻¹ for PEDOT and from 51.0 to 6.7 μV K⁻¹ for PPy. The thermal conductivity remains unchanged with the oxidation state of the film, κ ≈ 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 × 10⁻² for PEDOT and 6.8 × 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.