Doping effects on the thermoelectric properties of pristine poly(3,4-ethylenedioxythiophene)

Abstract

Electronic and thermoelectric properties of poly(3,4-ethylenedioxythiophene) (PEDOT) depend strongly on its geometric structure and the free charge concentration in the PEDOT crystals. In this work, pristine and doped PEDOT crystals with tosylate (Tos)/tosylate anion (Tos⁻) are studied using Density Functional Theory (DFT) methods. During Tos⁻ doping, charge transfer from PEDOT chains to Tos⁻ occurs with a structural transformation (from aromatic-like to semi-quinoid-like). These changes shift the Fermi level into the valence band and exhibit metallic character with a semiconductor-metal transformation. The influence of the doping concentration on the thermoelectric properties of the pristine PEDOT, such as electric conductivity, Seebeck coefficient and power factor, was carefully studied. We found that doping affects the geometric structure, free charge concentration, and eventually leads to changes in the electronic and thermoelectric properties of PEDOT. When the Tos⁻ doping concentration is around 12.5%, a high conductivity and Seebeck coefficient can be achieved at the same time (which means a high power factor). Our investigations also show that there is a compromise between doping and the thermoelectric properties of PEDOT(Tos/Tos⁻), and doping does not always work well to improve the electrical conductivity and Seebeck coefficient for organic thermoelectric materials. This study might be beneficial to the engineering realization of PEDOT for thermoelectric applications.