Enhanced thermoelectric properties of poly(3,4-ethylenedioxythiophene) thin films treated with H2SO4

Abstract

Poly(3,4-ethylenedioxythiophene)–tosylate–polyethylene glycol–polypropylene glycol–polyethylene glycol (PEDOT–Tos–PPP) films were prepared via a vapor phase polymerization (VPP) method. The thermoelectric (TE) properties of the films before and after treated with H2SO4 at different concentrations were measured at 295K. The TE properties of the films have been significantly improved by the H2SO4 treatment. For example, after treated with H2SO4 at 1M, the electrical conductivity of the film has increased remarkably from 944 to 1750Scm−1, the Seebeck coefficient of the film reduced slightly from 16.5 to 14.6μVK−1, and the thermal conductivity decreased from 0.495 to 0.474W/mK. Hence, the ZT value at 295K has increased from 0.016 to 0.024. The electrical conductivity (Seebeck coefficient) of the untreated and 1M H2SO4 treated PEDOT-Tos-PPP films decreases (increases) with increasing temperature from 295 to 375K. And the power factor of the films monotonically increases with temperature. The power factor at 375K of the 1M H2SO4 treated film is almost twice as high as that at 295K. Atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) analyses, and the thickness measurement of the films indicate that the tri-block copolymer PPP have been removed from the PEDOT-Tos-PPP films after the H2SO4 treatment, and the UV–Vis-NIR absorption spectroscopy and Raman spectroscopy analyses reveal an increasing in the doping level in the PEDOT chains after the H2SO4 treatment. Therefore, the TE properties enhancement may be attributed to the combined effects of the removal of the insulating PPP from the PEDOT-Tos-PPP film, increase the doping level and conformational change of the PEDOT chains resulted from the H2SO4 treatment.