Enhanced thermoelectric properties of poly(3,4-ethylenedioxythiophene) thin films treated with H_2SO_4

摘要

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 H_2SO_4 at different concentrations were measured at 295 K. The TE properties of the films have been significantly improved by the H_2SO_4 treatment. For example, after treated with H_2SO_4 at 1 M, the electrical conductivity of the film has increased remarkably from 944 to 1750 S cm~(-1), the Seebeck coefficient of the film reduced slightly from 16.5 to 14.6 μV K~(-1), and the thermal conductivity decreased from 0.495 to 0.474 W/mK. Hence, the ZT value at 295 K has increased from 0.016 to 0.024. The electrical conductivity (Seebeck coefficient) of the untreated and 1 M H_2SO_4 treated PEDOT-Tos-PPP films decreases (increases) with increasing temperature from 295 to 375 K. And the power factor of the films monotonically increases with temperature. The power factor at 375 K of the 1 M H_2SO_4 treated film is almost twice as high as that at 295 K. 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 H_2SO_4 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 H_2SO_4 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 H_2SO_4 treatment.