Simultaneously enhanced electrical conductivity and Seebeck coefficient in Poly (3,4-ethylenedioxythiophene) films treated with hydroiodic acid

• PEDOT films were prepared via a VPP method and treated with HI acid.
• Thermoelectric properties of the films were enhanced by the HI acid treatment.
• The maximum ZT value of the HI acid treated PEDOT films at 295 K is 0.04.
• The properties enhancement mechanism of the films was proposed.

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 various concentrations of HI acid have been studied. The electrical conductivity and Seebeck coefficient of the films simultaneously increased after treated with dilute HI acid solutions (<5 wt%). The electrical conductivity of the film treated with 5 wt% HI acid increased from 1532 to 1690 S cm−1 and the Seebeck coefficient increased slightly from 14.9 to 20.3 μV K−1. The maximum power factor value of the treated film is 69.6 μW m−1 K−2 at 304 K, which is about twice as high as that of the pristine film (33.8 μW m−1 K−2). Field emission scanning electron microscopy (FESEM), Raman spectroscopy analyses and the thickness measurement of the films indicate that the tri-block copolymer PPP have been removed by the HI acid treatment. The UV–vis-NIR absorption spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses reveal that iodine anions (I−, I3− and I5−) exist as dopants in the film after the treatment. The TE properties enhancement are, therefore, attributed to the combined effects of the removal of the insulating PPP and Tos− and iodine ions doping by the HI acid treatment.