Sodium formaldehyde sulfoxylate, an ionic-type, water-soluble reducing reagent to effectively improve seebeck coefficient of PEDOT:PSS film

Abstract

Conducting polymers such as poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) have attracted extensive attention for thermoelectric applications due to its solution-processability, mechanical flexibility, low thermal conductivity and tunable electrical conductivity. This work demonstrated a sequential post-treatment method with formamide and sodium formaldehyde sulfoxylate (SFS) to significantly improve thermoelectric properties of PEDOT:PSS film, particularly its Seebeck coefficients. Water-soluble and non-toxic SFS is an ionic-type reducing agent, which is used as an environmentally benign chemical reagent for the first time to treat PEDOT:PSS. First, the PEDOT:PSS film was soaked with formamide (F-PEDOT:PSS), and then was treated with SFS solutions with various molar concentrations (SFS–F-PEDOT). The Seebeck coefficient of F-PEDOT:PSS film treated with 100 mM SFS was steeply increased from 14.8 to 51.8 μV/K primarily due to the proper control of the doping level and the carrier concentration although the corresponding electrical conductivity of the film was reduced from 2,873 to 693 S/cm. The resulting power factor reached its maximum value of 185.8 μW/K2m, which was approximately three times that of formamide-treated film (F-PEDOT:PSS: 63.7 μW/K2m). The cross-plane thermal conductivity of the pristine PEDOT:PSS film was dropped from 0.59 W/mK for the pristine film to 0.29 W/mK for the SFS-F-PEDOT:PSS film, leading to an estimated ZT value of in the range of ~0.07–~0.14 at 300 K. Also, the stability of SFS-F-PEDOT:PSS film was examined under a harsh environment, and results showed that the film retained its electrical conductivity and Seebeck coefficient of more than 85% after continuous exposure under 70 °C and a humidity of 75% RH for 480 h, revealing the excellent long-term environmental stability.