Poly (3,4-ethylenedioxythiophene)/poly (styrene sulfonate) (PEDOT:PSS) is a new conductive film material. In order to improve the physical properties of the film, an effective secondary doping method is adopted. The conductivity of PEDOT:PSS film was improved by secondary doping with various inorganic acids. The physical properties of doped film were tested and analyzed by four point probe method and UV spectrophotometer. Atomic force microscope (AFM) was used to measure the surface morphology of the film. The internal structure of the film was analyzed by X-ray photoelectron spectroscopy (XPS) and Hall effect testing system. The conductivity of H2SO4-doped film was improved to 2 216 S/cm at the concentration of 0.08 mol/L, and the conductivity of H3PO3-doped film can be enhanced to 2 033 S/cm. The FoM of film was affected by the thickness, transmittance and quality of film. The film prepared by 0.1 mol/L H4P2O7 shows a transmittance of 82% at 550 nm and the FoM could reach 40, which had potential for commercial application. The conductivity of the film treated with H2SO4, H3PO3, H4P2O7 and H3PO4 was inversely proportional to the dissociation constant of the doped reagent. H+ could better combine with PSS, which promoted the separation of PEDOT and PSS. Therefore, conductive PEDOT wrapped by insulating PSS could be exposed to form PEDOT aggregates. The aggregation of PEDOT made the crimped PEDOT chain more linear and the electronic transmission easier. The PKa of doping reagent was not the only factor affecting the conductivity. The PKa of HClO4 was –10, but the conductivity was only 760 S/cm. It was attributed to the increase of the concentration of the HClO4-doped solution, which increased the thickness of the prepared film (361 nm), and the thickness was inversely proportional to the conductivity. AFM test results show that PEDOT and PSS are separated, and PEDOT fibers are formed in the film. XPS characterization results show that the peak intensity ratio of PEDOT and PSS is 0.34 for pristine PEDOT:PSS film. The peak intensity ratio of the HPF6-doped film is 0.35, indicating that the sheet resistance of the film decreases. The peak values of PSS for HPF6-doped and H4P2O7-doped films have shifted by 0.5 eV, but the peak of PEDOT does not change its position, which indicates that PSS combines with H+ from doped acid to form hydrogen bond, and the binding energy of hydrogen bond is greater than that of PEDOT. According to Hall coefficient test results, the carrier concentration of the H4P2O7-doped film is raised from 3.2×1019 to 6.2×1021 cm–3. Moreover, the HPF6-doped film has a higher carrier density from 3.2×1019 to 5.4×1019 cm–3, so H4P2O7-doped film has more advantages in increasing electrical conductivity. To sum up, inorganic acid doping reagents with low boiling point and low dissociation constant can effectively improve the conductivity of the PEDOT:PSS film, and can obtain films with high FoM. The doping of inorganic acids have two main effects on the improvement of conductivity. On the one hand, the doping of inorganic acid separates PEDOT and PSS, which increases the relative content of conductive PEDOT, thus reducing the sheet of the film. On the other hand, inorganic acid doping can reduce the coulomb interactions between PEDOT and PSS, making the curly PEDOT chain wrapped by PSS more linearly. The aggregation of PEDOT and its chain are more linear, which makes the conduction path longer and the contact points more, thus promoting the charge transmission in the film and increasing the conductivity. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.
