Modulation of the doping level of PEDOT:PSS film by treatment with hydrazine to improve the Seebeck coefficient

Abstract

As the most popular conducting polymer, poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is widely used for a variety of applications, including thermoelectrics. This paper reports the modulation of the doping level by treatment with hydrazine to improve the Seebeck coefficient of PEDOT:PSS films. PEDOT:PSS films were first treated with formic acid followed by hydrazine, leading to a significant increase in the Seebeck coefficient from 17.5 to 42.7 μV K^-1, about 2.5 times higher than that of the pristine film partially at the expense of electrical conductivity. An optimum power factor of 93.5 μW K^-2 m^-1, being 2.4 times that of the one treated with only formic acid, was achieved. The substantial improvement in the Seebeck coefficient and the power factor is collectively attributed to the removal of the PSS, and more importantly, the reduction of the doping level of PEDOT by the hydrazine treatment, which is evidenced clearly by UV-vis-NIR spectroscopy, XPS and Raman spectroscopy.

Fig. 1. Schematic of preparation and sequential post-treatment of PEDOT:PSS films with formic acid and hydrazine.

Fig. 2. Schematic illustrations of the setup of S measurement and the electrode geometry. Side view (left) and top view (right).

Fig. 3. TE properties of PEDOT:PSS films as a function of the hydrazine concentration. (a and b) Formic acid × 1 time and hydrazine × 1 time; (c and d) formic acid × 3 times and hydrazine × 1 time.

Fig. 4. UV-vis-NIR absorption spectra of the untreated, formic acid-treated, and formic acid–hydrazine treated PEDOT:PSS films using different concentrations of hydrazine: (a) 200–320 nm (b) 400–1600 nm.

Fig. 5. Schematic diagram of neutralization of chains of PEDOT:PSS by dedoping, i.e., bi-polaron (a di-cation charge carrier), polaron (a radical cation charge carrier) and neutral chain.

Fig. 6. XPS spectra of (a) pristine PEDOT:PSS, (b) formic acid treated PEDOT:PSS, (c) 0.15 wt% hydrazine treated PEDOT:PSS, and (d) combination of normalized pristine PEDOT:PSS, formic acid treated PEDOT:PSS, and PEDOT:PSS films treated with different hydrazine concentrations.

Fig. 7. (a) Raman spectra of the PEDOT:PSS films before and after treatment with formic acid and formic acid–hydrazine. (b) The zoom-in spectra for the wavenumbers ranging from 1300 to 1500 cm⁻¹.

Fig. 8. AFM height images: (a) pristine, (b) formic acid treated and (c) formic acid–0.15 wt% hydrazine treated. AFM phase images: (d) pristine, (e) formic acid treated and (f) formic acid–0.15 wt% hydrazine treated. All images captured with an area of 1 × 1 μm².

Fig. 9. Variation of carrier concentration (n) and mobility (μ) of formic acid–hydrazine-treated PEDOT:PSS films with different weight concentration of proportional to hydrazine. The σ is proportional to n × μ and the S is inversely.

Fig. 10. The effect of stability of σ and S at the optimum doping level with formic acid–0.15 wt% hydrazine treatment of PEDOT:PSS film as a function of exposure time (humidity: 75%; temperature: 70 °C).

Fig. 11. Chemical structures of PEDOT, PSS and schematic illustration of the mechanism of TE properties enhancement of PEDOT:PSS films caused by the sequential treatment of formic acid and hydrazine.