Abstract
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most popular conducting polymers and widely used as polymer thermoelectric materials, and its thermoelectric performance could be improved by a variety of post-treatment processes. This paper reported two series of post-treatment methods to enhance the thermoelectric performance. The first series method included pre-treatment of PEDOT:PSS film with formamide, followed by imidazolium-based ionic liquids. The second series method included pre-treatment of PEDOT:PSS film with formamide, followed by sodium formaldehyde sulfoxylate, and finally imidazolium-based ionic liquids. Two series of post-treatment methods significantly improved the power factor of PEDOT:PSS when compared to that of PEDOT:PSS treated with formamide only. For example, using the first series post-treatment method with 40 vol.% ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide, the Seebeck coefficient of the PEDOT:PSS film increased from 14.9 to 28.5 μV/K although the electrical conductivity reduced from 2,873 to 1,701 S/cm, resulting in a substantial improvement in the overall power factor from 63.6 to 137.8 μW/K2m. The electrical conductivity enhancement in the formamide-treatment process was in part ascribed to the removal of the insulating PSS component. Further treatment of PEDOT:PSS film with ionic liquid caused dedoping of PEDOT and hence increased in Seebeck coefficient. In contrast, second series post-treatment method led to the reduction in electrical conductivity from 2,873 to 641 S/cm but a big improvement in the Seebeck coefficient from 14.9 to 61.1 μV/K and thus the overall power factor reached up to ~239.2 μW/K2m. Apart from the improvement in electrical conductivity, the increase in Seebeck coefficient is on account of the substantial dedoping of PEDOT polymer to its neutral form and thus leads to the big improvement of its Seebeck coefficient. The environmental stability of ionic liquid-treated PEDOT:PSS films were examined. It was found that the ionic liquid treated PEDOT:PSS retained more than 70% Seebeck coefficient and electrical conductivity at 75% RH humidity and 70°C for 480 h. The improved long-term TE stability is attributed to the strong ionic interaction between sulfonate anions and bulky imidazolium cations that effectively block the penetration of water and lessen the tendency to take up water from the air.
Highlights
Thermoelectric (TE) materials are able to directly convert heat into electricity and vice versa (Bell, 2008; Snyder and Toberer, 2011)
We reported the enhancement of the S and ZT of PEDOT:PSS films with ionic liquids (ILs) treatment
The formamide treated PEDOT:PSS films were cooled to room temperature in air, and these films were rinsed with DI water and dried again on a hot plate at 140◦C for 5 min
Summary
Thermoelectric (TE) materials are able to directly convert heat into electricity and vice versa (Bell, 2008; Snyder and Toberer, 2011). Fan et al treated PEDOT:PSS films with sulfuric acid and different concentrations of sodium hydroxide They reported an improved σ of 2,170 S/cm, an S of 39.2 μV/K, and a PF of 334 μW/K2m at room temperature (Fan et al, 2017). The formamide treated PEDOT:PSS films were cooled to room temperature in air, and these films were rinsed with DI water and dried again on a hot plate at 140◦C for 5 min. The films were rinsed by dipping the films in DI water for three times and annealed at an elevated temperature of 140◦C under air to remove the residual solvent These treated samples were cooled down to room temperature before TE property measurements. The stability study was conducted using a constant climate chamber (Memmert HPP 110) in the temperature range from +0◦C to +70◦C, as well as the active humidification and dehumidification from 10 to 90% RH
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