Improved Thermoelectric Properties and Environmental Stability of Conducting PEDOT:PSS Films Post-treated With Imidazolium Ionic Liquids

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/K 2 m. 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 posttreatment 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/K 2 m. 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 Yemata et al. Improved Thermoelectric Properties of PEDOT:PSS 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.

Binary treatment of PEDOT:PSS films with nitric acid and imidazolium-based ionic liquids to improve the thermoelectric properties

“…S4, ESI †). 36,40,52 It was noteworthy that, compared with PSS, the relative intensity of S 2p in PEDOT significantly enlarged because of the loss of PSS in the N-PEDOT: Fig. 3.…”

“…Ionic liquids posttreatment was conducted at ambient temperature according to our reported method. 40 The details of PEDOT:PSS film preparation and subsequent post-treatment with HNO 3 and ionic liquids are summarized in Fig. S1 (ESI †).…”

“…The environmental stability of both N-PEDOT:PSS and [bmim]-[OTf]-N-PEDOT:PSS films is better or comparable to that of these reported in literatures (Table S3, ESI †). [38][39][40]45,73…”

“…37 Recently, we reported a series of work on post-treatment of PEDOT:PSS film using a combination of chemicals including a variety of reagents such as formamide, 8 formic acid, 38 trifluoroacetic acid, 19 trifluoromethanesulfonic acid, 24 hydrazine, 38 formaldehyde sulfoxylate 39 and ionic liquid. 40 For example, PEDOT:PSS films were post-treated with formamide and ionic liquids, and it was found that treatment only marginally had effect on the s but remarkably increased the S, achieving an overall PF up to B239.2 mW m À1 K À2 . 40 In addition to having outstanding TE performance, excellent long-term environmental stability is necessary for PEDOT:PSS film to be applied for useful TE modules, [41][42][43][44][45] but limited studies have been reported.…”

“…40 For example, PEDOT:PSS films were post-treated with formamide and ionic liquids, and it was found that treatment only marginally had effect on the s but remarkably increased the S, achieving an overall PF up to B239.2 mW m À1 K À2 . 40 In addition to having outstanding TE performance, excellent long-term environmental stability is necessary for PEDOT:PSS film to be applied for useful TE modules, [41][42][43][44][45] but limited studies have been reported. In this paper, we reported binary post-treatment with nitric acid (HNO 3 ) and ionic liquid to improve the s and S values of PEDOT-PSS films as well as its environmental stability.…”