Correlation of Thermoelectric Performance, Domain Morphology and Doping Level in PEDOT:PSS Thin Films Post‐Treated with Ionic Liquids

Abstract

Ionic liquid (IL) post‐treatment of poly(3,4‐ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thin films with ethyl‐3‐methylimidazolium dicyanamide (EMIM DCA), allyl‐3‐methylimidazolium dicyanamide (AMIM DCA), and 1‐ethyl‐3‐methylimidazolium tetracyanoborate (EMIM TCB) is compared. Doping level modifications of PEDOT are characterized using UV–Vis spectroscopy and directly correlate with the observed Seebeck coefficient enhancement. With conductive atomic force microscopy (c‐AFM) the authors investigate changes in the topographic‐current features of the PEDOT:PSS thin film surface due to IL treatment. Grazing incidence small‐angle X‐ray scattering (GISAXS) demonstrates the morphological rearrangement towards an optimized PEDOT domain distribution upon IL post‐treatment, directly facilitating the interconductivity and causing an increased film conductivity. Based on these improvements in Seebeck coefficient and conductivity, the power factor is increased up to 236 µW m−1K−2. Subsequently, a model is developed indicating that ILs, which contain small, sterically unhindered ions with a strong localized charge, appear beneficial to boost the thermoelectric performance of post‐treated PEDOT:PSS films.

In Situ Observation of Morphological and Oxidation Level Degradation Processes within Ionic Liquid Post-treated PEDOT:PSS Thin Films upon Operation at High Temperatures

“…First of all, σ dc ,0 of the post-treated PEDOT:PSS thin films increases with increasing EMIM DCA concentrations, being the highest for the 0.35 M sample with 1610 S cm –1 . This trend is in excellent agreement with previous studies showing the positive effect of IL treatments on the electrical conductivity . The 0.05 M EMIM DCA post-treated film heated at 100 °C shows with τ = 800 min the most rapid decay in electrical conductivity.…”

“…This increase might indicate that upon EMIM DCA post-treatment, the type of primary charge carrier transport is changing from a hopping-like semiconductor behavior to a more band-like metallic charge transport . In the literature, this kind of temperature-dependent charge transport is referred to as fluctuation-induced tunneling charge transport, which could result from the rearrangement in the PEDOT:PSS morphology toward smaller and more densely packed PEDOT domains upon IL post-treatments, like shown in our previous studies . This formation of closer conductive domains strongly decreases the energy barrier for interdomain charge carrier transport and improves the overall electrical conductivity. , While, for the untreated PEDOT:PSS thin films, the distances between conductive PEDOT domains are more significant, and the heating enhanced overcoming of the energy barrier for the charge carrier transport is the confining factor here.…”

“…Since PEDOT:PSS is very promising for the application as an organic thermoelectric material, some research also aims to improve its Seebeck coefficient ( S ). ,, Successful at simultaneously increasing σ dc and S is the combination of DMSO and salt treatment or the post-treatment of PEDOT:PSS thin films with ionic liquids (ILs). − Thus, IL post-treatment is especially beneficial as with only one treatment step, the thermoelectric properties of PEDOT:PSS thin films can be strongly improved. In previous studies, we post-treated PEDOT:PSS thin films with different ILs in varying concentrations and examined the resulting effects. , On the one hand, with investigations of the inner film morphology, we were able to show the before-mentioned removal of excess PSS and a thereby induced formation of an σ dc -conducive fine arrangement of small but densely packed PEDOT domains. On the other hand, we observed a doping level change of the PEDOT chains with spectroscopic studies, leading to a decrease of the charge carrier concentration, which is favorable to the Seebeck coefficient.…”

“…On the other hand, we observed a doping level change of the PEDOT chains with spectroscopic studies, leading to a decrease of the charge carrier concentration, which is favorable to the Seebeck coefficient. With this approach, the electrical conductivity increased from 1 to 1126 S cm –1 and the Seebeck coefficient from 22 to 46 μV K –1 as compared to those of the untreated film …”

“…To address this issue, in our present study, we investigate the thermal stability of ethyl-3-methylimidazolium dicyanamide (EMIM DCA) post-treated PEDOT:PSS thin films. We chose this ionic liquid because EMIM DCA contains small and sterically rather weakly impaired ions, with strongly localized charges, which we found in our previous studies to be beneficial for the optimization of thermoelectric properties . With measurements of the electrical conductivity of the film over the time of heating at elevated temperatures, we study the extent of thermal degradation dependent on the applied temperature and the EMIM DCA post-treatment concentration.…”