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

Organic thermoelectric thin films are investigated in terms of their stability at elevated operating temperatures. Therefore, the electrical conductivity of ethyl-3-methylimidazolium dicyanamide (EMIM DCA) post-treated poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thin films is measured over 4.5 h of heating at 50 or 100 °C for different EMIM DCA concentrations. The changes in the electrical performance are correlated with changes in the film morphology, as evidenced with in situ grazing-incidence small-angle X-ray scattering (GISAXS). Due to the overall increased PEDOT domain distances, the resulting impairment of the interdomain charge carrier transport directly correlates with the observed electrical conductivity decay. With in situ ultraviolet-visible (UV-Vis) measurements, a simultaneously occurring reduction of the PEDOT oxidation level is found to have an additional electrical conductivity lowering contribution due to the decrease of the charge carrier density. Finally, the observed morphology and oxidation level degradation is associated with the deterioration of the thermoelectric properties and hence a favorable operating temperature range is suggested for EMIM DCA post-treated PEDOT:PSS-based thermoelectrics.