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

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^-1 K^{- 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.