Abstract
Poly(3,4-ethylene-dioxy-thiophene) (PEDOT) is an inherently hydrophobic and conductive polymer which usually doped with poly(styrene sulfonate) (PSS) for higher dispersity in aqueous solution. Nevertheless, PSS, a non-conductive polymer, inevitably lessens conductivity performance of PEDOT:PSS film. Plenty researchers have discovered that the conductivity of PEDOT:PSS films could be strongly enhanced upon minor presence of an ionic-liquid additive (such as [BMIM]BF4) in the dispersion or after post-deposition treatment with volatile solvent (such as methanol). The present study concerns the origin of this conductivity enhancement. By use of small-angle X-ray scattering (SAXS), grazing incidence small-angle X-ray scattering (GISAXS), X-ray reflectivity (XRR) and surface potential microscopy (SPoM), here we report morphological differences in PEDOT:PSS solutions and corresponding films upon presence/treatment of selected additive/solvent. According to SAXS results, nano-ellipsoid (ca. 24 nm in length and 3 nm in width) are the dominant in aqueous solution and the existence of [BMIM]BF4 increases the population of big aggregates. GISAXS and SPoM results show that population of fractal clusters rises along concentration of [BMIM]BF4 in the film, which give rise to conduction paths in the film state. Fractal cluster formed as well after the film was treated with methanol, but with smaller correlation length (ca. 36 nm), in contrast to micron-size fractals in the former case. We conclude that instead of the apparent phase separation or gel formation, the increased conductivity is mainly due to enhanced formation of fractal structure. Successive little fractal structures, formed after methanol treatment, provide more effective conduction-pathways comparing to huge fractal structures in identical volume.