Abstract
Poly(3,4-ethylenedioxythiophene) (PEDOT) is an inherently conductive polymer, and usually doped with poly(styrene sulfonate) (PSS) in order to be well-dispersed in its aqueous solution, since PEDOT itself is hydrophobic. In this work, we aim to show the effect caused by PSS on the stability of PEDOT:PSS system by comparing PEDOT:PSS systems of PSS/PEDOT mass ratio r = 2.5 vs. 6. For both the colloidal dispersion and the solid thin film, structural models of PEDOT:PSS systems were constructed to facilitate analysis of synchrotron-based small-angle X-ray scattering (SAXS), grazing-incident small-angle X-ray scattering (GISAXS) and X-ray reflectivity (XRR) data. Colloidal dispersions with solid contents ranging from 1.2 to 0.2 wt% and the corresponding spin-cast films were observed in the attempt to emphasis the stability strength of PSS in severe condition, in which water is much more than PEDOT in its amount. According to SAXS results, PEDOT:PSS dispersion generally comprises colloidal particles (of a PEDOT-rich core ca. 5 nm in size) as well as their fractal aggregates up to ca. 200 nm in diameter. It was observed that the colloidal particles tend to aggregate with decreasing PSS content. As for structural characterization of PEDOT:PSS thin film, results from GISAXS analysis reveal a fractal structure composed of prolate particles (~45 nm) in the film, along with some degree of particle aggregation, which is more severe at r = 2.5. GISAXS analysis also suggests that, there is no significant structural differences between bulk layer and surface layer in the case of r = 2.5 whereas the surface layer of high-PSS-ratio system appears to be less compact in its structure than the bulk layer in the case of r = 6. Meanwhile, XRR result provides in-depth composition analysis of the thin film, and indicates higher surface roughness in films with high PSS ratio. Integrating all experimental observations, it is concluded that an adequately high PSS content in PEDOT:PSS greatly decreased the aggregation of PEDOT particles in both the aqueous dispersion and thin-film states.