The Morphological Power of Soap: How Surfactants Lower the Sheet Resistance of PEDOT:PSS by Strong Impact on Inner Film Structure and Molecular Interface Orientation

In the rapid development of organic electronics, there is a strong need for highly conductive and transparent electrode (TE) materials to act as charge transport layers. In this context, poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS) is a highly promising candidate, because it can act directly as TE. This makes the use of brittle, rare, and expensive indium tin oxide electrodes dispensable. Modification of the inner film morphology, e.g., by solvent additives can dramatically reduce the sheet resistance of PEDOT:PSS. In this work, it is investigated how the (fluoro)surfactant Zonyl and the co-solvent ethylene glycol influence the electrical and optical properties of the film, namely, the sheet resistance, the transmission, and the figure of merit for TEs. The electronic characteristics are then related to the morphological changes investigated with grazing incidence small angle X-ray scattering (GISAXS) and polarized resonant soft X-ray scattering (P-SoXS). Using GISAXS, structure evolutions are related to sheet resistances and device characteristics in organic solar cells. Further, the influence of (fluoro)surfactant on the phase separation and relative molecular orientation at polymer interfaces is investigated utilizing P-SoXS. Transparent PEDOT:PSS films with low sheet resistance are essential for market introduction and mark the next milestone for the success of future organic electronic materials.