Substrate-Dependent Morphology and Its Effect on Electrical Mobility of Doped Poly(3,4-ethylenedioxythiophene) (PEDOT) Thin Films

Deposition dynamics, crystallization, molecular packing, and electronic mobility of poly(3,4-ethylenedioxythiophene) (PEDOT) thin films are affected by the nature of the substrate. Computational microscopy has been carried out to reveal the morphology-substrate dependence for PEDOT thin films doped with molecular tosylate deposited on different substrates including graphite, Si3N4, silicon, and amorphous SiO2. It is shown that the substrate is instrumental in formation of the lamellar structure. PEDOT films on the ordered substrates (graphite, Si3N4, and silicon) exhibit preferential face-on orientation, with graphite showing the most ordered and pronounced face-on packing. In contrast, PEDOT on amorphous SiO2 exhibits the dominant edge-on orientation, except in the dry state where both packings are equally presented. The role of water and the porosity of the substrate in formation of the edge-on structure on SiO2 is outlined. On the basis of the calculated morphology, the multiscale calculations of the electronic transport and percolative analysis are performed outlining how the character of the substrate affects the electron mobility. It is demonstrated that good crystallinity (PEDOT on graphite substrate) and high content of edge-on (PEDOT on SiO2 substrate) are not enough to achieve the highest electrical in-plane mobility. Instead, the least ordered material with lower degree of the edge-on content (PEDOT on silicon substrate) provides the highest mobility because it exhibits an efficient network of pi-pi stacked chain extending throughout the entire sample.