Film formation, surface character, and relative density for electrochromic PEI/(PSS:PEDOT) multilayered thin films

Thin films of alternating layer composition were constructed from the polyelectrolyte complex PEDOT:PSS and the polycation PEI, using ionic self assembly (ISA). The PEI/PEDOT:PSS system displays a consistent trend in film growth, as evidenced by UV–visible spectroscopy and ellipsometry. We find that the overall density of PEDOT increases with increasing number of layers. The density of PSS during multilayer deposition differs from PEDOT, with a sharp drop in density between the 3rd and 6th bilayers. Combining film deposition estimates with contact angle measurement, we distinguish three regions of growth, separated by the 3rd and 6th layers. We ascertain that a constant level of interpenetration between PEI and PEDOT:PSS is reached by the 6th layer. Results from kinetics experiments and pH variation reveal a local increase in pH for the PEDOT species as it comes into contact with the PEI surface. Electrochemical characterization indicates that our films have an interpenetrated PEDOT network and a relatively hydrophilic surface. We demonstrate that ISA can be used to generate robust thin films, stable over a large pH range, whose coloration and conductivity may be manipulated on a large scale using applied voltage, and may be fine-tuned by changing the pH. The films exhibit electrochromic properties similar to other PEDOT derivatives, with a change in transmittance of 51% for 16 bilayers at 643nm.