Rapid electrochromic switching in ionically self-assembled multilayers

The electrochromic effect results from the motion of ions into and out of an active material, resulting in reversible changes in electronic structure and absorption spectra. We report studies of electrochromic devices fabricated from ionically self-assembled multilayers (ISAMs), in which fast switching rates are achieved by nanoscale control of film composition. ISAMs, fabricated by alternate adsorption of cationic and anionic components, yield exceptionally homogeneous thin films with subnanometer control of the thickness and relative spatial location of the component materials. ISAM films were adsorbed onto indium tin oxide-coated glass substrates using electrochromic polymers, such as polyanaline(emeraldine base) (PANI), with the ionic conductor poly(2-acylamido 2-methyl propane sulfonic acid) (PAMPS). Solid-state electrochromic devices, fabricated by sandwiching identical ISAM films with a conducting interlayer (Figure 1), exhibited large, reversible absorbance changes with application of 1.0 V. Rise and fall times shorter than 20 milliseconds have been observed in the devices. We describe systematic studies of the magnitude of the absorbance change and the switching speed as a function of nanoscale thickness and composition.