Roll-to-roll processed PEDOT:PSS thin films: application in flexible electrochromic devices

In this paper, poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) denoted as PEDOT:PSS, a common material used as electrode in organic electronics or as charge balancing layer in electrochromic devices, was processed in thin film by roll-to-roll technique onto flexible substrate. Improved wettability was achieved when the ITO/PET substrate received a pre-treatment with isopropyl alcohol (iPrOH) and small amount of this alcohol was mixed with the PEDOT:PSS, as demonstrated by the reduction of contact angle. Besides allowing the preparation of uniform films in larger areas, this procedure also resulted in PEDOT:PSS films with increased conductivity, from similar to 2 x 10(-4) Scm(-1) up to similar to 2.5 x 10(-4) Scm(-1). This arises from the effect of the alcohol that spreads out the isolating PSS moietes from the surface of conducting PEDOT moieties and/or change the PEDOT chain conformation, resulting in increased conductivity. These roll-to-roll processed PEDOT:PSS films were tested in electrochromic devices having a polymer:solid electrolyte composite based on poly(ethylene oxide) and lithium perchlorate. This device was sealed by lamination procedure in air. Contrast between oxidized (light blue) and reduced (dark blue) state of PEDOT:PSS was monitored by UV-Vis transmittance. The use of iPrOH as additive resulted in improved electrochromic response with coloration efficiency (eta(d)) of 377 C-1 cm(2) after 0.3 s of switch time.