Electrochromic device response controlled by an in situ polymerized ionic liquid based gel electrolyte

Polymer electrolytes were synthesized by two different approaches and applied to electrochromic\uddevices based on electrodeposited tungsten oxide (WO3) or poly(3,4-ethylenedioxythiophene)\ud(PEDOT) films as the cathode, and a Prussian blue (PB) film as the anode. The first method involved\udthe entrapping of an ionic liquid in a polymer host (poly(methylmethacrylate) or PMMA) and the\udsecond approach relied on the in situ thermal polymerization of methylmethacrylate (MMA) in the\udhydrophobic ionic liquid, yielding a solidified transparent gel. The effect of in situ solid polymer\udelectrolyte formation on device performance characteristics was realized in terms of a larger\udcoloration efficiency of 119 cm2 C21 (l = 550 nm) achieved for the WO3–PB (MMA) device, as\udcompared to a value of 54 cm2 C21 obtained for the WO3–PB (PMMA) device. Similar enhancements\udin electrochromic coloring efficiency, reflectance contrast, and faster switching kinetics were obtained\udfor the PEDOT–PB (MMA) device. The strategy of introducing an electrolyte to the electrochromic\uddevice in a liquid state and then subjecting the same to gradual polymerization allows greater\udaccessibility of the electrolyte ions to the active sites on the electrochromic electrodes and superior\udinterfacial contact. As a consequence, larger optical contrast and faster kinetics are achieved in the\udMMA based devices. While PEDOT films were amorphous, PB films were semi-crystalline but only in\udthe case of WO3; the hexagonal structure of WO3, equipped with three/four/six-coordinated voids was\udfound to affect bleaching kinetics favorably. The performance of PMMA based electrolyte is limited\udby high resistance at the electrode–electrolyte interface, and a smaller number of ions available for\udoxidation and reduction. Large area (y10 cm 6 4 cm) devices were also fabricated using this simple\udwet chemistry method and their ability to color uniformly without any pinholes was demonstrated