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

Abstract

Polymer electrolytes were synthesized by two different approaches and applied to electrochromic devices based on electrodeposited tungsten oxide (WO₃) or poly(3,4-ethylenedioxythiophene) (PEDOT) films as the cathode, and a Prussian blue (PB) film as the anode. The first method involved the entrapping of an ionic liquid in a polymer host (poly(methylmethacrylate) or PMMA) and the second approach relied on the in situ thermal polymerization of methylmethacrylate (MMA) in the hydrophobic ionic liquid, yielding a solidified transparent gel. The effect of in situ solid polymer electrolyte formation on device performance characteristics was realized in terms of a larger coloration efficiency of 119 cm² C⁻¹ (λ = 550 nm) achieved for the WO₃–PB (MMA) device, as compared to a value of 54 cm² C⁻¹ obtained for the WO₃–PB (PMMA) device. Similar enhancements in electrochromic coloring efficiency, reflectance contrast, and faster switching kinetics were obtained for the PEDOT–PB (MMA) device. The strategy of introducing an electrolyte to the electrochromic device in a liquid state and then subjecting the same to gradual polymerization allows greater accessibility of the electrolyte ions to the active sites on the electrochromic electrodes and superior interfacial contact. As a consequence, larger optical contrast and faster kinetics are achieved in the MMA based devices. While PEDOT films were amorphous, PB films were semi-crystalline but only in the case of WO₃; the hexagonal structure of WO₃, equipped with three/four/six-coordinated voids was found to affect bleaching kinetics favorably. The performance of PMMA based electrolyte is limited by high resistance at the electrode–electrolyte interface, and a smaller number of ions available for oxidation and reduction. Large area (∼10 cm × 4 cm) devices were also fabricated using this simple wet chemistry method and their ability to color uniformly without any pinholes was demonstrated.