Plastic electrochromic devices based on viologen-modified TiO2 films prepared at low temperature

In this study, flexible plastic electrochromic devices (ECDs) based on viologen-modified TiO2 nanostructured films thermally treated at only 120 °C are reported. Even if a sintering process at typically above 450 °C was avoided, the resulting mesoporous TiO2 films were highly transparent on both TCO-coated glass and plastic substrates while presenting adequate conductivity and high active surface area suitable for anchoring electro-chromophores. The synthesis of a viologen specifically designed to provide colorless-to-blue switching while being more compatible with plastic substrates than the traditional ethyl viologen is reported as well. The effect of the TiO2 layer thickness and pore characteristics as well as the grafting time on the device performance were also investigated. Transparent ECDs based on these viologen-modified TiO2 films have been assembled on flexible plastic substrate (ITO/PET) offering transmittance changes (ΔT) above 60% and switching speeds of 6-8 s. The change in transmittance is further enhanced up to 71% in dual ECDs using PB on ITO/PET as the counter electrode and viologen-modified TiO2 films on FTO/glass. Finally, operational large-area all-solid devices (40 cm×30 cm) are presented, demonstrating the suitability of the present technology for the production of large-area plastic ECDs.