Fast-response electrochromic (EC) cell composed of EC dye-modified TiO₂-nanoparticle-based transparent porous electrode was reported by R. Cinnsealach et al. (Sol. Energy Mater. Sol. Cells, 55, 215, 1998). From then, TiO₂ porous electrodes modified with EC dyes have been regarded as a promising candidate for fast-response EC display (ECD) applications. Our research purpose is the realization of flexible fast-response ECD applications. However, TiO₂ porous electrodes require high sintering temperature (>400 °C) to use as EC electrode because TiO₂ has high resistivity. This high process temperature required by TiO₂ porous electrode was an important obstacle for their application as transparent electrodes in plastic substrate-based flexible ECDs. To solve this problem, we substituted transparent conductive oxides (TCO) for TiO₂ as the porous electrode material. TCO materials have high electrical conductivity than TiO₂, so they have the potential to work as the EC electrode even if they are sintered at low temperature.

Previously, we reported that the porous electrodes consisting of indium tin oxide (ITO) nanoparticles exhibit good optical transparency and high electric conductivity. Moreover, EC response characteristics of EC-dye modified ITO porous electrode sintered at 150 °C was improved than that of conventional TiO₂ porous electrode due to its high conductivity (AIP Adv., 6, 065121, 2016). Therefore, TCO porous electrodes have potential for application in porous electrode for flexible ultrafast-response ECDs. To achieve such applications, it is necessary to analyze the factors controlling the EC response speed in TCO porous electrodes.

In this study, we investigated the response characteristics of EC dye-modified ITO, antimony-doped tin oxide (ATO), and TiO₂ porous electrodes. ITO and ATO porous films have low electric resistivity due to their high electric conductivity. As mentioned above, ITO porous electrode exhibited fast response. On the other hand, low-resistivity ATO porous electrode exhibited slower response than that of a conventional high-resistivity TiO₂ porous electrode. This clearly indicates that the EC response speed of the ATO porous electrode is dependent on a factor other than the electric resistance of the porous film. From the structural comparisons, it is revealed that the pore size of the film consisting of small nanoparticles is one of the key factors controlling the EC response speed of porous electrode. The details about the effect of porous film structure on EC response characteristics will be discussed.