Viologen-based flexible electrochromic devices

Electrochromic technology has gained significant attention in various fields such as displays, smart windows, biomedical monitoring, military camouflage, human-machine interaction, and electronic skin due to its ability to provide reversible and fast color changes under applied voltage. With the rapid development and increasing demand for flexible electronics, flexible electrochromic devices (FECDs) that offer smarter and more controllable light modulation hold great promise for practical applications. The electrochromic material (ECM) undergoing color changes through electrochemical reactions is one of the key components in electrochromic devices. Among the ECMs, viologens, a family of organic small molecules with 1,1'-disubstituted-4,4'-dipyridinium salts, have garnered extensive research interest. Viologens exhibit well-reversible redox reactions, excellent electron acceptance ability, and the ability to produce multiple colors. Notably, viologen-based FECDs demonstrate color changes in the liquid or semi-solid electrolyte layer, eliminating the need for two solid electrodes and thus simplifying the device structure. Consequently, viologens offer significant potential for the development of FECDs with high optical contrast, fast response speed, and excellent stability. This review aims to provide a comprehensive overview of the progress and perspectives of viologen-based FECDs. It begins by summarizing the typical structure and recent exciting developments in viologen-based FECDs, along with their advantages and disadvantages. Furthermore, the review discusses recent advancements in FECDs with additional functionalities such as sensing, photochromism, and energy storage. Finally, the remaining challenges and potential research directions for the future of viologen-based FECDs are addressed.