An overview of electrochromic devices with electrolytes containing viologens

Viologen, known as 4-4 ' bipyridine salts, is one of the most widely used electrochromic materials (ECMs) because of its diverse colour changes, high optical modulation and colouration efficiency. In recent years, researchers have made great efforts to investigate electrochromic devices (ECDs) with electrolytes containing viologens. This review focuses on the differences in the preparation and properties of ECDs with viologens in detail, including all-in-one ECDs with solution-phase electrolytes, all-in-one ECDs with gel electrolytes, and ECDs with electrolytes containing viologens and EC films. Firstly, the review introduces the concept, classification, working principle and application of ECDs. Then with an emphasis on materials choice, device fabrication and performance, a thorough overview of the research development of the three types of ECDs with electrolytes containing viologens is presented. Finally, the current problems and the development prospects of ECDs with electrolytes containing viologens are discussed.