Double‐layered Black Electrochromic Device with a Single Electrode and Long‐Term Bistability

This article describes a new electrochromic device (ECD) designed to achieve improved black color and long‐term bistability. We achieved these improvements by fabricating a double‐layered nanostructure of two different electrochromic materials stacked in layers on a single electrode. The fabricated ECD maintains 90% of the light transmittance of the colored state even after 1 h if powered off in the colored state. To achieve an improved black, green and blue viologens were successively immobilized in the bottom and top layers of materials on an electrode. The green viologens are initially colored at low voltage, and the purple viologens become colored when the voltage increases. This is because the reduction potential of the green viologens is less than that of purple viologens. Finally, an improved black color is obtained through a mixture of the two colors.