Metal-oxide films for electrochromic applications: present technology and future directions

Many transition metal-oxide films exhibit an electrochromic (EC) effect as they change their optical transmittance upon charge insertion or extraction. These materials may be integrated into multilayer devices, and the optical modulation is then produced by application of a small electrical voltage. Electrochromic films are therefore being developed for application in dynamic or ‘‘smart’’ windows that are at the forefront of emerging energy-saving advances in building technologies. Here we will describe the state-of-the-art technology that is being implemented in commercial applications. It predominantly relies on the use of tungsten oxide-based films (coloring with ion insertion) and nickel oxide-based films (coloring with ion extraction). We also suggest future research directions that are motivated by the need to reduce the production costs of large-area EC windows. Specifically, we describe the possibility of alternative less expensive manufacturing processes, as well as the development of flexible EC devices that allow for an inexpensive ‘‘retrofit’’ installation to existing structures.