Film Deposition of Electrochromic Metal Oxides through Spray Coating: A Descriptive Review

Electrochromism induces reversible changes of coloration in specific organic and inorganic materials through electrical charge/discharge reactions. When processed into thin films, electrochromic metal oxides can be integrated into glazing applications such as displays, rearview mirrors, goggles and, most notably, smart windows in energy-efficient buildings. Over the years, the use of spray coating as a liquid-based approach has been acknowledged for its cost-efficient, high-throughput samples production with a low volume consumption. It represents an interesting alternative to vacuum processes and to other wet methods, suitably responding to the current limitations of electrochromic thin films production by offering improved control over deposition parameters and capacities of up-scaling, together with lowered energetic and economic costs. The present review summarizes the main theoretical and practical aspects of spray coating, notably distinguishing room-temperature methodologies from pyrolysis-based, under heating protocols. The main families of functional electrochromic metal oxides are then screened and discussed, establishing how spray processing can challengingly lead to higher levels of optical contrast, commutation kinetics, coloration efficiency and cycling durability, and how low-toxic and environment-friendly precursors can be favored while sustaining large deposition areas.