Electrochromism in nickel oxide films containing Mg, Al, Si, V, Zr, Nb, Ag, or Ta

Electrochromic “smart windows” are able to change their transmittance of light and solar energy by electrical charging/discharging, thereby being able to provide a comfortable indoor climate together with large energy efficiency. Electrochromic multilayer coatings with nickel oxide and tungsten oxide films joined by a polymer electrolyte have several good properties, but their optical absorptance has remained undesirably high. This paper reports on electrochromism in films based on nickel oxide with a variety of additives (Mg, Al, Si, V, Zr, Nb, Ag, or Ta). Emphasis is put on the perceived color under irradiation from different types of light sources. The films were prepared by reactive DC or RF magnetron co-sputtering. The bleached-state absorptance could be significantly lowered when the additive was Mg, Al, Si, Zr, Nb, or Ta, i.e., a type known to form oxides with large optical band gaps. The charge capacity was not impaired by this doping and, furthermore, the addition to the nickel sputter target rendered it non-magnetic, or weakly magnetic, which is a significant asset with regard to practical thin film deposition.