Electrochromic W_1–x–yTi_xMo_yO₃ Thin Films Made by Sputter Deposition: Large Optical Modulation, Good Cycling Durability, and Approximate Color Neutrality

Tungsten oxide thin films are used in electrochromic devices such as variable-transmittance “smart windows” for energy efficient buildings with good indoor comfort. Two long-standing issues for WO₃ thin films are their limited durability under electrochemical cycling and their blue color in transmission. Here, we show that both of these problems can be significantly alleviated by additions of titanium and molybdenum. We found that ∼300 nm-thick films of sputter deposited W_1–x–yTi_xMo_yO₃ are able to combine a midluminous transmittance modulation of ∼70% with good color neutrality and durability under extended electrochemical cycling. The Ti content should be ∼10 at. % in order to achieve durability without impairing transmittance modulation significantly, and the Mo content preferably should be no larger than 6 at. % in order to maintain durability. Hence, our results give clear guidelines for making three-component mixed-oxide thin films that are suitable for electrochromic “smart windows”.