Transparent photochromic Fe-doped W18O49 films with ultrahigh solar energy modulation for smart windows

Recently, tungsten oxides have attracted extensive attention in the field of photochromic smart windows due to the naturally ultraviolet response, high luminous transmittance (Tlum), and substantial solar light modulation (ΔTsol). To promote its applications, limitations such as slow bleaching speed and poor reversibility need to be addressed urgently. Here, we proposed a Fe3+-doping mediated rapid electron transfer strategy to improve the overall photochromic performance of W18O49. By increasing the Fe3+ doping concentration, the bleaching speed of W18O49 films was significantly improved. Notably, films doped with 6 at% and 8 at% Fe3+ could be fully bleached within 5 min, with bleaching rates twice that of undoped films. This improvement is attributed to Fe3+ doping, which creates more oxygen vacancies in W18O49, facilitating the reduction of W6+ to W5+. Furthermore, the 6 at% Fe3+-doped W18O49 bilayer film exhibited excellent photochromic performance, with the ultrahigh Tlum and ΔTsol of 82.9% and 60.2%, respectively, fast photochromic speed (within 2 min), and nearly complete near-infrared light blocking in the colored state. These findings demonstrate the potential of Fe3+-doped W18O49 to significantly improve the performance of photochromic smart windows, offering promising avenues for energy-efficient building technologies.