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

Abstract

Recently, tungsten oxides have attracted extensive attention in the field of photochromic smart windows due to the naturally ultraviolet response, high luminous transmittance (T_lum), and substantial solar light modulation (ΔT_sol). 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 T_lum and ΔT_sol 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.