Optimization of electrochromic Mo doping WO3 films: A study on dual-phase stacked structures for energy-efficient smart windows

Amorphous tungsten trioxide (a-WO3) films were prepared on ITO conductive glass via electrodeposition and subsequently crystallized to obtain crystalline WO3 (c-WO3) films by heating a-WO3. A dual-phase stacked WO3 film was fabricated by covering Mo-a-WO3 film onto c-WO3/ITO substrate using electrodeposition and thermal-assisted electrodeposition methods, respectively. Optimization of electrochromic performance was achieved by varying Mo doping levels (0 similar to 5 atom%). Results demonstrate that appropriate Mo doping (3 atom%) enhances the electrochromic properties of a-WO3 films. Mo doping introduced structural distortions that reduced energy barriers and enhanced ion mobility, leading to improved electrochemical and electrochromic properties. The intermediate c-WO3 layer improves adhesion between a-WO3 top film and ITO glass substrate, while the porous structure of a-WO3 layer increases the number of active sites for electrochromic reactions. Mo3-a-WO3/c-WO3 dual-phase stacked film with doping 3 atom% Mo shows an optical modulation range of 83.4 % at 633 nm, a coloration efficiency of 74.3 cm(2)/C, rapid response time (bleaching/coloration: 3.4 s/6.1 s), and 86.6 % retention of its maximum current density after 2000 cycles, respectively. The high oxidation ion diffusion coefficient (3.53 x 10(-10) cm(2)/s) and reduction diffusion coefficient (1.55 x 10(-10) cm(2)/s) were also observed. This dual-phase stacked film shows significant improvements in electrochromic performance due to the synergistic effects between the dual phases and Mo-doping. Electrochromic device (ECD) assembled with Mo3-a-WO3/c-WO3 dual-phase films as the working electrode, ITO glass as the counter electrode, and 1 mol/L LiClO4/PC solution as the electrolyte exhibited an optical modulation range of 74.2 % and response time (bleaching/ coloring) of 6.8 s/3.7 s. These findings confirm that ECD with Mo-a-WO3/c-WO3 dual-phase films offer excellent electrochromic performance.