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

Abstract

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∼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 cm²/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 × 10⁻¹⁰ cm²/s) and reduction diffusion coefficient (1.55 × 10⁻¹⁰ cm²/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.