Compositional Gradient–Engineered Ti–WO ₃ Films for Simultaneous Enhancement of Coloration Efficiency and Mechanical Robustness

Electrochromic devices offer immense potential for energy-saving and adaptive optics, yet their advancement is hindered by slow ion diffusion and low charge utilization, which critically limit the development of next-generation flexible optoelectronic technologies. In this study, a gradient-engineered Ti-doped WO₃ architecture is developed to enable robust electron-ion coupling, leading to enhanced coloration efficiency and mechanical robustness. By dynamically modulating the sputtering powers of TiO₂ and W metal targets, a continuous Ti concentration gradient was established, forming a self-built internal electric field that promotes electron-ion synergy and accelerates Li⁺ transport. The optimized gradient film delivers a large optical modulation of 78.9% and a high coloration efficiency (CE) of 137.4 cm² C^-1, outperforming uniformly doped counterparts. The gradient structure suppresses abrupt band offsets and induces smooth energy band bending across the film, facilitating fast redox kinetics and enhanced reversibility. Furthermore, after 500 bending cycles, the film retains over 82% of its modulation amplitude and exhibits an increased CE of 213.73 cm² C^-1, confirming outstanding flexibility and stress adaptability. This gradient doping strategy unites the structural continuity of homojunctions with band engineering of heterojunctions, offering a universal design paradigm for high-performance flexible electrochromic and photoelectronic systems.