Harnessing 4f-orbital rare-earth ion chemistry in WO₃ for next-generation electrochromic windows with tunable modulation kinetics and enhanced ion–optical dynamics

Electrochromic (EC) smart windows offer a dynamic and energy-efficient solution for managing solar radiation in buildings by reversibly modulating light transmittance in response to an applied voltage. Among EC materials, amorphous tungsten oxide (WO₃) is widely recognized due to its broad optical modulation and well-defined W⁶⁺ to W⁵⁺ redox transitions. However, its practical deployment is hindered by slow switching kinetics, limited coloration efficiency, and structural degradation during long-term cycling. In this study, rare-earth (RE = Neodymium (Nd), terbium (Tb), and gadolinium (Gd)) doping was employed to tailor the structural, electronic, and ionic transport properties of amorphous WO₃ thin films synthesized <i>via</i> a peroxo-assisted electrodeposition method. Structural and spectroscopic analyses confirmed uniform dopant incorporation, elevated oxygen vacancy concentrations, and the preservation of the amorphous network. Among all compositions, the Tb-doped WO₃ (WTb) film exhibited the most favorable EC performance, achieving an optical contrast of 78.34%, coloration efficiency of 138.82 cm² C^-1, fast switching times of 16 s and 11.4 s, and a lithium-ion diffusion coefficient of 9.249 × 10^-11 cm² s^-1. A large-area EC device based on WTb demonstrated strong optical modulation, excellent reversibility, and long-term cycling durability. These findings establish RE doping as a powerful design strategy for engineering high-performance WO₃-based EC materials for next-generation smart windows.