Charge redistribution in NiO induced by Cu2+ doping for enhanced electrochromic kinetics and stability

Nickel oxide (NiO) is an ideal candidate for electrochromic smart windows due to its neutral color appearance.   However,   its practical application is hindered by narrow spectral modulation, low electrical conductivity, and poor cycling stability. While strategic doping offers a promising solution to these limitations, the underlying mechanisms remain insufficiently elucidated. In this work, we strategically employ Cu 2+ doping to investigate how controlled doping optimizes charge transport and enhances electrochromic performance. The optimized NiO film with 3 % Cu 2+ doping exhibits remarkable improvements, achieving broadband optical modulation (82.7 % at 550 nm and 55.1 % at 1000 nm), faster switching kinetics (1.90/4.11 s), and excellent cycling stability (negligible degradation after 3,000 cycles). Theoretical and experimental analyses reveal that these improvements originate from doping induced charge redistribution, which stabilizes Ni 3+ species with oxygen vacancies, increases charge carrier density, and reduces OH − binding energy. These findings establish a clear structure-performance relationship between electronic structure modulation and electrochromic behavior. Furthermore, this mechanistic understanding guided the fabrication of a bifunctional device that integrates optical modulation with energy storage functionality. The elucidation of the charge redistribution mechanism provides a targeted doping strategy for designing high-performance electrochromic materials and offers a practical pathway toward advanced multifunctional smart windows.