Niobium–Molybdenum Oxide Thin Films Grown on Flexible ITO-Coated PET Substrates

Mixed transition metal oxides, known for their superior electrochemical performance, are widely used in lithium batteries, supercapacitors, and other energy storage devices. In electrochromics, which have evolved from single to multifunctional applications, these oxides are particularly valuable. Integrating charge storage in electrochromic devices represents a promising advance in energy technology. In this study, transparent niobium–molybdenum oxides were deposited on rigid and flexible substrates by using DC magnetron sputtering. Samples were prepared with Mo/Nb atomic ratios up to 0.62, with a fixed O₂/Ar flow ratio of 0.52 and a 15 min deposition time. Film thickness ranged from 195 nm (0 Mo) to 620 nm (0.62 Mo), displaying a featureless and amorphous morphology. Surface roughness (Sq) was on the nanometric scale, reaching a maximum of 18 nm. Optical transmission in the visible range was up to 82% for Mo-free films. Dielectric response increased with molybdenum content, peaking at ε’ = 294 (dielectric constant) at 300 K and 1 kHz for the 0.62 Mo sample. Films on flexible substrates showed improved mechanical durability in the bending tests. The electrochromic behavior was evaluated under Li⁺ intercalation, demonstrating strong reversibility and reproducibility, achieving a maximum coloration efficiency of 30.49 cm²/C, a maximum reversibility of 83%, and a relative optical modulation of up to 79%.