Niobium oxide thin films were deposited onto flexible ITO-coated PET substrates for electrochromic applications. The films were prepared by reactive DC magnetron sputtering, and three sets of samples were prepared:

i) first series: different O2/Ar flow rate ratios were used to prepare films with different Nb oxidation states;

ii) second series: the oxygen content was fixed and the time of deposition was modified;

iii) third series: the oxygen content and time of deposition varied.

A structural threshold as a function of the O2/Ar flow rate ratio was found, ranging from crystalline to amorphous nature, and from a nontransparent appearance with metallic-like conductivity to transparent and dielectric behaviour.

The transparent NbOx nanometer-thick films present a compact/dense and featureless morphology. The films surface roughness is in the order of a few nanometers and the maximum optical transmission, in the visible range, is 83%. The mechanical performance of the thin films grown on flexible substrates were evaluated by automated bending tests. Electrochemical cyclic voltammograms were performed and showed colour reversibility and reproducibility on flexible substrates opening new framework possibilities for new and high-technological electrochromic devices.