Electrochromic properties of inkjet printed vanadium oxide gel on flexible polyethylene terephthalate/indium tin oxide electrodes.

Vanadium oxide gel was synthesized and formulated for the assembly of solid-state electrochromic cells on flexible and transparent electrodes using inkjet printing. FTIR, Raman, and X-ray diffraction spectroscopic measurements showed that the vanadium oxide gel here synthesized consisted of V(2)O(5)·6H(2)O, microstructures similar to orthorhombic V(2)O(5), while Raman spectroscopy also shows the presence of amorphous domains. Atomic force microscopy (AFM) images of the thin films printed using an inkjet shows a ribbonlike structure, which is in accordance with previous results of the vanadium oxide gels in solution. Solid-state electrochromic devices were assembled at room temperature using the inkjet printed films, without any sinterization step. The electrochemical properties of the vanadium oxide gel were characterized by cyclic voltammetry and spectroelectrochemistry by visible/NIR absorption spectroscopy (in both liquid and solid-state). Several redox steps are observed, which gives rise to a variety of color transitions as a function of the applied voltage. The different optical properties of the vanadium oxide gel are assigned to different intercalation steps of Li(+), leading to different crystalline phases of the gel. The final result is a solid-state electrochromic cell showing excellent contrast between the redox states, giving rise to colors such as yellow, green, or blue. Color space analysis was used to characterize the electrochromic transitions, and while absorption spectra showed rather long switching times (up to 100 s), in L*a*b* color space coordinates, the switching time is smaller than 30 s. These electrochromic cells also have an excellent cycling stability showing high reversibility and a cyclability up to more than 30,000 cycles with a degradation of 18%.