טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentOz Lavi
SubjectElectrochromic Properties of WO3/ITO Films
DepartmentDepartment of Materials Science and Engineering
Supervisors Professor Emeritus Siegmann Arnon (Deceased)
Full Professor Frey Gitti
Full Professor Ein-Eli Yair
Full Thesis textFull thesis text - English Version


Abstract

Electrochromic materials change their color under an external electric field. The color change is commonly between a transparent and a colored state, or between two colored states.

Among the electrochromic materials, WO3 has been commonly used for smart windows, automotive mirrors, information displays, etc. When alkali ions, such as Li+, intercalate the WO3 structure, W+6 is reduced to W+5 and the yellow/green material turns blue. This process is reversible; upon extraction of the ions from the WO3 structure, the W+5 ions are oxidized back to W+6 and the material returns to its original color. The spectral-purity and switching-time between the colors is limited by the rate of ion drift in- and out-of the WO3 grains under the applied field.

In this thesis, Indium Tin Oxide (ITO) nanopowder was added to a polymer film containing WO3·H2O particles to enhance Li ion conductivity. WO3·H2O was synthesize from acid decomposition of solid Na2WO4·2H2O. Paste containing WO3·H2O, ITO nanopowder and a dissolved polymer was sprayed over a conductive transparent substrate.

Electrochemical characterization of the film was carried out by cyclic voltammetry measurements in a three electrode glass cell with 1 M LiPF6 in 1:1 EC:DMC (ethylene carbonate:di-methyl carbonate) as the electrolyte. The electrochromic effect was studied by performing electrochemical and optical measurements simultaneously. The electronic properties, chemical structure and morphology of the film were investigated by resistance measurements, X-ray diffraction, Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy, respectively.

Upon decreasing the film potential from OCV (open circuit voltage) at ~0.5 V vs. Ag/AgCl by 1.2 V, the color changed from greenish to strong blue, and upon increasing the voltage back to the OCV the film returns to its original color. It was found that film conductivity increases with the content of ITO nanopowder, with a noticeable reduction of the film resistance at 2-5 wt% ITO, suggesting that conductive networks are formed through the film at this content range. Furthermore, the largest difference in the transmittance between the two color states was found for the 8 wt% ITO nanopowder films, indicating that this content provides enhanced electrode's conductivity leading to rapid and intense color changes.