Studies on electrochromatic materials and devices
This thesis investigates electrochromic thin films needed to construct a variable transmission electro chromic device. Such a device is made of 5 layers sandwiched between 2 pieces of glass: two electronic transparent conducting layers, an optically active electro chromic layer (W03), a ion-conducting polymer electrolyte and an ionstorage layer (NiOx, TiOx, VOx, VzTiyOx) . Electrochromic NiOx thin films were produced by R.F. magnetron sputtering and electrodeposition techniques and studied under proton intercalation. A visible transmittance modulation of 0.70 and 0.80 and a visible coloration efficiency of 35 and 100 cm2.C-1 for a thickness of 300 and 200 nm were obtained for sputtered and chemically-deposited NiOx films respectively. Anodic films are extremely porous and soft. Under the mechanical stresses of ionic insertion/extraction they degrade more quickly than the compact nanostructure of physically deposited films. When studied under lithium intercalation, sputtered NiOx films exhibit a nucleation loop observed in cyclic voltammetry indicating the growth of a new phase and are seen to degrade quickly. NiOx films were not seen to be potential candidates for EC applications using Lt intercalation. W03, TiOx and VOx thin films were deposited by R.F. magnetron sputtering and studied under Lt intercalation/deintercalation. Optimised W03 films exhibited good electro chromic properties: a visible transmittance modulation of 0.82 and a visible coloration efficiency of 49 cm2 . C-l for a thickness of 450 nm. Electrochromic properties of TiOx films were seen to not strongly depend on the sputtering process parameters whereas VOx films showed a stronger dependence. TiOx films are able to store a limited quantity of charge Q = 13 mC.cm-2 for thicknesses greater than 13 nm. They are transparent in both charged and uncharged states T V,u and Tv,ch> 0.80, and are stable upon charge insertion/extraction. VOx films can store a much larger quantity of charge Q = 35 mC.cm-2 for a thickness of 70 nm. They are yellow in the uncharged state and bluish in the charged state: Tv,u and Tv,ch > 0.70, and the charge insertion/extraction process is seen to evolve during the initial cycles. Both TiOx and VOx films did not show all the required electrochromic properties for EC applications. The main achievement of this work was the development of highly durable vanadium/titanium mixed oxide thin films. Work was carried out on different VITi ratios using specific deposition techniques developed for that purpose. Films with a vanadium to titanium ratio of about 50 % showed optimum performance characteristics for passive ion storage layer applications. Such layers deposited on ITO exhibited high visible transmittance: Tv,ch > 0.62, and a relatively low visible modulation (0.20), with high storage capacity Q > 40 mC.cm-2 for a thickness of80 nm. The laminated W03IPAAUAlVzTiyOx EC device was assembled and exhibited under specific switching conditions encouraging properties: a visible transmittance modulation > 0.50 over more than 105 cycles.