Electrochromic (EC) property is the continuous reversible changing of optical transmission, absorption or reflection of materials by applying electric voltage. Tungsten oxide is one of the most applicable EC materials due to its high response velocity, appropriate coloring performance and good chemical stability near electrolyte solutions. EC devices are extensively used in smart windows, rear-view mirror s in automobile, digital displays, photo-electrochemical cells, etc. These devices play an important role in controlling and saving energy. The aim of this study is to enhance the efficiency of WO 3 - based EC devices. The crystal structure of tungsten oxide is one of the effective factors in EC performance of WO 3 thin films. WO 3 crystals transform structurally under heat treatment. Therefore, first the effect of heat treatment on EC characteristic of WO 3 thin films was investigated.It was observed that heat treatment leads to the reduction of ionic species penetration into the structure by compacting the crystal structure of WO 3 and hence cuases a decrease in EC efficiency of films. Since electrochemical reactions have significant influence on EC efficiency, the selection of a suitable electrolyte solution in the presence of an ion storage material is of interest. Phosphoric acid electrolyte solution is an appropriate ion conductor because of its small light H + ions. Moreover, Palladium (Pd) is a well-known hydrogen storage metal with high speed of storage. So, the effect of Pd on H + absorption rate in WO 3 thin films and EC response was explored. Our findings show that although Pd is capable to store electric energy by decreasing the reduction energy of tungsten oxide, it can act as a barrier against H + penetration into WO 3 structure. Furthermore, the effect of aging on electrochemical characteristics of pure and Pd doped samples was tested. Our results indicate that Pd increases the chemical stability of WO 3 thin films as well as EC durability. To prepare pure and Pd doped WO 3 thin films, the obtained tungstic acid via sol-gel route was doped with Pd in different molar ratios. The final sol was deposited on traarent conducting glass substrates (FTO) by dip coating method. The characterization of samples was achieved using cyclic voltammetry (CV) technique, X-ray diffraction (XRD), atomic force microscope (AFM), field emission scanning electron microscope (FE-SEM), Fourier transform infrared spectroscopy (FTIR), UV-Vis spectroscopy and a designed setup for recording optical transmittance. It has been demonstrated that the presence of pd in the WO 3 sol in the vicinity of phosphoric acid won’t increase the EC efficiency of films generally.
