Kinetic behavior of WO 3 -doped Nb 2 O 5 electrochromic thin films

An electrochromic material (EC) reversibly changes its optical characteristics response, coloring and bleaching states when a small voltage or current is passed through it. This phenomenon is used to develop electrochromic devices like smart windows, which control the amount of heat and light entering in a building and optimize energy consumption. The change of the transparency of these devices involves the injection and extraction of small cations and electrons into the EC material and study of the kinetics of ions injection implies on operation understanding of these devices. Pure and doped niobium oxides (Nb 2 O 5 ) are promising cathodic electrochromic materials and their electrooptical performance depends strongly of its structural morphology. The sol-gel process allows for facile fabrication of large area coatings at a low cost and offers advantages of controlling the composition and microstructure of the films. In order to study the solid sate diffusion of lithium into Nb 2 O 5 , Nb 2 O 5 :Li + and Nb 2 O 5 :WO 3 , two electroanalytical techniques have been used i.e. galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). GITT have been applied in order to obtain the chemical diffusion coefficient of Lix in Nb 2 O 5 doped and undoped films, where the values approaching were of the 2.5x10 -11 cm 2 s -1 at x=0,83, 7.4x10 -13 cm 2 s -1 at x=1.65 and 1.6x10 -10 cm 2 s -1 at x=0.33 for Nb 2 O 5 , Nb 2 O 5 :Li + and Nb 2 O 5 -WO 3 respectively. From these measurements it was also observed that within each film, D increases as x increases.