AN ELECTROCHROMIC DEVICE WORKING IN ABSENCE OF ION STORAGE COUNTERELECTRODE

The fabrication and the characterization of a full solid state electro chromic device are discussed. The peculiarity of the realized device i s the absence of a specific ion storage counterelectrode, in fact a fi lm of an electrochromic material (EC) is interfaced and coupled with a particular ion conducting solid polymer and a second conducting layer . This kind of structure greatly simplifies the procedure of fabricati on. The EC is Tungsten trioxide doped with Molybdenum oxide, electroch emically deposited on ITO coated glass. The electrolyte is a polymer f ilm of Nafion-H(TM) (Trade Mark of E.I. Du Pont de Nemours). The prese nce of Nafion-H (hydrogenated form of Nafion), which has a very high p roton concentration, totally avoids the necessity of presence of anoth er component. Moreover, in the assembling procedure of the device, it is not necessary to charge preventively the electrochromic and/or the ion storage counterelectrode. The second electrode is constituted by a nother ITO coated glass. The realized device switches between the dark blue and transparent state with a low value of applied potential rang ing from -4 upto 2 V. Different waveform potentials have been tested i n the past in order to find the optimum bias conditions to operate bet ween stable phases. Previous experimental results indicate the trapezo idal waveform potential as the most convenient one. The visible optica l transmittance in the bleached and coloured state has been measured a nd has given the following values: T-v(bleached) = 57.7% and T-v(colou red) = 8.4%. Taking also into account the magnitude of the exchanged c harge (similar or equal to 12.8 mC/cm(2)), these values of transmissio n give a very good coloration efficiency. Dynamical measurements evide nce an acceptable switching rate, in fact the typical switching time f or the bleaching process is about 20 s, while a somewhat longer time i s necessary for coloration. The device exhibits quite good long memory properties and chemical stability.