Experimental Results on Electrochromic Devices

Impedance Spectroscopy. Impedance spectroscopy has been carried out on devices with WO3 as the cathodic electrochromic layer, counter electrodes of iridium oxide, polyaniline or Prussian blue, and polymers as electrolytes (Katsube et al [1986], Friestad et al [1997]). The equivalent circuit for a whole device becomes very complicated. In the works quoted above simplified, Randles-type circuits were used for the two electrochromic layers, while the ion conductor was modeled by a pure resistance, or neglected. Extraction of device parameters from the data fitting was reported. However, it is clear that in many cases it will be difficult to distinguish the contributions from the different layers in a device, in particular if the migration impedances, ion diffusion impedances, etc. are of the same order of magnitude. When it comes to characterizing electrochromic devices, impedance spectroscopy is a very time-consuming process, since a spectrum down to low frequencies should be taken at a number of equilibrium potentials. Thus we believe that transient current measurements in many cases offer a faster alternative that sometimes allows a simple determination of diffusion coefficients. 

Transient Ion Currents. Transient current measurements, according to Section 4.3.4.3, have been performed on laminated devices (Jonsson et al [2004]) as well 

The transient current versus time response for all-solid-state devices and laminated devices show some interesting common features. The initial response is similar in the two devices although the response is slower in the all-solid-state device, hi both types of devices the ion transport is markedly quicker from WO3 into NiO than in the opposite direction. Also, pronounced peaks and shoulders are only present when ions are transported from the NiO layer towards the WO3 layer. This asymmetric behavior indicates that barriers of different magnitude are present at the interfaces between the oxide layers and that these barriers strongly affect the transient current response. 

The investigation of transient ionic currents in electrochromic devices is still in the initial stage. The theory of Frenning et al. [2003] constitutes a good starting point for detailed comparisons with experiments, but further developments are probably needed in order to reach a thorough understanding. 

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