Electrochromic devices response times

Metal-oxide-based electrochromic systems are especially interesting for the development of electrochromic windows because they mostly switch from a transparent state to a dark colored state [38,39]. In addition, their relatively slow response times are acceptable for this kind of application, possibly even preferable from an aesthetic point of view. Again, W03 has seen the most use in the development of actual devices. Several different deposition techniques have been applied. For example, a prototype electrochromic window based on W03 with reasonable dimensions (0.7 X 1 m) has been assembled that reduces light transmission by a factor of 4 in its colored state [28]. 

A second important aspect of electrochromism is the temporal response under alternating potentials ( 0.55 V). The DG showed sharp and distinct transitions between the colored/oxidized and bleached/reduced state across the entire visible spectrum (Fig. 6.10b). This time-resolved switching behavior was analyzed in more detail at A = 630 nm (Fig. 6.10c). The DG device showed short characteristic response times of 53 ms for the bleaching step and 63 ms for the reverse process, determined by fitting exponential functions to the switching curves. These short response times which are close to video rate (24 frames per second) are enabled by the short ion diffusion distance through the only >= 5 nm thick NiO nanotube wall. 

The electrochromic performance of conducting polymers has been investigated since 1983 [7, 11-23], most of the studies having been carried out in liquid electrolyte. However, the data reported in the early papers are not exhaustive enough to evaluate the parameters necessary for the use of the electrochromic material in device technology. These data are generally of electrical response time only (i.e. the time the polymer takes to store charge) and may not be representative of electrochromic response time. 

On the other hand, oxyalkyl-substituted polymers are also potentially attractive for the known cation coordinating properties of their ether groups, which are expected to improve ionic transport in the polymers and, hence, to give faster electrochromic response time. The ether-group compatibility of these polymers with polyethylenoxide-based polymer electrolytes is also expected to improve the electrode-electrolyte contact in solid-state electrochromic devices. 

Figure 4.3.27. Transient current versus time response of laminated tungsten oxide-polymer electrolyte-nickel oxide electrochromic device for different directions of the ion movement. The break in the curve at 9 s is due to measurement artifacts.

In any case, energy to write or erase devices based on such electrochromes will be typical of the necessary energy used in wet electrochemical display devices such as those using metal plating or polyviologen (U8). Operation of these devices require from 1 to 10 mJ/cm with response times in the tens of millisecond range. 

Alternative proton-conducting electrolytes, for which device-related work has been reported, are PEO-H3PO4, PVP-H3P04, 9 PE1-H2S04, > and PEI-H3PO4. 0> > The polymers based on PEO and PVP have a strongly temperature-dependent conductivity, which leads to a corresponding variation of the c/b response time for electrochromic devices. Hence... 

The first demonstration of a PEM with electrochromic properties was disclosed by SchlenofFand coworkers [66], using poly(butanylviologen)/ PSS films. While this film exhibited strong electrochromic response, it still required the use of an outer electrolyte solution. DeLongchamp and Hammond disclosed for the first time a solid-state device comprised of two electrochromic PEM-modified ITO electrodes separated by a 200-p,m thick poly(2-acrylamido-methane-2-propanesulfonic acid), proton-conducting PAMPS membrane (see Eigure 2.30) [196]. Both PEMs used in... 

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