Electrochromic contrast

Hahn, R., Ghicov,A,Tsuchiya, H.,Macak,]. M., Munoz,A G.,and Schmuki, P. [2007]. Lithium-ion insertion in anodic Ti02 nanotubes resulting in high electrochromic contrast. 5 International Conference on Porous Semiconductors — Science and Technology. Physica Status SolidiA —Applications and Materials Science., 204, pp. 1281-1285. 

Not only do oxidatively doped polypyrrole films show significant environmental stability, but a number of these films can also be electrochemically prepared and switched in both organic and aqueous solvents [90]. While poly(N-methylpyrrole) was the only polymer of the series that exhibited a comparable electrochromic contrast in both solvents, N-benzyl (19a), N-tolyl (19b), and the parent polypyrrole showed a decrease in electrochromic contrast when comparing results obtained using organic and aqueous electrolytes. Polymers prepared using N-phenyl and N-benzoylpyrrole (20) showed no electrochromic switching in aqueous solutions. 

Mishra, S.P., K. Krishnamoorthy, R Sahoo, and A. Kumar. 2004. Synthesis and characterization of monosubstituted and disubstituted poly(3,4-propylenedioxythiophene) derivatives with high electrochromic contrast in the visible region. / Polym Sci Part A Polym Chem 43 419-428. 

Girotto, E.M. and M.-A. De Paoli. 1998. Polypyrrole color modulation and electrochromic contrast enhancement by doping with a dye. Adv Mater 10 790—793. 

Stability. Another major facet in the overall performance of electrochromic materials is the stability of the system. Many potential issues, such as degradation of the active redox couple due to irreversible oxidation or reduction at extreme potentials, iR loss of the electrode or the electrolyte, detrimental side reactions due to the presence of water or oxygen in the cell or heat release due to the resistive parts in the system, can result in the eventual loss of electrochromic contrast and function [7]. One effective approach towards the improvement of electrochemical cycling stability is the use of boron fluoride ethyl ether (REEF.) during the electrochemical synthesis of some heterocyclic compounds (including thiophene and bithiophene) [20]. For example, Camurlu et al. have shown reasonable stability and a switching speed of less than 1.5 s for the homopolymer of hexanedioic acid bis (2-thiophen-3-ylethyl) ester (HABTE) [21],... 

Polythiophenes and their derivatives have been intensely studied due to their interesting electronic properties. Owing to the combination of their electronic properties, environmental stability, stmctural versatility, low bandgap, low cost and ease of preparation, polythiophene and its derivatives have been utilized in the development of many new electrochromic devices. Here, we focus on the use of polythiophenes for electrochromic applications in terms of their basic properties bandgap and its relation to stability, chain length of substituted functional groups and optical properties such as electrochromic contrast (with some examples from the literature). 

Currently, PSS is used as a polyelectrolyte to stabilize the PEDOT dispersion. However, use of PSS in these dispersions, has certain drawbacks such as its nonconducting nature and also it limits the shelf life as it releases sulfuric acid (desulfonation of styrene sulfonic acid side chain) with time. Various efforts need to be directed towards possible replacements of PSS or to synthesize PEDOT powders that can be dispersed easily in water for final processing. Another direction could be to develop new additives or formulations to further improve the properties of the existing PEDOT dispersion such as conductivity, transparency and electrochromic contrast. Recent efforts in improving the conductivity of Clevios-P ( 3000 S cm while retaining the transparency (>85%) in the visible region are very significant. These results open up a whole new area of applications of PEDOT as a potential replacement of ITO in the field of transparent conductors. 

An example of a case where the additive itself is electrochromic is the encapsulation of the redox indicator dye indigo carmine within a polypyrrole matrix (93,94). More recently, 3-D graphs have been constructed from absorbance data acquired during cyclic voltammetry. The enhancement and modulation of the color change on indigo carmine insertion into polypyrrole or polypyr-role/dodecylsulfonate films was shown (95). As expected, the use of indigo carmine as dopant improves the films electrochromic contrast ratio. 

Table 1 shows typical emittance data for two devices. The emittance data may be compared with the variation observed in extant mechanical louvers, ca. 0.16 to 0.56 Ae = 0.4) (3). We also note that, to the best of our knowledge, these dynamic range (i.e. light/dark electrochromic contrast) values, e.g. > 50% Reflectance at 3 to 12 ftm (Figure 3) and Ae> 0.5 (Table I), represent the largest dynamic (i.e. switchable) IR signature variation in arsy material... 

A detailed investigation of PEDOT-S has been performed by Reynolds and coworkers. PEDOT-S— both chemically and electrochemically prepared— was characterized and studied regarding its electrochromic and its hole trans-portproperties. The electrochromic contrast was found tobelowerthaninmany other electrochromic polymers, but rapid switching could be performed. 

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