Stability electrochromic devices

Further research on the substitution of the thiophene 3-position with phenyl groups containing electron-withdrawing or electron-donating groups (such as methyl, methoxy, fluoro, chloro, bromo, trifluoromethyl, sulfoxy) in the para position have lead to polymers with unique features [57]. The electron-withdraw-ing groups allow the formation of a radical anion and thus stabilize the n-doped state. As a result, such conducting polymers can be reversibly oxidized and reduced and electrochromic devices can be built with identical anode and cathode materials [58]. 

Stability tests performed in sandwich-type cells containing the dyes adsorbed on Sn02/Sb electrodes demonstrated a high stability, with optical density changes lower than 2% after cycling the electrochromic device 20000 times between -0.5 and +0.5 V. 

The presence of alkali metal ions is crucial for the stabilization of excess charge trapped within the nanopartides. Intercalation of metal ions within the nanoparticle thus becomes a limiting factor as the rate of transport of these ions becomes slower in thicker metal oxide films. This in turn controls the rate of coloration and recovery of the electrochromic effects. Limited efforts have also been made to employ mixed Ti02/W03 [145], WO3/V2O5 [146], and WO3/M0O3 [147] systems to enhance the efficiency of electrochromic effects. The beneficial aspect of these nanostructured semiconductor films in electrochromic devices is yet to be explored in a systematic way. 

Among the conjugated polymers, polypyrrole (PPy) is the most representative one for its easy polymerization and wide application in gas sensors, electrochromic devices and batteries. Polypyrrole can be produced in the form of powders, coatings, or films. It is intrinsically conductive, stable and can be quite easily produced also continuously. The preparation of polypyrrole by oxidation of pyrrole dates back to 1888 and by electrochemical polymerization to 1957. However, this organic p>-system attracted general interest and was foimd to be electrically conductive in 1963. Polypyrrole has a high mechanical and chemical stability and can be produced continuously as flexible film (thickness 80 mm trade name Lutamer, BASF) by electrochemical techniques. Conductive polypyrrole films are obtained directly by anodic polymerization of pyrrole in aqueous or organic electrolytes. 

The combination of favorable properties of PANI and TiO opens the possibility for various applications of PANI/TiO nanocomposite materials, such as piezoresistivity devices [41], electrochromic devices [99,118], photoelectrochemical devices [43,76], photovoltaic devices/solar cells [44,50,60,61,93,119], optoelectronic devices/UV detectors [115], catalysts [80], photocatalysts [52,63,74,75,78,84,87,97,104,107,121,122,125], photoelectrocatalysts [122,123], sensors [56,61,65,69,85,86,95,120,124], photoelectrochemical [110] and microbial fuel cells [71], supercapacitors [90,92,100,109,111], anode materials for lithium-ion batteries [101,102], materials for corrosion protection [82,113], microwave absorption materials [77,87,89], and electrorheological fluids [105,106]. In comparison with PANI, the covalently bonded PANI/TiO hybrids showed significant enhancement in optical contrast and coloration efficiency [99]. It was observed that the TiO nanodomains covalently bonded to PANI can act as electron acceptors, reducing the oxidation potential and band gap of PANI, thus improving the long-term electrochromic stability [99]. Colloidal... 

PEDOT is dark opaque blue in its reduced form and transmissive light blue in its oxidized form [ 107]. On this basis, efforts directed to the application of EDOT-based polymers in electrochromic devices have been focused on the improvement of the solubility, stability, switching time, and optical contrast [58,291,292]. 

The interest in electrochromic devices lies in the fact that they have a number of specific advantages, such as high optical contrast with continuous variation of the transmittance and no dependence on viewing angle, optical memory, UV stability, and wide operation temperature ranges. These favourable characteristics may ultimately overcome the well known deficiencies of liquid crystal displays and thus place electrochromic devices in a prominent position for the production of large visual angle panels. 

Poly (thiophene)s are of particular interest as electfochromic materials owing to their chemical stability, ease of synthesis and processability. For the most part, current research has been focused on composites, blends and copolymer formations of several conjugated polyheterocyclics, polythiophene and its derivatives, especially PEIX)T. In one example, poly(3,4-ethylenedioxythiophene) (PEDOT)/poly(2-acrylamido-2-methyl-l-propanesulfonate) (PAMPS) composite films were prepared by Sonmez et al. for alternative electrochromic applications [50]. Thin composite films comprised of PEDOT/PAMPS were reported to switch rapidly between oxidized and neufial states, in less than 0.4 s, with an initial optical contrast of 76% at A.max. 615 nm. Nanostructured blends of electrochromic polymers such as polypyrrole and poly(3,4-ethylenedioxythiophene) were developed via self-assembly by Inganas etal. for application as an electrochromic window [26]. Uniir etal. developed a graft-type electrochromic copolymer of polythiophene and polytetrahydrofuran for use in elecfiochromic devices [51]. Two EDOT-based copolymers, poly[(3,4-ethylenedioxythiophene)-aZ/-(2,5-dioctyloxyphenylene)] and poly[(3,4-ethylenedioxythiophene)-aft-(9,9 -dioctylfluorene)] were developed by Aubert et al. as other candidates for electrochromic device development [52],... 

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). 

A new alkylenedioxythiophene derivative, spirobipropylenedioxythiophene [poly(spiroBiProDOT)], was reported by Reeves et al., which exhibited three color states and a luminance change of 30 % at intermediate potentials, leading to electrochromic devices with greater stability and longer lifetimes (Figure 20.11) [61]. 

Dual-type polymer electrochromic devices based on copolymers of 2-benzyl-5,12-dihydro-27f-pyrrolo [3, 4 2,3] [1, 4]dioxocino[6,7-6]quinoxaline (DPOQ) and 5,12-dihydrothieno[3, 4 2,3] [1, 4]dithiocino [6,7- >]quinoxaline (DTTQ) with bithiophene were developed. P (DPOQ-co-BT) and P(DTTQ-co-BT) were used as the anodically coloring and PEDOT as the cathodically coloring electrochromic materials [81]. Each device performed with a favorable switching time, optical contrast, open-circuit memory and stability. 

Dual-type absorptive/transmissive polymer electrochromic devices based on poly[thiophen-3-ylacetic acid 4-pyrrol-1-ylphenyl ester (TAPE)-co-A -methylpyrrole (NMPy)] and PEDOT have been assembled, which exhibit good optical memory, stability with moderate switching times and light yellow and green colors upon doping and dedoping, respectively [82],... 

Transition metal oxides that do not change their transparency, or color very little, under ion/electron insertion and extraction can also be used as a counter electrode in electrochromic devices anploying tungsten oxide as a cathodic material. There has been particular interest in oxides based on vanadium pentoxide and cerium oxide. Pure V2O5 as well as a mixture of vanadium and titanium oxide are of interest. Cerium-based mixed oxides, in particular cerium-zirconium oxide (Veszelei et al. [1999]), exhibit less optical absorption, but the stability is not sufficient for many applications. 

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