Electrochromic materials types

The physical classification dilTerentiates three types of electrochromic materials. Type I electrochromic materials are always in solution. MetaUic ions belong to this class. Type II electrochromic materials are colourless and in solution at one state and coloured and solid at the other state. Heptyl viologen is type II. Type in electrochromic materials are always solid. Most electrochromic materials are type HI, including conducting polymers or metal oxides [59,60]. 

Metal Oxides Tungsten trioxide, undoubtedly the most widely studied electrochromic material, is used in several types of commercial electrochromic devices. 

Other Inorganic Compounds Prussian Blue represents another type of inorganic mixed valence electrochromic materials. This material is known in three states ... 

It is important, however, to realize that whilst many types of chemical species exhibit electro-chromism, only those with favorable electrochromic performance parameters1 are potentially useful in commercial applications. Thus, most applications require electrochromic materials with a high contrast ratio, coloration efficiency, cycle life, and write-erase efficiency.1 Some performance parameters are application dependent displays need low response times, whereas smart windows can tolerate response times of up to several minutes. 

Stabilization of the redox cycle is relatively important in construction of potentially useful electrochromic materials, because the molecules needed for application require high redox-stability. Recently, S. Hiinig et al. proposed the concept of violene-cyanine hybrid to produce stabilized organic electrochromic materials (3). The hybrid is constructed by a violene-type redox system containing delocalized closed-shell polymethine dyes as end groups. The hybrid is expected to exhibit the color of a cyanine dye, by an overall two-electron... 

Electrochromic materials of interest can be classified according to three types of color change (1) from a bleached state (transparent) to a colored state, (2) between two colored states, and (3) between several colored states, if more than two redox states are accessible. This behavior is called polyelectrochromic... 

Electrochromic materials are of three basic types [i]. In a given -> electrolyte solution, type I materials are soluble in both the reduced and oxidized (redox) states, an example being l,l -di-methyl-4,4 -bipyridylium ( methyl viologen ), which, on reduction, switches from the colorless di-cation to the blue radical cation. Type II materials are soluble in one redox state, but form a solid film on the surface of an electrode following electron transfer. An example here is l,l -di-heptyl-4,4 -bipyridylium ( heptyl viologen ). In type III materials, such as -> tungsten oxide, - Prussian blue, and electroactive conjugated polymers, both... 

Several classes of organic electrochromic materials are known. The triphenylamine unit can be used to be impart electrochromism into PAI resins. In particular, AA -bis(4-aminophenyl)-A -diphenyl-1,4-phenyl-enediamine and 4,4 -diamino-4 -methoxytriphenylamine can be condensed with bis(trimellitimide)s to get electrochromic PAI types. ... 

Similarly, introduction of carbazole in 174 also leads to a polymer presenting distinct redox processes and hence optical states [297,298]. Unlike electrochromic materials based on pure PEDOT, these compounds are colored in the oxidized state and colorless in the neutral one. The exploitation of the complementary properties of these two types of polymers has led to electrochromic devices exhibiting a large variety of colors [122],... 

The absorptive/transmissive-type ECD operates with a reversible switching of the electrochromic materials between a colored state and a bleached state. Both working electrode and counter electrode are transparent so that light can pass through the device [4,5,15,250]. For flexible devices, ITO, SWNT, or PEDOT/PSS deposited onto a plastic such as poly(ethylene terepthalate) (PET) have been used [258,259]. When deposited in the doped form and dried, PEDOT/PSS films, to a thickness of 300 nm, are relatively transmissive in the visible region ( 75% T), have a relatively low resistivity (500 fi/D), and adhere to the plastic substrate in most common electrolyte solutions. The polymer films were demonstrated to be useable over the operating range of the device with no loss in conductivity or transmissivity. 

Lampert and coworkers [36] have used a modified amorphous PEO-LiCFaSOs electrolyte for the realization of WO3 laminated windows using several types of counter-electrodes, such as niobium oxide, nickel oxide and a new class of solid redox polymerization electrodes [63]. These latter electrodes have an advantage over inorganic layers in that they can be tailored to the electrochromic material and ion specifically. Figure 8.18 illustrates the optical transmittance of a EW made of WOa/modified a-PEO/ion storage polymer [63]. 

Through the blending of different types of nanocomposite particles, as well as through control of the relative amounts of the different electrochromic materials combined in a single ink, the properties of the inks can be easily tuned to exhibit the desired electrochromic effect. 

The alteration of the optical properties for an electrochromic material involves the insertion or extraction of charge. These polymers can be classified into three types, depending on their specific optical states (1) absorption/transmission-type materials made of metal oxides, viologens or polymers such as PEDOT, including at least one colored and one bleached state for smart windows, (2) display-type materials made of polythiophenes without a transmissive state and (3) materials composed of blends, laminates and copolymers including more than two colored states [7],... 

In another study, Argun and Reynolds also devised a reflective-type lateral ECD using PEDOT as the electrochromic material with both cross-patterning and line-patterning methods, as shown in Figure 20.10 [48],... 

A new bipropylenedioxythiophene, poly(spiroBiProDOT), has been reported with dual cathodically and anodically coloring properties, displaying three different colors in the oxidized, neutral and reduced states [61, 73]. (l-Phenylethyl)-2,5-di(2-thienyl)-17f-pyrrole [P(PETPy)] was used as the anodically coloring material and PEDOT as the cathodically coloring electrochromic material for dual-type ECDs [4]. 

The production of patterned, rapid-switching, reflective ECDs has been demonstrated by Aubert et al. with active electrochromic materials such as PEDOT, (PProDOT) and the dimethyl-substituted derivative PProDOT-Me2, whose resulting switching times were 0.1-0.2 s (5-10 Hz) [51, 56]. In another dual-type polymer PEDOT and PBEDOT-B(OCi2H2s)2 reflective device, a 2 x 2 pixelated lateral configuration has been shown. 

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. 

Determination of Parameters from Randles Circuit. Electrochemical three-electrode impedance spectra taken on electrochromic materials can very often be fitted to the Randles equivalent circuit (Randles [1947]) displayed in Figure 4.3.17. In this circuit R /denotes the high frequency resistance of the electrolyte, Ra is the charge-transfer resistance associated with the ion injection from the electrolyte into the electrochromic film and Zt, is a Warburg diffusion impedance of either semi-infinite, or finite-length type (Ho et al. [1980]). The CPEdi is a constant phase element describing the distributed capacitance of the electrochemical double layer between the electrolyte and the film having an impedance that can be expressed as... 

Cathodic Electrochromic Materials—Fluorinated Ti Oxide. Figure 4.3.18 shows two electrochemical three-electrode impedance spectra taken at different temperatures on a heavily intercalated Li containing flnorine doped Ti oxide film (Str0mme Mattsson et al. [1997]). The impedance response corresponds to that of the Randles circuit with a Zd of finite-length type. Details about the film preparation and the measurement conditions can be obtained from Strpmme Mattsson et al. [1996c, 1997]. The high frequency semicircle clearly has a center below the real... 

Cathodic Electrochromic Materials—Tungsten Trioxide. Figure 4.3.20 shows electrochemical impedance spectra on both amorphous and crystalline Li containing WO3 films together with fits to the Randles circuit (Strpmme Mattsson [2000]). For the amorphous film the high frequency semicircle overlaps with the diffusion response. In the case of the crystalline film, only a part of the semicircle due to Cdi and Ra, can be observed. As is obvious from the displayed spectra, the charge transfer resistance is much larger for the crystalhne sample than for the disordered one at an equilibrium potential of 2.9 V vs. the Li reference electrode used in the experiment. Impedance spectra were taken at several equihbrium potentials, and in all cases the impedance response corresponded to that of the Randles circuit with a Zd of semi-infinite type. 

---