Electrochromism and electrochromic devices pdf

Viscosity of electrolyte, electrical potential To be commercially viable, ECDs must be able and dimensions of display are optimized to obtain to compete with LCD displays. LCDs are fabricated the best possible display characteristics. Electrochromes like viologen, shutters or light modulators. Such modulators are methylene blue and phenolsafranine dyes may be in- employed in fiber optic front ends as color filters. However, this tectors. Hence, an adjustable ECD is inserted in the display is based on phosphate groups chemisorbed to optical path before the detector which matches the a transition metal oxide surface.

The reduced form output laser intensity to that required by the detector. Since the ECD is to be used only as a filter, it remains more or less constant during transmission. Hence, the operation is not limited by response time. In contrast, if electrochromic filters are used as on- off shutters in fiber optic applications, they require sub nanosecond response time.

Hence, redox ECDs cannot be used for this application. For optical data storage, pixels do not need to 4. Electrochromic substances lose or gain 4. The electrodes here are Research in the field of electrochromic paper is replaced by chemical species to be analyzed.

The environmentally motivated. If a paper can be elec- color change can be correlated to amount of charge trochromically colored to print a pattern and then taken up and therefore to the concentration of sub- bleached, it can virtually be reused for ever. The idea stance being analyzed. A number of other electrochromic paper and use of an inert electrode as a pen. Organic elec- trochromes are by far most researched and patented Electrochrome Analyte for use in printing.

In Rosseinsky and Monk in- Chromium oxide Ozone vestigated whether voltametry in paper was possible. Metalloporphyrins Chlorine The study revealed marginal problems associated Nickel oxide Ozone with IR drop across the paper and variations in its Phthalocyanine Chlorine, NO2, toluene internal humidity.

The ma- fabricated by IBM in The device is capable of expressing 5 types of display patterns simply by varying the voltage in the range of All materials remain in solid state Such devices are used in refrigerators to indicate and reversibly change color. Therefore they can be whether food in under safe temperature limits.

If easily reused by applying a bias potential to bleach electrochromic indicator changes color, it indicates color. Tungsten trioxide is most sensitive to gas con- that food may be contaminated by bacterial activity.

Another novel application of electrochromes is explored by Eveready Battery Company who used an 4. Electrochromic layers of Electrochromes can be used in coloring of ac- PEDOT have been used as multi information layers cess cards, credit card labels, vouchers, tokens and in optical disks manufactured by Kojimo and Terao. Such applications can replace conventional Such disks have large capacity, high sensitivity in punch and paper and is advantageous in terms of recording and supporting hardware requirements are reusability.

The speed of coloring is dependent on temperature. At low temperature, the device is more or less passive and does not In a more recent application, electrochromes are change color even when a bias potential is applied. In a very simplified model, Such a device uses an ionic electrolyte with high a sheet of WO3 is intermittently coated on top of a activation energy for movement of counter ions.

The counter Since the activation energy is high, at lower tem- electrode is coated with a uniform sheet of WO3. The coloration of each square in the matrix is pro- Example of such a device is WO3 in conjugation portional to the charge passed by electrode. The with electrolyte containing poly ethylene oxide and color intensity can be observed by eye, or analyzed dissolved LiClO4.

Materials by doping with other metal oxides. For application in ECDs, amorphous forms are preferred. However, 5. Oxides of a number of transition metals like 5. They are widely used for thin Metal oxides are deposited on ECD electrodes film applications such as smart glass because they as a thin film that may be crystalline, polycrystalline do not undergo photochemical degradation like or- or amorphous.

Oxides which show very intense Chemical Vapor Deposition color changes are termed as primary electrochromes A volatile metal hexacarbonyl or alkoxide is in- e. The color change is high temperature and may contain carbon and hy- caused due to redox reaction in the species.

Presence of such impurities adversely affects the properties of device. Tungsten oxide in particular dissolves in film which is later annealed to give a crystalline water to form an aqueous acid. They are photochem- structure. The metal hydroxides degrade on heating to form ECDs made from these oxides have solid state oxides.

Some of these reversible chemical changes are a If residual hydroxides are present, the film is called part of electrochromic phenomena. The mechanical oxyhrydoxide MO. If more than one oxide is stresses in oxides of iridium, nickel and tungsten can to be deposited in a single film, electrodeposition be studied by deflection of laser beam incident on is governed both by kinetics diffusion of cations the material.

For small amounts of charge, stresses and thermodynamics partition of cations between developed are found to vary linearly with charges solution and oxide layer leading to a complicated inserted. Other characterization techniques include elec- Alternatively, ultrasound energy is used to agi- trochemical quartz-crystal microbalance EQCM , tate particles leading to formation of microbubbles X-ray diffraction and Raman vibrational spec- which later collapse into microstructures.

The first evenly spread the oxide layer by centrifugal force. Usually these are metal alkoxides Other methods or peroxometallate compounds that decompose in Apart from the 3 most commonly employed tech- liquid phase to form oxide particles comprising a niques, other methods used are electron beam splut- sol.

Sol aggregates into a 3 dimensional network to tering, radio-frequency rf spluttering, thermal de- form a gel. Oxides of cerium, cobalt, nickel and tungsten are deposited by spray pyrolysis, where gel is sprayed 5. The film formed may be subse- quently annealed to burn away organic components Electrochromic property of WO3 was first re- of gel.

It is one of the most commonly used In dip coating, the electrode is immersed in gel material in ECDs. WO3 film may be amorphous and then removed to form a thin oxide film. This a-WO3 or crystalline c-WO3 or a mixture of 2 process is useful for making mixed oxides. To pro- phases. Properties of WO3 are very electrolytic solvents, however, this also leads to dis- sensitive to the method of preparation.

Difference solution of oxide film. A microporous material also in morphologies of flims can be determined by X- has a better response time but is again susceptible to ray diffraction studies. Pure WO3 is pale yellow higher dissolution. This problem can be overcome whereas if thin films are deficient in oxygen, they by using anhydrous acids DMSO and alkali metal are pale blue. The oxide film can be covered WO3 can be prepared by number of techniques.

A Thermal vapor deposition was modified by Sun and very thin film of gold not only enhances the response Holloway. The evaporation is carried out in high time, but also increases durability. Spluttering with argon ions in low pres- property on reduction or addition of cations.

These sure oxygen leads to a chemically robust film. WO3 include Group VI metals like molybdenum and vana- may be electrodeposited by using carboxylate or per- dium. Group VIII metal oxides exhibit transition oxo precursors. For fabricating large area devices from colorless to colored state on oxidation or addi- like electrochromic windows, sol gel process is used.

Hydrated iridium oxide is one such example. Redox Properties When a reductive potential is applied to the film, 5. This Prussian blue or ferric ferrocyanide or iron III leads to a change in color.

The general formula of these compounds At low x, the electrochromic transition in the film is : is from colorless to blue. The optical properties is still not very well understood. Crystalline WO3 is optically absorbent as well as partially reflective,but amorphous form is mostly absorbent.

Fe III is rors and smart windows. Often the devices having in high spin and Fe II is low spin. Both atoms are WO3 as an electrochrome are fabricated with sec- coordinated octahedrally by N or C atoms. In several applications it is used in Films prepared by electrodeposition are partially conjugation with other electrochromic materials like oxidized to Prussian Green PG or may be totally prussian blue and conducting polymers.

Pure PX may The response time of these devices can be en- be prepared by electro-oxidation of a thin film of PB. All the oxidation states are 5. PB films can be electrochemically deposited on The film is maintained electrically neutral by a a number of electrode substrates. The electrolytic balance in movement of electrons from electrode solutions contain iron III and hexacyanoferrate III substrate to film and counter ions from electrolyte to ions.

Most practical electrochromic devices utilize the rate determining step in this process. The solu- PB to PW transition. However, other transitions may tion is in ionic equilibrium and is brown-yellow in also be suitably used to give a multicolored ECD.

Principles of nanofabrication may be utilized to produce thin films. SCE ] with KCl 0. Wavelengths abscissa are in nm. This is called early Source: Mortimer, R. Such process was first used by Tieke and co-workers to fabricate of multilayers of Prussian blue on gold surfaces.

A layer of PB is placed between two electrodes. On applying a potential, oxidation and reduction takes place at respective electrodes to form PX and PW respectively. Thus the device can differentially change color at different electrodes. However, the bulk material still remains as PB. PB can be used with WO3 to produce a com- plimentary coloring effect. They are used as thin films separated by a conducting material.

The two films work symbiotically, as in one case reduction yields blue color, whereas in other oxidation yields blue. Similarly it can be combined with conducting Of the three common redox states dication vi- polymers to change color from blue to light green.

This device counter ions. The intensity of cycles between redox states, and a pixel colour exhibited by di reduced viologens is very low integrated device with 8 levels of gray tone on a the obvious reason is no optical charge transfer. Use of long chain viologens avoids this problem because long chain viologen radical cation is insoluble.

In this system tin- erally produce blue coloured radical cation here this doped indium oxide In2 O3 :sn glass surface ITO crimson colour implies incorporation of red radical- with conductive side inward and a reflective metal- cation dimer in the solid deposited on the surface of lic surface are spaced a fraction of millimeter apart, electrode. This problem tively charged thiazine, possibly phenylenediamine can be avoided or overcome by addition of a redox as the anodically colouring electrochromic material.

During the opera- species migrates to their respective electrodes. Once tion hexacyanoferrate III is electro generated, this the dual electrochromic coloration process begins generated ferrate III chemically oxidizes deposited the species start diffusing away from electrodes and radical cation salt.

Alternate way to inhibit this crys- meet in the intervening solution where the mutual tallization process is use of asymmetric viologens. II, , replenishment of the colored ectroactive species lost controlling sensoron seeing daylight, which thus re- by their mutual self-erasing solution redox reaction. Because the oxide crystals are so small, such films have an extraordi- Bleaching occurs at short or open circuit by ho- narily high internal surface area. The ratio between mogeneous electron transport in the bulk of the so- the internal surface area and the smooth geometrical lution A photosensitive detector is placed facing areaof the electrode approaches for a film that rearward to monitor any dazzling incident light but, is only 4 mm thick.

The colored redox states of the electrochromic materials are shown in bold type. Source: Organic electrochromic materials review paper by Roger J. Mortimer ficiency. Furthermore, as electrochromic viologen phonated phenothiazine molecules. Nanostructured titanium dioxide in tionic viologen to its blue radical cation, and oxi- its anatase form can be deposited as a thin film of dizes the phenothiazine from its weak yellow colour high surface area.

Viologens are strongly adsorbed to red. The overall colour change is therefore from on its surfaceowing to their electron deficiency. Placing a Based on the above principles NTera Ltd have diffuse reflector between the electrodes, e. An assembled tanium dioxide, gives on coloration the visual effect NanoChromics electrochromic device uses two of ink on pure white paper. Without the intermediate metal-oxide films - one at the negative electrode TiO2 layer the display is transparent while retaining and, unusually, another at the positive electrode.

In readability. In such devices, many thou- film 4 mm thick, followed by a monolayer of self- sands of switches are possible before there is signifi- assembled, chemisorbed, phosphonated viologen cant degradation of performance. Some open-circuit molecules.

The positive F-doped tinoxide conduct- memory persists, the colour remaining for more than ing glass counter electrode i. Electrodes can be micro-patterned for tin oxide SnO2:Sb 3 mm thick, followed by a display applications.

The vertical pores of the elec- trode can support effective diffusion of leuco dyes perpendicular to the electrode and can prevent the diffusion of the dye around the electrode. Mortimer 5. The yellow-green undoped form undergoes re- Chemical or electrochemical oxidation of var- versible oxidation to the blue-violet conductive ious conjugated or resonance stabilized aromatic form, with insertion of charge-compensating an- compounds such as pyrrole produces conducting ions.

The succeeding mechanism is believed to Source: Electrochromic organic and polymeric involve either coupling between radical cations or materials for display applications Roger J.

Mor- reaction between radical cation and the monomer. Dyer, John R. Reynolds In undoped state parent poly-pyrrole films are in 5. As for dialkoxy substituted thiophenes, The reaction scheme for preparation is shown in addition of oxygen at three positions lowers the band figure 7. Since dimers are more easily oxidizable compared to Poly[3,4- ethylene dioxy pyrrole] PEDOP shows monomers, it results in further coupling of dimer rad- bright-red color in its neutral state and upon oxida- ical cation through proton loss and re-aromatization.

Films of good quality can be obtained on Furthermore, by increasing the ring size of the the surface of electrodes. Poly[3,4- propylene dioxy pyrrole] charge carriers electroactive conducting polymers PProDOP , in neutral state is orange in color are charge balanced, doped with counter ions P- and, by intermediate doping, passes through brown doping and have a delocalized p-electron band and finally upon full oxidation to light gray-blue structures. Such a P-doped polymer on reduction color.

Such poly electrochromism can also be ob- with counter anions removes electronic conjugation served in the substituted PProDOPs and poly[3,4- which results in un-doped insulating form.

Both are coupled bipotentiostatically. These were not polymers also exhibit multiple colored states at observed when they are studied individually.

These two poly- mers are thus anodically coloring polymers, in that Higher band gap polymers can be prepared they change from a colorless state to a colored one by substituting at nitrogen in poly 3,4-alkylene upon oxidation, in contrast with cathodically col- dioxypyrrole , which can retain their lower oxida- oring polymers that are colored in their reduced tion potentials.

Effective pi-conjugation will be de- state and become colorless upon oxidation. These creased by substitution at nitrogen which induces N-substituted polymers have been shown to work twist in polymer backbone, in-turn increasing poly- effectively in dual-polymer high-contrast absorp- mer band gap.

Mortimer Rauh and co-workers. The electro-coloration occurs exposure to such electrolytes. The solubility in- during reduction to form lithium fulleride, Lix C60 : creases with higher insertion coefficient, x. A band the insertion coefficient x: reversibility is lost if x is also forms in the UV. Figure 9 shows the spectrum of too high, as found with tungsten oxide Steep concen- C60 as a function of applied potential. Electrochem- tration gradients form in the fulleride films during ically formed Lix C60 is identical with the fulleride electrochromic operation.

Analysis is complicated salt formed by exposing C60 to alkali-metal vapour. Reaction with water table. The electrochrome Lix C60 catalyses , The four redox states of Polyaniline are: leucoemeraldine base LB, yel- low , emeraldine salt ES, green , emeraldine base 5.

The LB to ES transformation occurs under a very The synthesis of Polyaniline films is generally low driving potential and with high color contrast. The This is the most widely used method of prepar- adsorption maxima wavelengths of Polyaniline and ing Polyaniline in the industry. Peroxydisulfate is the conditions in which the synthesis is performed.

In most commonly used oxidant, and its ammonium neutral or basic conditions, the film prepared was salt was preferred to potassium counterpart because black and didnt exhibit any electrochromism. When of its better solubility in water. Since the reaction is prepared under acidic conditions, these films show exothermic, temperature can be used as an indicator reversible multicolor electrochromism.

The cyclic voltammogram and the color changes under continuous scanning of the applied potential from It shows reversible color changes from monium peroxidisulfate yields Polyaniline yellow, yellowish green, green, dark green, bluish EB hydrochloride green, blue and violet to brown, depending on the applied potential.

The UV-visible absorption spectrum measured after applying a potential to a PANI film coated on a Nesa glass can be seen in figure 10 b. They show two major absorption regions around - nm and above nm. The absorption at the wavelength above nm shifts to a shorter wavelength when the applied potential is changed from This blue shift of the absorption corresponds to Temperature profile of the chemical oxidation color changes of yellow If the Source: A typical temperature profile of the pro- absorption maximum shifts to around nm, as is cess recorded at the Institute of Macromolecular expected when the applied potential is above 0.

However, application of a potential above this commences and the temperature of thereaction mix- value indicated an irreversible color change of the ture increases; it passes through a maximum after PANI film. Whilst electrochemical polymerization is a very suitable method for the preparation of relatively low surface area electrochromic CP films, it may not Electro-polymerization be suitable for fabricating large area electrochromic A much more efficient method of producing displays.

Efforts have been made to synthesize solu- Polyaniline film is using electro-polymerization. Ag-AgCl electrode [12]. A large no. Some ITO surfaces. Slight modifications to thiophene monomer can 5. Advanced materials. Non-emissive electrochromic devices ECDs , which can be operated under a wide range of viewing and lighting conditions, are especially attractive in applications that include smart windows, … Expand.

Oxide electrochromics: An introduction to devices and materials. The paper … Expand. A review of organic electrochromic fabric devices. Electrochromic fabric devices represent a further extension to the plethora of available literature on conductive fabrics. This article contains a brief overview of electrochromic devices, … Expand.

Prominent electrochromism through vacancy-order melting in a complex oxide. Nature communications. Establishing dual electrogenerated chemiluminescence and multicolor electrochromism in functional ionic transition-metal complexes. Journal of the American Chemical Society. Development and Manufacture of Polymer-based Electrochromic Devices. Electrochromic symmetric and non-symmetric disubstituted oxo-bipyridinium ionic liquids are developed in high purities and yields.

Electrochromism and electrochromic devices of new extended viologen derivatives with various substituent benzene. Abstract New electrochromic devices based on six extended viologen derivatives with various aromatic substituents were manufactured and systematically investigated. With the introduction of various … Expand. Polymer-based electrochromic devices. Abstract Electrochromic phenomena related to p -doping-undoping process of thiophene-based polymers are briefly described.

Electrochromic performance data of poly 2,2'-bithiophene and … Expand. Electrochromic organic and polymeric materials for display applications. Polymer-based electrochromic devices—I.

Poly 3-methylthiophenes. The most important requisites for an electrochromic device are … Expand. New Electrochromic Materials. Science progress. Review on electrochromic devices for automotive glazing. Materials Science, Engineering. Electrochromic materials have been intensively studied for applications of various switchable optical systems.

These materials exhibit adjustable optical absorption upon reversible … Expand. Electrochromic Systems and the Prospects for Devices. Many inorganic and organic materials exhibit redox states with distinct electronic UV-vis absorption bands.

When the switching of redox states generates new or different visible region bands, the … Expand. Electrochromism of conducting polymers. Abstract An overview of the electrochromism of conducting polymers is given. Conducting polymers are promising materials for electrochromic applications, not only as a colorant but also as a … Expand.

Electrochromism and smart window design. Abstract An electrochromic device ECD has in its center an electrolyte or ion conductor, which is in contact with films that provide optical modulation, ion storage, and transparent electrical … Expand.

Electrochromic paper : utility of electrochromes incorporated in paper. Abstract Investigation of the electrochemical behaviour of the inorganic electrochromes tungsten oxide, molybdenum oxide, and Prussian blue PB , each within mashed paper, shows that WO3 and MoO3 are … Expand.