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Electrochromic Materials and Devices: Present and Future Prakash R Materials Chemistry and Physics 77 (2002) 117–133 Electrochromic materials and devices: present and future Prakash R. Somani a,∗, S. Radhakrishnan b a Photonics and Advanced Materials Laboratory, Centre for Materials for Electronics Technology (C-MET), Panchawati, Off Pashan Road, Pune 411008, India b National Chemical Laboratory (NCL), Polymer Science and Chemical Engineering, Pune 411008, India Received 17 May 2001; received in revised form 10 September 2001; accepted 26 September 2001 Abstract The increase in the interaction between man and machine has made display devices indispensable for visual communication. The informa- tion which is to be communicated from a machine can be often in the form of color images. Electrochromic display device (ECD) is one of the most powerful candidate for this purpose and has various merits such as multicolor, high contrast, optical memory, and no visual dependence on viewing angle. A large number of electrochromic materials are available from almost all branches of synthetic chemistry. In this review, we have tried to describe the fundamentals of such electrochromic materials and their use in EDDs. The most important examples from major classes of electrochromic materials namely transition metal oxides, Prussian blue, phthalocyanines, viologens, fullerenes, dyes and con- ducting polymers (including gels) are described. Examples of their use in both prototype and commercial electrochromic devices are given. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochromism; Electrochromic displays (ECD); Conducting polymers; Optoelectronic materials 1. Introduction multicolor-electrochromism. This optical change is effected by a small electric current at low dc potentials of the order Theoretical considerations suggest that the absorption of a fraction of volts to a few volts. and emission spectra of certain dyes may be shifted by Electrochromic materials are generally first studied at a hundreds of angstroms upon application of a strong electric single working electrode, under potentiostatic or galvanos- field. This effect is called “electrochromism”, in anal- tatic control, using three electrode circuitry. Electrochem- ogy to “thermochromism” and “photochromism” which ical techniques such as cyclic voltammetry, coulometry, describe changes of color produced by heat and light. chronoamperometry, all with, as appropriate, in situ spec- This definition of electrochromism does not, however, fit troscopic measurements are employed for characterization. within the modern sense of the world. An electrochromic An electrochromic device is essentially a rechargeable bat- material is the one that changes color in a persistent but tery in which the electrochromic electrode is separated by a reversible manner by an electrochemical reaction and the suitable solid or liquid electrolyte from a charge balancing phenomenon is called electrochromism. Electrochromism counter electrode, and the color changes occur by charging is the reversible and visible change in transmittance and/or and discharging the electrochemical cell with applied po- reflectance that is associated with an electrochemically tential of a few volts. After the resulting pulse of current has induced oxidation–reduction reaction. It results from the decayed and the color change has been effected, the new re- generation of different visible region electronic absorption dox state persists, with little or no input of power, in the so- bands on switching between redox states. The color change called “memory effect”. Fig. 1 illustrates an electrochromic is commonly between a transparent (“bleached”) state and device configuration. a colored state, or between two colored states. In case more The electrochromic electrode of these devices, which than two redox states are electrochemically available, the can work either in the reflective or transmissive mode, electrochromic material may exhibit several colors and may is constituted by a conductive, transparent glass coated be termed as polyelectrochromic or can be said to possess with electrochromic material. The counter electrode can be of any material that provides a reversible electrochem- ∗ Corresponding author. Tel.: +91-20-5899273/5898390; ical reaction in devices operating in the reflective mode fax: +91-20-5898180/5898085. (like electrochromic displays); by contrast, in variable light E-mail address: psomani [email protected] (P.R. Somani). transmission electrochromic devices (like electrochromic 0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0254-0584(01)00575-2 118 P.R. Somani, S. Radhakrishnan / Materials Chemistry and Physics 77 (2002) 117–133 Fig. 1. Schematic diagram of the EDD. windows). In variable light transmissive electrochromic is generally accepted that the injection and extraction of devices the counter electrode substrate also has to be electrons and metal cations (Li+,H+, etc.) play an impor- transparent indium–tin-oxide (ITO) glass, with the counter tant role. WO3 is a cathodically ion insertion material. The electrode chemical species being either colorless in both its blue coloration in the thin film of WO3 can be erased by redox forms or electrochromic in a complementary mode the electrochemical oxidation. In the case of Li+ cations to the primary electrochromic material, since the entire sys- the electrochemical reaction can be written as Eq. (1) tem is in the optical path. Hence, transparent electrolytes and the generalized equation can be written as Eq. (2). are required in the transmissive devices. For applications + − VI V WO3 + x(Li + e ) → LixW(1−x) Wx O3 (1) that are designed to operate in a reflective mode, such as (transparent) (blue) displays, the counter electrode can be of any material with + − a suitable reversible redox reaction. WO3 + xM + xe → MxWO3 (2) The purpose of this review is to give a flavor of the diver- (transparent) (blue) sity of this fascinating subject by the description of some of The fractional number of sites which are filled in the WO3 the most important examples from the major classes of elec- lattice is indicated by the subscript x in the general for- trochromic materials, and to give some examples of their mula MxWO3.Atlowx the films have an intense blue color use in both prototype and commercial electrochromic dis- caused by photoeffected intervalence charge transfer (CT) play devices (ECDs). between adjacent WV and WVI sites. At higher x, insertion irreversibly forms a metallic “bronze” which is red or golden in color. The process is promoted by cathodic polarization 2. Transition metal oxides (TMOs) which induces ion insertion and electron injection: the in- serted ions expand the lattice of the guest oxide while the Recently, oxides of many transition metals (in the film compensating electrons modify its electronic structure and form), e.g. iridium [1,2], rhodium [3], ruthenium [4], tung- in turn its optical properties. It can be stated simply that sten [5], manganese [6], cobalt [6], etc. have been shown the injected electrons are trapped by a W6+ forming a W5+ + to possess electrochromic property. This class of elec- while M remains ionized in the interstitial sites of the WO3 trochromic (EC) materials have been classified under inor- lattice. This gives rise to the formation of tungsten bronze ganic EC materials. The TMO films have been deposited having electrical and optical properties different from those by several techniques such as vacuum evaporation [7], of the pristine oxide. In fact, the pristine state WO3 is a sputtering [7], spray deposition [8], electrodeposition [9], pale-yellow and a poor electrical conductor, while in the in- electrochemical oxidation of tungsten metal [10], chem- tercalated MxWO3 state it becomes highly conducting and ical vapor deposition (CVD) [10], sol–gel method [10], blue in color with absorption spectra around 0.5–0.6 ␮m. etc. The TMO films can be electrochemically switched The above mentioned W6+/W5+ intervalance transition to a non-stoichiometric redox state which has an intense model implies a certain delocalization of electrons which electrochromic absorption band due to optical intervalence is consistent with the enhancement of the conductivity that charge transfer [10–13]. A typical and most widely studied accompanies the insertion processes. However, although example is the tungsten trioxide (WO3) system, since its this is the most accepted theory, other models, including a electrochromism was first reported in 1969 [10–13]. Tung- non-localized electron model are proposed to explain the sten oxide has a nearly cubic structure which may be simply electrochromic mechanism of tungsten oxide, in particular, described as an “empty-perovskite” type formed by WO6 and of inorganic ion insertion compounds in general. octahedra that share corners. The empty space inside the The characteristics of the electrochromic process can be cubes is considerable and this provides the availability of a conventionally examined by directly comparing the optical large number of interstitial sites where the guest ions can be and electrochemical response of the WO3 electrode. Fig. 2 inserted. Tungsten trioxide, with all tungsten sites as oxida- shows the cyclic voltammetry and the optical transmittance VI + tion state W , is a transparent thin film. On electrochemical ofaWO3 electrode measured in a cell having Li ion con- reduction, WV sites are generated to give the electrochromic ducting electrolyte and a Li metal counter electrode. The (blue coloration to the film)
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