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Development of Electrochromic Devices Agnieszka Pawlicka* Departamento de Físico-Química, Instituto de Química de São Carlos, Universidade de São Paulo, C.P. 780, CEP 13560-970, São Carlos-SP, Brazil Received: May 15, 2009; Accepted: July 15, 2009; Revised: July 17, 2009
Abstract: Electrochromic devices (ECD) are systems of considerable commercial interest due to their controllable transmission, absorption and/or reflectance. For instance, these devices are mainly applied to glare attenuation in automobile rearview mirrors and also in some smart windows that can regulate the solar gains of buildings. Other possible applications of ECDs include solar cells, small- and large-area flat panel displays, frozen food monitoring and document authentication also are of great interest. Over the past 20 years almost 1000 patents and 1500 papers in journals and proceedings have been published with the keyword “electrochromic windows”. Most of these documents report on materials for electrochromic devices and only some of them about complete systems. This paper describes the first patents and some of the recent ones on ECDs, whose development is possible due to the advances in nanotechnology.
Keywords: Electrochromic devices, history, patents. 1. INTRODUCTION Scientific and industrial research into the domain of electrochromic devices (ECDs) has gained a great deal of attention over the last decades, after the first Deb´s publication [1] and patent [2], evidenced by numerous scientific papers, books and chapters in books as well as popular articles. Over the last 20 years 944 patents and 1450 papers in journals and proceedings have been found on Web of Science after introducing the keyword “electrochromic devices” [3]. However, as can be rapidly observed, most of these documents report on materials for electrochromic devices and only some of them describe complete electrochromic windows. The great interest in this subject of scientific and technological research is due not only to the practical application of these devices, but also to the complexity of the problem to be resolved to obtain the satisfactory practical functionality at low investment cost. The solution to many problems associated with ECDs is due, principally, to the multi-layer ECDs structure composed of many materials and different component combinations. A proof of these different possibilities is the fact that the described ECDs almost always have different compositions. Also the complexity of the problem can be confirmed by the existence of few papers about ECDs in comparison with the quantity of the articles about thin films, i.e., nanometric electrochromic and ion storage coatings as well as ionic conductors.
Fig. (1). Five-layer electrochromic device scheme composed of a glass-ITO substrate (GS-TC), electrochromic (EC) and counter electrode (IS) films and electrolyte (IC).
To introduce this subject, the typical ECD composition is a five-layer structure visualized in many papers and described as GS/TC/EC/IC/IS/TC/GS, where GS is a Glass Substrate, TC is a Transparent Conductor, EC is an Electrochromic Coating, IC is an Ion Conductor and IS is an Ion Storage coating [4]. This structure can be seen in Fig. (1).
Depending on the application, the GS-TC can be a reflective (platinum or silver) or transparent coating leading to a reflectance or transmittance mode of operation, respectively. As already described by Granqvist et al. [5] and Heusing and Aegerter [6], this multi-layer structure provides numerous possibilities to manufacture the devices.
*Address correspondence to this author at the Departamento de FísicoQuímica, Instituto de Química de São Carlos, Universidade de São Paulo, C.P. 780, CEP 13560-970, São Carlos-SP, Brazil; Tel: (+55)16 33739919; Fax: (+55)16 33739952; E-mail:
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Electrochromic material (EC), principal component of ECDs, has the property of changing color after applying a voltage or a current under electrochemical cells conditions. This color change should be reversible when the polarity of
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either the voltage or the current is reversed. Electrochromic materials have been known for many years. As described by Monk et al. [7] in 1704, Diesbach (1761) discovered Prussian blue (hexacianoferrate – pigment), whose coloration changes from transparent to blue. In 1815 Berzelius observed coloration changes of slightly heated WO3 in H2 atmosphere and 9 years later, in 1824, Wöhler described coloration changes of WO3 after reaction with Na. In 1930 Kobosew and Nekrassow [8] demonstrated that WO3 could be colored electrochemically in acid solution (reduction process) and in 1951 Brimm et al. [9] described a reversible coloration reaction of tungsten bronze. The important mark of this research was Krauss’ work, which provided a detailed description of the electrochromism of WO3 in 1953. However it was only a laboratorial report and was never published. Few years later Platt [10] proposed the term electrochromism and Deb [1, 11] described the electrochromic reaction. So far these two works have been the most referenced ones. Also it was Deb who invented and patented the first electrochromic device [1,2]. At present electrochromic materials, i.e., molecules or thin films with electrochromic properties, can be divided into three types: (i) those with species permanently in solution, as methyl viologen, (ii) those with species that are initially in solution, but after electron transfer start to form solid and colored products, as heptyl viologen and (iii) those which are always solid materials in the film form, as Prussian blue or some of the thin films of transition metal oxides. Transparent electronic conductors (TC) are generally ITO (indium tin oxide), FTO (fluor tin oxide) or ATO (antimonium tin oxide). All of them are thin films deposited on glass substrates by sputtering, physical or chemical vapor deposition (CVD). TCs can be also deposited on polymeric materials, as poly(ethylene terephtalate) (PET), allowing for the manufacture of solid and flexible electrochromic devices. These substrates are commercial products. Ion storage coating (IS) is used to protect the ITO coating from ions insertion and irreversible coloration. Depending on the material applied, this thin film can also work as a secondary EC, which improves the primary electrochromic material coloration [7]. The central part of the device is occupied by an electrolyte, which can be liquid, gel or solid. Electrochromic coatings and ion storage layers are generally deposited separately on the transparent conductor coatings and then jointed with the electrolyte and sealed. The EC and IS are thin films that can be deposited by sputtering, CVD, spin- or dip-coating from sol-gel precursors, etc., therefore using nanotechnology. Electrochromic devices are very interesting due to their possible application in architecture for an efficient use of energy in modern buildings where it is possible to control the flow of light and heat passing through the building glazing. ECDs can also be applied as glazing of vehicles, trains, aircrafts [12] and also as rearview mirrors already used in vehicles, as well as temperature control of frozen food [13]. As showed above, the ECDs even constituted by two electrodes and an electrolyte between them can have many configurations as a result of different layer compositions, therefore their comparison becomes difficult. Nevertheless the size, transmittance, reflectance, color/bleach cycles and
Agnieszka Pawlicka
switching time data of ECDs investigated between 1998 and 2001 have been presented in recent papers of Granqvist et al. [5] and Heusing and Aegerter [6]. From these publications it is possible to observe that the ECDs with solid electrochromic coatings of approximately 100-300 nm thickness, such as WO3 or Nb2O5 and counter electrodes as CeO2-TiO 2 have been produced with various electrolytes, like allinorganic solid-state materials [14], organic compound-based materials [15] or organic-inorganic (Ormolytes) based systems [16] that contain mobile charge species, such as lithium or proton [7]. In the case of Ormolytes and organicor polymeric-based electrolytes, lithium salts, usually LiClO4, are generally used as conducting species. Among these different possibilities, the polymeric electrolytes are very interesting due to their low production cost and more appropriate mechanical properties than inorganic materials. Different solid polymeric electrolytes (SPEs) have been proposed, most of them based on poly(ethylene oxide) (PEO) systems [17, 18], but also those with natural polymers have been recently published [19-21]. 2. SOME HISTORICAL PATENTS The first patent describing the electro-optical device having variable optical density was made by Deb and Shaw [2] in 1970. In this work the authors describe an electrooptical device with a sandwiched structure composed of a 0.1-100 m thick electrochromic film based on tungsten, molybdenum, niobium or vanadium oxides. This film was separated from the electrode by a permeable insulator film composed of silicium oxide, calcium or magnesium fluoride, which are permeable for a current carrier. It is also mentioned that a transition metal has different stable oxidation states between -50 oC and 125 oC with different electromagnetic radiation absorption characteristics and the state is changed by applying a potential across the electrodes. The presented device shows a coloration and bleaching upon changes of polarity of an applied electric field at ambient temperature. Moreover suitable substrate materials including glass, wood, paper, plastic among others, which can be transparent or opaque, are proposed. In 1975 Berets from American Cyanamid Company proposed and patented a device with WO3 as primary and secondary electrochromic materials separated by gel electrolyte [22]. In 1978 Deb and Witzke [23] patented another invention concerning electrochromic devices entitled “Electrochromic device including protective layer - preventing electrolyte degradation of electrochromic layer”. The authors propose a protective layer of tin oxide deposited over electrochromic WO3 coating to avoid its degradation by phosphoric acidbased electrolytes. As mentioned by the authors, this layer protects the electrochromic coating effectively and increases the lifespan of the device. The first patent concerning practical ECD was made by Bechtel and Bayker [24], from Gentex Corp. in 1992. The authors describe a solution-phase electrochromic rearview mirror for automotive vehicles which has a range of grayscale electrical potential controllable reflectance from greater than 70% to less than 10%. This type of device has been already installed in many cars.
Development of Electrochromic Devices
3. RECENT PATENTS In 2003 Rukavina and Lin [25] patented an idea for switchable electrochromic devices for use in aircraft transparency windows. In this work they present a scheme of the window for use in airplans composed of an outboard and fog preventing electrochromic pane assemblies separated by one or two chambers filled or not with insulating gas, as air, argon or krypton. They also propose the use of curved glass substrates to follow the overall shape of the aircraft. In 2005 Callahan and Schafer [26], from The Boeing Company, patented an electrochromic system, which was applied two years later to their aircraft DreamLiner 787, presented during Le Salon de l’Aéronautique et l’Espace Paris Le Bourget 2007. In this paper one can read that the electrochromic system for use in aircraft has a window dimming control system which utilizes the existing wiring to distribute electronic control signals to dimmable windows throughout the passenger cabin of the aircraft. The difference in this invention compared with the previous one is the complexity of the whole system, where there are individual and main control modules besides the description of the electrochromic system. This four-layer device is composed of two transparent electrodes, a multi-color electrochromic coating based on poly(3,3-dimethyl-3,4-dihydro-2H-thieno [3,4-b][1,4]dioxepine) (PProDOT-(CH3)2) and a gel electrolyte based on gamma-butyrolactone (GBL) and salt. This invention also includes a description of the construction of an informative panel which can be installed inside the aircraft as sections separating walls. Many different pictures of devices as well as aircraft installations are shown in this paper. A more recent patent was published in 2008 [27]. In 2005 Gentex Corp. [28] patented an invention concerning color-stabilized electrochromic devices, where they proposed a two-electrode assembly separated by a solution with electroactive (anodic and cathodic) species (at least one of them electrochromic), solvent and colorstabilizer additive to maintain colorless devices in open circuit conditions. Depending on the substrate these devices can work in transmittance or reflectance mode. For the transmittance mode the authors proposed ITO, FTO or doped zinc oxide coated glass. Reflector substrates can comprise rhodium, silver, silver alloys and chromium, among others. As cathodic species they proposed viologen-type molecules, as methyl and octyl viologen tetrafluoroborate or 1,1’,3,3’tetramethyl-4,4’-bipyridinium tetrafluoborate. However, they did not exclude the use of WO3 as an electrochromic material. The anodic material may comprise ferrocene family, as di-tert-butyl-diethylferrocene. The concentration of these reagents should be in the range of 5 to 50 mM in one of many commercial solvents, such as acetonitrile, dimethyl formamide, dimethyl sulfoxide, alcohols, etc. The solution may also comprise different additives, as light and thermal stabilizers, anti-oxidants, viscosity modifiers and others. This invention was improved one year later, with the publication of a patent concerning a self-healing, cross-linked polymer matrix as a gelled electrochromic medium [29]. In 2007 Sotzing and Seshadri [30] patented an invention concerning five-layer ECDs having a dual polymer configuration, where the conducting polymers based on tiophenes are proposed. These polymers have already been
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known for over 25 years, have a very low band-gap of about 1.5 eV principally after acid or basic doping and also show electronic conduction and color changes, depending on their oxidation state. This principle was used and proposed to manufacture electrochromic devices based on conducting polymers. These polymers can be deposited from solutions onto ITO coated glass by spin- and spray-coating, ink jet, roll-to-roll or screen printing, among others. After that a drying procedure at either room or higher temperature should be performed. In this work the authors also mentioned the addition of other additives, like anti-oxidants, UV stabilizers, viscosity modifiers, etc. As electrode materials they proposed the use of inorganic thin films, like glass-ITO, platinum, gold, etc. or again organic conducting polymers. As electrolytes they suggested the use of liquid aqueous or organic solutions, polymers or solid electrolytes comprising poly(ethylene oxide) or gel electrolytes, which include lithium salts dissolved in a polymer matrix. In 2007 Donnelly Corporation [31] invented ECDs for use as automotive mirrors. The main component of this invention is the electrochromic monomer, which polymerizes to polychromic solid films. As described in this paper the color of the mirror changes uniformly from a silvery appearance to blue or bluish purple, depending on the electrochromic monomer used, after applying a potential of 1.3 V to the glass-coated ITO electrodes. At the same time the reflectance of these ECDs change from 60% to 20% in 2 to 10s and the bleaching process at 0 V from 10% to 50% in 3 to 56s. Saint-Gobain Glass France [32] presented an invention concerning the means of sealing and peripheral reinforcing against the liquid and vapor water penetration using thermosetting polymers from families of ethylene vinylacetate, poly(isobuthylene), butylic rubber and polyamide. The paper describes different electrochemical compositions for electrochemical devices using solid thin films with thicknesses of about 100-500 nm, liquid crystals, viologens, photovoltalic and electroluminescent systems. The versatility of the application of electro-optical devices can be confirmed through the Eastman Kodak Company patent [33], where the authors describe an electrochromic display transmissive or reflective device configuration. This device is constituted by many individual cells separated by walls; each cell is considered a pixel and independently controlled by an electric field. The cells are small electrochromic devices, which contain electro-optical fluid constituted by colored oil droplets or particles dispersed in continuous fluid carrier also called oil-in-oil emulsion. The advantage of this invention is the high mobility of the droplets or particles, their stability, consistent behavior with time resulting in excellent aging behavior. In 2008 Samsung Electronics [34] also showed an interest in ECDs and patented an invention of a device displaying various gray scale levels depending on the color and concentration of the black electrochromic material. They also proposed the use of cyan, magenta and yellow ECs, which can be distributed on the secondary EC layer, and red, green and blue as the first EC layer. There are no limitations for the use of transparent or reflective electrodes and the use
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of different electrolytes with lithium, sodium or potassium salts dissolved. Karmhag et al. [35] patented a method to manufacture electrochromic devices for use in architectural windows, information displays, sunroofs and windows in vehicles, eyewear, etc. This method consists practically in the separated deposition of coatings on two plastic substrates, where at least one of them is transparent. Over the first substrate are deposited subsequently, the electronic conductor, electrochromic film, and electrolyte. The electronic conductor film, counter electrode film and electrolyte are also deposited on the second substrate. Both parts are jointed and sealed. The objective of this invention was to provide an improved method to obtain ECDs with uniform thickness of coatings and avoid the edge delamination during production and prevent vacuum or external gases intervention and consequent degradation of the devices. The present patent mentions many different plastic substrates, such as polycarbonates, polyacrylates, polyesters, etc., different electronic transparent conductors, different electrochromic and counter electrode coatings and many possibilities of electrolytes. The film coatings can be obtained by magnetron sputter-deposition, spray-pyrolisis, sol-gel, electrochemical deposition, chemical vapor deposition, etc., where the thicknesses of these coatings are generally much smaller than those of the substrates. In the same period Widjaja et al. [36] published an interesting paper describing a roll-to-roll process to produce electrochromic devices. This continuous process using magnetron seems to be much more favorable for industrial production, although so far the authors have obtained only PET/ITO/WO3 coatings. 4. PRACTICAL APPLICATIONS As showed above the development of ECDs is a very interesting research area due to their possible application in architecture, vehicles, trains and aircrafts glazing [12], rearview mirrors already used in vehicles, different displays and also to the possibility of controlling the temperature of frozen food [13]. The main objective in the ECDs research for architectural application is to develop a new “smart system” that can control alone, depending on the weather, the interior temperature and luminosity and therefore the energetic consumption of the building. The energetic control of the edification is a very important problem to be solved in the country in the winter and tropical weather. The main problem is the consumption and waste of energy with the heating and conditioning of life and work places. For the external opaque surface, there already exist very good solutions, such as synthetic foams and mineral wool [37]. However a great part of infrared and visible radiation, corresponding to the temperature and visual conditions of the buildings, crosses the windows. Thus, the windows are not only the transparent surfaces; they also become very important in the energy waste calculation. In some countries the solar problem is partially solved by the use of colored and reflective glasses. These types of glasses diminish the transmission of some of the wavelengths from ultraviolet to near infrared [38], but due to their constant composition the
Agnieszka Pawlicka
reduction of incident radiation is always the same. However, the weather frequently changes from cold to warm and from sunny to cloudy. Based on this idea there has been an attempt to develop a new “smart system” that can be modulated depending on the weather, implying a decrease in the energetic consumption. This is the main objective in the ECDs research for architectural, automotive or aircraft applications. Currently these electrochromic windows that change color when voltage is applied are already installed in the Stadtsparkasse bank in Dresden, Germany [39]. This was the first commercial installation of electrochromic windows in a building. Contrarily to the smart windows for architecture, electrochromic devices for car rearview mirrors are already widespread commercially available [13]. The main advantage of these systems is the glare attenuation during night driving, as seen on the Gentex web site [40]. The construction of these devices is similar to the construction of electrochromic windows, but one of the electrodes (back electrode) is composed of reflective material instead of transparent, enabling the ECD to act as a normal mirror when bleached and as a darkened mirror when the device is colored. For instance only liquid or gel-state devices have been sold, although there are several reports about all-solid mirrors in the literature. Other practical applications can be found in luxurious cars, such as Maybach [41] or Ferrari Superamerica [42], whose sunroof is made by Saint Gobain Sekurit and launched in 2004. The use of electrochromic glass means that the driver can completely control the level of light entering the cockpit. There are five tint levels available and the glass can range from dark to light in less than a minute, by the touch of a button. However, for instance the commercial ECDs have a high price and are installed only in exclusive products. Sage Electrochromics, Inc. For Electrochromic Skylights Windows [43], through its partner, Velux America, also offers electrochromic windows for various dimensions, with price starting from U$1000 for 57cm x 57cm ECDs. A larger market penetration is expected to be achieved after decreasing the price level to U$100–$250/m2. 5. CURRENT & FUTURE DEVELOPMENTS The development of electrochromic devices consisting in nanotechnology aspects is a very interesting subject, mainly from the point of view of the possibility of numerous applications, such as smart windows, small- and large-area displays, solar cells, frozen food monitoring and document authentication. As showed above many people and also very important international companies have tried to develop different opto-electrochemical devices which change color, i.e., their transmission/reflection properties, after the application of electric field. The main reason for interest in this type of devices is the decrease in energy consumption, when applied as windows in buildings or in automotive vehicles or aircrafts, promoting regulation of light and heat transmission. However, as also showed above, although many patents have already been published, there are not many practical applications available, due principally to the complex fabrication processes, which result in very high price, only accessible for high-technology exclusive pro-
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ducts. Even with many published patents, this area is still open for new and innovative solutions using nanotechnology advances, which can result in the applications of smart devices.
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ACKNOWLEDGMENTS
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The author is indebted to FAPESP, CNPq, and CAPES for the financial support given to this research.
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