This book deals with chromogenic materials and their uses primarily in large-area optical technology. The word "chromogenic" is new and is used here to denote materials which are able to alter their optical properties in response to a change in ambient conditions such as illumination intensity, temperature, applied electric field, etc. The changeable optical properties embrace transmittance, reflectance, absorptance and emittance.
Glass plays a significant role in the design of building envelopes today. Since its emergence during the last century as a major building material, glass has evolved into an ubiquitous and versatile building design element, performing functions today that would have been unimaginable a few years ago. The optical clarity and transparency of glass that we take for granted is one of its most unique features. Glass windows keep out the cold wind and rain without blocking the view, but also perform many more complex functions which require variable properties and tradeoffs between conflicting conditions. The glazing that provides view must also provide visual privacy at other times and must sometimes become totally opaque (for audiovisual shows, for example). Transparent glass admits daylight, providing good color rendition and offsetting electric lighting energy needs, but it can also create discomfort and disability glare conditions. The sun provides desirable warmth in winter but its heat is unwelcome in summer when it contributes to thermal discomfort and cooling energy requirements. And glass is an important element in the appearance and aesthetics of a building, both interior and exterior.
Automobiles present both opportunities and challenges for large-area chromogenics. Opportunities include optical and thermal control of vehicle glazing along with optical control of rearview mirrors and privacy glass. Challenges include cost-effectively meeting automotive safety, performance, and reliability standards. Worldwide automobile production' for 1987 is listed in Table 1. Of the roughly 33 million cars produced annually, approximately 8% are luxury models which are candidates for features such as auto- matically dimming rearview mirrors or variable opacity sunroofs. Thus copious commercial opportunities await whatever chromogenic technologies qualify for use in automobiles. This review will describe the performance, safety, and reliability/durability required for automotive use. Commercial opportunities and challenges will be discussed including cost factors and specifications. Chromogenic technologies such as electrochromism, liquid crystals and thermochromism will be reviewed in terms of how publicly announced technical developments match automotive needs and expectations. Construction and performance of ex- isting or imminent chromogenic devices will be described. Finally, how opportunities and challenges of the automotive environment translate to other applications for chromogenic materials such as architectural or information display devices will be discussed. The objective is to generally review the applications, the technologies appropriate to these applications, and the automotive chromogenic devices available at the time of writing to match these applications.
Photochromic glasses are mainly used for selfÃ¢â‚¬â€adjusting sunglasses. Thus, the photochromic materials have to fulfil several specifications in order to be useful for the human eye. At low light intensity they should be transparent enough, whereas at full solar power their transmittance should be reduced to about those values known for conventional sunglasses, e. g., values in the range between 20 and 30 %. The transmittance should be reduced in the whole visible spectral region uniformly as much as possible in order to avoid too large a shift of the color stimulus. Since these glasses should darken especially under solar irradiation, the darkening mechanism must be induced in the UV range of the solar spectrum which is not seen by the human eye. The IR is not to be used for that purpose, since incandescent lamps are emitting in that spectral range and the photochromic glasses are to remain in the transparent state under that illumination. The time constants for the darkening and regeneration kinetics should be fast enough to make these glasses useful. There are additional prerequisites, such as chemical durability, optical homogeneity, low light scattering and low sensitivity of the optical properties to variations of the ambient temperature.
A variety of materials and compounds change color when exposed to light and revert to their original color when placed in the dark. This reversible color change phenomenon induced by light is called photochromism. Since many compounds exhibit this phenomenon over A spectral range wider than the visible region, a broader definition for photochromism has been established as a reversible change of a single chemical species between two energy states with distinct absorption spectra and such a reversible change is induced in at least one direction by the action of the electromagnetic radiation.
The color of a substance in general depends upon its state and upon the external forces it experiences, both past and present. One of the simplest methods of attempting to change the state of a material is to vary its temperature. Thermochromism is a noticeable dependence of the color of a substance on temperature. This is thus one of the easier chromogenic effects to detect. Since the changes triggered by temperature variation often are indicative of the effects that can be induced by other means, it is convenient to use the observation of thermochromism as an indication of the possible existence of other chromogenic behavior.
This chapter discusses the technological application of a class of materials with a chameleon-like nature, that is, they exhibit the properties of metals under certain conditions of temperature and pressure, and semiconductor-to-dielectric properties under other conditions. Many materials exhibit this behavior, most notably the transition metal oxides and sulfides. Typically, the transition from one state to another in transition metal oxides is accompanied by a sharp change in electrical conductivity (as large as 107 in some oxides of vanadium), as well as changes in other physical properties such as crystalline symmetry. The changes in electrical conductivity alter, in turn, IR transmittance, and some of these effects extend into the visible spectrum. A material such as this, whose transition occurs at the appropriate temperature, would be useful for solar energy control in buildings. For example, a coating of thermochromic (TC) material on glass would transmit solar energy at temperatures below its transition temperature (Tt), and when the temperature rises above Tt, the TC material would reflect the incident solar energy. Thus, solar influx would be high at low ambient temperature and low at high temperature. Though very few of these materials have Tt in the range required for such an application, we can adjust Tt by using dopants.
Thermochromism offers interesting possibilities for controlling the radiative throughput through windows, as well as for numerous other applications.1-3 The state of the art is summarized for organic and inorganic thermochromic materials in the previous two chapters by Day and by Jorgenson and Lee. The purpose of this brief paper is to point at the potential and prospects of using sputter-deposited vanadium oxyfluoride coatings.4
Electrochromism can be generally defined as the phenomenon of color change brought about by the passage of an electric current. The current causes a chemical change, an oxidation or reduction reaction. Such a general definition covers a good deal wider range of substances than is usually understood by the term "electrochromisrn". Under this definition any color change accompanying an oxidation/reduction reaction, one of the most common reaction types in chemistry, could be said to be electrochromic. As the term is commonly used, one of the states (oxidized or reduced) is typically colorless, thus the process of producing the colored state is "electrochromic" or "chromogenic". If the reduced state is colorless and color appears upon oxidation, the material is said to undergo "anodic coloration". If the oxidized state is colorless, color appears upon reduction and it is "cathodic coloration". A moment's reflection will reveal that if a material has an absorption in, let us say, the UV in its reduced form, and an absorption in the visible in its oxidized form, then it is "anodically coloring" in the visible, but "cathodically coloring" in the ultraviolet region of the spectrum. We will continue to use the terms "cathodically coloring" and "anodically coloring" because it is convenient to do so, and the usage is widespread in the literature, but it should be borne in mind that these are limited concepts in a general sense.
Large area coatings such as infrared-mirrors and transparent heaters are widely used in buildings and vehicles. The spectral transmission and reflection of such thin films are time independent. Electrochromic (EC) systems are a new class of optical filters with voltage controlled transmission. One can choose the desired optical characteristics using a small voltage. This time dependence is very useful for different applications.
During the past decade a great interest has been shown in the study of transition metal trioxide (W03 and MoO3) thin films. The reason is that these transition metal oxides present a number of interesting optical and electrical properties.1-3 Among these properties the electrochromic effect in the films is of particular interest. By the use of this phenomenon it is possible to alter the transmission and reflection properties of glasses to regulate the radiated energy throughout (smart windows).4 Another possible application of this effect is in display .
Recently a new technique for making electrochromic coatings has been presented. In the semiconductor industry, plasma enhanced chemical vapor deposition (PE-CVD) is routinely used to produce oxide and nitride coatings and photovoltaic a-Si devices . At the Solar Energy Research Institute the technique has now been developed to produce electrochromic coatings of tungsten and molybdenum oxidel-4 .The results are very promising and coatings show the same characteristics as tungsten and molybdenum oxides produced by evaporation. The main difference is that the coatings are produced at a much higher rate using PE-CVD. Deposition rates greater than 40 nm/s have been achieved for tungsten oxide. This points at the possibility to produce electrochromic devices at a more competitive price than with competing technologies.
From the viewpoint of improvement of energy efficiency in buildings, the so-called " smart window" with dynamic control of solar radiant energy throughput is very attractive, especially for the glass manufacturers. Several types of smart windows using chromogenic compounds have so far been proposed. Among them, the electrochromic device is believed to be the most suitable for the smart window owing to its controllability of the solar transmittance as well as the memory effect. One of the key technologies to fabricate the electrochromic smart windows is to form electrochromic films with large area. The electrochromic film represented by tungsten trioxide has been prepared mainly by the physical vapor deposition methods; however, these are generally expensive and are not suitable to make a uniform and thick film over a large area.
Much attention has been paid, over the last twenty years, to the electrochromic properties of W03 and related materials. It is known that W03 (Ref. 1) and Mo03 (Ref. 2) films are colored by the simultaneous injection of protons (or cations) and electrons.
The preparation of any large area system is going to require a deposition system for the thin film elements of the device which not only have the properties required but also can be made in the large areas, and for the low cost, that will be needed for any device which is to have any large-scale applicability. Techniques for the large-scale production of multi-layer optical filters for use in windows for buildings and cars have appeared over the last few years, and it has been demonstrated' that planer magnetron sputtering can give the large area uniformity, low cost and high rate required for such a system. Part of this process includes the reactive deposition of an oxide. The properties of this oxide are not critical, but research is proceeding into the techniques that need to be used to manufacture oxides with much better control of their properties while maintaining the advantages of large area availability and low cost.
The electrochemistry of hydrated nickel oxides (NiOxHy) has been an active field of research , principally due to the use of these materials as the positive electrode in secondary nickel/cadmium batteries. Sintered plate materials", as well as electrochemically' and chemically' grown thin films have been studied; the published literature is reviewed in refs. 4 and 5. However, and in spite of the great effort in this direction, some important aspects of the electrochemical behaviour of these materials remain still unclear. Among then'', one could emphasize the mechanism of the electrochemical reaction as a whole, the mechanism by which diffusion of the ions into the film occurs, the role of water molecules incorporated in the film structure, and the dependence of the electrochemical properties on the film microstructure.
This Chapter treats the physical properties of electrochromic nickel-oxide-based coatings prepared by sputter technology. It discusses optical and structural properties and gives support to a model for electrochromic coloration following upon hydrogen extraction.
Studies of the electrochromic properties of nickel hydroxide and related nickel oxide materials have revealed promising characteristics for smart window applications including a high coloration efficiency throughout the visible region' 8. In addition, good durability in electrochromic switching (over 5000 coloration/bleaching cycles with no significant degradation in response) has been reported for sputtered nickel oxide films7. However, there have also been some unfavorable reports about the durability of electrochromic nickel hydroxide films3'5'8. For example, a significant deterioration in the electrochromic response of cathodically deposited nickel hydroxide films has been observed in less than 500 cycles3. Estimates of the number of coloration/bleaching cycles needed for automotive smart window applications are in the range of 105 cycles--enough to allow switching once per mile during a 100,000 mile car lifetime.
Interest in vanadium pentoxide (V205) as an electrochromic material has increased with the realization that V205 has suitable properties for counter electrode applications in variable transmittance electrochromic devices. Vari- able transmittance devices require a charge balancing counter electrode whose optical properties either contribute to, or at least do not interfere with, the overall transmittance modulation. Preliminary studies have shown that V205 exhibits anodic coloration in the blue and near-UV for lithium intercalation/deintercalation and is thus a suitable counter electrode to cathodically coloring electrochromic materials such as tungsten trioxide (W03).
The synthesis of highly conducting polyacetylene in the late seventies inaugurated an era of extensive studies of electronic structure and transport in organic polymers. The recent attainment of conductivities in metallic polyacetylene comparable to that of copper is the outcome of major advances in the theoretical understanding and chemical synthesis of these materials. In parallel to this quest for understanding of the electronic properties of "one-dimensional" organic polymers, there has also been an active search for applications of these materials. A few applications are now reaching extensive use, among which should be mentioned polymer secondary batteries exploiting polymer materials as electrodes. Many new techniques for preparing the conductive polymers in forms suited for practical application have been proposed. For application as conductive coatings, anti- static surface treatments and electromagnetic shielding, practical use can be foreseen in the near future. One of the applications that has received considerable study, electrochromic devices utilizing electroactive polymers as active materials, has not reached that stage of maturity yet. In this short review, we present the basic physics of electrochromism in electroactive polymers, also named conductive polymers; point out the requirements for use of electroactive polymers in electrochromic devices (which includes displays, smart windows and electrooptical modulators); and describe some of the materials and processing aspects that are of particular relevance with reference to thin film technology.
The first conducting polymer was reported in 1977 when Shirakawa, MacDiarmid, Heeger and other coworkers1 discovered that polyacetylene (Fig. 1), an organic polymer and electrical insulator, could be converted into an electrical conductor by absorbing a small amount of iodine. Since then, active research has led to the synthesis of new conducting polymers. Examples are: polyphenylene2, polypyrrole3, polythiophene4, poly-(phenylenesulfide)2 and polyaniline5. Their chemical structures are shown in Fig. 1.
Physical and chemical properties of solvents and solutes are extremely important. For electrochromic applications,the electrolyte has sufficient amount of solute to ensure conductivity and mass-balance of ions between electrolyte and electrochromic materials. For instance ,an electrochromic device based on W03 and lithium electrolyte should have enough amount of Lit ion to achieve high modulation ratio of bleached to colored state, which requires about 20mC/ cm2.1 Furthermore,the ion-exchangeable hydroxyl sites in W03 consumes about 4x10-7M/cm2(i.e.,40mC/cm2) of Lit for W03 film with the density of 5.5g/cm3 and thickness of 0.55p m. Therefore the electrolyte should have at least 60mC/cm2(i.e.,6x 10 Mice) of Lit. If we use an electrolyte with 1M/9_ lithium salt, the necessary thickness of the electrolyte should be 6/Lm and finally the thickness of lithium-based electrolyte would be about 50- 60//m in order to avoid the change in conductivity before and after electrochromic reaction.
The advantages of applying polymeric ion conductors to electrochromic(EC) devices' were mentioned in relatively early studies of the EC research. The electrochromic devices based upon amorphous W03 in the past used the liquid protonic electrolytes such as sulfonic acid. However, the W03 films were found to be dissolved in contact with aqueous media, forming poly- tungstate. Such degradation of the W03 film in electrochromic devices was partly overcome by replacing the sulfonic acid with proton conducting polymers. In the meanwhile, reduction of underlying transparent conductive oxides were found as a result of proton attack. Thus appeared the non- aqueous electrolytes,e.g. solutions of an alkali salts in aprotic solvents.
Solid state materials exhibiting ion conduction properties are usually referred to as "solid electrolytes", "fast ion conductors (FIC)", "superionic solids" or simply as "ion conductors", depending on the author's preference or the magnitude of electrical conductivity. Since the pioneering work by Yao and Kummer' in 1967 on sodium conduction in B-aluminas, a large number of studies have been made on alkali ion conduction in inorganic solids. Applications of these conductors can be found in various areas such as solid state batteries, fuel cells, electronic displays or memory devices. More recently, in the early 80's, suggestion of applying electrochromic (EC) materials to the fabrication of smart windows (SW) has prompted increased interest in ion conductor studies, especially those aiming at finding transparent ion conductors.
Materials with both high transparency and high electriTa conductance have recently attracted growing technological interest . Applica- tions of such materials include coatings for windows with ability of deicing and demisting, coatings for electromagnetic shielding and antistatic coatings. Today, their application has been extended to- ward optoelectronic devices. They are being used for the fabrication of a variety of devices such as photovoltaic devices, display devices and light control devices which include electrochromic devices. In the above devices, transparent electronic conductors are almost always used as electrodes, where at least one electrode is required to feed current, while at the same time it allows incident light into the device or emissive light from the device. In other usages, transparent electronic conductors play an active part, such as in Schottky junc- tions in photovoltaic devices.
Over the last few years, hydrated nickel oxide or hydroxide has gained interest as an electrochromic materiall. It has many important features. First it shows a transparent to bronze coloration which is suitable for a number of architectural, vehicle and aerospace glazing applications. This material is stable in an alkaline environment and colors and bleaches at potentials below 1.0V and below the oxygen and hydrogen evolution potentials. It can be deposited by a wide range of processes, including anodic and cathodic electrodeposition, r.f. and d.c. sputtering, vacuum evaporation, chemical vapor deposition, and solgel processes. In this work we will discuss films made chiefly by anodic electrodeposition. Some of the best characteristics are seen in films made by electrodeposition. The properties of films made by other deposition processes will be discussed in subsequent chapters. Much of the electrochemical knowledge about hydrated nickel oxide comes from studies of metallic nickel electrodes used in batteries. The study of hydrated nickel oxide based electrochromic devices is just emerging, and so far very few devices have been reported2-4. Some of the work by my group on nickel based devices will be outlined later in this chapter.
Proc. SPIE 10304, Optical techniques for the study of electrochromic phenomena application of Raman spectroscopy and optical multichannel analysis (OMA) to the coloration of oxides and polymer films, 103040S (28 March 1990); doi: 10.1117/12.2283631
Electrochromism in order to be well understood, requires an in-situ study during changes of color. The usual analytical techniques, even if they give excellent information, are not generally able to be used during the polarization of a sample in an electrolytic medium. Three techniques will be described here, none of which have been commonly used up to now, but all of which can have a noticeable effect in the field of electrochromism.
We describe recent research aimed at developing an all-solid-state smart window for energy-efficient buildings. The basic concept is to connect two pieces of glass, with complementary electrochromic coatings, by a transparent polymeric ion conductor which serves also as a lamination material. This Chapter reports data on the components of one specific design of this type, as well as some initial device results.
This article reviews the electrochemistry and optical switching performance of variable transmittance electro- chromic devices based on the a-W03/a-Ir02 (a=amorphous) combination of electrochromic materials. The review concentrates on past research at EIC Laboratories on a-W03/a-IrO2 devices containing polymeric proton (HÃƒÂ·) conductors with an ancillary discussion of devices using c-1(Ã¢â‚¬Å¾W03,(,4) and the mixed oxide a-MoxWi-x - 03 as the primary electrochromic material. Approximately one half of the data presented has not been published pre- viously, with the remaining data taken from articles in earlier SPIE volumes and the journal Solar Energy Materi- als.
Electrochromism is a phenomenon of color change including resultant changes in the optical transmission and reflection. This phenomenon is induced by the redox reactions of the electrochromic (EC) material. Ordinarily the redox reactions are caused by the injection(or ejection) of electrons and ions into(or from) electrochromic materials. For any electrochomic device to manifest sufficient cycle life time, a strict charge balance' has to be accomplished within every color/bleach cycle. In addition we must be careful in material research for the examination of whether the device can retain its initial characteristics when put in the environmental conditions where the device should serve for a long time.
The glazed areas in automobiles are increasing in size.' As the glass area grows larger, we need better solar control to maintain comfort for the occupants and to reduce the need for larger air, conditioners. For solar control, we need to filter out infrared energy that causes heat buildup,2 control visible light and block out ultraviolet energy. To control visible light, devices using photochromism, liquid crystal, thermochromism and electrochrornism have been studied.3 We developed a transmissive type of electrochromic (EC) device (for antiglare purpose) by using cathodically coloring tungsten oxide (W03) and anodically color- ing Prussian blue (PB). Recently a practical antiglare mirror for automotive use has been put on the market,'" but large scale EC device for automobiles is still under development.
Electrochromic (e.c.) thinÃ¢â‚¬â€layer systems are well suited for the construction of optical devices with controllable, directed, reflectivity. The application of the e.c. effect to reflecting systems is even more efficient than its use in transparent devices since the light travels twice through the absorbing layer of the electrochromic material, which, in this case, is positioned between light source and reflector as well as reflector and receiver or observer. This application, in addition, offers a greater choice of materials for constructing the devices in that some of the layers of the e.c. system can be arranged behind the reflector or reflecting electrode and can thus be lightÃ¢â‚¬â€ absorbing, reflecting, or can have a variable transmittance, e.g. electrochromic materials can be used as storing layers. Several papers on the modes of construction of reflecting e.c. systems have appeared, 1-4 and the worldÃ¢â‚¬â€wide state of affairs was recently reported.
The studies related to electrochromic phenomena performed in the seventies were mainly aimed at the development of information displays. Such applications require sm211 electrode sizes, i.e. with active surface areas of between about 0.01 to 10 cm . The development of large information devices and chiefly smart windows require much larger switching areas. This paper deals with the influence of increasing the active surface area on the response time. The latter depends on both properties of the cell components (transparent conducting layer, electrochromic film, electrolyte and counter electrode) and structure of the cell (size, shape, gap, resistivity of the busbar). Experimental devices were constructed with given components and cell geometry. The effect of a series resistance arisen mainly from the cell size was investigated and explained by the effect of the additional series resistance on the response time of a diffusion-controlled process. The study indicates that the scaling-up of W03 devices will be limited by an increase of the response time with increasing active area.
Liquid crystals have been widely used for many years in electro-optic displays. We describe here selected, fundamental properties of liquid crystals which make them useful in electro-optic appli- cations. Our aim is to provide the background material needed for understanding the guest-host systems described in Chapter 10.2 and the polymer-dispersed liquid crystal films discussed in detail in Chapter 10.3. We also review the major liquid crystal (LC) electro-optic effects which have been used in display applications and comment on their potential advantages and disadvantages for large area transmittance control. For more information, we refer the reader to several excellent texts1-3 which discuss in detail the fundamentals of LC physics and chemistry and the basic electro- optic effects observed in these materials. Comprehensive reviews of the material characteristics and electro-optic effects most useful for LC displays have also been published.4,5
The first guest host (GH) display was suggested in 1968 by Heilmeier and Zanoni . They proposed dissolving pleochroic dye molecules in a nematic liquid crystal (LC). A pleochroic dye is characterized by unequal absorption of two perpendicularly polarized beams of incident light. This anisotropic absorption can be described by two absorption constants. In the case of linear dichroism these absorption cons- tants are related to directions of polarization, respectively parallel and perpen- dicular to the transition moment of the dye molecule.
Thin polymer films containing microdroplets of liquid crystalline (LC) material are potentially use- ful for a variety of electro-optic applications because they can be switched electrically from a light scattering state to a transparent state.1-20 In this chapter, we shall describe the structure, opera- tion, and preparation of these films and review their electro-optic properties. Although electronic information displays are a very promising application of these films, we will not discuss topics such as full color capability19 or addressing schemes, which are unique to display applications. Rather, we shall emphasize those electro-optic characteristics which are important for control of solar radiation in buildings and automobiles; some of these, of course, are also important for display applications. We shall also restrict our discussion to films containing nematic LC droplets.