University of Patras
Department of Physics
George LeftheriotisRealised by:
TOC o “1-3” h z u I.Introduction PAGEREF _Toc523751187 h 3II.Structure and Working Principle PAGEREF _Toc523751188 h 5III.Materials PAGEREF _Toc523751189 h 7IV.Methods for the deposition of electrochromic thin films PAGEREF _Toc523751190 h 11V.Applications of electrochromic materials PAGEREF _Toc523751191 h 12VI.Electrochromic “smart” windows PAGEREF _Toc523751192 h 14VII.Electrochromic auto-dimming mirrors PAGEREF _Toc523751193 h 16VIII.Conclusions PAGEREF _Toc523751194 h 18Bibliography PAGEREF _Toc523751195 h 19
IntroductionOne of the main problems faced by mankind is the exaggerated consumption of energy. This problem is trying to be solved by finding new ways to produce energy. The main solution that is believed to be more efficient is using of nuclear energy, but this involves many risks and great costs. Nowadays, more and more scientists support the idea that in order to solve energy problem is better not to find new ways to produce energy, but to find methods how to reduce energy consumption. One of the methods is using of electrochromic devices.
The history of colouration goes back to 1704 when Diesbach discovered Prussian blue (hexacyanoferrate), which changes the colour from transparent to blue under oxidation of iron. In the 1930s, Kobosew and Nekrassow first noted electrochemical colouration in bulk tungsten oxide. Between the late fifties and mid-nineteen sixties there was considerable interest in the proprieties of tungsten bronzes. It was observed that metal ions could be preparatively incorporated into the lattice structure of tungsten oxide (WO3), and the resulting non-stoichiometric compounds exhibited strong colour centers and metallic proprieties. While working at Balzers in Lichtenstein, T. Kraus provided a detailed description of electrochemical colouration in thin film of tungsten trioxide (WO3) on 30 July 1953 1. In 1969, S. K. Deb reported that an electrical field could be used to form colour centers in thin films of tungsten oxide, producing colours similar to those of tungsten bronzes. Deb reported his results as “A Novel Electrophotographic System”, where a WO3 film was incorporated with a photoconductive (CdS) layer. A picture could be reversibly recorder on WO3 film by the application of an electrical potential across the film combined with the projection of light (through a photographic negative). The light projected through the negative increased of conductivity of the CdS layer allowing colouration of the tungsten oxide under the applied potential. In fact, the real birth of the electrochromic technology is usually attributed to S. K. Deb’s seminal paper of 1973, wherein he described the colouration mechanism in WO3.
Electrochromism is a reversible change in a material’s optical properties (transmittance, absorbance and reflectance) under an applied voltage 2. The simplest way to understand how an electrochromic device can reduce energy consumption is to give an example. Electrochromic window is a very good example (later will be discussed about electrochromic applications). It helps to maintain desired temperature in a room by changing window’s optical proprieties in such a way that solar radiation is absorbed more and more depending on applied voltage. In this
case air conditioning unit can be used less and so energy consumption is reduced. One can say that instead of using such a complicated device it is simpler to use a curtain or a louver which are cheaper and can also don’t allow the entry of solar radiation. It is true that in this case the temperature will decrease, but also the room will become darker and the outside landscape will be hidden. Darkening the room will imply the necessity to use energy for illumination so we don’t do much with that.
An electrochromic device can be viewed as a thin-film electrical battery whose charging state is manifested in optical absorption, i.e. the optical absorption increases with increased state-of charge and decreases with decreased state-of-charge. To successfully predict and optimize the performance of electrochromic systems without large investments, a cost-efficient method capable of predicting and optimizing the performance of large area electrochromic devices has to be developed. 3 Such methods are created, but none of them can predict all that is needed, so there is still a lot of work.
It is obvious that thanks to the proprieties of electrochromic devices, domains in which they are utilized should be investigated increasingly. Knowing that there are a lot of parameters which can affect the operation of an electrochromic device (method of preparation, used materials, etc.) or parameters which are asked by consumers (design, cost, etc.), application areas that use electrochromics become larger. So, a greater number of specialists are needed and a more in-depth study should be done.
Structure and Working PrincipleThe details of the electrochromic process in solid-state devices are not completely understood. However, a fair amount of work has been done in recent years to elucidate the basic physical principles involved. It is generally believed that the colouration occurs as a result of the injection and trapping of a large density of electrons in the electrochromic oxide layer (whatever might be the nature of the trapped electrons, it is imperative that such a large density of injected carriers have to be charge-compensated either by hole injection or by ion injection). 4
A standard electrochromic construction that allows basic features and operating principles to be introduced conveniently is illustrated in the figure below (Fig. 1).
Figure 1. Basic design of an electrochromic device, indicating transport of positive ions under the action of an electric field.
The design embodies five superimposed layers on one substrate or positioned between two substrates in a laminate configuration. The substrates are normally made of glass or flexible polyester foil. The central part of the five-layer construction is a pure ion conductor (i.e., electrolyte) that can be organic (an adhesive polymer) or inorganic (often based on an oxide film). The ions should be small in order to be mobile; protons (H+) or lithium ions (Li+) are normally preferred. This ion conductor is in contact with an electrochromic film (tungsten oxide being a typical example) capable of conducting electrons as well as ions. On the other side of the ion conductor is a film serving as ion storage, ideally with electrochromic properties complementary to those of the first electrochromic film (nickel oxide being a typical example). This central three-layer structure is positioned between electrically conducting transparent films. The best material in terms of optical and electrical properties—or at least by far the most well-known one—is In2O3: Sn (indium tin oxide, or ITO).
When a voltage of the order of one volt is applied between the transparent electrical conductors, ions will be shuttled between the ion storage film and the electrochromic film. The electrons extracted from or injected into the transparent conductors then alter the optical absorption. A reversal of the voltage, or short-circuiting, brings back the original properties. The colouration can be stopped at any intermediate level, and the device exhibits open-circuit memory so that the optical changes take place only when charge is moved. It is then apparent that the electrochromic device can be viewed as an electrical battery with its charging state manifested as optical absorption. 5
Typically, electrochromic devices are of two types depending on the modes of device operation, namely the transmission mode and reflectance mode. In the transmission mode, the conducting electrodes are transparent and control the light intensity passing through them, this mode is used in smart-window applications. In the reflectance mode, one of the transparent conducting electrodes (TCE) is replaced with a reflective surface like aluminum, gold or silver, which controls the reflective light intensity, this mode is useful in rear-view mirrors of cars and electrochromic display devices.
Figure 2. Modes of electrochromic device operations
As was said above the design of an electrochromic device embodies five superimposed layers on one substrate or positioned between two substrates in a laminate configuration. The layers from the edges are electrically transparent conductors (TCs). TCs are a major issue for all electrically activated devices because of the need for a high-quality TC and their costs. TCs need a high transparency in order to let the active part of the used device regulate as much as possible. Furthermore, the TCs need a high electronic conductivity to provide a low voltage drop along the conductor surface.
The most widely used transparent conductor in all kind of devices is tin-doped indium oxide In2O3(Sn) (ITO), which may be replaced by heavily doped conductors such as SnO2: F (FTO), ZnO: Al or ZnO: Ga. ITO is in short supply. In fact, one predicts that we could run out of indium, a silvery metal produced as a by-product of zinc mining, in the next 10 years. The price of the metal has raised from around US $100 per kilogram to nearly $1000 in the past six years 6.
The basis of an electrochromic device is a glass or plastic covered by a transparent conducting film (ITO) on which one (or multiple) cathodic electroactive layer(s) are affixed. These are followed by a layer of ion conductor, on its turn followed by an ion-storage film or one (or multiple) complimentary anodic electroactive layers and another transparent conducting film.
The electroactive layers are often denoted as electrochromics. Most favorable are electrochromics that are reflecting in their coloured state instead of absorbing, but this has been found very difficult and most electrochromics are absorbing. By combining different type of electrochromics, ion-storage films and ion conductors, different properties can be obtained for the device, where the modulation range, durability and switching speeds can be optimized.
Many of these electrochromics are well-known today. Most important are the metal oxides, of which tungsten oxide is the most well-known, but also electrochromic polymers are applied in electrochromic windows and devices.
The electrochromic phenomenon of materials was originally discovered in tungsten oxide WO3 thin films, and remains until now the most promising, most studied and most applied electrochromic material in electrochromic windows and devices.
Electrochromism of tungsten oxide is a complex phenomenon and is still not yet completely understood, but it may and can be represented by the simple reaction:
transparent WO3+xM++xe-?MxWO3 (deep blue)where M+ can be H+, Li+, Na+ or K+; 0 ; x ; 1 and where e- are denoting electrons. WO3 turns blue, while doping the oxide with molybdenum, Mo provides colour neutrality.
Many other electrochromic metal oxides are known besides WO3 and applied in prototype electrochromic devices, e.g. Bi2O3, CeO2, CoO, CuO, FeOOH, Fe2O3, Fe3O4, FeO, MnO2, MoO3, P2O2-y, RhO3, RuO2, 6 SnO2, Ta2O5, TiO2 and V2O5, but most interest lately goes towards nickel oxide NiIIO(1-y)Hz, iridium dioxide IrO2 and niobium pentoxide Nb2O5.
Films based on NiO have enjoyed much interest lately because they combine a reasonable cost with excellent electrochromic properties, which even can be improved by mixing NiO with wide band gap oxides such as MgO or Al2O3 (Garcia-Miquel et al. 2003; Avendaño et al. 2004, 2006, 2007; Penin et al. 2006).
NiO:X, where X is Mg, Al, Si, V, Zr, Nb, Ag, Ta, Li, Al or B, has been found complementary with WO3:X in the visible and near IR: pairing them in a complete device result in a very dark colour neutral system (Michalak et al. 1999). The main electrochromic effect takes place in the UV and VIS spectra and reaches a very high colouration efficiency between 100 cm²/C at 340 nm and 25 cm²/C at 800 nm.
(transparent) Ni(OH)2?NiOOH+H++e- (grey)transparent NiOH+ Ni(OH)2?Ni2O3+3H++3e- (brownish)Iridium oxide
Also, films based on IrO2 and Ir2O3 have enjoyed an increased interest. While IrO2 – based films are excessively expensive, good electrochromic properties are obtained after dilution with the much cheaper Ta2O5.
(transparent) Ir2O3 · xH2O ? Ir2O4 · (x-1)?H2O + 2H+ + 2e- (brown)Niobium oxide
The interest in niobium pentoxide (Nb2O5) has increased in the last decade because of its promising electrochromic properties. Pure Nb2O5 and doped Nb2O5:X, where X is Sn, Zr, Ti, Li, Mo, WO3 or TiO2, layers change colour by insertion of H+ or Li+ ions from transparent to brown, blue or grey depending on the crystallinity of the layer.
(transparent) Nb2O5 + xM++ xe- ? MxNb2O5 (blue, brown or grey)The disadvantage of the Nb2O5 layers is their small colouration efficiency.
Besides the oxide films there are also organic films available with EC properties, but most of them show UV degradation and are hence less likely for possible energy -related applications. Many different polymers have been incorporated in prototype EC devices, e.g. poly- and monomeric pyrrole, viologens, 4,4′-diaminodiphenyl sulfone, poly(3-metylthiophene) or diclofenac, but most interest lately goes towards polyaniline (PANI) and poly(3,4-ethylene-dioxythiophene) (PEDOT). As for the inorganic electrochromic materials, the electrochromic polymers do also need to have a transparent state for many application (e.g. smart windows, rear view mirrors).
Polyaniline is a conducting polymer which may undergo colour changes from a transparent state to violet by both a redox process and proton doping. A simplified formula for PANI consisting of reduced and oxidized units can be written as (-B-N(H)-B-N(H)-)x(-B-N=Q=N-)1-xy with benzenoid and quinoid units (see Fig.3). This conversion of benzenoid units into quinoid units results in a typical absorption peak around 600-700 nm.
Figure 3. A simplified formula of PANI consisting of reduced and oxidized units with benzenoid (B) and quinoid (Q) units
PANI is one of the most extensively researched electrochromic material till date, owing to its good electrochemical cycling stability in non-aqueous electrolytes above 106 cycles, its low cost and ease of processing by electrodeposition or liquid casting techniques.
Electrochromic applications based on ?-type polymers have also drawn a lot of attention due to their ease of colouring, high electrochromic contrast and fast response times, of which poly(3,4-ethylenedioxythiophene) (PEDOT) and its derivates are most researched. PEDOT switches from blue in the neutral state to transparent in the oxidized state, but has a rather weak electrochromic contrast.
Polythiophene(s) are of particular interest as electrochromic materials due to their chemical stability, ease of synthesis and processability. Polythiophene thin films are blue (?max=730 nm) in their doped (oxidised) state and red (?max=470 nm) in their “undoped” form. Tuning of colour states is possible by suitable choice of thiophene monomer, and this represents a major advantage of using conducting polymers for electrochromic applications. Subtle modifications to the monomer can significantly alter spectral properties. For example, the colours available with polymer films prepared from 3-methylthiophene-based oligomers are strongly dependent on the relative positions of methyl groups on the polymer backbone. Colours available include pale blue, blue and violet in the oxidised form, and purple, yellow, red and orange in the reduced form. The colour variations have been ascribed to changes in the effective conjugation length of the polymer chain.
Methods for the deposition of electrochromic thin filmsThere are many methods employed for the deposition of the electrochromic thin films. Those which have attracted widespread use an attention for electrochromic materials as follows:
Physical Vapour Deposition (PVD)
Evaporation (Thermal, Cathodic Arc Deposition, Electron Beam (EB), Pulsed Laser Deposition (PLD))
Sputtering (DC Sputtering, RF Sputtering)
Chemical Vapour Deposition (CVD)
Electro-deposition / polymerisatiom
Of those techniques described above, PVD and CVD techniques have established themselves as widespread industrial and scientific methods of depositing thin films. Typically, these techniques are performed under vacuum, and any process which does this is known as a vacuum deposition process. Advantages of deposition thin films under vacuum include:
Increased free collision path for target material due to low particle density (the air in normal atmospheric conditions is very particle-dense in comparison);
Ability to closely control deposition conditions such as gas or material composition, deposition rates and reduce potential contamination;
Ability to generate plasma (where required).
Physical vapour deposition is a group of processes whereby free atoms or molecules of a target material are released into vacuum by physical bombardment or evaporation before condensing on the desired structure to form the deposited film. Prior to deposition, these free atoms or molecules may react with other atoms or molecules present in the vacuum which are usually introduced as a reactive process gas (oxygen), to form the desired product. Where the target material is reacted with a process gas or other material it is known as a reactive deposition.
Chemical vapour deposition is another group of deposition processes which result in the formation of thin films upon the desired substrate. Often, but not always, this is once again performed under vacuum using gaseous chemical precursors or a vaporized from of the target material introduced into the chamber. Here they react to form a thin film, often as a result of thermal means; on the surface of the substrate, with the surface of the substrate, or before condensing upon the strubstrate.
Applications of electrochromic materialsIt was not long after the initial discovery of electrochromics that suggestions for the commercial uses of electrochromic devices came about, the earliest example which is widely cited being the British patent filled by F.H. Smith in 1929 where in a clear solution containing an iodide and dye precursor develops a colour when an electrical current is passed through it, and returns to its bleached state upon removal of the electrical current. Following this there were a number of early patent applications whereby electrochromism was employed to provide changes of colour, level of light transmittance and even reflectivity. 8
Since these early patents, thousands more have followed, along with a very large and increasing number of publications. The most commonly cited applications of recent times being that of dimmable mirrors, and smart windows.
Generally speaking, all of these applications will involve a device operating in a reflective mode, transmissive mode or derivative thereof.
Figure 4. Electrochromic devices operating modes. (Arrows indicate path and intensity of incident electromagnetic radiation (light))
Above image (Fig. 4) provides an illustration of electrochromic devices operating in various modes. Device permutations utilising these traits may be realised to provide different functionality, e.g.:
Purely transmissive electrochromics may be used for various technologies such as optics, eyewear and smart windows;
Transmission varying electrochromes with reflective electrodes as in the reflective case, 2 above, may be used for dimmable mirrors;
Transmissive / reflective electrochromic devices, such as those constructed with Cu and Bi may be used for switchable mirrors;
Display devices may be realised by combining transmission varying electrochromes with a diffuse white backing or electrode.
Other potential applications which have being identified and have subsequently attracted attention include: display devices of various types, printing mediums, including security marks, detection of latent fingerprints, battery charge indicators, thermal exposure indicators for frozen food, motorcycle visors, sunglasses and even coatings for stealth military aircraft. It is obvious that these are not the only possible applications, there are many others and there is a lot of work to produce new fields of applications, due to the fact that electrochromism is a relatively new domain, all that is needed are specific knowledge and a little bit of imagination.
In next paragraph will be discussed about the most studied and of great interest applications which are electrochromic windows and electrochromic auto-dimming mirrors.
Electrochromic “smart” windowsWindows can be considered one of the most important components to improve the energy efficiency of existing and new buildings. With this purpose, various material and solutions have been adopted to enhance both their thermal and optical characteristics. Among these solutions, electrochromic films can be considered one of the most promising technologies to achieve energy saving as well as occupant visual comfort.
A solid state electrochromic smart window has the configuration glass/TC/EC/IC/IS/TC/glass, where TC is a transparent conductor, EC is an electrochromic coating, IC is an H+ or Li+ ion conductor and IC is an H+ or Li+ ion storage manner. Application of small voltage at the TC electrodes changes optical transmission of the device in a persistent reversible manner 9. Electrochromic windows darken when voltage is added and are transparent when voltage is taken away. Like suspended particle devices, electrochromic windows can be adjusted to allow varying levels of visibility 10.
As it derives from the name it’s clear that the main part of an electrochromic window is the electrochromic coating. As was said before, “electrochromic” describes materials that can change color when energized by an electrical current. Essentially, electricity kicks off a chemical reaction in this sort of material. This reaction (like any chemical reaction) changes the properties of the material. In this case, the reaction changes the way the material reflects and absorbs light. In some electrochromic materials, the change is between different colors. In electrochromic windows, the material changes between colored (reflecting light of some color) and transparent (not reflecting any light).
The chemical reaction at work is an oxidation reaction – a reaction in which molecules in a compound lose an electron. Ions in the sandwiched electrochromic layer are what allow it to change from opaque to transparent. It’s these ions that allow it to absorb light. A power source is wired to the two conducting oxide layers, and a voltage drives the ions from the ion storage layer, through the ion conducting layer and into the electrochromic layer. This makes the glass opaque. By shutting off the voltage, the ions are driven out of the electrochromic layers and into the ion storage layer. When the ions leave the electrochromic layer, the window regains its transparency.
It must be added that maintaining a particular shade does not require constant voltage. You merely need to apply enough voltage to make the change, and then enough to reverse the change – making this pretty energy-efficient.
-28257537445800An intuitive image of how things work is shown in the figures below (Fig.5, Fig.6):
Figure 5. Electrochromic window Figure 6. Electrochromic window switched OFF switched ON
A dynamic switching sequence of an electrochromic laminated window is shown in next figure (Fig. 7):
Figure 7. Switching sequence of an electrochromic laminated glass
We’re surrounded by windows every day, but we probably don’t stop to think about them very often. With advances in smart window technologies, we will start to see windows in a whole new light.
Electrochromic auto-dimming mirrorsElectrochromic devices have fascinating possibilities to be used in the automotive industry, too. Electrochromic glass represents an important step forwards in the use of glass, both in the field of building construction and in the car industry, as glass of this kind can modify its transparency and reflectivity as required, when an electric field is applied for a specified time.
Headlight glare in the mirrors from trailing vehicles can make it very difficult to see the road in front. This is particularly dangerous on dark rural highways with cross-traffic. Drivers who have driven at night on dark roads know that the light from incoming traffic is not the biggest problem, but the seconds after that light has disappeared, leaving utter darkness in its wake. Even after the glare is removed, an after-image remains on the eye’s retina that creates a blind spot for the driver. This phenomenon, known as the Troxler effect, postpones driver reaction time by up to 1.4 seconds. At 100 km/h, a car will travel about 38 m in this amount of time. Human factors studies relate driver discomfort to incident glare and the results are incorporated into a fully automatic mirror whose reflectivity varies dynamically to suit changing driving conditions 11.
Auto dimming rear-view mirrors, both the interior mirror at the top of the windshield and the exterior side mirrors, are intended to counteract the Troxler fading effect and increase driving safety by eliminating glare that can impair vision. Electrochromic mirrors have variable reflectivity and this offers an opportunity to select a reflectance level that avoids glare, but that maintains rear vision. In automatic mode, the electrochromic mirror responds appropriately to all driving situations, and mirror operation is transparent to the driver with smooth response and no unexpected change.
A dimming mirror is just a piece of glass, but with some interesting characteristic. The glass main feature is the ability to turn from a clear (uncoloured) to a tinted, colored state, when subjected to light (Fig. 8).
Figure. 8. Regular and auto-dimming rearview mirrors
In order for the dimming mirror to be effective, something must tell it when it’s time to act. Dimming mirrors used in the automotive industry are fitted with sensors to detect the intensity of the light. Usually, there are two sensors, one pointed to the front and the other to the rear. The interior mirror’s sensors and electronics control the dimming of both interior and exterior mirrors. The sensors, when active, are constantly looking for low ambient light. Poor lighting tells the sensors that the driver is driving at night and they begin looking for a glare source which may impair driver’s vision. When they detect a change in light intensity, they trigger an electrical charge to be applied to the glass through a low-voltage power supply. The electricity travels through an electrochromic gel placed between two pieces of glass during manufacturing, which have been treated with an electrically conductive coating. As a result, the mirror darkens automatically and proportionally to the light detected by the sensors (proportionally to how bright the glare is). When the glare is no longer detected, they revert to their idle (clear, uncoloured) status 11.
Many of the components of the automatic-dimming mirrors (sensors, circuit boards, micro-controllers, etc.) can be shared with other advanced features to save cost and space, while reducing part counts and overall vehicle complexity. They’re also easy to install, allowing vehicle manufacturers to bring the new features to the market quickly and efficiently across different vehicle platforms. Automatic-dimming mirrors are easily accessible, simplifying diagnostics and service. If necessary, they can easily be replaced.
ConclusionsThe evolution of technology does not stop, it is constantly developing. Electrochromics are a good discovery which has a great contribution to this development. Among the merits of this domain (that uses electrochromic materials) the most highlighted are: reductions in all aspects of cooling and lighting electricity usage (reducing of energy consumption), improving driving safety and increasing of thermal and visual comfort for building occupants.
Eletrochromism is a relatively new field, so it has a poor theoretical background, but there are a lot of models which gives very good result for specific applications. These models can be improved to develop a stronger theoretical background and can be used to create completely new devices that will improve people’s life.
Another important characteristic of electrochromic devices is that they can be fabricated either from inorganic materials or from organic materials. It is obvious that the proprieties and the cost of a specific electrochromic device depend on materials from which they are made. This gives possibility for consumer to choose what he needs according to his requirements (cost, quality, proprieties, etc.).
The future of electrochromics has a promising development. This is so thanks to its unique propriety, i.e. material can change color when energized by an electrical current. This propriety must be harnessed because it gives birth to many interesting applications and it have to do with energy reduction, which is one of the most discussed actual problem.
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10 How smart windows work (link)
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