Electro Chromic

Electrochromic Description

Electrochromic materials are materials that change colour upon application of an electrical potential. Electrochromic systems have been used successfully in the past in mirrors and windows for anti-glare and anti-reflective applications. However, other potential applications, including large area privacy/security glazing and high contrast angle-independent dynamic displays have yet to reach the market. This is due to the limitations of existing electrochromic materials.

To date, commercial electrochromic technologies have relied on either of two, broadly defined, types of architectures.

Firstly, there are solution based systems, which rely on organic electrochromic species dissolved in the electrolyte compartment of an electrochemical cell comprising at least one transparent electrode. The most commonly used electrochromophores, salts of 4,4’-bipyridines, also called viologens, are synthetically tunable which allows for different colours, and have intrinsically high extinction coefficients, yielding excellent colouration intensities. The switching speed depends on the diffusion of these and other redox active species in the electrolyte to the electrodes and is typically in the order of seconds. Because the redox active species are dissolved in an electrolyte these mobile molecules will diffuse to both electrodes once an appropriate electrical potential has been applied to the circuit. the electrodes and is typically in the order of seconds. Once the potential has been removed, the charged species mix, transfer their charges, and the colour dissipates from the system. Therefore there is no open circuit memory in these devices and power must be applied continuously to maintain colouration.

The other approach relies on the intercalation, or insertion and bonding, of ions into the crystal lattice of materials. This technology is based on electrochromic cells in which at least one of the electrodes is a thin but compact layer of a metal oxide, typically tungsten oxide WO3, which exhibits electrochromic properties. Colouration is achieved when ions, typically H+ or Li+, and electrons are injected electrochemically into a WO3 layer by means of an applied voltage. A strength of this intercalation electrochromic technology compared to solution based technologies is that they offer a memory capability in devices on the condition that the complementary redox species are bound to the counter electrode surface or a charge storage layer is available to support the charging of the device without dissipation of its charge upon removal of the applied potential. The intercalation process is slow due to the diffusion of the ions in the electrolyte and into the thin film. In addition, the metal oxides used do not form very highly coloured complexes so the contrast ratio is intrinsically lower than for solution based organic systems. Due to the specific weaknesses of each of these earlier systems, neither technology is suitable for the development of paper quality displays.

The key to transforming electrochromics into a high contrast angle-independent dynamic display is the use of chemically modified nanostructured electrode materials which is the core of NTERA’s patented technology “Nano Chromics”™

• http://www.ntera.com/technology/elec_displays.asp

Electrochromism is defined as the electrochemical generation of colour in accompaniment with an electron-transfer (or 'redox') reaction [1]. The research aim here is to prepare materials for inclusion within electrochromic display devices akin to, say, liquid-crystal displays but better in that a 'memory' effect is available: the colour persists even after the current has been switched off. The goals of this world are to prepare thin-film metals oxides which are electrochromic and for which the colour can be 'tuned'; and to tailor the rates of colouration.

Electrochromic materials are being utilised within novel applications such as the electrochemically self-darkening mirror, such as that marketed by Gentex.

• http://www.chem-mats.mmu.ac.uk/Research/Monk/Research.htm

Chromogenic Materials used in glazing and windows

Glazing materials that selectively control the spectral aspect of radiation are now commonplace. Low-emittance coatings supress infrared radiation transfer thereby imparting additional thermal insulation. Modified low-emittance coatings can also reject unwanted heat gain due to solar infrared. Additional energy savings result with dynamic control over the spectral characteristics of the glazing. There are a variety of technologies that can produce the desired effect:

ElectroChromic: Considered to be the most suitable chromogenic technology for energy control in buildings, ElectroChromic are the subject of intensive research. ElectroChromic materials undergo a reversible change in optical properties upon injection of light ions. Typically they consist of two electrodes separated by an ion conductor. Transparent conductors form the contacts. We have an active research program focussing on complementary counterelectrodes in lithium-ion systems. Association with the Berkeley Electrochemical Research Center provides a constant flow of new ideas for materials.

Measurement of optical properties is a specialty, and we have an extensive library of optical indices of electrochromic materials. Realistic images generated by Radiance have been used to visualize an electrochromic office and determine light levels under various conditions. DOE also supports an Electrochromics Initiative whose purpose is to accelerate the development of a window product.

Hydrides: These materials can be classified as electrochromics, but they are different in several ways from conventional oxide electrochromics. Originally deposited as a metal, they can be converted to a partially transparent hydride by injection of hydrogen from the gas or solid phase. Thus, they switch to a reflective state which has several potential advantages in terms of energy performance and durability, Read more about the newly discovered transition-metal hydrides.

Liquid Crystals: This familiar technology was commercialized for window use and later discontinued. Liquid crystal windows switch quickly from  a transparent state to a diffuse white state.  The primary function is to provide privacy and control glare as a substitute for conventional shading devices. In the diffuse state liquid crystals are primarily forward scattering so there is little control over solar heat gain.   back to menu

Suspended Particle Displays: Like electrochromics, SPDs are a promising energy control technology. They are reported to have a number of advantages over electrochromics. More information is available from Research Frontiers Inc. 

Photochromics: As the name implies, these materials darken under the direct action of sunlight. They are not considered as versatile as electrochromics becuase they cannot be manually controlled and because optimum energy performance requires consideration of temperature conditions as well as solar radiation. For example, a photochromic window may darken on a cold sunny day when more solar heat gain is desireable. They are used widely for automatically darkening sunglasses.  back to menu

Thermotropics: As photochromics respond primarily to light, thermotropics respond to heat. Again this is not as versatile a response as electrochromics. Daylight or view may have a higher priority for the occupant, at least temporarily, than reduction in solar gain.

• http://home.howstuffworks.com/framed.htm?parent=smart-window.htm&url=http://windows.lbl.gov/materials/Chromogenics/default.htm#Electrochromics

Related Nodes

• DisplayTechnologies
• WearAbleTechnologies
• ChromicMaterials
• SmartandInteractive
• SmartTextiles

-- RachelWingfield - 14 Nov 2004
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