GREEN DISPLAYS

De ontwikkelingen in display technologie zijn de laatste jaren bijzonder uitgebreid. Plasma, LCD en LED zijn natuurlijk allang letterlijk household names. Maar 3D schermen, holografie en reflectieve schermen staan voor de deur. Deze schermen zijn al een aantal jaar te zien op beurzen en  evenementen, de komende periode zijn ook deze te vinden in winkelcentra, winkels als in de huiskamer.

Een andere belangrijke ontwikkeling is die van het flexibele beeldscherm. Tot op heden zijn beeldschermen vooral bekend in glas. Nieuwe technieken maken het echter mogelijk schermen te verpakken in transparante kunststoffen. Deze zijn oprolbaar , verbuigbaar en soms te veranderen in kleur. De vorm van het beeldscherm veranderd en daarmee het type toepassing.

Tot voor kort gaven alle beeldschermen licht, denk maar aan de blauwe gloed in de huiskamer bij de oude beeldbuis, maar ook bij het licht van een LCD monitor. Een aantal nieuwe beeldschermen zijn ook lichtgevend, we hebben het dan vooral over OLED’s schermen gemaakt van organische kunststoffen, de zogenaamde organic polymeren. Deze OLEDs kunnen verwerkt worden in dunne lagen plastic die in zijn geheel licht geven.

Licht heeft echter als nadeel dat het relatief veel energie kost. En voor communicatie moet het concurreren met het licht in de buitenwereld. Vooral buiten zijn bepaalde schermen daardoor onbruikbaar, denk maar eens aan het gebruik van een laptop op een zomers terras. Er bestaan wel buiten schermen, maar deze moeten een lichtgevendheid hebben van over de 1500 nits of lumen, een maat voor licht. daarnaast moeten ze weer bijzonder goed geventileerd zijn en gekoeld en geïsoleerd voor vocht. Een reden waarom outdoor displays aanzienlijk duurder zijn dan “indoor.”

Voor ons is echter relevant een reeks schermen die reflectief zijn. Ze geven zelf geen licht maar verkleuren alleen en geven contrast. En ze reflecteren het licht, weerkaatsen het. Hoe meer zonlicht erop staat, hoe beter ze te zien zijn. Vergelijk het met drukwerk, wat immers ook in het licht te zien is, en in een donkere kamer niet. Deze groep schermen wordt dan ook e-paper genoemd, en een van de meest vooraanstande merken is e-ink.

Veel van deze reflectieve schermen zijn ook bi-stabiel. Ze kunnen schakelen van de ene afbeelding naar de andere op basis van een stroomimpuls. Maar wordt de spanning eraf gehaald dan blijft het beeld staan. Deze schermen blijven dus hun afbeelding tonen als er geen energie meer aan wordt geleverd. Daarom zijn deze schermen extreem energie- zuinig.

De snelheid van ontwikkelingen is overigens lang niet zo groot als bijvoorbeeld in digitale content, gadgets of software. Het duurt vaak jaren om een schermtechniek te ontwikkelen, testen, bouwen en op grote schaal te produceren. De schermen zoals we ze nu in het straatbeeld tegenkomen hebben soms een voorbereidingstijd gehad van decennia. Toch vindt ook hier versnelling plaats. Duurde bij televisie de sprong van zwart wit naar kleur nog meer dan 30 jaar, in epaper ligt dit eerder in de 5 tot 10 jaar.

In aparte posts zullen we verder ingaan op ontwikkelingen in schermtechnologie.

Voor een overzicht van E paper technologieen en verschillende aanbieders, verwijzen we naar de e-paper technology guide op e-papercentral.com.

Hiervan komen ook onderstaande quotes die een overzicht geven van de verschillende reflecterende beeldscherm technieken.

E-paper Technologies Reference Guide

For almost three decades, electronic paper technologies have been evolving to combine the flexibility of digital information with the familiarity, quality, and convenience of a paper-like substrate. More than a dozen companies have announced work on active e-paper programs, and there are a number of start-ups coming to existence as well.

The production structure of electronic paper is fairly complex. E-paper is based on IP/technology developed by a handful of technology developers. In many cases this manufacturing is contracted out. In addition, E-paper generally needs some kind of backplane that is manufactured by another group of firms. It is important to note that there is an additional group of firms—consumer product firms—who design and market the product into which the e-paper display fits. For example, the e-readers marketed under the Sony brand have incorporated e-paper technology from E Ink and backplane technology from Polymer Vision.

This guide is designed to provide a background in both e-paper frontplane technologies and the current backplane technologies used to manufacture such displays. It is divided into two sections. The first will explain the various e-paper technologies that exist today and provide analysis on the pros and cons to each of them. The second section will discuss the various backplane technologies used to power the e-paper frontplanes.

Electrophoretic Technology

Electrophoresis is a process, which enables separating molecules according to their size and electrical charge by applying an electric current. In an electrophoretic frontplane small, charges submicron particles are suspended in a dielectric fluid that is enclosed into a sub-pixel size cell or microcapsule. When an electric field is applied across this cell or capsule, the ink particles will move towards the electrode with the opposite charge.

With a transparent electrode the cell or capsule surface takes on the color of the ink when current is applied. The contrast is improved by using opposite colored particles. such as black and white—and charging them with opposite polarities. When current is applied, all the black particles will migrate to one side, and all the white to the other. Switch the field, and the capsule will change color. This enables switching between all black particles and all white particles on the transparent front electrode of the cell or microcapsule. This is how the high contrast ration of electrophoretic displays is created. The difference between the various electrophoretic frontplane technologies lies simply in the method of encapsulation for the charged particles and fluid medium. Some versions use a “microcup” rather than a particle.

E Ink
SiPix
Bridgestone

Cholesteric Liquid Crystal Display Technology

Cholesteric materials are modified liquid crystals and extremely suited for reflective, bistable displays. A cholesteric liquid crystal is a type of liquid crystal with a helical (smooth curve like a spiral) structure.. Cholesteric liquid crystals are also known as chiral nematic liquid crystals.While solids have molecules that maintain their orientation., molecules in liquids change their orientation and move anywhere in the liquid. Some substances exist in an odd state that is similar to both liquid and solid. When they are in this state, the molecules tend to maintain their orientation, like solids, but can also move like a liquid. Liquid crystals are such materials. However, in essence they are more like a liquid and require only a little heat to move from this odd state to a liquid state.

A feature of liquid crystals is that they are affected by electric currents. Depending on the temperature and particular nature of a substance, liquid crystals can be in one of several distinct phases, including nematic phase and the cholesteric phase. LCDs use these types of crystals because they react predictably to electric current in such a way as to control light passage. The use of a cholesteric liquid crystal means that the display has a far better readability than a display using conventional nematic liquid crystals and can be made thinner, since it reflects 50 percent of certain wavelengths, removing the need for color filters and polarizing layers. This in turn means that the background color is more vivid and the contrast much better than conventional reflective-type LCDs.

Fujitsu
Hitachi
Kent Display
Kodak
Nemoptic
ZBD Display

Electrowetting Technology

Electrowetting is based on controlling the shape of a confined water/oil interface by an applied voltage. With no voltage applied, the (colored) oil forms a flat film between the water and a hydrophobic (water-repellent), insulating coating of an electrode, resulting in a colored pixel. Applying voltage between the electrode and the water causes the interfacial tension to change, which causes the water to move the oil aside. The result is a partly transparent pixel; if a reflective white surface is used under the switchable element, a white pixel results. This forms the basis of the reflective display.

Displays based on electrowetting have several advantages. The switching between white and colored reflection is fast enough to display video content–supposedly pixels can switch states in around 10 milliseconds-fast enough to generate 100 new images in a second. TV-quality video only requires 25 images per second. In addition, the high reflectivity and contrast of the wetting displays make them clearer: color displays are four times as bright as LCDs and twice as bright as other e-paper technologies. Electrowetting displays reflect around 40 percent of light.

Since it is a low-power/low-voltage technology, displays can be flat and thin. Reflectivity and contrast are claimed to be better or equal to those of other reflective display types and approach those of paper. It can be used as a basis for high-brightness full-color displays. Such displays are claimed to be four times brighter than reflective LCDs and twice as bright as other emerging technologies. While it is low power, electrowetting is not bistable, so some electricity is needed for image retention. One of the advantages of electrowetting technology is that it can be integrated into existing manufacturing structures for LCD systems.

Electrofluidic Technology

Images of pigment droplets and an overlay of three CMY prototypes. (Image courtesy of Nature Photonics)

Electrofluidic displays have a polymer layer with very small cavities which are filled with aqueous pigment dispersion. The underlying physics is based on electrowetting technology, but the device principles and performance are quite different. The name electrofluidic describes the mechanism which involves movement of liquids through microfluidic cavities as a result of an applied charge. Because the cavities only comprise about 5-10% of the visible area, the pixels are hidden from view. When an electromechanical voltage is applied, the pigment dispersion is pulled out of the cavities and begins to expand over the layer to encompass 90-95% of the visible area. Because the process uses pigment dispersions, it has more brightness and color saturation than currently available e-reader technologies and more closely simulates the look of traditional ink on paper. Current e-paper technologies have 40-50% reflectance. The new technology has 55%, but researchers feel an 85% reflectance of ambient light is possible (the same as that of paper). The technology also claims faster switching speeds. At present a black-and-white prototype has been demonstrated, but the inventors claim that color is not an issue. Two electronic layers would be used for color displays, which would enable a CMY subtractive approach similar to printing ink on paper. While this would need double the power of a single plate, it would still be sufficient to be attractive to consumers. An additional advantage is the fast switching speed. By tweaking variables such as geometry, surface tension and viscosity, the speed could be maximized to a sub-millisecond. The displays can be manufactured using existing processes for producing LCDs.

Gamma Dynamics

Electrochromic Technology

Electrochromism refers to the characteristic color change of a material associated with the materials’ reduction/oxidation state. Polyaniline and polyethylenedioxythiophene (PEDOT) are examples of electrochromic materials. An EC display element consists of at least two conductors, an electrochromic material and an electrolyte combined on a carrying substrate. The optical contrast is a result of the contrast between the white paper surface and the electrochromic materials switched to its colored state. These displays are
fully flexible and the printed devices are less than 100 microns thick.

One of the claimed advantages of electrochromic displays over other technologies is the high contrast, the vibrant, rich looking color image of the display against the white background. This is due to the fact that electrochromic materials absorb some light spectra and reflect others, similar to pigments used in printing. Other technologies use light scattering techniques.

Acreo
Aveso
Ntera
Siemens

Interferometric Modulator Display

An Interferometric modulator display (IMOD) uses a technology made up of subpixels which are actually miniature Fabry-Perot interferometers (etalons). An etalon, which is an optical term, reflects light at a specific wavelength and gives pure, bright colors like those in a butterfly’s wings. Moreover it consumes no power. Microelectromechanical systems (MEMS) are used to switch the display on and off.

Qualcomm

Photonic Crystal Technology

Another technology, which has recently entered the limelight, is based on photonic crystals, which are nanostructures arranged in a regular pattern. Changing the pattern causes a change in the color of light that the crystals reflect. Artificial opals are similar to those occurring naturally, with one exception—artificial opals can be stimulated electrically to change color. These opals can then be integrated into a layer of millions of tiny silica spheres, which are embedded into an electroactive polymer. The layer is then sandwiched between transparent electrodes. When current is applied, it causes the polymer to swell, which in turn changes the spacing of the crystals. If this movement is controlled, the crystals can be maneuvered to produce the entire light spectrum. Such layers can then be arranged into a display similar to a traditional LCD screen. The advantage of this technology is that the pixels can be individually tuned to any color, and the color is purported to be brighter and more intense.

The technology is very new, however, and commercial products are still years away. However, recently the speed at which the crystals are able to change color was improved dramatically, as were the available spectrum colors that could be achieved with the technology. The increased speed was made possible by dissolving the nanospheres into a porous polymer structure, eliminating the silica. The pores are filled with electrolyte and then the material is sandwiched between electrodes.

Nevertheless, there are still challenges to be resolved. One of the disadvantages of the photonic crystal approach is its dependence on the flow of electrolyte in response to electricity. This could mean a decrease in efficiency after repeated cycles, similar to rechargeable batteries. Furthermore, when pixels change from long wavelength colors to shorter ones the speed decreases. In addition, pixels need more color contrast. Adding nanoparticles to the polymer might improve the contrast.

Opalux

REED Technology

Reverse Emulsion electrophoretic Display (REED) uses nano-droplets of a polar liquid, which are composed of a blue dye and surfactants in a measured ratio. These droplets are dispersed in a non-polar liquid. When energy is applied, the droplets reassemble in the liquid. The technology is purported to use less energy than currently available electrophoretic technologies, and have faster switching speeds, which would make video possible. Moreover, it can be produced using existing LCD manufacturing techniques.

Zikon Corp


Bistable LCDs

Liquid Crystal Displays offer excellent picture quality with brilliant color and video, however, they require a brushing process on the inside of a glass sandwich to lock the twisted molecules. These twisted molecules are necessary for bistable displays, which offer low voltage advantages. Bistable LCDs use a photo-alignment process to eliminate the brushing of the glass. The process, a roll-to-roll technology, uses a new photo-alignable polymer—an azo-dye, which has an anchoring energy that can be adjusted by changing the UV exposure time. The layer is stabilized by heat polymerization after the azo-dye monomers are photo-aligned. The liquid crystal is then deposited on top.

According to researchers this arrangement is inherently low cost, likely to give much better colors than electrophoretic technology, be more robust and operate without need of a transistor active matrix backplane or ITO or alternative transparent electrodes with all their problems of cost and of cracking when bent. The display is also optically rewriteable by means of light emitting diodes (LEDs). However, currently there is no optical writing mechanism, which is small enough for the display, which is 500 nanometers thick. Commercialization is not expected for several years.

Backplanes for E-paper

As mentioned previously, there are various technologies currently being used to manufacture the backplanes to be used in e-paper displays. Although the initial offerings were thin film transistors (TFT), some of the newer introductions are rather unique technologies for backplanes. Following is a list of these manufacturers and some explanation of the technologies they are using.

HP
NEC
Plastic Logic
Polymer Vision
Prime View International
Ricoh
Samsung
Seiko Epson

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