Electronic paper (e-paper) has emerged as a radically new display technology, using reflective rather than emissive technology to create images. Unlike paper, e-paper allows for constant information updates via a network. Conversely, with low power usage and the ability to be read in daylight, it challenges more traditional display technologies. It has found application in several low-end display applications such as smart shelves, smart cards and most notably in e-book readers. However, for it to become a serious competitor to traditional display technologies-or even to move beyond very low end applications-it must move from monochromic into full-color screens.
Brighter Color Needed
Color displays are currently available; the challenge is to achieve multicolor displays with higher brightness so that market acceptance can become easier. While there are several colorization techniques that might be explored, the most direct route would be conventional red, green, blue (RGB) color filters. However, theses can rob the display of light, leaving unsaturated colors as a result. For example, the R filter does not admit green and blue light, thus reducing the amount of available light by two-thirds. Spatial RGB filtering also reduces the resolution. So clearly, there is work to be done.
Current Color Displays
Despite the challenges, there is a widespread move to color versions of e-paper displays with multiple companies introducing prototypes. LG Phillips LCD, Korea has introduced a 14.3-inch e-paper display, with a screen resolution of 1280 by 800 pixels, which is a significant improvement over its predecessors. Using metal foil and plastics for a substrate, it is both flexible and bendable and capable of displaying 16.7 million colors. The display uses E Ink’s electrophoretic display technique, which is RGB-based color. E Ink has also worked on color with Toppan Printing since 2002. Another Asian company, NEC, has demonstrated an e-paper display using E Ink’s electrophoretic technology. Rather than monochrome screens, the latest prototypes— A3 (297mm by 420mm) and A4 (210mm by 297mm) sheets of paper—feature 16-tone (greyscale) displays that can be combined with as many as eight screens. It also features white reflectivity of 43% and a contrast ratio of 10:1. While a step above black and white, it is not really color.
As part of a joint development project with Unidym, Samsung Electronics has also demonstrated a 14.3-inch full-color active matrix electrophoretic display (EPD). This e-paper device uses a carbon nanotubes (CNT) transparent electrode. Researchers claim that CNT materials can demonstrate conductivity comparable to the incumbent indium tin oxide (ITO) technology, uniformity over large areas in films and compatibility with different display technologies and fabrication processes.
Taiwanese PVI has demonstrated a 9.7-inch color e-paper display with a plastic background, while Polymer Vision in Europe has introduced a new prototype color rollable display with a roll radius of only 6 mm. The 65,000 color display has a resolution of 127 ppi. (The new Polymer vision monochrome display has a resolution of 254 ppi, illustrating the road that must be traveled before color becomes a commercial reality.)
RGBW Color Filter
Nemoptic, based in France, is also researching the development of multi-color and flexible products. It uses standard liquid crystal display manufacturing technology for its “bistable nematic” displays. Nemoptic has shown a prototype of a new version of its full-color display, the BiNem(R) Demonstrator (BD) 1000, which uses a RGBW (Red-Green-Blue-White) color filter. An important feature of BD 1000 is its fine subpixel size-127 x 127 micrometers, leading to a resolution of 100 dpi, considered high for bistable color displays. Earlier prototypes of BiNem color e-paper displays were processed using basic RGB color filters, which lead to limited brightness at acceptable color saturation. As a result of switching to RGBW color filters, the company claims this version has improved brightness up to 20 percent, among the highest for existing color e-paper displays, while maintaining the same color saturation level.
Rather than having the pixel colors side-by-side, some companies have stacked the color layers on top of each other. This means the color effect is less spread out, giving a clearer image when viewed straight on, although the colors can shift at an angle. Fujitsu Frontech’s Super Frontech Vision EP series indoor display board has three layers on a flexible substrate which contain cholesteric LCD (ChLCD) material colored red, green and blue, avoiding the need for separate filters. It claims a color range from 8 to 4096 colors.
Although RGB and RGBW filters might be the easiest route for the new technology, CMYK (cyan, magenta, yellow, black), which predominates in the printing industry, is also an option. LiquiVista, a spin off from Philips Research, uses “electrowetting” technology for its stacked combination of the CMYK approach. The company (in conjunction with Plastic Logic) recently received a $12 million research grant from the U.K. government’s Technology Strategy Board to develop full-color flexible electronic displays with video capability.
Recently, another technology has been introduced based on photonic crystals. These are nanostructures arranged in a regular pattern, which when changed cause different light colors to be reflected. In this case, the photonic crystals used are artificial opals, which can be stimulated electrically to change color. Opalux, based in Toronto, Canada, is a recent start-up focusing on photonic crystal technology, sometimes called P-Ink. It was co-founded by André Arsenault, a recent PhD graduate of the University of Toronto. These opals are integrated into a layer of millions of tiny spheres, which are embedded into an electroactive polymer. By applying a controlled current, 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.
Although the speed at which crystals change color was improved dramatically, the technology is still new and commercial products are years away. There are other challenges also. One of the disadvantages of the photonic crystal approach is its dependence on the flow of electrolyte in response to electricity, which could mean a decreased efficiency after repeated cycles.
So while the commercialization of full-color e-paper products may be down the road a bit, the journey has definitely begun.
By Linda M. Casatelli