In the wake of Display Week 2008, which showcased an explosion in reflective displays and e-paper products, it is clear that it is only a matter of time before this growing sector of the display market will take off.

by Robert Zehner

THE CONTINUING GAIN in popularity of reflective displays was in full evidence at Display Week 2008 in May, as at least 15 companies and organizations exhibited some form of electronic-paper product or concept. Technologies that were still in the laboratory 5 years ago are now entering mass production, spawning a variety of new display applications. Once the domain of a handful of start-up ventures, reflective-display suppliers have by now shipped tens of millions of units to big-name customers such as Amazon, Motorola, Sony, and Tesco. While smaller active-matrix manufacturers such as Prime View International have staked an early claim to the field, larger players such as LG Display and Samsung have clearly taken notice, showing prototypes of their own future products. In discussions with the many exhibitors of reflective displays, the key question for this market segment is no longer "if," but "when."

Particle- and Dye-Based Reflective Displays

Particle- or dye-based reflective displays accounted for a large majority of the e-paper exhibitors, led by materials developers E Ink,SiPix Imaging, and Bridgestone, as well as electrowetting-display designers Liquavista (which did not exhibit its products publicly), and adt Deutschland GmbH.

Fig. 1: Samsung's prototype 14.3-in. color electrophoretic display on a flexible plastic substrate. The substrate is curved and shows a picture of a window opening to the sea.

Samsung and LG Display appear to have extended their long-standing rivalry into the electrophoretic-display (EPD) space as well.Samsung's booth included a prototype 14.3-in. color electrophoretic display on a flexible plastic substrate (Fig. 1), while LG Displayshowed off an 11.5-in. UXGA monochrome EPD on thin stainless-steel foil (Fig. 2), as well as samples of a production 6-in. SVGA monochrome AMEPD module. Immediately across from LG Display, Prime View International (PVI) promoted an array of EPD product sizes from 1.9 to 9.7 in. on the diagonal, both glass-based and on flexible plastic substrates (Fig. 3). Eindhoven-basedPolymer Vision showed off a 127-ppi color rollable display as well as a 254-ppi monochrome display. The color rollable display had a roll radius of 6 mm and compared very favorably alongside a color front-page photo from the Los Angeles Times newspaper (Fig. 4).

All of the aforementioned products were built using E Ink's Vizplex^™ electrophoretic imaging film. In fact, E Ink's products were featured in nine different booths, from driver-IC manufacturer Solomon Systech, to Arizona State University's Flexible Display Center, to Epson Research's stunning 13.5-in.-diagonal 385-ppi e-newspaper (Fig. 5). E Ink's product consists of a film of microencapsulated "electronic ink," coated onto a thin plastic substrate. When laminated onto a pixelated backplane, the particles within the microcapsules can be rearranged with an applied electric field to display an image.

"With over 10 different manufacturers in Asia, Europe, and North America launching over 15 different eBook products, E Ink has become the de facto display standard for e-reader applications," said Sriram Peruvemba, E Ink's Vice President of Market-ing. "E Ink's Vizplex^™ film, launched in 2007, offers a maximum reflectivity of nearly 40% and a response time significantly faster than that of E Ink's previous-generation product.

"E Ink has also had an opportunity to fine-tune its products and processes by shipping millions of segment-type displays to customers such as Motorola, for the Motorola F3C phone, and Lexar, for its Mercury series of USB drives with integrated capacity meter. These features clearly appealed to companies such as Sony, iRex, Amazon, Jinke, Bookeen, Neolux, Polymer Vision, and Plastic Logic."

Not to be outdone, Taiwan-based SiPix Imaging showed improved versions of its microcell-based electrophoretic display material, including a large number of active-matrix prototype displays. Like E Ink, SiPix generates images by moving charged particles in an electric field; instead of containing the particles within microcapsules, SiPix forms an array of embossed "microcups," which it then fills and seals to make its display film.

"We have a bigger focus on active matrix this year," said Bryan Chan, Director of Marketing for SiPix Imaging. "Through constant improvement of our materials, we have achieved white-state reflectivities of between 30% and 35% in our latest prototypes." This improvement, achieved by optimizing the composition of their electrophoretic fluid, makes for brighter displays. While SiPix Imaging was not advertising its partner for production of active-matrix products, a SiPix AMEPD module was also on display in the booth of Taiwanese manufacturer Wintek (Fig. 6).

Fig. 2: LG Display's 11.5-in. UXGA monochrome EPD on thin stainless-steel foil.

Fig. 3: Prime View International (PVI) showcased several EPD product sizes from 1.9 to 9.7 in. on the diagonal, both glass-based and on flexible plastic substrates.

Fig. 4: Polymer Vision's 127-ppi color rollable display with a roll radius of 6 mm compared favorably alongside a color front-page photo from the Los Angeles Times. The display is stored in a rolled up position and then is unrolled to present the image.

For its part, Bridgestone's offerings of its QR-LPD^™ air-gap electrophoretic display focused on signage products, ranging from 20-in.-diagonal (A3 paper size) color and monochrome displays with 1920 x 2560 pixels down to postcard-sized electronic-shelf-label (ESL) modules. While Bridgestone displays also move charged particles to form an image, the QR-LPD^™ uses air, not liquid, as the carrier for the particles. According to Bridgestone's Natao Satake, Bridgestone and ESL module integration firm Pricer are currently conducting field trials with the Japanese supermarket chain Mainichi. Bridgestone and Dai Nippon Printing also unveiled an active-matrix QR-LPD^™ display fabricated using organic semiconductors (Fig. 7). Hiroki Maeda, a DNP researcher responsible for developing the matrix backplane, explained that organic TFTs are well-suited for this application because they can support the large drive voltages (80 V or more) required by the QR-LPD^™ material. While this was only an early prototype, adding active-matrix capability would presumably help Bridgestone to shorten the switching time of its large matrix displays, which took more than 10 sec to change from one image to the next.

Electrowetting-display manufacturer Liquavista did not participate in this year's exhibition, but Liquavista CTO Rob Hayes gave an excellent overview of the current status of the company's product development in an invited symposium talk (Paper 44.1). Liquavista's electrowetting displays work by electrically adjusting the wetting properties of colored oil drops on an electrode surface. Depending on the applied voltage, the oil drops shrink or spread to cover more or less of the viewing surface. Hayes explained that Liquavista plans to launch single-color segmented-type displays in the latter half of this year, followed by monochrome active-matrix displays in 2009 and full-color displays formed by using black dye droplets with a color-filter array in 2010. Back at the exhibit hall,adt Deutschland GmbH was demonstrating an electrowetting-display device that functions by moving a dye droplet to opposite ends of an hourglass-shaped container. The unique shape of adt's display physically traps the dye droplet at one end of the cell or the other, generating bistability. One drawback to this structure is that each display pixel must be paired with a hidden storage well where the droplet can hide in the "off" state, limiting the fill factor of the display device. Still, the engineers at adt believe that this may not be a big issue for large signage products or for mobile devices where a single row of droplets could be used to show wireless signal strength or battery capacity.

Bistable Reflective LCDs

Particle/dye displays were not the only reflective technologies represented at Display Week; bistable reflective LCDs continue to move forward in developing new markets and applications. In a tour of the booth of cholesteric-LCD manufacturer Kent Displayswith CEO Albert Green, I was surprised to see that two of the products he chose to highlight were not displays at all (or perhaps they are single-pixel displays). The first is an electronically erasable message board that exploits the well-known ability to change the state of a cholesteric film with pressure to write images with a fingernail or stylus. In order to erase the image, the control electronics drive the film back to the fully transparent state. The second product, which Kent has dubbed eGo^™, is a 60-μm-thick multilayer film that can be wrapped around the exterior of a cell phone or other gadget (Fig. 8). At the touch of a button, the film is addressed to one of 4000 possible colors to create, in effect, an electronic skin. While not a display in the traditional sense, the eGo^™ concept uses cholesteric's strength at creating bright colors to appeal to industrial designers – and, in the age of the iPhone, industrial design matters. In the past, Kent was primarily known for licensing its patents to display companies seeking to make cholesteric-display modules, but these two products will be produced and sold directly by Kent. "We are not a licensing company any more," said Green. "We are a manufacturing company."

Fig. 5: Epson Research's stunning 13.5-in.-diagonal 385-ppi e-newspaper was one of myriad EPD products at Display Week 2008 that utilize E Ink's Vizplex^™ film.

Fig. 6: This SiPix AMEPD module was also on display in the booth of Taiwanese-manufacturer Wintek.

British display maker ZBD began 8 years ago with a new method for creating bistability in a twisted-nematic LCD. In that time, the company has not only brought a new display technology to commercial reality, but has also developed a vertically integrated ESL product it calls epop^™. To bring this point home, ZBD created a mosaic of 560 epop^™ display units, each fully functional and controlled wirelessly from a nearby laptop PC (Fig. 9). The overall effect was quite impressive, especially after approaching the array up-close and watching individual signs update wirelessly. Owing to their bistability, these modules can run for up to 5 years off of the enclosed batteries, making them practically maintenance-free. Cliff Jones, ZBD CTO, explained that the display cells are manufactured through a partnership with Chinese display manufacturer Varitronix and that ZBD has landed a contract with Tesco, a leading international retail chain, to outfit one of its stores with several thousand of the ESL devices.

Although Nemoptic uses a different approach for generating bistability in its displays, the French display developer is equally focused on display modules for retail applications, according to Daniele Cognolato, the company's Sales and Marketing Manager. Nemoptic's ESL and point-of-purchase display devices can be updated using infrared or wireless signals to display product information and pricing. Display cells are produced through a partnership with Seiko Instruments, Inc. In addition to its focus on the retail market, Nemoptic has also received a grant of 2 million euros from the French government to support its SYLEN e-book reader project. SYLEN aims to deliver not only a reader device, but also a content-delivery system to fill the e-reader with books, newspapers, and magazines. Although the prototypes on display were of high quality, Nemoptic did not share details of any volume customers for their product.

MEMS-Based Reflective Displays

Last, but certainly not least, Qualcomm's mirasol^™ displays belong in a category all their own. Instead of particles, dyes, or liquid crystals, Qualcomm's display technology (microelectromechanical systems, commonly known as MEMS) uses arrays of microscopic reflectors that move under the influence of an electric field to increase or decrease their reflectivity. The micromirrors can change position in microseconds, allowing extremely fast switching; at the same time, the mirrors can be held in place by applying a bias voltage to the display, creating an ultra-low-power image stable state.

Fig. 7: Bridgestone and Dai Nippon Printing unveiled an active-matrix QR-LPD^™ display fabricated using organic semiconductors, which are well-suited for this application because they can support the large drive voltages (80 V or more) required by the QR-LPD^™material.

Fig. 8: Kent Displays' eGo^™ is a 60-μm-thick multilayer film that can be wrapped around the exterior of a cell phone or other gadget. At the touch of a button, the film is addressed to one of 4000 possible colors to create, in effect, an electronic skin.

Qualcomm acquired the fundamental tech-nology behind mirasol^™ when it purchased Iridigm, a 3-year-old display start-up, in 2004. Four years later, Qualcomm's customers are launching seven different products with mira-sol^™ displays, including a stereo Bluetooth headset, three different mobile handsets, a cellular video transmitter, and a "Dick Tracy"-inspired watch-phone. All of these devices feature small displays (the largest, in the phone, is 1.2 in. on the diagonal) that make good use of the sunlight readability and low-power capa-bilities of mirasol. Brian Galley, Engineering Director for Qualcomm MEMS Technologies, explained that a cell-phone handset maker is able to deliver two additional days of standby time by replacing an AMLCD with a Qualcomm display. From a visual perspective, these black-on-silver monochrome mirasol^™ displayshave a very similar appearance to more con-ventional reflective STN-LCD modules, albeit with substantially better power consumption and low-temperature performance.

Qualcomm also exhibited samples of color mirasol modules (Fig. 10). The on-off nature of the MEMS mirrors limited the color palette for these demonstrators, but Galley explained that future product generations will take advantage of subpixel dithering to produce a full range of colors.

Conclusion

With customer interest growing across a wide variety of applications and technologies, it is clear that reflective displays have found their place in the overall display market. It is interesting to note that, in contrast to the transmissive-LCD segment that is dominated by a handful of multi-billion-dollar corporations, the average reflective-display company is small, privately held, and has been in business for a decade or less. This small handful of companies is managing to turn out an increasing number of products each year, targeting entirely new markets such as color-changing skins for cell phones, electronic-book readers, and credit cards with integrated displays. With demographics such as these, it seems certain that each year will continue to bring surprising developments in reflective displays. •

Fig. 9: ZBD dramatically demonstrated its vertically integrated ESL product, "epop," with a mosaic of 560 epop display units, each fully functional and controlled wirelessly from a nearby laptop PC. Their bistability allows these modules to run for up to 5 years off of the enclosed batteries, making them practically maintenance-free.

Fig. 10: Qualcomm's mirasol^™ display technology uses arrays of microscopic reflectors that move under the influence of an electric field to increase or decrease their reflectivity. The micromirrors can change position in microseconds, allowing extremely fast switching; at the same time, the mirrors can be held in place by applying a bias voltage to the display, creating an ultra-low-power image stable state.