E-Paper Primer Part I

Introduction

E-paper displays are reflective devices that deliver crisp text and images using very little power. They are also typically thin, lightweight devices that are conducive to implementation on flexible substrates. Some are printable, and most are bistable, that is, they can retain screen contents without consuming power. E-paper is, in short, very much like ink and paper, with a high contrast ratio, outdoor readability (a major appeal of these reflective devices), and the potential for high resolution and low cost. E-paper's key value proposition is that it combines some of the benefits of paper with the dynamism of displays.

However, the bulk and cost of e-paper more or less ensure that it will never be a complete substitute for paper. And for the time being, poor color quality and the inability to support video also limit the ability of e-paper to compete directly with more general purpose display technologies such as LCD or OLEDs. Nonetheless, there are markets-principally in signage-related applications-where e-paper has advantages over real paper because it can be electronically updated on the fly. And there are other markets-principally in mobile communications-where e-paper has advantages over other kinds of displays, because (like paper) it can be read in bright sunlight. There are also almost certainly other opportunities for e-paper-many of which still wait to be discovered-that lie outside the usual list of applications for displays. Electronic book readers are the obvious example here and they have become almost synonymous with e-paper.

E-Paper Capabilities

E-paper alternatives improve on conventional passive LCDs in a number of ways. Their bistability, for example, expands the potential pixel capacity of a screen by eliminating the need for electrical refresh. It also raises the threshold at which an AM (active matrix) is required to deliver an acceptable image.

Optical Performance

The fundamental metric of a reflective display is reflectivity, representing the percentage of incident light that can be reflected back as useful information for a user. Reflectivities of 30 percent to 50 percent are common among e-paper screens, and levels as high as 70 percent have been reported. As far as optical performance is concerned, e-paper contrast is equal to or better than conventional passive LCDs, and far better in a few cases. Particle e-paper displays in particular are inherently high contrast, based as they are on pigmented materials, essentially tiny paint chips. Also, alternative LCDs based on light scattering have far superior contrast to conventional transmissive LCDs, which use light polarization as a control medium.

Color

The next big opportunity for e-paper needs to be color. There can be no doubt that very large markets are currently closed to e-paper because all commercial e-paper products are monochrome with some gray scales.

The signs are that these markets will open bit by bit, rather than all at once. There are a growing number of demonstrations of color e-paper displays, which, although fairly primitive, may be good enough for some low-end applications. No one would claim, however, that this type of color would be good enough for mobile video or advertising signage application. NanoMarkets believes that the first firm that could offer high quality color and (eventually) video on an e-paper platform at a reasonable price would gain a lot of attention and most probably a lot of orders.

There are a number of different colorization techniques available to e-paper developers, but the most straightforward is to use conventional RGB (red, green, blue) color filters. These, however, rob the display of light, leaving unsaturated colors as a result. The R filter, for example, throws away GB, representing two-thirds of the available light. Spatial RGB filtering also effectively reduces the resolution of a display by one-third. So there are clearly opportunities for improvement; higher reflectivity levels and optimized color filters should improve the situation in days to come. Several e-paper makers have also improved the picture by using a quad subpixel arrangement that adds a white subpixel to the conventional RGB trio.

One alternative to RGB filters is the RGB subdisplay stack. Here, three discrete displays are configured, one on top of another, with each responsible for a single primary color. One obvious benefit of this more costly and complex approach is that the display's resolution is not spatially subdivided, as it is with RGB filters. RGB colorization is additive; displays can also make use of CMY (cyan, magenta, yellow) primaries in a subtractive color stack. Particle displays have several other approaches available to them for providing multi-color capabilities. MEMS, in turn, are capable of patterning RGB pixels as part of their basic processes.

Power

The high cost of energy and the lack of very high density power sources for mobile electronics turn out to be powerful drivers for e-paper because e-paper displays use little power; bistable varieties use even less.

To cite one example, Fujitsu Frontech estimates that its ChLCDs require only about 1/100 the power of a conventional LCD, and only 1/10,000 the power when screen contents are stable. ESL (electronic shelf label) makers say that under typical retail pricing scenarios, their e-paper devices operate from battery power for as long as five years. In the e-book arena, estimates are that the owner of a Sony Librie ebook, based on a 6-inch E Ink EPD, can expect to read between about 7,000 and 7,500 pages of text before having to think about a recharge. Matsushita, in turn, says that the two AA batteries in the spine of its dual-page Panasonic Sigmabook will keep it and its two 7.2-inch Kent ChLCDs working for three to six months. For those who have tried to use their c onventional PDA as a reader of sorts this will be welcome news.

Speed

Most e-paper is relatively slow, taking between about a quarter second and a full second to update the screen. This is fine for some applications. ESLs, for example, have no need for action video, and having to wait a full second to "turn the page" of an e-book is no hindrance to technology adoption. But for other applications, video speed is a requirement; or at least it soon will be because video is becoming a growing part of many forms of communications.

As with the absence of color, the absence of video is a limitation on the markets that can currently be attacked by e-paper. The need to upgrade to video is probably not as urgent, although several e-paper makers do see it as an opportunity. These include Bridgestone, which cites a 200 microsecond response time for its EPDs, and QMT whose MEMS respond in under 10 microseconds.

Manufacturing Cost

Some parts of the e-paper market are fairly cost sensitive. This is especially true of the ESL segment, which is seen by many as one of the "low hanging fruits" for the e-paper business. It is probably not as true as for the e-book reader segment, but when one considers the mobile display market as a whole, cost sensitivity rears its head again.

One way of getting to the pricing needed in this marketplace will clearly come through improved manufacture. There are at least two kinds of thinking about this. The one that frequently gets the most emphasis is the use of printing, which offers all the advantages of additive process that uses relatively inexpensive equipment. NanoMarkets believes that there are some real advantages that will flow from the use of printing in the e-paper segment and to some extent it can even be said to have proven value; after all, the e-paper mindshare leader (and probably the market share leader, too) E Ink produces exactly what its name suggests: electronic ink.

Printing is also directly conducive to reducing manufacturing costs through R2R processes on flexible substrates. But the whole notion of printing as the complete solution to creating electronic devices has become a little tarnished as printed electronics firms have moved into full production mode. Many printed electronics firms (and not just in the e paper space) have found that as a matter of pragmatics, it is better to combine printing with other forms of manufacturing processes, typically those borrowed from the semiconductor or LCD industries. Plastic Logic, which has been singing the praises of printed electronics for many years, is now apparently using classical deposition techniques along with printing in producing its flexible backplanes.

Other firms are adopting classical fabrication techniques not just as a matter of pragmatics, but more as a matter of strategy. They take the position that by leveraging existing manufacturing infrastructures and technologies, better economies can be achieved than through the-still largely theoretical- advantages that printing is supposed to offer. As it happens, Plastic Logic's closest competitor, Polymer Vision, has chosen this road, emphasizing spin coating where Plastic Logic has emphasized ink-jet as a way of fabricating the transistor arrays on backplanes.

Polymer Vision is especially important in the context of e-paper, because it has used its flexible backplane technology to become the first firm to commercialize a flexible book reader product. But the idea that low costs would best be served by using existing technology is also to be found in the e-paper market itself; several firms emphasize that they can use existing lines - especially LCD lines - in their processes. PVI, which has produced most of the backplanes for e-book readers to date, uses more or less conventional TFT technology for its offerings.

It is important to note that such differences between manufacturers do not necessarily reflect a difference in opinion about the long-term viability of the printable electronics opportunity. Rather the difference lies in when manufacturers think the printing opportunity will emerge as an option that will contribute to the bottom line and not prove too troublesome from an operational point of view.