E-readers: beyond black and white

Today's e-book reader may be lightweight, easy-to-read, touch-sensitive and use little power, but already consumers want more. A color, flexible screen that hosts video is top of the wish list, but can display developers deliver?

U.S.-based electronic paper pioneer, E-Ink, has been quick to respond by placing a color filter over its established paper-like black and white display. Snapped up by China-based e-reader supplier, Hanvon, the color E-Ink e-reader is due to ship, in China, in May of this year. But all is not well.

Critics describe the display's colors as muted and have complained that it can't handle full-motion video, only simple animations. So what's the problem? The technology.

Like most of the screens in today's e-readers, E-Ink displays use electrophoretic technology. Here, black and white, charged, sub-micron particles, floating in a dielectric fluid are enclosed in a micro-capsule. When an electric field is applied, the particles move toward an electrode, making the surface appear black or white at that spot (Figure 1).

Figure 1. E-Ink microcapsules: Applying an electric field forces the charged particles to move to the electrode with the opposite charge, making the micro-capsule surface appear black or white.

Simple and effective, this technology has made E-Ink display market leader for two key reasons. First, its display is bi-stable and able to hold text and images without draining power from the battery. Power is consumed only when an image is changed, or a 'page is turned,' so the battery can last for weeks on a single charge. Second, the display is reflective and doesn't require a backlight to illuminate its pixels, which means it can be viewed in direct sunlight.

But for providing bright color and video motion, the technology has reached it limits. With the current particle and fluid chemistry, color is only achievable with a filter, which blocks light, making the display look dull and washed out. Meanwhile, the display's response rate is only 250ms as the particles suspended in fluid just can't be forced to move between electrodes at video-rate speeds.

E-Ink's vice-president of global sales and marketing, Sriram Peruvemba, is unperturbed. "We are not trying to produce a more colorful picture and this product is not going to replace LCDs or OLEDs in devices that do video and gaming," he asserts. "We intend to offer this to our existing market, the electronic publishers trying to replace books with these devices."

At the same time, however, E-Ink is developing a display based on an entirely different chemistry, which according to Peruvemba, provides bright color and the all-important video speeds. "Every sub-component has been examined and re-created with new materials," he says. "We currently have displays with refresh rates of 50ms running 'Cars' and 'Finding Nemo', in the laboratory...and it will take up to three years to transport the processes from here to mass production."

But while E-Ink perfects its new chemistry, several companies are getting ready to challenge the industry incumbent. One key contender, Japan-based Bridgestone, is working on bi-stable, electrophoretic displays, but with a difference.

Dubbed 'Quick Response - Liquid Powder Display', the Bridgestone display uses charged powder particles nestling between ribs, rather than encapsulated ink particles suspended in fluid (see Figure 2). The displays can be mounted on plastic substrates and fabricated using a cheap and efficient roll-to-roll manufacturing process. And importantly, this particular chemistry yields displays with response times of only 0.2ms, enabling video-speed applications.

Figure 2. The Bridgestone QR-LPD: When a negative voltage is applied to the upper transparent electrode, the positively charged black particles move to the upper electrode showing a black appearance. If a positive voltage is applied, white particles move upwards, leaving a white appearance.

"Our powder material behaves like a liquid, so we named it 'liquid powder'," explains Ryo Sakurai, from Bridgestone's Electronic Paper Development department. "It shows a high fluidity due to its morphology and electronic properties, and because the particles are dispersed in air, not liquid... so the [liquid powder] moves very quickly in the presence of electric fields."

Like E-Ink, the company has developed color displays using filters but has also added colored liquid pigments to the liquid powders to produce a display with a reflectivity close to recycled paper. While Sakurai says the company is deciding which technology to exploit for commercialization, it intends to launch a black-and-white, flexible electronic paper display this year, with a color version following in 2014.

Alternative technologies

Widespread in myriad mobile devices, color liquid-crystal displays that host video-speed applications are not typically used in e-readers as the screen cannot be read in sunlight and its backlight drains battery power. However a few companies are working around this.

U.S.-based Pixel Qi has developed a hybrid display that runs in two modes; the first as an LCD providing full color and video motion, and the second as a reflective, e-reader display preserving precious battery power. At the same time, Kent Displays of the U.S. is producing color plastic displays based on bi-stable liquid crystals. Already found in writing tablets, the company says the technology could also be used in e-readers.

Clearly hybrid LCDs hold promise for future e-readers, but two very different technologies promise the same vivid color and refresh rate of an LCD as well as the low battery consumption of a traditional e-reader. Both are close to commercialization.

The 'Mirasol' display, developed by U.S.-based Qualcomm MEMS Technologies, works by using interferometric modulators to reflect light so that specific wavelengths interfere with each other to create color (Figure 3). The company refuses to comment on a commercial product, but recently revealed plans to build a billion-dollar fabrication plant in Taiwan. Scheduled to start operations in 2012, a device should follow soon.

Figure 3. The Mirasol display: Each modulator comprises two plates; a thin film stack on a glass substrate and a self-supporting reflective membrane. Applying a voltage collapses the membrane, altering the size of the air gap between the plates. This determines the phase difference of the wavelengths of light reflected by each plate, whether constructive or destructive interference take place, and so the color. (Click to enlarge image.)

Meanwhile, Dutch company, Liquavista, has taken electro-wetting techniques pioneered by Philips last decade and using standard LCD manufacturing processes is fabricating color displays with full-motion video. Each display pixel comprises a transparent electrode, hydrophobic insulator, colored oil layer and water, sandwiched between glass or plastic substrates. The colored oil forms a film between the water and insulator until a voltage difference is applied, moving the water into contact with the insulator to expose the underlying reflecting surface, and changing the color of the pixel (Figure 4).

Figure 4. The Liquavista display: When a voltage difference is applied across the hydrophobic insulator, water moves into contact with the insulator and the oil is pushed aside to expose the reflecting surface. By altering the voltage, the optical properties of the pixel are tuned between a colored off-state (top) and transparent on-state (bottom).

According to company founder and CTO, Johan Feenstra, the liquids can be moved very quickly so the response time of the displays is only 10ms, making video content feasible. "We're not bi-stable but the battery will last for days," he adds. "You will see a device -- possibly color -- in the marketplace towards the end of 2011."

Liquavista has also been working with UK organic electronics pioneer, Plastic Logic, since 2008 to produce flexible e-readers that show color images and host videos. "Getting both technologies ready to produce fully flexible, color displays is not an easy challenge," says Feenstra. "Nobody has done this before, but we are making good displays and moving ahead."

The holy grail

But while these businesses race to deliver the future generations of e-reader, one academic has his sights firmly set on the horizon. Andrew Steckl, a professor at the Nanoelectronics Laboratory at the University of Cincinnati, hopes to use electro-wetting techniques to fabricate e-readers that you can roll or fold up, and put in your pocket.

After studying different papers with a range of surface finishes, his team has demonstrated electro-wetting action at video-rate speeds on a paper substrate, with results that, in his words, "exceeded expectations." His team's next goal is to produce electro-wetting pixel arrays on paper, and then move onto developing a monochrome display.

"We are aiming initially for black and white -- it will have a good contrast and look appealing to the human eye," he says. "Color is somewhere beyond the horizon, I just can't quite see it yet."

The final crucial step to fabricating a truly flexible e-reader will be to integrate all additional functionalities, from antenna to keyboard, into a sheet of paper. Can Steckl do this?

"We'll give it a shot," he laughs. "You could have multiple layers of paper, so underneath the paper you read is another layer that contains the antenna, and underneath that layer is another that holds the battery. I think the science is easier than the engineering, but its going to be a lot of fun to integrate the form and function of paper."

Figure 5. E-readers (left to right) from EInk, Samsung, and Qualcomm.

Note: Several of the technologies in this article will be explored in presentations at SPIE Photonics West in Conference 7956A, E-papers and Flexible Displays.

Rebecca Pool is a UK-based science and technology writer.