Jan. 6, 2004 — Royal Philips Electronics has developed a thin electronic-paper display using a process called “electrowetting,” a technology commonly used in labs-on-a-chip but is also fast enough to make electronic video paper possible.
“While the amount of information that we digitally process ever increases, more printers are sold each year. This contrast goes to show we still prefer reading from paper rather than from electronic displays,” said Peter Kurstjens, general manager of Electronic Ink Displays at Philips.
“But these last steps in the information chain, printing on paper and distribution, are also the most expensive,” he said. So, multinationals like Philips, Lucent, Xerox, and DuPont recognize the market potential for e-paper displays that keep the optical and physical properties of paper while combining them with the high-density storage capacity of electronic media.
Stacked inside the Dutch electronics and semiconductor company’s prototype is a 100-micron layer of water with a droplet of colored oil, a 1-micron-thick hydrophobic (water repellent) Teflon layer, a 15-nanometer transparent indium tin oxide (ITO) patterned electrode layer and a white polymer background.
When the device is switched on, with 20 volts between the electrode and the water layer, the polar water molecules are electrostatically attracted to the Teflon layer, wetting its surface. Hence the name “electrowetting.” In this “on” state, water pushes the colored oil aside, making the pixel white. When the device is off, the water contracts to a low-surface-area droplet. That signals the colored oil to spread out over the pixel area to a layer of 10 microns, darkening it.
Philips launched its thin polymer organic LED (PolyLED) displays in 2002. But like liquid crystal displays (LCDs), they are still emissive — meaning you can’t use them in broad daylight. The goal is a display that is reflective, just like real paper.
Philips said it will introduce reflective e-ink displays in hand-helds based on electrophoresis — movement of electrically charged black-and-white ink particles — this year with Massachusetts-based E-Ink Corp. as the inventor and Philips with the license to produce. Meanwhile, Philips is also working on reflective LCD technologies.
Why look at yet another reflective technology like electrowetting? “Because currently this is the only technology that is fast enough for video applications, and at the same time allows color screens with brightness and contrast similar to real paper,” said Johan Feenstra, one of the electrowetting display’s inventors.
“The question that always needs to be answered for a new display technology is whether its special characteristics are sufficiently compelling to overcome the already developed and very good transflective LCD technology, and the rapidly developing OLED (organic light-emitting device) technology, both of which have a substantial infrastructure in place to support them, and both of which, especially LCD, are very cost-effective,” Werner said.
He said that while it’s too early to tell whether the technique is commercially viable, “my personal feeling is that the electrowetting technology is deserving of further work, but I would not want to bet that I will see it in a cell phone or eBook any time soon.”
The technology that most closely resembles actual paper comes from E-Ink Corp. “But first-generation displays are still rather slow,” Kurstjens said. “It takes almost one second to attract the charged ink particles to the front electrodes to build up the image.” Typical video applications require an image refresh rate of 25 frames a second; the electrowetting display switches in 10 milliseconds, allowing 100 frames a second — four times faster than the average VCR.
“More importantly, at the moment the route to full color E-Ink screens is not yet clear. We lose too much brightness with the straightforward approach using color filters,” said Kurstjens.
Philips’ latest prototype is the size of a postage stamp, with 160-by-50-micrometer pixels. Feenstra said it is “difficult to say” when can we can expect electrowetting displays on store shelves. “Every self-respecting display company is working on a reflective technology, so I’m sure it’ll arrive eventually. But it’s too soon to determine which one, or which ones for that matter, will win.”
Then there is the manufacturing issue. “Current prototypes are made using similar lithographic processes as in the LCD industry. By doing so, we reduce the costs of introducing our technology,” Feenstra said.
“Often, discussions on new flexible e-paper technologies concentrate on integration of pixels and thin film transistors (TFTs), forgetting that the required materials are far from off-the-shelf,” said Bill MacDonald, researcher at DuPont Teijin Films, a company that makes polyester for flexible displays. These materials must meet strict demands. They must, for example, keep their dimensions even when heated to 300 degrees Celsius, have a smooth surface and shield active components from oxygen and humidity — the two main evildoers that limit the lifetime of these displays.
“With the integration of switching electronics and pixels, ideally, you would like to make the TFTs from semiconducting polymers using cheap inkjet printing to put them down, but that’s still under development. I don’t expect commercial displays of this type before 2005,” McDonald said.
Philips also has a down-to-earth view of the market and doesn’t expect electrowetting to hit the market within five years. “You have to be realistic, ” said Krustjens. “Sometimes, the press writes about new e-paper technologies as if they’ll become available next month.” So, we need to hang on to our printer for a little while longer.