New Twist on LCD Displays

Over the next 10 years, thin-film polymers and other flexible substrates could change how people think about and use displays. In the future, you may "print out" reports to sheets of e-paper: flexible polymer displays about as thick as a sheet of paper that can be spread out on a desk for easy comparison and analysis and then reused when the work is done. Your PDA or cell phone may incorporate a roll-up display that extends to let you view maps or Web pages on a larger screen. Your laptop may have a secondary display on the back of the case that can maintain any image you choose, such as your schedule and to-do list -- and you'll be able to refer to it even when the laptop is turned off. Some displays may be embedded on a shirt sleeve or curve around a watchband.

"We're talking about electronics we can wrap around a pencil," says Jim Brug, imaging materials department manager at HP Laboratories. He says he expects such technologies to evolve into real product designs within five years.

While today's flexible display prototypes are relatively small, Hewlett-Packard and many other companies are working on flexible screens that measure as large as 14 inches diagonally. But the goal is to complement current LCDs rather than provide a substitute for them. "It's a mistake to think of this as a replacement for a [desktop or laptop] display," says Brug. "This is really a new way of using surfaces for information." That might include interlocking flexible panels, like sheets of wallpaper, to create a single, wall-size screen, he says.

Making traditional displays flexible presents several challenges. An active- matrix LCD consists of two layers of glass with several components in between: a thin-film transistor (TFT) layer embedded in amorphous silicon and etched onto the bottom glass, which produces the light pixels, and a liquid-crystal layer on top that acts as a light shutter. A backlight sits beneath the display, while a color filer and polarizers sit above the LCD. Creating a flexible display involves eliminating the backlight and replacing the glass layers with a flexible substance such as a thin polymer film.

The problem is that the liquid crystals in LCDs don't like to bend. "The quality of the image depends on the cell gap" between the polymer layers, says Kimberly Allen, director of display technology and strategy at iSuppli. The LCD will distort the image if the gap between the two layers isn't uniform when the substrate flexes. Also, an LCD with contoured surfaces can be difficult to view because of the angle.

A number of companies are working on developing alternative technologies to enable the production of flexible displays, including reflective "e-paper" and emissive organic light-emitting diode (OLED) technologies.

Researchers are looking for flexible alternatives to amorphous silicon, the semiconductor material used to construct the TFT and embed it on a glass substrate. Traditional manufacturing techniques require high temperatures that work on glass but would melt plastic substrates. Researchers are experimenting with "ink jet" printing of the transistors onto a thin polymer sheet. This requires moving from inorganic silicon to soluble, organic materials. HP is also testing imprint lithography, where a circuit pattern is pressed onto the polymer. Researchers at Palo Alto Research Center are also working with a stainless-steel foil substrate that can withstand high temperatures.

E-paper displays are called "bistable" because they can maintain an image when the power is turned off. Reflective displays don't require a backlight, as LCDs do, and can be read outdoors. The first generation has been used in signage, store-shelf price labels and e-book readers.

OLEDs emit their own light. They use more power than today's active-matrix LCDs but offer faster performance and richer colors. But manufacturing OLEDs on a flexible substrate presents challenges.

"OLEDs are probably further away than [e-paper] because OLEDs require a strong barrier against moisture, and plastic lets that right through," says Allen. Researchers have also had problems with display life spans, particularly with OLEDs that produce blue light, although some say that life spans have improved in the past few years.

Flexible displays are still under development or in the prototype stages for both e-paper and OLED technologies. "There are no displays that are dynamically flexible that are currently being used," says Allen. But she predicts that the market will ramp up from virtually nothing today to about US$338 million annually by 2013.

Among vendors of bistable displays, E Ink, is the best known. Its technology consists of a thin film atop a layer of electronic ink -- a series of black and white charged particles, or "pigments," suspended in a fluid that move up or down to create a black, white or gray image. So far, display manufacturers have used E Ink's technology to create e-book readers and what Mike McCreary, vice president of research and advanced development, calls "conformative displays" that are initially contoured to fit the shape of an object but remain rigid in the final product. Seiko has developed a watch display, for example, and Lexar Media has embedded a capacity meter for USB memory sticks using the technology. E Ink displays are also being used in a flexible electronic newspaper that's being tested in 200 households in Belgium.

Sipix Imaging, uses a similar technology to produce black, green or blue colors on a white background. Polymer Vision is working with Sipix and E Ink to create a rollable display prototype called the Readius. "We are enabling very small devices with large displays," says Edzer Huitema, program manager at Eindhoven, Netherlands-based Polymer Vision. The 5-in. pull-out display will offer 16 levels of gray. The device refreshes about once per second -- too slow for menu navigation or video but acceptable for portable navigation systems or as a Web news or e-mail reader. Huitema predicts that Sipix will ship the device in the first half of 2007, with color and touch-screen capabilities available by 2010. E Ink is working on a color filter for its technology that it expects to be ready in the same time frame.

NTera is developing a flexible, conformable bistable display based on its nanochromic technology. Instead of moving particles, NTera's RGB display technology determines colors based on each particle's charge. The design uses a 200-dpi passive- matrix transistor array, which is typically slower than active matrix because it updates each row on the screen instead of individual pixels, as active matrix does. However, NTera claims that its technology is fast enough to support video speeds for short intervals. The company says its partners will produce conformable displays early next year for uses such as side displays on cell phones or input tablets. "That is our primary focus," says Alain Briancon, chief technology officer at NTera.

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NTera's technology adds another twist: The metal oxide display material is transparent when not charged, laying a foundation for transparent displays. "An overlay on top of a window could be realized," similar to what moviegoers saw in Minority Report, says Briancon. But NTera's current products are still built on glass.

Kent Displays, is developing a bistable LCD based on cholesteric technology. The supertwisted nematic LCDs used today twist constituent liquid-crystal molecules about 270 degrees, says sales and marketing manager Tony Emanuele. Kent's technology twists the molecules 16 or 17 times. When twisted that tightly, the molecules don't readily unwind, making the display bistable.

In the lab, Kent has demonstrated its technology deployed on plastic, paper and even fabric substrates. "The chemical recipe for cholesteric lends itself more readily to plastic substrates," Emanuele says, because it has 1/1,000th the barrier requirement of standard LCDs. Kent's current displays use a relatively slow passive matrix and are best suited for applications such as e-books and signage. The technology includes two-color combinations of either yellow/black or blue/white. Early units are fairly small, at 2.5 by 1.5 in., and offer 100-dpi resolution. Kent is working on a faster, active-matrix display and expects to be able to manufacture its passive-matrix version on flexible substrates by year's end. "Two to three years from now, flexible displays on plastic will be commonplace," Emanuele predicts.

In addition to the technical challenges, flexible displays face several other hurdles before they can become commercially viable. "Making [displays] on glass is hard enough," says iSuppli's Allen. Jet printing requires an entirely different, if potentially cheaper, manufacturing process that is still in the early stages of development. Until volume production is possible, jet-printed displays will be expensive relative to alternative technologies. For example, in the e-paper market, simple paper shelf labels and signage can't be updated electronically, but they're far cheaper, Allen says.

The key, says Jeremy Burroughes, CTO at Cambridge Display Technology, is to find a profitable niche for early designs. "The hurdle is always to find early areas to get into first," he says, "and gradually build up the knowledge and revenue profile to go into more advanced areas."

The economies of printed electronics

Jet printing enables flexible displays, but what will drive manufacturers toward ink-jet- and polymer-based substrates are the potential manufacturing cost savings, says Jim Brug, imaging materials department manager at HP Laboratories. "One of the big things going on in material science is moving beyond the inorganic thin-film technologies [such as amorphous silicon] to solution-based materials where you can actually spray these things out of an ink jet," he says. Unlike inorganic substances like amorphous silicon, organic semiconductor materials can be applied in liquid form.

HP Labs has developed an ink-jet printing and lamination process that could replace the vacuum deposition and photolithography techniques used to make today's active-matrix LCDs.

Printing on thin polymer sheets allows for a more efficient, automated process than the discrete manufacturing techniques used to produce individual silicon chips on a production line. Electronics can be printed on continuous polymer sheets, a process called roll-to-roll printing. "These are primarily issues of cost," says Bob Street, senior research fellow at Palo Alto Research Center. "The idea is one can use printing presses rather than silicon fabs to make these devices."

"Printing a display using roll-to-roll processing will allow it to be a very low cost, even disposable type display," says Brug. HP Labs is researching two processes: ink jetting and imprint lithography, where fine circuit patterns are pressed onto a plastic substrate.Xerox Corp.is working on processes for both polymers and stainless-steel foil substrates.

"This world of digital fabrication will open up very low-cost manufacturing plants that don't cost an arm and a leg to build. Does that mean you don't need cleanrooms? That's right," says Brug.

While the potential manufacturing economies are large, don't expect to run down to your local Staples to purchase organic ink cartridges and polymer sheets for printing electronics on your ink-jet printer. "We're not going to be making displays on our desks anytime soon," Brug says.

From flexible displays to bendable PCs

If ink-jet printing technologies can be used to lay down transistors for flexible displays today, what's to prevent other electronics -- perhaps an entire computing device -- from being embedded in that same polymer sheet? Jim Brug at HP Labs thinks the development of flexible computing devices is just a matter of time, as laptops and wireless PDA phones converge. "Will there be a world where your wristwatch will be your PC? One can imagine it coming out," he says. It's also possible that as more electronics are printed on polymer, they could be absorbed into the phone display, allowing contoured or even flexible, wearable designs.

Vendors are already working on embedding display driver circuitry onto polymer substrates, something that's done with today's glass-based displays. In addition, battery vendors such as Solicore, have developed flexible batteries that can be embedded into items such as radio frequency identification (RFID) tags and displays for smart cards. "We're taking the materials that are used for flexible electronics and embedding those into our battery," says Michael Mahan, vice president of business development at Solicore. Such devices, however, aren't strong enough to power a cell phone.

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The ultimate goal, says Darren Bischoff, senior marketing manager at E Ink, is "having this lightweight, rollable display that has next-generation electronics built into it so you can include the proc-essor, memory -- everything you need -- into this display material."

Building an entire PC on a flexible substrate would be tricky, although Brug thinks it's technically possible. Some of HP's earliest research on flexible electronics was done on memories, he says. "We won't be doing microprocessors anytime soon, but it won't be too long before the pieces start coming together," he predicts. Bob Street, a senior research fellow at Palo Alto Research Center says the issue isn't whether it can be done, but whether such a device can be created cost- effectively. He notes that attempts to print the antennas and transistors for RFID tags on a single sheet have so far been more expensive than using silicon and bonding the antenna to the chip. A flexible PC would be much more complex to engineer. "I'm sure you can do it," Street says. "The question is whether it is cost-effective versus doing it with crystalline silicon."

Bending OLEDs

OLEDs could be a replacement for traditional color LCDs, but most research with OLEDs is focused on rigid displays. U.K.-based Cambridge Display Technology is putting jet-printed OLEDs on polymers, but Chief Technology Officer Jeremy Burroughes says the primary benefit isn't flexibility but the fact that such displays are thinner, lighter and less breakable than traditional LCDs. "If your laptop weighed less and was less likely to break if you dropped it, you would be a happy person to have it," he says, adding that his company has no plans for flexible displays. But don't expect to see OLEDs in laptops any time soon. While the product life span has improved, blue LEDs are still only good for about 12,000 to 15,000 hours, researchers say. That's enough for a PDA or cell phone display but not for laptop screens or desktop monitors.

Universal Display Corp. (UDC), however, is focused exclusively on flexible OLED technology using an active matrix on a thin foil substrate. "Plastics aren't quite ready for use with OLEDs," says Janice Mahon, vice president of technology commercialization. UDC has demonstrated flexible quarter-VGA (320-by-240-pixel) color displays and is working on a rollable display for a universal communication device funded by the U.S. Department of Defense. UDC is also experimenting with wearable displays embedded in clothing, but it is starting with conformable displays. "The next step is to get it so you can bend \[the display\] a thousand times," he says. While others think flexible OLEDs could be 10 years off, Mahon thinks UDC's technology will be viable much sooner. "In a couple of years, there will be a flexible OLED in the marketplace," she predicts.

Hewlett-Packard is working on both flexible bi-stable and OLED displays on polymer. Its first bi-stable display, a 125-color, 128-by-96-pixel passive-matrix LCD developed in its lab in Bristol, England, is nearing production for use in consumer devices such as digital photograph viewers and e-book readers.

HP's "emissive ink" technology is a variation on the OLED that uses nanocrystals called "quantum dots." The crystals, 2 to 10 nanometer in size, are mixed into polymers and emit a color that's more pure than traditional active-matrix LCDs can deliver. "Because they're so small, they can only emit one color. We put these into the polymer, which conducts the electrons and holds the nanocrystals, and they emit a photon," says Jim Brug, imaging materials department manager at HP Laboratories.

"Fabricating good quality transistors on plastics is one major development. The other is to show that quantum dots can emit very high quality light in polymer materials," Brug says.

Beyond flexible, stretchable computing

If flexible displays sound like a stretch, John Rogers' research will sound even more fantastic.

The materials scientist at the University of Illinois Urbana-Champaign has already created electronic transistors and diodes that stretch. Now he is busy working on stretchable circuit boards that could lead to wearable electronics. "The next generation is to go beyond flexible to stretchable," he says.

Products created from stretchable electronics could be used in "any kind of system where you need stretchability," Rogers says. One example: smart surgical gloves with embedded sensors that provide feedback on the patient's condition for any point on the body that the surgeon touches. Another application, he says, would be for a structural health monitor that wraps around an airplane wing to measure stresses and strains.

To achieve stretchability, Rogers combines a relatively inflexible substance for the circuits, silicon, with the ultimate stretchy substrate: rubber. Rogers' process requires first stretching the rubber and then bonding an ultrathin (0.1-micron) silicon circuit to it using a stamping process. When the rubber returns to its unstrained length, the circuit compresses and buckles into an accordion-like pattern, but it doesn't break. Although silicon is normally inflexible, the extreme thinness of the circuit allows it to bend.

"What we've done is achieved bendability in a material that's inherently not bendable. We've made it bendable because we've made it thin," Rogers says. When combined with rubber, the bendability translates to stretchability. The material can be stretched and relaxed over and over again.

Rogers helped found Printed Silicon Technologies Inc. to commercialize the technology. So far, he has demonstrated stretchable transistors and diodes -- the basic building blocks of circuits -- but Rogers doesn't think circuit boards will be much of a stretch.

"There's no fundamental challenge to that, just tricky engineering. We'll have a demo by end of year," he says. But actual products built with stretchable electronics are still at least three to five years away, he says.