Thin Displays as Wristbands

WristFlex: This prototype made for the U.S. Army is
worn on the wrist and incorporates a thin, lightweight
flexible OLED display.   Credit: Universal Display

The U.S. Army is evaluating full-color flexible displays that can be worn on the wrist

MIT Technology Review, October 21, 2010, by Katherine Bourzac  —  The U.S. Army is testing a prototype “watch” that’s lightweight and thin and has a full-color display. This display is built on flexible materials encased in a rugged plastic case and can be worn on a wristband to display streaming video and other information. It uses newly developed phosphorescent materials that are efficient at converting electricity into red, blue, and green light, which means the display needs less power to work.

Most phones, laptops, and TVs today use liquid-crystal displays (LCDs) controlled by electronics built on glass. To make more energy-efficient displays that are controlled by flexible electronics, which are lightweight and won’t shatter like glass, many companies are turning to organic light-emitting diodes (OLEDs). The pixels in OLED displays replace the layers of electronics and filters in LCDs with organic dye molecules that emit light in response to electrical current.

For consumers, flexible OLEDs promise portable electronics with beautiful screens that don’t drain battery life and won’t shatter when dropped. But so far, no companies have developed economically viable manufacturing methods for producing flexible OLEDs with long enough lifetimes and consistent quality. The U.S. military has been funding development with the aim of providing soldiers with rugged, thin communications devices that can display maps and video without adding too much weight to their load.

The new display prototypes use efficient OLED materials developed by Universal Display of Ewing, New Jersey, and are built on foil-backed electronic controls developed by LG Display, headquartered in Seoul, South Korea. The devices were designed by L-3 Display Systems of Alpharetta, Georgia. The display is 4.3 inches. As part of military demonstration tests, the device has been used to stream real-time video from unmanned air vehicles.

“These prototypes represent not so much one major advance but continued progress on many fronts,” says Janice Mahon, vice president of technology development at Universal Display. Those fronts include the OLED materials themselves, the electronics that control them, and the integration and packaging of the device.

The first generation of OLED materials, used today in glass-backed cell-phone displays and some small TVs, can convert only 25 percent of electrical current into light; the rest is lost as heat. Universal Display is designing and developing materials that work by a different mechanism and that have a theoretical efficiency of 100 percent. The prototypes for the Army use a full set of phosphorescent materials; the companies have not released specifications about power consumption, but Mahon says displays made with these materials use one-fourth the power of a conventional OLED.

Samsung Mobile Display, the biggest maker of OLED displays, currently uses Universal Display’s red phosphorescent materials in its products; Samsung and other companies are currently evaluating green materials. Phosphorescent materials that work with higher energy light such as blue tend to be less stable over time and have been slower in coming. The companies have not disclosed information on the expected lifetime of the all-phosphorescent displays.

Universal Display applied the light-emitting layer to electronic controls made by LG Displays. The electronics are an array of amorphous-silicon transistors built on stainless steel foil instead of glass. Other companies, including Hewlett-Packard and Samsung, are developing flexible amorphous-silicon transistor arrays, mostly on sheets of plastic. Working with metal poses some challenges because the surface is rough, which can disrupt the structure of the transistors, but metal can withstand higher processing temperatures than plastic can. That’s an important trait when it comes to laying down the silicon. High-temperature processing results in silicon crystal that’s not only higher quality but also more stable over time.

“The broader story is that we’re starting to see some good-looking demos of flexible OLED displays,” says Nicholas Colaneri, who heads the Flexible Display Center at Arizona State University. Sony and Samsung Mobile Display have both demonstrated flexible displays built on sheets of plastic; both companies have been tight-lipped about these technologies. But, Colaneri notes, “just because you can do it doesn’t mean you can afford to do it.”

A major hurdle remains before displays like the prototype made for the Army will arrive on store shelves. Amorphous-silicon transistor arrays can be made at temperatures suitable for flexible electronics, and the LCD industry has created a lot of infrastructure for making them. But over time, they’re not the best electronics for controlling OLEDs. The electrical currents required to switch OLED pixels burn out these transistors; the pixels that are on most frequently start to malfunction.

Canadian startup Ignis Innovation is developing software and other controls to extend the lifetime of the transistor arrays by ensuring that no single pixel is on too often. Colaneri says its initial prototypes are promising. In the meantime, Colaneri and other researchers are developing alternative transistor materials such as metal oxides to make OLED electronics that won’t burn out.

The companies that made the Army prototype are not disclosing the metal-silicon electronics used to run it, but say they have met the Army’s specifications.

Print out: This plastic material is used as the backing for Phicot’s
amorphous silicon electronics. Credit: Phicot

A new company puts silicon transistors on plastic for flexible displays

MIT Technology Review, by Kate Greene  —  Engineers and technophiles have long dreamt of plastic-based displays that are flexible, lightweight, and rugged compared to their glass-backed counterparts. But plastic screens still aren’t widely available, partly because they’re so hard to manufacture reliably in large numbers.

Now a company called Phicot has adapted a technique for printing amorphous silicon electronics onto plastic that could finally make such displays practical. The manufacturing technique, already used to make cheap solar cells, involves depositing chemicals on long sheets of plastic as they are fed through a series of rollers. Phicot is a subsidiary of PowerFilm of Ames, IA, which already makes amorphous silicon solar cells using this roll-to-roll process.

“The basic technology of roll-to-roll can bring the price down and make plastic an excellent option for the back half of the display,” says Frank Jeffrey, cofounder and CEO of PowerFilm.

Most modern displays rely on transistors made of amorphous silicon on glass. The problem with amorphous silicon on glass is that is deposited at high temperatures that melt plastic. So Phicot turned to low-temperature chemical-vapor deposition of amorphous silicon, which doesn’t melt plastic and yet produces transistors fast enough to control the pixels of electrophoretic displays such as E-Ink, and eventually those in an organic light-emitting diode (OLED) display.

At Phicot’s facility, layers of amorphous silicon and insulating materials are deposited onto plastic. These rolls of plastic are then sent to a facility at HP Labs, where engineers use a novel kind of lithography, called self-aligned imprint lithography (SAIL), to etch transistors onto the plastic’s surface.

Once the transistors have been deposited, the screen itself must be added. HP has tested its transistors using E-Ink and with its own reflective display technology, capable of showing color and video. According to Carl Taussig, director of HP’s information surfaces labs, the amorphous silicon transistors could be replaced with those made of other semiconductors that could drive OLEDs.

Phicot isn’t the only company trying to make plastic-based displays. Polymer Vision, a spin-off of Philips, and Plastic Logic are both promising products in the near future. However, these devices will rely on transistors made of organic materials, which are easy to deposit on plastic at low temperatures, but operate more slowly than those made of amorphous silicon. While organic transistors are good enough to power electrophoretic displays, they are incompatible with OLEDs. Another company, Kovio, is aiming to print silicon on plastic using technology that resembles an ink-jet printer; the main applications at this point are RFID tags.

See through: Researchers have created a flexible graphene sheet with silver electrodes printed on it (top) that can be used as a touch screen when connected to control software on a computer (bottom). Credit: Byung Hee Hong, SKKU

MIT Technology Review, by Nidhi Subbaraman  —  Sheets of atom-thick carbon could make displays that are super fast.

Graphene, a sheet of carbon just one atom thick, has spectacular strength, flexibility, transparency, and electrical conductivity. Spurred on by its potential for application in new devices like touch screens and solar cells, researchers have been toying with ways to make large sheets of pure graphene, for example by shaving off atom-thin flakes and chemically dissolving chunks of graphite oxide. Yet in the thirty-some years since graphene’s discovery, laboratory experiments have mainly yielded mere flecks of the stuff, and mass manufacture has seemed a long way away.

“The future of the field certainly isn’t flaking off pencil shavings,” says Michael Strano, a professor of chemical engineering at MIT. “The large-area production of monolayer graphene was a serious technological hurdle to advancing graphene technology.”

Now, besting all previous records for synthesis of graphene in the laboratory, researchers at Samsung and Sungkyunkwan University, in Korea, have produced a continuous layer of pure graphene the size of a large television, spooling it out through rollers on top of a flexible, see-through, 63-centimeter-wide polyester sheet.

“It is engineering at its finest,” says James Tour, a professor of chemistry at Rice University who has been working on ways to make graphene by dissolving chunks of graphite. “[People have made] it in a lab in little tiny sheets, but never on a machine like this.”

The team has already created a flexible touch screen by using the polymer-supported graphene to make the screen’s transparent electrodes. The material currently used to make transparent electronics, indium tin oxide, is expensive and brittle. Producing graphene on polyester sheets that bend is the first step to making transparent electronics that are stronger, cheaper, and more flexible. “You could theoretically roll up your iPhone and stick it behind your ear like a pencil,” says Tour.

The Korean team built on rapid advances in recent months. “The field really has advanced in the past 18 months,” says Strano. “What they show here is essentially a monolayer over enormous areas–much larger than we’ve seen in the past.”

Last year, Rodney Ruoff and his team at the University of Texas in Austin showed that graphene could be grown on copper foil. Carbon vaporized at 1,000 degrees would settle atom-by-atom on the foil, which was a few centimeters across. Byung Hee Hong, a professor at Sungkyunkwan University and corresponding author on the paper, says the use of a flexible base presented a solution to the graphene mass-manufacturing dilemma.

“[This] opened a new route to large-scale production of high-quality graphene films for practical applications,” says Hong. “[Our] dramatic scaling up was enabled by the use of large, flexible copper foils fitting the tubular shape of the furnace.” And the graphene sheets could get even bigger. “A roll-to-roll process usually allows the production of continuous films,” says Hong.

In Hong’s method, a sheet of copper foil is wrapped around a cylinder and placed in a specially designed furnace. Carbon atoms carried on a heated stream of hydrogen and methane meet the copper sheet and settle on it in a single uniform layer. The copper foil exits the furnace pressed between hot rollers, and the graphene is transferred onto a polyester base. Silver electrodes are then printed onto the sheet.

The technique shows some potential to be scaled up for mass production. “They particularly show that they are able to grow the graphene [in a way] that is compatible with manufacturing,” says Strano. “It’s a very economical way to manufacture materials.”

Hong sees application for the method in the production of graphene-based solar cells, touch sensors, and flat-panel displays. But he says products will be a while in coming. “It is too early to say something about mass production and commercialization,” he says. Current manufacturing processes for indium tin oxide use a spreading technology that is different from roll-to-roll printing. “However, the situation will be changed when bigger flexible-electronics markets are formed in the near future,” Hong says.

HP and Arizona State have announced the first flexible display prototype, by Shane McGlaun  —  Lots of research time, effort and money is going into products that could make revolutionary changes in the electronic products in use around the world. So many products today take advantage of displays from cell phones and computers to refrigerators and even outdoor grills that the market for displays is large and varied.

HP and Arizona State University have teamed up to develop a flexible display at the Flexible Display Center (FDC) at the university. Today the FDC announced that it has developed the first prototype of an affordable, flexible electronic display.

The flexible display is paper-like, but constructed totally out of plastic. The plastic construction allows the display to be easily portable and consume less power than typical displays available today. Potential uses for the new flexible display according to the FDC are in electronic paper and digital signage.

The technology could also make its way into future electronic devices like smartphones and notebook computers. The displays are claimed to be unbreakable and use up to 90% less materials by volume when compared to traditional LCD displays.

HP and the FDC created the flexible displays by using self-aligned imprint lithography (SAIL) technology that was invented by HP Labs. HP says SAIL technology is considered self aligned because the pattering information is imprinted on the substrate in a way that perfect alignment is maintained regardless of distortion.

Displays built using SAIL technology can be fabricated on thin film transistors on flexible plastic material in a roll-to-roll manufacturing process. This allows for continuous manufacturing rather than batch manufacturing used to create current displays.

HP Labs’ Carl Taussig said in a statement, “The display HP has created with the FDC proves the technology and demonstrates the remarkable innovation we’re bringing to the rapidly growing display market. In addition to providing a lower-cost process, SAIL technology represents a more sustainable, environmentally sensitive approach to producing electronic displays.”