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Market report: The future looks bright for flexible organic displays

17 Jun 2002

European businesses are up there with the Americans and Japanese in the development of flexible displays. Rob van den Berg looks at the work of the pioneer firms in this field.

From Opto & Laser Europe March 2001

"I want to go where liquid crystals can't." E Ink Corporation CEO Jim Iuliano doesn't want to leave any misunderstanding about the intentions of his company, which has just launched the first application of its novel electronic ink: large signs - not much thicker than a piece of paper - that can be updated electronically.

Iuliano is determined, but he faces fierce competition from other players in the fast-moving display industry. Hardly a day passes without an announcement of yet another alliance, the unveiling of a novel prototype or the start-up of another pilot-line. And all of the companies have a common goal: to manufacture lightweight and flexible displays, with high brightness and contrast, wide viewing angles, low power consumption and at a low cost.

The potential of such third-generation displays seems limitless. They will appear not only in cell phones and digital video cameras, but also as paper-thin wall-mounted television screens, billboards, reprogrammable books and even wallpaper. This multibillion-dollar market for flat-panel displays can be divided up according to the technology used: electronic ink, which relies on an outside light source; and emissive displays, which are based on electroluminescence - the emission of light by organic molecules in response to an electric field. A voltage applied across a thin layer of such a material generates negative and positive charge carriers that migrate from the contacts and emit their energy in the form of visible light when they recombine.

This relatively old technology was pioneered and patented in the 1980s by Eastman Kodak. The company was also the first to market organic light-emitting diodes (OLEDs) in a Pioneer car-stereo display.

The first product to use a colour OLED display was a Motorola cell-phone that was made by Kodak licensee Pioneer. In May last year, in alliance with Sanyo Electric, a 5.5 inch active-matrix display - with a resolution of 240 ¥ 320 pixels - was unveiled at the annual meeting of the Society for Information Display in California.

Kodak is far from being the only player in this field. Research from the universities of Princeton and Southern California, in the US, led to the founding of another company, Universal Display Corporation (UDC). Among its products is an award-winning novel-pixel architecture in which the individually controlled RGB pixels are vertically stacked. This allows for higher resolution and enhanced colour quality displays.

Both Kodak and UDC rely on emissions from relatively small organic molecules, unlike UK company Cambridge Display Technologies (CDT), which owes its existence to a discovery that was made at the University of Cambridge, UK, in 1990.

Richard Friend and Jeremy Burroughes developed the first electroluminescent polymer, polyphenylene vinylene. They demonstrated that sandwiching a thin layer of this material between a pair of electrodes - one of which was transparent - was enough to make the polymer glow.

In the last 10 years the efficiency of polymer LEDs has increased. Different plastics emit at various wavelengths and their stability is improved by making use of better sealing techniques, which keep out air and moisture more efficiently. Some polymer LEDs have demonstrated continuous-working lifetimes of more than 50,000 h.

These devices have a major advantage over other types of LED because they can be easily manufactured from solution and do not require vacuum deposition or sputtering techniques that are necessary for small-molecule OLED processing.

To obtain a thin polymer film of typically 0.1 µm thickness, a drop of polymer solution is placed on a rapidly rotating disc - a technique called spincoating. However, to create colour displays the spincoating method cannot be used because different polymers need to be placed next to each other to make up the three differently coloured pixels. So CDT teamed up with printer manufacturer Seiko Epson to develop an inkjet printing process to deposit the individual polymer pixels. In May last year they presented their display. Keith Bergelt, in charge of business development at CDT said: "We are still only 12 to 15 months behind [small-molecule OLEDs] in terms of commercialization. Don't forget that there has been four times as much investment in small molecules as there has been in polymers. Seiko Epson has taken an ambitious route to leapfrog to a full-colour display. Before the end of this year we will have equivalence, and licensees will start bringing products to the market."

CDT has started building a USD 25 million pilot production line in Godmanchester, UK, to test materials and manufacturing techniques. It will be followed by a limited production line for specialized customers who will start putting the displays into real devices. The factory should be finished by the first quarter of 2002, with production beginning a couple of months later.

Bergelt said: "Inkjet technologies will evolve into roll-to-roll flexible-substrate manufacturing processes. That's where the great win is: a process that offers the potential to move away from Asian manufacturing. However, we know our limitations and will ultimately partner with an established LCD manufacturer."

Electronics giants Philips and Hewlett Packard are supporting CDT's technology. Philips has just started a polymer-LED pilot production line in Heerlen, the Netherlands, to manufacture graphic displays for cell phones and automotive applications.

Philips has begun manufacturing polymer LEDs on glass. "It is still difficult to find a suitable barrier material to block out moisture and oxygen when using flexible substrates," said marketing manager Jan-Willem Vogel. "Apart from their higher brightness and wider viewing angle, polymer LEDs also have a much faster switching speed than LCDs. Our first 65 ¥ 100 monochrome display, which will be introduced onto the market by a cell-phone manufacturer in the middle of this year, is capable of displaying video images."

Vogel also plays down the immediate importance of flexibility: "It would be a big step if we could supply designers with a display that is curved. For now they are forced to work large flat areas into their designs."

However, it is by no means clear that emissive displays will be the way of the future. Some argue that the natural way of displaying something is via reflected light, because the human eye has evolved to deal with that. We are used to seeing ink on paper, so several companies have worked hard to come up with an electronic alternative. They have all used some form of bistable component that switches between black and white, for example.

"Thus, for third-generation displays a situation has arisen with an obvious parallel in the LCD industry," said Herman Schoo of the TNO Institute of Industrial Technology in Eindhoven, the Netherlands. "In an LCD either backlighting or a simple, cheap, reflective technology that requires an outside light source is used. That's exactly what has happened in OLEDs, polymer LEDs and electronic inks."

The first company to launch an electronic-ink display was E Ink Corporation, which grew out of the Media Lab of MIT in Boston, in the US. Its technology is based on 100 µm-diameter capsules that are filled with dye and negatively charged white-pigment chips, which are placed between a pair of electrodes. Depending on the charge of the upper electrode, the pigment chips are attracted to the top of the capsules, making them appear white.

A competing technology is Gyricon, which was invented at Xerox's Palo Alto Research Center. It uses a thin layer of plastic in which small beads are randomly dispersed. The opposing hemispheres of the beads are oppositely charged and have contrasting colours. Depending on the voltage applied, the beads rotate to reveal either their white or black side.

Schoo said: "These displays are not affected by moisture or oxygen, which makes flexible displays viable using currently available substrate and packaging technology. Moreover, they can be easily applied to a large surface area and have an extremely low power consumption. Once the individual balls have been switched, they remain fixed in that position. However, switching speeds are too slow to allow video-rate applications, and it is not easy to make a full-colour display." However, there are many applications at the low-end of the market - which is still valued at more than USD 10 billion - ranging from screens and billboards to newspapers.

Whereas Gyricon still has to bring out its first product, E Ink's Immedia indoor signs have been tested extensively by a number of businesses in the US, for example, department store J C Penney, and newspapers, including The Arizona Republic.

Recently, E ink and Lucent Technologies unveiled a 25 square inch, fully plastic, active-matrix display, with polymer transistors fabricated using a low-cost printing process with high-resolution rubber stamps. Ultimately, electronic-ink displays will become smaller for integration into everything from wrist watches and hand-held computers to cell phones.

Both Schoo and Bergelt are convinced that these different technologies will coexist into the next decade. For Vogel this is a necessity: "To build up a proper industrial infrastructure we need other companies. We should not be afraid of some competition. The market is big enough for all of us, united as we are against LCDs."

Philips clearly has great confidence in the technology. According to Vogel, an investment decision will be taken soon to build a large manufacturing plant, which should be in operation within the next two years. Bergelt commented: "These displays are maturing at the right time. There is an intense desire for content and information and the display becomes the medium through which it all happens."

art Photonics GmbHECOPTIKDIAMOND SAOmicron-Laserage Laserprodukte GmbHAlluxaSPECTROGON ABHyperion Optics
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