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Printed displays flex their muscles

Analysts are predicting that 2008 will be a landmark year for flexible displays.

Jennifer Colegrove, senior analyst at iSuppli, is in no doubt as to the importance of this year to flexible displays. "Why is 2008 year one of the flexible display?" she asks rhetorically. "Well, before this year, you could only see demonstrations."

At the recent Printed Electronics Europe conference in Dresden, Germany, Colegrove listed the companies who are gearing up for production of displays that can fold or roll away. This summer, Polymer Vision will start selling its Readius portable e-reader. Plastic Logic is in the middle of fitting out its fab, and expects to go into volume production by the end of the year. And, according to Colegrove, Prime View International and LG Display are readying production of flexible screens. Those are just the big names. Because the production of flexible displays calls for a number of new technologies, she estimates that some 200 companies are now involved in flexible displays in one way or another.

There are not just a lot of companies vying for a position in flexible displays. The vendors are trying out many different designs and production methods in the hope that the one they pick is commercially successful. About the only thing that is consistent between different approaches is the choice of technique for putting a circuit onto a flexible substrate: most of them use some form of printing.

Part of the reason for the spread of technologies lies in the target applications for flexible displays. Companies such as Polymer Vision are focusing on handheld devices - intended to replace printed paper with something more interactive. Eastern manufacturers such as LG Display and Sony have their eyes on much lighter TVs for the living room.

Jong-Kwon Lee, chief research engineer at LG Display, points to the flexible display as a further step in the evolution of large display panels. Over time, LCDs and plasma screens have supplanted cathode-ray tubes because they are much thinner. Now, companies such as LG are looking to screens that you can just hang on the wall - not something you can do with a plasma unless you have very strong brackets.

"The future display will have flexibility, portability and interactivity," says Lee. "We expect flexible displays will be mainstream in the future."

Jiro Karahara, R&D director at Sony's Fusion Domain Laboratory, agrees: "I think the evolution is from the panel to the sheet. It is not an unreachable target: it can be achieved within ten years."

For wall-hanging displays, flexibility in itself is a secondary concern, although it will make them easier to store and transport. The key issue for the home display is one of weight: in principle, printed electronic displays can be made much lighter than LCD or plasma technologies.

"Only with these technologies can we have a wall-hanging display," claims Kasahara. "That is the big difference between this technology and LCDs and plasma."

For Polymer Vision's target market, it is all about portability. The display on the company's Readius wraps around the phone-sized case. The result is a device that fits easily in a pocket but with a display that is bigger than that of most PDAs. For Edzer Huitema, CTO of Polymer Vision, there is even an environmental angle: "The reason is paper. More and more information is being consumed but we have not reduced paper consumption. Paper is still the dominant reading medium. There is a real environmental need to reduce printing, as printing will reduce the trees in the rain forest to zero." By switching to roll-up displays, the hope is that people will finally switch from paper to computerised screens. 

Then there is the cost argument. By switching to a printed technology rather than the processes derived from chipmaking that dominate today's displays, it should be possible to produce displays in massive volume much more cheaply than is possible today. For these, flexibility is far from being the biggest concern. It is in these cheap displays that iSuppli sees a lot of the promise in the near term. "Most displays in the early years will not be truly flexible," claims Colegrove.

Carl Taussig, programme manager at HP Laboratories, says the main aim of his team's project to develop a roll-to-roll process for making display was cost: "The aim was not to create a flexible display but to create a cheap display.

Why go roll-to-roll?

Substrates can be less expensive and we can do more things with substrates. A big opportunity for the roll-to-roll process is the hope that we can use the core [of the roll] as a part of the clean-room, and maintain cleanliness when the roll is transferred from station to station," Taussig explains.

HP's comparison of costs between roll-to-roll and batch production based on semiconductor technology, such as photolithography, found that the roll-to-roll process is much cheaper in principle. "Most of the cost in production has to do with the photolithographic steps," says Taussig.

One big problem with making flexible displays is that they stretch during manufacture, and not always the same way, which further complicates the use of photolithography. "Even if you could find a way to make alignment at one point, you can't guarantee alignment everywhere," claims Taussig.

Taussig says HP went for an imprint-based technique that, in one action, can define the major features that need to be left on the substrate. It meant that, effectively, the mould stretches with the substrate, he says. "We deposit all of the materials at once, using vacuum deposition tools. Then we do a single printing process. After that, it is only subtractive steps to produce the rest of the backplane. After imprinting, we etch the stack a number of times. On the downside, it has never been done in a roll-to-roll environment before."

However, there are still big hurdles to overcome before roll-to-roll printing becomes the norm for flexible, cheap displays. "We have issues dealing with defects over large areas," says Taussig. Although the process broadly works, yield remains an issue. "It is all about yield. The thing that surprised me when we got through initial development were the problems that you face with yield. You deal with one issue, then the next rises to the top."

But Taussig is undeterred: "We feel that it is feasible to make electronics in a roll-to-roll environment. But I am a little anxious that it has taken so long because people have gone to production with batch manufacturing. But we believe the cost advantage lies with roll-to-roll and we are now scaling to pilot production."

Because roll-to-roll processing is so new, Polymer Vision and Plastic Logic have, among others, opted for more traditional, batch-manufacturing techniques. To deal with the inherent stretchiness of plastic substrates, Plastic Logic has developed a direct-write approach to lithography that adjusts to deal with distortion. However, people such as Polymer Vision's CTO Edzer Huitema believe that the industry will ultimately move to roll-to-roll processing because of the cost advantage - as long as the yield issues can be overcome.

There remain plenty of other problems to overcome before flexible displays can begin to displace existing flat-panel screens. "There are so many technology issues," says Lee.

To deal with the problem of thermal expansion, LG has used metal foil as the display's substrate rather than plastic. "Plastic is transparent and light and reliable. But it has poor thermal stability and permeability. So, we used metal foil, but we had to develop a multibarrier structure to reduce surface roughness," says Lee.

Then there is the issue of what you make the electronics from. The original plan with printed electronics was to concentrate on organic molecules such as pentacene and polymers. However, the performance of these chemicals in electronic circuits is very poor: often not as good even as amorphous silicon and nowhere near that of the polycrystalline silicon used in the thin-film transistors (TFTs) found on LCDs. If the transistors are slow, then display response speeds will not be good. Printed displays have response times of hundreds of milliseconds: good enough for e-books, but too slow for video. This is where a lot of research is now concentrating: to improve the performance of the flexible display to the point where it can compete almost head-to-head with the LCD.

Huitema says work on yield will result in larger, rollable displays and there will be parallel work on versions that have touchscreens, as well as colour (see 'Glorious technicolour' on facing page). "With those you can have the ultimate compact device," Huitema claims. "We see this as the next wave of disruption." 

Glorious technicolour

The first products to hit the market are monochrome 'e-paper' displays, mostly using the technology developed by e-Ink, although an electronically controlled powder developed by Bridgestone is likely to debut soon in flexible displays.

The advantage of e-paper technologies lies in power and relative manufacturing simplicity. The e-paper displays are electrophoretic - moving charged coloured particles around a capsule by applying a voltage difference to contact in front of and behind the display. Typically, dark particles are designed to carry an opposite charge to white particles, so they move in opposite directions.

This type of display has two advantages, which are not confined to flexible displays. One is that the electronic ink needs very little power to maintain an image. The other is that the display is reflective: it uses ambient light rather than light emission to form an image. Again, that reduces power. It also makes the display look more like paper, which should reduce eye strain. However, some users of the Amazon Kindle, which uses an e-paper display on a rigid screen, have complained of not being able use the device for more than short periods. The contrast is rated as better than newsprint in measurements but some users have been put off by the light-grey tint of the background.

The other option is the combination of a thin-film transistor and organic light emitting diode (OLED). This is likely to be a favoured technology for wall-hanging TVs. The advantage of the OLED for TV-like displays is that they emit light rather than relying on reflected light. Flexible displays based on OLEDs are running behind those based on e-paper in terms of development - but colour e-paper displays still face big problems.

Jiro Kasahara, R&D director of Sony's Fusion Domain Labora-tories, says his team has created a display that you can bend to a radius of about 2cm. "The picture doesn't degrade at all," he says.

"But it is lab work, and you can see several defects," says Kasahara, pointing to the long dark lines that run across parts of the display: defects mean that some of the lines needed to control the OLEDs in the display do not work.

"We have demonstrated a fully printed technology," says Kasahara. "But still we have issues to overcome before practical commercialisation."

LG Display opted for the e-paper approach for its prototype colour display. "E-paper is a good candidate because it has a paper-like look, it is lightweight and has low power consumption," says Jong-Kwon Lee, chief research engineer at LG Display.

Lee says that colour electrophoretic capsules can easily be made: you just have to alter the colour of the pigment inside. The tricky part is making a full-colour display with them. "Pixel-by-pixel colour is very difficult," says Lee.

People such as Edzer Huitema, CTO of Polymer Vision, say the first step will be to displays that have fixed colours at certain points because that is easier to make than a matrix of coloured capsules that can be driven individually.

The cheapest way to make electrophoretic displays is to scatter the capsules across the display: the electrodes underneath determine the pixel size. To make a colour display, you need to place the different coloured capsules with high precision. That is not the path to a cheap process.

One option, Lee claims, is to put colour filters over segments of the array of electrophoretic capsules. "But the filter absorbs light, so you need to improve the reflectivity and the contrast factor," says Lee.

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