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Printing Flexible Displays

With organic semiconductors and inkjet printing, Xerox PARC looks to create cheap, flexible displays.

REGISTRATION IS a key concept in printing, whether it's an inkjet printer in your office or a newspaper press: Each layer of color ink has to be deposited in exactly the right place. If any one is just a hair off, the resulting image is out of register, producing a fuzzy-looking picture.

Now imagine trying to keep registration on target when printing a transistor. That's one of many issues that Xerox' Palo Alto Research Center (PARC) in California faces as it refines plastic semiconductors for applications such as large-area displays.

The project's roots can be traced back to the mid-1970s, when PARC researchers looked at using amorphous silicon (a-Si) for photocopiers. A mixture of silica and hydrogen, a-Si can be deposited as a thin film on substrates such as glass and plastic. A decade later, PARC had learned enough about a-Si to create diodes and transistors. In 1996, PARC spun out a company called dpiX, which used a-Si in products such as flat-panel displays.

Hungry for a new field to explore, PARC turned to organics, which was — and still is — a hot area. “We quickly decided that we wanted to combine ink jetting and solution-processed semiconductor material to see if we could inkjet transistors,” says Raj Apte, a research staff member at PARC.

Today, PARC has two parallel projects: One focuses on a-Si, but replaces conventional lithographic manufacturing techniques with inkjet printing, and with an eye toward using flexible substrates. The other uses organic materials rather than silicon.

Why inkjet printing? One reason is that, at least in theory, it's cheaper than conventional techniques. “Photolithography is one of the most expensive steps in manufacturing an LCD,” Apte says.

Scaling up

Working with partners such as the Xerox Research Centre in Canada, PARC developed an organic semiconductor ink. The research also leverages 600 dots-per-inch (dpi) print heads manufactured by PARC's parent company Xerox. “They're much better than anyone else's, so that's a huge advantage,” says Apte, who points to precision as their biggest strength for printing displays. “Even if it's off a pixel or two, you don't notice it, but you notice it like crazy when you're printing transistors. They can't be off.” (More information about PARC's inkjet printing systems is available at www.parc.com/research/dhl/projects/dropletdispensing/printheads.html.)

“PARC's approach aims to use inkjet for all stages of producing the active-matrix backplane,” says Craig Cruickshank, founder of cintelliq, a Cambridge, England-based research firm that tracks the organic semiconductor market. “All gate, source, and drain, along with data lines would be fabricated by ink-jetting the appropriate materials onto the surface of the substrate.”

PARC currently is refining the technology and manufacturing techniques to enable large-scale, large-capacity printers, as well as looking at making a move from rigid substrates to flexible ones. “We're now able to print displays with our inkjet printer and our special ink,” Apte says. “We've demonstrated 128-by-128 pixel electro-paper-type displays, and I've just built a prototype to do 480-by-480. We're learning a lot about how to scale up these processes.”

The price is right

Scalability is important, particularly for PARC's research to have a shot at being commercialized into large-area displays, such as retail signage or video screens like something out of the movie Minority Report. But as much as size matters, it's not a make-or-break factor for selling into markets where price is just as important.

“What we're making isn't better than what anyone else can make at this size,” Apte says. “What's exciting is that you may be able to make it cheaper. The whole point of printing isn't that it allows you to build things that couldn't have been built before. It allows you to build them cheaper and faster and easier.”

Apte says that large-area displays based on PARC's technologies and manufacturing techniques would cost significantly less than conventional systems. “In an optimistic case, we're talking about a factor of 10 improvement in the price of displays,” Apte says. “Our conservative estimate puts it at 50 percent. So somewhere between a half and a tenth of the current price of displays is what we'd be looking at.”

OLEDs are out

Apte expects that in about five years, PARC's technology will be commercially viable for printing large-area displays in quantity. One unresolved issue is developing a business model. Options include building the printers and then selling them to display manufacturers, selling the semiconductor ink or manufacturing backplanes, and then selling them to display vendors.

Another issue is determining which companies it could partner with or sell to. One factor in that decision is that plastic electronics aren't compatible with organic LEDs (OLEDs), which require high current and thus have to be paired with semiconductor materials such as a-Si.

“We're doing work with partners on OLEDs using amorphous and polysilicon on flexible substrates, but we can't do it with organics,” Apte says. “So OLEDs are out. What's in are cheap reflective displays like E Ink or Gyricon. Those could be used in low-power applications.”

Gyricon is an Ann Arbor, MI-based PARC spin-off whose SmartPaper displays are currently used in applications such as convention centers and hotel lobbies. A SmartPaper display looks like a monochrome cross between a whiteboard and the screen of a late 1970s computer, with blocky characters of a fixed size. The display consists of millions or billions of polyethylene balls, each about half the diameter of a human hair, all nestled against a backplane. One side of the ball is black and positively charged, and the other side is white and negatively charged. When the backplane applies power in a specific pattern, each ball that receives a positive charge rotates its black half to face out, while those that receive a negative charge show their white face.

With enough black sides facing out and in the right places, letters and numbers are formed. Gyricon says that future generations of SmartPaper will be fully pixilated, enabling graphics and variable text sizes. (For more details about Gyricon's technology, see “The Paper Chase” in the January 2005 issue of Pro AV.)

“They have an active-matrix prototype based on a-Si, but I think they're excited about work we've been doing with them to build an organic backplane,” Apte says. “We've demonstrated it at 128 by 128.”

Road maps and road blocks

Many academic researchers and companies —including established players such as DuPont, Lucent, Pioneer, and Samsung — have demoed displays that use organic active-matrix backplanes. So although the market has a lot of potential, it's already shaping up to be a relatively crowded one. That activity stems from organics' low cost.

“Organic thin film transistors (OTFT) is probably the only technology at present that can address the flexible display market at the expected cost structures,” Cruickshank says. “There is nothing inherent about organic semiconductor technology that will stop it from being a suitable candidate for flexible displays. The level of technology performance is rapidly improving, and the quality of demos and prototypes are impressive.”



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