Xerox takes step towards roll-up plastic displays

The day when computer displays and TVs roll up are a step closer to reality as Xerox researchers announced last week that they'd cracked one of the biggest problems in putting plastic circuits on flexible materials. The problem: ink.

According to Beng Ong, a Xerox fellow at the Xerox Research Centre of Canada (XRCC) outside of Toronto, his team has developed both an inkjet-like ink and the process to print semiconductors -- one of the three elements of a circuit -- using that ink onto flexible stock.

Circuits are traditionally made of silicon, a process which doesn't work on flexible materials. Xerox, as well as other companies, has been trying to come up a low-cost alternative to silicon that could print flexible plastic transistors as easily as printing a newspaper.
'The technology could lead to backplanes for LCDs,' said Ong, 'that would roll up, or to 'electronic paper,' like a newspaper.'

The key to the new ink, and its printing technique, is that it can be used in the open air and at low temperatures, an important consideration when the goal is to drive down costs, said Ong.

The electrical properties of most liquid-based organic semiconductors degrade when exposed to oxygen in the atmosphere, which makes it tough to build functional transistors in ambient air, rather than under inert gas environments and at high temperatures.

The Xerox polythiophene-based ink has molecular characteristics that allow it to be processed into structurally-ordered semiconductor nanoparticles. When dispersed in a liquid, these nanoparticles form a stable ink that can be sprayed in an inkjet printing fashion to form organic transistor channel layers.

The ink could also be printed onto materials using other techniques, such as offset, screen, or stencil printing, added Ong.

Ong's team has also come up with ways to print flexible conductor and the dielectric components. The former, for instance, can be put onto flexible stock with a metal-based ink, then heat-treated to make it conductive.
'We now have all the components to print a complete plastic circuit,' said Ong.

One step still to be taken, however, is to meld the printing of the three components of a plastic transistor -- semiconductor, conductor, and dielectric -- at one time. So far, said Ong, they've been able to print only one at a time.

The three-year research effort by Ong and his XRCC team could also lead to low-cost RFID tags. 'Tags now are extremely expensive, and impossible with current technology to place on low-ticket items, such as those in grocery stores,' said Ong.

Printing to plastic tags -- especially at the low cost envisioned by Xerox with its low-temp, open-air ink -- could solve that problem.

Under a National Institute of Standards and Technology Advanced Technology Program (ATP) grant, Xerox is working with Motorola and Dow Chemical to develop these plastic integrated circuits (ICs).

Using XRCC's materials and processes, Xerox's Palo Alto Research Centre (the famous PARC, known for such landmark innovations as the graphical interface and the mouse) is inkjet printing active-matrix addressed arrays as backplane circuits for displays.

Motorola, meanwhile, is fabricating plastic circuits for various applications using commercial printing technologies.

Ong wouldn't commit to a definitive date when the XRCC ink and printing processes would make it into commercial products, saying only that he expects them 'in the near future' to appear commercially. 'The next step after research is to take it into development,' Ong said of his technology. 'That's a major step, and not always easy.'