Jet Set Displays
Jet set displays

Want to make a colour video screen? Then just print one out

COLOUR displays that can be made using a simple "printing" process will provide ultra-cheap video screens for the next generation of cellphones and palmtops, say British researchers.

In London last week, a team from Cambridge Display Technology and Seiko Epson of Nagano demonstrated the world's first full-colour video screen to use arrays of red, green and blue light-emitting polymers (LEPs). Unlike complex liquid crystal displays, LEPs have a wide viewing angle. And because they need no backlights, colour filters or polarisers, they are far simpler to make, says CDT's technical director Jeremy Burroughes, a member of the team that developed LEPs at the University of Cambridge in 1989. (Click on thumbnail graphic for diagram.)

The team found that a polymer called PPV will emit light when a voltage is applied to them. Because PPV molecules contain benzene rings, which allow electrons to move through the molecule, the polymer can act as a semiconductor--and form the basis of a light-emitting diode.

Seiko Epson, which makes ink-jet printers, has adapted ink-jet technology to build cheap LEP displays. Using solutions of PPVs as the "ink", Seiko Epson can deposit individual pixels of red, green and blue LEPs directly onto a silicon substrate. The PPVs are electrostatically charged and sprayed onto the base, which has an opposite charge. Electrodes are then laid down so that voltages can be applied to light up individual pixels. But this process requires conventional chip-making technology.

Last week's demo showed a mobile-phone-sized 6.3-centimetre LEP screen, displaying a colour video of TV quality on a 270 000-pixel screen. CDT claims it will be able to manufacture its display cheaply enough to make colour video affordable on future wireless Internet devices, such as palm tops, webpads and mobile phones.

Until now, a major problem with semiconducting polymers has been that they oxidise and become discoloured when exposed to light. CDT has previously only been able to make green monochrome video displays. Now, CDT claims the red, green and blue LEPs in its new display are the first to have lifetimes long enough for use in consumer products. CDT's red LEP will work for 100 000 hours, green for 30 000 hours and blue for 1000 hours. But the company isn't saying how it has achieved these improvements.

"Seiko's research in Japan shows that a cellphone will only actually get 200 hours use per year," says Burroughes. "People replace their mobiles well before the current blue polymer lifetime of 1000 hours is reached." And further improvements in LEP lifetimes are expected, Burroughes says.

The companies expect the ink-jet printed video display to begin appearing in Seiko's mobile phones in two years' time, at prices "significantly less" than rival models with colour LCDs.

But perhaps the most tantalising development on the horizon for LEP technology, says Burroughes, is the possibility of creating video displays for, say, wireless Internet access, that can be rolled up. CDT is working on this research with the DuPont chemical company, which has itself acquired another light-emitting polymer research company called Uniax.

Paul Marks

From New Scientist magazine, 01 July 2000.