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.
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.
Advertisement
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.
