Preparing for the age of total mobility

July 7, 2000

Internet-friendly mobile phones will soon be everywhere. Wendy Barnaby explains how they work

At the end of this year, the electronics giant Philips launches new mobile phones. Proud owners will be able to download pages from the internet, which will appear on the phone's display.

Bigger and clearer displays will be easier to read - but also more expensive. The present display technology, photolithography, is cumbersome and time-consuming, and soon after the new phones appear, Philips will be changing to a simpler and cheaper one: printing with giant molecules as though they were ink.

The idea of printing like this has an impish appeal to its developer, Richard Friend, Cavendish professor of physics at the University of Cambridge. "We'd like to go back to Caxton," he grins.

The giant molecules are polymers, the molecular shapes behind plastics - and us: they give rigidity to our bones, skin, and flesh. They are huge strings shaped like stars, chains and trees. The fact that they are made of so many atoms connected together gives them strength, toughness, chemical variety, desirable electronic and biological properties and a huge spectrum in costs. They are also on an industrial roll: worldwide demand for major plastics has been rising steadily from 1985. Richard Friend and his colleagues at Cambridge Display Technologies (CDT) are exploiting the electronic properties of polymers - using them as semiconductors.

Processing silicon, the current semiconductor material, is complicated. It needs a vacuum, and there is no way of simplifying working with it. New processing needs a new paradigm, and carbon-based polymers supply that.

They can be grown from solvents and processed from solution, which is easier and cheaper than working with silicon.

In a polymer light-emitting diode (LED), a thin layer of polymer (the semiconductor) is painted onto a substrate and sandwiched between two electrodes. A voltage is passed across these, and where the charges meet, light is produced.

CDT has turned this basic diode into a display in which individual pixels can be picked out to make the desired pattern. The device turns out to be more efficient than tungsten lightbulbs and most inorganic LEDs.

Working with a Japanese firm, Seiko-Epson, CDT has produced a 5cm diagonal screen with 180,000 pixels to show EastEnders at video rate. This is in black and white; but the polymers can be tuned chemically to produce specific colours of emission. CDT has produced blue, green and red in this way.

Although the phone displays will be in black and white, the technology is expected to be used for flat television screens in future.

The cost of producing displays in this way may be cheaper than current technology by factors of two to five. This is because cost is affected by the whole packet, not just the semiconductor.

There is great interest in taking electronics off glass, which is needed for liquid crystal displays, and putting them onto plastic, which is lighter and not so fragile.

Plastic substrates, however, cannot cope with the high temperatures needed to process inorganic semiconductors. But organic polymers can be processed at low temperatures, which plastics can stand.

Low-temperature processed, printed polymer displays would change the whole manufacturing structure even down to the casing around the product, which would no longer need to be so protective or provide rigidity.

"The next generation of plastics won't be mixed in a bucket to see what happens; it will be engineered from molecules to give specific, desired properties," says Friend. "The new LEDs are simply a taste of what's to come."

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