August 22, 1996. Copyright, 1996, Graphic News. All rights reserved

THIN FILMS SET TO REVOLUTIONISE TV SCREENS
By Laura Spinney, Science Editor

LONDON, August 22, Graphic News- Large, foldable TV and computer screens with improved colour quality could soon be here, thanks to new technology based on thin films of organic molecules that emit light.

Visual displays currently rely on light-emitting diodes (LEDs) made from inorganic crystals. Because they are composed entirely of crystals, these tend to be inflexible and even brittle. But organic LEDs are potentially more versatile because they consist of thin films of small organic molecules or long chain polymers Ð sometimes only a few atoms thick Ð which can be sprayed on to a flexible surface or substrate. When these films are placed in an electric field, they convert electrical energy into photons of light.

According to Dr Karl Pichler, who heads the device technology group at Cambridge Display Technology (CDT), a company specialising in light-emitting polymers, this kind of LED also has the potential to display a wider range of colours than the inorganic variety. ÔThere is a much wider variability in terms of colours because itÕs very easy to change the colours by changing the polymer chemistry,Õ he says. And because the length of a polymer chain is easier to modify than the size of an inorganic crystal, displays made from organic polymers can be larger.

Organic LEDs are not yet commercially available because until recently they have been short-lived, sometimes burning out after only days. But according to a report in this weekÕs issue of the journal ÔScienceÕ, researchers all over the world are beginning to design longer-lasting devices. At CDT, for example, Pichler and his colleagues have pushed the lifespan to 7,000 hours Ð almost a year of continuous operation. It is their long-term goal to produce organic TV screens, but within the next year or two Pichler believes organic backlights for mobile phones and other forms of communications could be on the market.

At CDT, thin films are created by spin-coating, where a drop of polymer is placed on the substrate and spun. But for other applications Ð high-temperature superconductors (HTS), for example Ð thin films can be made by blasting blocks of the relevant ingredients with a laser beam, in the presence of the substrate. An HTS is a ceramic that conducts electricity with no resistance when cooled only by liquid nitrogen, rather than by the more expensive liquid helium needed to reach much lower temperatures. In the form of thin films these are being used, among other things, as highly sensitive magnetic field detectors in magnetic resonance imaging (MRI) for medical diagnosis.

Sources: Science, CDT, The Royal Society