Innovation: Plastics show slim TV screen in a new light

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TELEVISION has come a long way since the small, hazy monochrome cathode-ray tubes of the 1950s: today you can wear a liquid-crystal colour TV screen on your wrist. And the next generation of design may bring a screen as thin, light and flexible as a sheet of paper, if a product being developed at Cambridge University realises its potential.

The flexible 'sheet screen' would be based on plastics that can be made to emit light. These materials, discovered in 1990, can now produce light in all the colours of the spectrum and are tough and pliable enough to withstand being filed away in a sheaf of papers.

Light-emitting plastics are the fruit of a collaboration between physics and chemistry; specifically, between the teams of Richard Friend and Donal Bradley in the physics department at Cambridge, and Andrew Holmes in the university's chemistry laboratories.

The new plastics developed from the work of this group, which carries out research into plastics that conduct electricity. The researchers recently set up a company, Cambridge Display Technology, with backing from a local venture capital firm, Cambridge Research and Innovation (CRI), to commercialise their discoveries. The university has also taken a stake.

Plastics are formed from substances called polymers, which consist of large, often chain-like molecules built by linking many smaller, identical units. The most familiar plastics, polyethylene (better known as polythene) and polyvinyl chloride (PVC), are good electrical insulators - the former is widely used for coating electrical cables.

But during the 1970s, a polymer was discovered that acted as a good conductor of electricity. Numerous polymeric materials have since been devised with this property, and in some cases their conductivity is comparable to that of copper. In the course of studying one of these materials, polyphenylene vinylene (PPV), doctors Friend, Bradley and Holmes found that the plastic emitted light when a voltage was set up across it.

Electric current passes through conducting polymers, as through metals, in the form of mobile electrons: in polymers, they may be considered to run along the polymer chains as if these were tiny molecular wires. By applying an electric field across PPV, electrons can be injected into the material at the negative terminal and removed at the positive terminal.

The injected electrons fall into the 'holes' left by those removed, losing energy, which is radiated in the form of visible light. The amount of energy lost determines the colour of the light, and this can be adjusted by subtle changes in the chemical make-up of the polymer.

This is where the chemists in the Cambridge partnership come into their own: they have been able to design and synthesise variants of PPV to emit different colours.

This technology offers a potential alternative to the current approach to flat-screen televisions, based on liquid crystals. Liquid crystal displays are ideal for small-scale applications, such as the face of a wristwatch, but it is difficult to make big screens with them.

Making electrical connections to each pixel (single dot) of the screen is a serious problem, because it results in a lot of tangled wiring. The switching requirements for pixels made from light-emitting polymers would be much simpler, making them easier to wire and reducing power consumption.

Colour displays based on liquid crystals require an array of filters to extract the appropriate colours from the light source, whereas polymer displays would generate their own coloured light. Moreover, this would make the polymer displays brighter than their liquid crystal counterparts.

A further advantage is that polymer displays could be viewed from any angle, whereas anyone familiar with liquid crystal screens on laptop computers will know that the image tends to vanish when viewed from an oblique angle.

Another attractive aspect of displays based on light-emitting polymers is their toughness and flexibility. Polymers are as strong as polythene, and can be bent without detriment.

In 1992, researchers in California demonstrated a plastic light-emitting device that could be bent backwards while shining as brightly as ever. The electrical contacts to the luminescent plastic were made with transparent electricity- conducting polymers, so the flexible device was all plastic. Liquid crystal displays, in contrast, tend to be brittle and inflexible.

The biggest obstacle to making these devices marketable is that the polymers degrade with use. But the Cambridge researchers are confident that they can overcome this problem.

Similar concerns plagued applications of electricity-conducting polymers, but batteries are now made from these materials with longer shelf-lives than their conventional counterparts.

And the latest results for polymer-based, light-emitting devices (LEDs) are promising: operation without failure for up to 700 hours has been demonstrated.

Cambridge Display Technology has what it describes as a 'fundamental patent position' on light-emitting polymers. Its principal competitor is Uniax, a Californian company established by Alan Heeger of the University of California at Santa Barbara; but the primary focus of Uniax is on electricity- conducting polymers, which are currently used in applications ranging from anti-static coatings to microelectronic components.

Devices based on light- emitting polymers are also being developed in Sweden, the Netherlands and Germany, but so far the Cambridge group is ahead of the field.

Dr Holmes stresses, however, that the polymer flat-screen TV is a distant prospect; even rudimentary display devices are some way from production at present.

Cambridge Display Technology will first concentrate on simple light-source applications and demonstration devices aimed at raising industrial awareness of the potential of these plastics. Dr Holmes expects the first of these devices to be on the market in six months.

(Photograph omitted)