THE SWEDES seem to have stolen a march in the international race to build flat, flexible, full-colour screens for PCs or TVs using the potentially cheaper technology of polymer light-emitting diodes rather than liquid crystal displays.
Research by a Swedish team shows that more than one colour can be produced from a single LED by making it from a mixture of polymers and then simply varying the voltage applied to it. If in future a single LED could be made to produce red, green and blue light, it could become the basic building block for a flat and flexible TV screen.
Each of the polymers commonly in use in LEDs emits a characteristic wavelength of light when a voltage is applied, so they can only be used for single-colour displays. The Swedish researchers first blended pairs of these polymers, and found that the light emitted was a mixture of the characteristic wavelengths of the two polymers. The team then discovered that they could vary the colour of the blended polymer LED by varying the applied voltage.
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鈥淚t was a chance discovery by one of my students,鈥� says Olle Ingan盲s, who heads the team at the applied physics laboratory at the University of Link枚ping. 鈥淲e were quite excited by it, because people had not foreseen this.鈥�
The team鈥檚 paper, published in Nature (1 December, p 444), suggests it is easy to make LEDs that generate mixtures of either red and blue, green and red, or orange and blue. But the key to building the multicoloured display screens lies in combining the three colours needed to generate natural-looking colour pictures 鈥� red, green and blue. Once this has been achieved, the colour of each element of the screen could be adjusted individually just by varying the voltage applied to it.
鈥淚t鈥檚 a very pretty piece of research,鈥� says Richard Friend, a physicist at the Cavendish Laboratory at the University of Cambridge, who has also been working on light-emitting polymers for visual displays. 鈥淚t highlights the potential for polymers鈥� versatility in use and illustrates the fun that鈥檚 to be had with this category of materials.鈥�
Friend is working with Cambridge Display Technology, a company trying to develop commercial applications for polymer LEDs. 鈥淭he early uses are likely to be in low-level lighting using one colour, probably in consumer products,鈥� he says.
Worldwide, there is intense competition to incorporate polymer LEDs into commercial products, because theoretically they could be cheaper than the liquid crystal displays commonly used in the screens of portable PCs. Researchers in the US and Japan are also exploring the potential of the polymers.
One of the biggest obstacles to further development is that the polymers have short useful lives: the Swedish team found that their blends failed after a couple of days of use, although they were stable for much longer when stored. 鈥淚t鈥檚 still difficult,鈥� says Ingan盲s, 鈥渂ut we might find materials that are stable, and the ways to make them. Stability problems are common to all these sorts of materials.鈥�
The polymers that the team used are derived from chains of thiophene, which contains rings of four carbon atoms and a sulphur atom. 鈥淎ny organic chemist could synthesise these products in a couple of months,鈥� says Ingan盲s. However, the key to the success of the technology is not only in making stable versions of thiophene but also those versions which emit light at the necessary wavelengths.
The Engineering and Physical Sciences Research Council in Britain has launched a six-year, 拢6 million research programme to fund work in the same area, which is expected to lead to applications such as flat TV screens that can hang on walls, miniature head-mounted video screens and lighter portable PCs.
