Flickering beginnings

Although printing flexible screens in this way will help to make them affordable, they still have a long way to go to catch LCDs. For that, two things need to happen. One is that the displays must turn from black-and-white to colour. The other is that they must be able to refresh their images at a rate fast enough to show moving pictures. Researchers at the Flexible Display Centre and elsewhere are working on ways to do that, and there seems little doubt it will happen. Yet even with their present limitations, flexible screens have some important advantages over LCDs.

For a start, LCDs are difficult and costly to make. Most are produced in huge, ultra-clean factories using batch processes similar to those for making silicon chips. This is a fractious, finicky process and tiny defects in the materials, or failures in the alignment of the different layers, can result in 20% or more of a batch being scrapped. Moreover, the glass means LCDs are heavy and easily broken, as anyone who has dropped a laptop knows to his cost.

Another drawback is that LCDs consume a lot of power because they are lit from behind. It also means that the image can be hard to see in bright sunlight.

Electrophoretic displays work in a different way, using a form of electronic ink that has been under development since the 1970s. E Ink’s version employs tiny capsules filled with a clear fluid containing positively charged white particles and negatively charged black ones. The capsules are arranged as pixels and electric charges applied to each pixel pull either the black or the white particles towards the top of the capsule (and the opposite colour to the bottom). Unlike an LCD’s, this image does not require backlighting. Instead, the user relies on reflected light, as he would if he were reading a sheet of printed paper. Moreover, once the particles in the capsules have settled down they stay put. That means the image remains on the screen without drawing power. A further dose of electricity is required only when the image changes; when a user “turns” to the next page, for example. Not only does this mean that electrophoretic displays are cheaper to run, the lack of constant refreshment makes them more comfortable to read—as comfortable, it is claimed, as printed paper.

Colour section The one feature these screens do not yet offer is colour and, though colour versions will surely come to market, no one is yet sure which version will succeed. Electrophoretic displays can use coloured particles and filters to produce red, green and blue subpixels, but as each colour occupies only a

third of a pixel’s area, the brightness of the image is correspondingly reduced. Liquavista, a spin-off from Philips, is trying something called “electrowetting”. This uses an electrical field to modify the surface tension of coloured oils and water within pixels that are mounted on a flexible Teflon base. As each pixel is activated, the wetting properties of the oil and water change, making colours visible.

Another approach is to use materials that emit light. Some firms, such as Sony, are looking at organic light-emitting diodes composed of thin films of organic molecules which generate light in response to an electric current.

Photonic crystals are a further alternative. These are tiny particles that have a crystal structure which influences the flow of photons, the particles of light. By changing the structure of such a crystal slightly, using an electric charge, the colour of the light reflected by that crystal will change too. Tune the crystals appropriately and you can create different colours.

There are also hybrid methods, like that used by Adrian Geisow at HP Labs’ campus in Bristol, England. He has taken a conventional approach to generating colour, using liquid crystals and red, green and blue filters. However, he has done so in a plastic film produced in a printing-type process. The screen can be backlit, like a standard LCD, but it is capable of retaining its image because the material the liquid crystals sit on encourages the pixels to stay transparent or opaque once they have been switched. However it is eventually done, Dr Geisow is convinced that putting colour into flexible screens is what will turn them into a very big picture indeed.

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