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Self-powered display technologies

New Scientist

Screens that not only display images but also generate their own power are on the horizon.

One of the new display technologies will be suitable for cellphones, making their batteries last far longer than they do now. The other could lead to selfpowered electronic billboards.

The development of the novel screens is revealed in two recent patent documents. Motorola of Schaumberg, Illinois, has been granted a patent (US 7206044) on a liquid crystal display that incorporates a solar panel. The self-powered billboard appears in a patent application filed by Nokia of Espoo, Finland (US 2007/0080925). In this device the picture cells of the display perform twin duties: as well as forming the image, they also generate power.

Motorola has developed its solar-powered display to meet the rising power demands of mobile phones. As more and more features have been incorporated into cellphones - such as wireless internet access, video cameras, music players and GPS location finding capabilities - their lithium-ion batteries have started to struggle to keep up. To give batteries a longer lifetime between charges without adding to their size and weight, manufacturers have tried fitting solar cells behind phones' LCD displays. Till now this has not been successful, because the LCD absorbs most of the incoming light before it can reach the solar cell, says Motorola engineer Zili Li.

Li's answer has been to propose building the LCD with colour filters made from a polymer film that reflects only narrow bands of red, blue and green light. This is enough to provide an adequate colour picture, while allowing through enough energy at other wavelengths for the solar cell to generate power to charge the phone's battery. Motorola will not say when the technology might reach the shops, but Li says the patent takes the company a step closer to developing mobile devices that need charging far less often than they do now.

Meanwhile, Nokia has already built a working 200-pixel-square prototype of its monochrome self-powering display, according to its inventor Zoran Radivojevic. The key to this device is the use of titanium dioxide nanoparticles both to generate the image and to harvest power from light.

The cells that make up the display are packed with these particles, which can be switched from a colourless to a black form by applying a voltage to them. When the particles are in the colourless state, they generate a voltage when struck by light, and this can be used to drive a current to charge a battery. To turn the pixel black, the screen's control electronics reverse the current and apply a voltage from the battery to the nanoparticles.

Because each image cell is packed with a large number of nanoparticles, the resolution of the display can be anything from ultra-fine, for small, highresolution displays, to very coarse for billboard-type displays. "We can scale pixels from submicrometre level to centimetres or more in size, so it will be most suitable for large still-image billboard displays whose images change slowly," says Radivojevic. Electronic billboards like this will cost businesses nothing to run, he says, and because they will not draw any power from outside sources they will not contribute to carbon emissions.

One expert in display technology, who preferred to remain anonymous, says the idea of such self-powered displays is a compelling prospect. But he warns that the displays will only succeed if the developers can match the image quality of today's displays.

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THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 5 MAY 2007

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