Thursday, May 03, 2007

Self-powered displays keep gadgets alive

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 self-powered electronic billboards.
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.
It has been proposed to build 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.
Meanwhile, Nokia has already built a working 200-pixel-square prototype of its monochrome self-powering display. 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, high-resolution displays, to very coarse for billboard-type displays. "We can scale pixels from sub-micrometre level to centimetres or more in size, so it will be most suitable for large still-image billboard displays whose images change slowly,". Electronic billboards like this will cost businesses nothing to run and because they will not draw any power from outside sources they will not contribute to carbon emissions.


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