Motorola has patented a way of charging mobile phones by sunlight. The company was recently issued a patent for an LCD that includes solar cells capable of charging the battery in a mobile phone or other portable device.

The basic premise has been proposed before: a display screen is stacked over one or more solar cells, which are charged by the light passing through the display. But earlier designs allowed a relatively small amount of light to reach the solar cells, resulting in very little power being generated even under the best light conditions, Motorola researchers said in the patent.

The ultimate goal is to develop a device that could remain charged indefinitely, without requiring users to plug into a socket or carry external chargers with them when they travel. Until now, the major obstacle has been the LCD's polariser and reflective screen, which sends light back to the viewer. In earlier designs, the reflective screen allowed less than 6 percent of the available light to reach the solar cells, Motorola said.

To solve this problem, Motorola proposed using either cholesteric liquid crystal or polymer-disbursed liquid crystal in the display, instead of super-twisted nematic liquid crystals. This change in materials eliminates the need for both a reflective screen and polariser in the LCD screen. As a result, Motorola claims as much as 75 percent of available light is able to reach the solar cells, providing a sufficient amount of power to charge the battery of a mobile device.

Motorola also found a way to increase the amount of light that passes through screens based on super-twisted nematic liquid crystals, by using a selective colour reflector. These reflectors only reflects one color, such as green for a green display, and allows other colours to pass through, the patent said. While not as effective as designs using cholesteric liquid crystal or polymer-disbursed liquid crystal, these displays still allow around 30 percent of available light to reach the solar cells, Motorola said.

The patent, which offered no hint of commercial product plans, also outlines how solar cells can be added to OLED (organic light-emitting diode) and touchscreen displays.