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Friday, March 8, 2013

Solar Cells May Supplement Smartphone Batteries

Why don’t power-thirsty smartphones incorporate solar cells, to reduce the reliance on batteries Because in general, the kind of solar cell that can be fabricated in a lightweight, flexible and durable form does not capture enough energy per square inch to make it worthwhile.

But a Silicon Valley solar power company thinks it has found a way to adapt the highest-efficiency solar cell chemistry, gallium arsenide, into a form that can be attached to a portable electronic device like a smartphone or a tablet. And a panel the size of a cellphone or a tablet would add measurably to the device’s longevity.

It wouldn’t do away with the battery. But depending on the light level where the device was carried, it could add 80 percent to the battery life. The main benefit would be outdoors or on a windowsill, because sunlight has about 100 times more energy than the light typically provided by fluorescent or incandescent lamps. Indoors, it might add only 10 to 15 percent. But the efficient type, gllium arsenide, is not only better overall at capturing energy; it is also better suited to capturing energy in low-light conditions that the ordinary silicon solar cells.

The problem has been that gallium arsenide, used heavily in the space program because it captures a high fraction of the light it receives, usually comes in an awkward form: rigid, heavy, fragile crystals. Solar cells already used on portable devices, like the ones on pocket calculators, are “thin film” devices, lighter and flexible, but they capture only a small fraction.

The solar company, Alta Devices, says that it can make gallium arsenide cells with the physical characteristics of thin films, and that they will convert 30.8 percent of the energy in the light to electric current.

Making a thin film out of gallium arsenide is a complicated process. Once upon a time, the photovoltaic world was divided into cells made from silicon that was grow! n into crystals, and cells made from gases that were deposited in thin films on special glass, called amorphous silicon because they were not crystalline.

Alta starts with a silicon wafer, crystalline in structure, heats it to 800 degrees Celsius, and deposits gases on it. These organize themselves into a crystal form, somewhat the way eggs, placed into an egg carton, line up in a uniform pattern. The cell is formed in layers, including a first layer that can later be dissolved chemically, so the finished cell can be peeled away from the silicon wafer. When it is peeled off, it retains its crystalline structure, a little like a waffle being pulled off a waffle iron.

The gallium arsenide layer is extremely thin, about 1 micron, or about one fortieth the width of a human hair.

The Alta cell is actually a stack of two working cells, which is now a common technique in high-end cells. The gallium arsenide captures energy from light in the wavelengths that include most of the visible spectrum. Bt it lets some other wavelengths pass through.

The second layer is a different material, indium gallium phosphide, which gets energy from the shorter wavelengths. That combination was pioneered for satellites in 1991, according to the National Renewable Energy Laboratory.

For hand-held electronics, the technology still has a way to go. Christopher S. Norris, the chief executive of Alta Devices, acknowledged that so far, the only customer is the military; soldiers use the cells to minimize the amount of batteries they have to carry on the battlefield. They are also used at fixed bases to reduce consumption of diesel fuel for generators, simplifying logistics.

But Alta has built a prototype cover for a Samsung Galaxy phone and has designs that could be incorporated in other smartphones, Mr. Norris said. Incorporating the film would be fairly easy, he said, because much of the electronic hardware and the soft! ware need! ed to connect the cell to the battery is already on the phone.

A smartphone would probably take a patch of film with a peak output, in full sunlight, of 1.5 watts, he said, which is probably only about $3 worth of materials. (A cellphone plugged into a wall outlet generally draws 3 to 5 watts, he said, and an iPad, about 10 watts.) “If you’re in full sun, a watt and a half for10 minutes will give you an hour of talk time,” he said. The company posted an on-line calculator to estimate the benefits in various kinds of light.

He said there was probably a strong market in North America, Europe and Japan, but that the market could be even stronger in Third World locations where wireless has arrived in advance of an electricity grid.

Another potential market is the automobile. The surface area of a car will not allow enough electricity production to run the motors that power an electric car. But increasingly, some ancillary functins, like power steering or power brake pumps, or air-conditioning, are performed by electric motors, to lower the load on the internal combustion engine and improve gas mileage. Cells on the car roof could help with that, he said.