Record-breaking solar cells are tailored to their location

The burning hot sun at the equator is a far cry from the weak sunlight that reaches higher latitudes. To make the most of such different conditions you need specially tailored solar cells, according to UK firm Quantasol.

So the company has come up with a new solar cell design that can be tuned to the light at a particular latitude, and in the process broken a 21-year-old efficiency record for one type of solar cell.

Semiconductor materials such as gallium arsenide (GaAs) are more efficient at converting light to electricity than the cheaper silicon cells most common today. First used in space, GaAs solar cells are beginning to find uses on Earth too.

But the uniform light conditions in space aren't matched on the ground. The atmosphere acts as a filter, so the light reaching Earth varies from place to place and with changing atmospheric conditions.
Tuned in

Quantasol has now created GaAs solar cells that can be tuned to the prevailing light conditions of a particular place, to get the most out of the cells wherever they are.

To do that, the firm added indium gallium arsenide (InGaAs) to pores just a few nanometres across on the surface of their cells, called quantum wells. Like the GaAs that makes up the rest of the cell, they can absorb light to produce electric current. But they do so at very specific frequencies.

The pores can be tuned to absorb light at the frequencies that are most common in a particular place but aren't absorbed well by GaAs. Over time this strategy should extract more energy than an off-the-shelf solar cell.
World record

After the quantum wells have been tuned, the GaAs solar cell absorbs more of the incoming light than previous devices. The peak efficiency of the new cell is 28.3 per cent when exposed to light 500 times as strong as normal sunlight, a figure that has been confirmed by the Fraunhofer Institute of Solar Energy in Germany.

That may only be one-tenth of a percentage point higher than the previous world-record holder, but it's the first advance in 21 years.

Commercial silicon solar cells are much cheaper than GaAs, but have an efficiency of just 10 to 12 per cent and are also bulkier. The Quantasol device can cope with much brighter light without becoming overloaded, making it possible to use a very small solar cell to absorb light collected by a system of cheap lenses and mirrors.

But more important than the peak efficiency is that the new cells can generate more electrical energy over the course of days and weeks, says Kevin Arthur, Quantasol CEO.

"The commercial market doesn't just want high efficiency, they want the device to be optimised to the environment," he says. "In the past we measured performance in dollars per watt. Now it's cents per kilowatt-hour that's more important."

Quantasol will showcase its new device at the UK's Royal Society Summer Science Exhibition in London this week.

Source: www.newscientist.com