Wednesday, October 3, 2012

PV cells keep getting better

There are precious few reasons for optimism these days.  But one that stands out is that photovoltaic cell production keeps making significant improvements.  Not only are they becoming more affordable by the day, their performance keeps improving.  Here we see a story of how the Germans are closing in on the maximum theoretical efficiency for such devices.

High-efficiency solar cell grabs top prize

Date 01.10.2012
Author Richard Fuchs

The 2012 German Environmental Award goes to companies and researchers in the solar industry for developing technology that utilizes sunlight more efficiently than ever before.

For Andreas Bett, the future will fit onto a tiny, pinhead-sized chip. In one smooth motion, the physicist pulls a small metal plate the size of a matchbox off the production line at his laboratory in Freibrug, Germany. He proudly points to the little, nearly invisible photovoltaic cell attached to it.

This solar cell is the stuff of Bett's visions for the future. After all, he speaks from experience. As deputy director of the Fraunhofer Insitute for Solar Energy Systems (ISE), he's been working on these multi-junction solar cells for over 25 years. Bett shares this year's German Environmental Award, presented by German Environment Foundation (DBU), with his teammate Hansjörg Lerchenmüller of Soitec Solar, and with Günther Cramer, co-founder of SMA Solar Technology.

The conventional silicon solar cells of today are only able to transform a small portion of sunlight into energy. But Bett's solar cells can do a whole lot more. They use almost the entire spectrum of sunlight. To achieve that, the scientists working in Freiburg use three different materials to create a high-efficiency multijunction photovoltaic cell. Instead of silicon, they employ semi-conductors gallium-indium-phosphide, gallium-indium-arsenide and germanium, with each of the three materials transforming a different part of the light spectrum into electrical energy.

"We adjust our three solar cells stacked on top of one another - or semi-conductor structures - to the colors of the rainbow," explains Bett. The top layer uses the short blue wavelengths of light, the middle layer the green wavelengths, and the lower layer the long, infrared wavelengths. Only in this way can efficiency levels peak.

"One can calculate that with a semi-conductor material such as silicon, one can achieve a maximum theoretical efficiency of 33 percent," says Bett. Multijunction photovoltaic cells built at the Fraunhofer Institute, on the other hand, achieve over 41 percent efficiency. That's a world record. Never before has a solar cell transformed more light into electrical energy under laboratory conditions.

Various sectors can use such photovoltaic cells for specific products. AZUR Space Solar Power, based in Heilbronn, Germany, for example, applies the ISE technology to create high-efficiency multifunction solar cells primarily for supplying power to communications satellites.

Some 1,000 solar chips fit onto a round disc, or "wafer," that's about the size of a DVD. Klaus-Dieter Rasch, managing director of AZUR Space Solar Power, is proud that around 150,000 such wafers roll off the conveyor belt at his company each year, but such mass production was complex in the making. "We had to use standard systems from LED production, but found they were not good enough," said Rasch. So production lines and software were altered to better accommodate multifunction solar cell production - with the result that over 400 nonmilitary satellites tap their power via the photovoltaic cells bearing the "Made in Heilbronn" label.

But only an additional trick developed by the ISE scientists make the space solar cells cost-effective enough for use on Earth. Andreas Bett demonstrates the trick in the Freiburg laboratory. He places a metal plate covered with several photovoltaic cells into a rectangular-shaped box. He then clamps a matte silicon wafer over it. "The trick is to concentrate the sunlight with an optical lens and direct it onto a very small semi-conductor surface," he explains. more

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