Better Yield from Solar Harvest

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The field of solar energy is changing rapidly, with new research, ideas, and results abounding on all sides. One of the many frontiers is the Swiss Federal Institute of Technology Zurich (also known as ETH Zurich, for brevity), where researchers are exploring nanotechnology and the production of hydrogen directly from sunlight.

The researchers at ETH Zurich believe they can significantly increase energy conversion efficiency with new nanomaterials — the current average for the solar cells shimmering on rooftops all over the world is under 20%.

ETH Zurich’s team is working with tiny little structures distinctly smaller than the wavelength of the light they’re converting to energy. The problem as they see it is the presence of so-called “hot electrons.”

When sunlight hits the semi-conductor material, it results in charge carriers — electrons — and the more the better, because that generates more electric current. The wavelength of the light in question directly affects the amount of power that can be generated. The most efficient conversion is at a wavelength of 1000 nanometers, or infrared light. If the wavelength is shorter — like, say, daylight — the electrons respond by carrying an extra jolt of energy (this is what makes them “hot”).

Hot electrons are BAD. They “cool off” — let that extra energy go — within a few billionths of a second, but that extra energy sticks around and just heats up the solar cell. This is no good.

The ETH Zurich team’s idea is to keep the electrons hot until they’ve left the solar cell gridline. In order to force them to retain their extra energy, the team uses nanostructures composed of lead selenide coated with titanium dioxide to expand the energy bandgap the electrons must jump across within the semiconductor material. The larger gap forces the electrons to keep the energy or they won’t be able to pass the barrier.

“Hot film solar cells would be the ultimate solar technology,” says David Norris, professor in the Department of Mechanical Engineering and Technology. “66% conversion efficiency is the theoretical upper limit.”

Wish him luck — 66% sounds pretty impressive from here.

Researchers are working on other ideas as well, including a nanocrystal bilayer for tandem catalysis and thermophotovoltaics. The full text of the article is available in the ETH GLOBE, Nr. 2/June 2011, on page 22.

Source | Picture: Sonnenseite

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