06 February 2008

Look At All That Surface Area! NextGen Nanowire: Solar Cells "On Steroids"

New gallium arsenide nanowires grown on a silicon substrate by McMaster University researchers promise to deliver a broader spectrum photovoltaic return.
"Most of the nanowire work to date has focused on silicon nanowires," says LaPierre, explaining that McMaster's approach relies on nanowires containing multiple layers of exotic Group III-V materials, such as gallium arsenide, indium gallium phosphide, aluminum gallium arsenide, and gallium arsenide phosphide. "It creates tandem or multi-junction solar cells that can absorb a greater range of the [light] spectrum, compared to what you could achieve with silicon. That's one of the major unique aspects of our work."

When used in conventional crystalline solar cells, Group III-V materials are known to have much higher efficiencies than silicon, but the great cost of these materials has limited their use. LaPierre says that cost becomes less of an issue with nanowires because so little material is needed. This is in part because the structure of the nanowires provides a more efficient way to absorb light and extract electrons freed by the light. In conventional solar cells, which are made of slabs of crystalline material, greater thickness means better light absorption, but it also means that it's more difficult for electrons to escape. This forced trade-off is overcome with nanowires. Each nanowire is 10 to 100 nanometers wide and up to five microns long. Their length maximizes absorption, but their nanoscale width permits a much freer movement and collection of electrons. "The direction in which you absorb the light is essentially perpendicular to how you collect electricity," explains LaPierre. "The dilemma is overcome."___TechReview

The sheer surface area is amazing. Animal organs such as lungs, kidneys, intestines etc. take advantage of huge surface areas for gas exchange, nutrient and fluid absorption, waste elimination, etc. Nano-scientists are learning how to do the same thing in a number of devices--including the above photovoltaics approach.

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