From Copper to Solar Cells

Friday, April 25, 2014 @ 10:04 AM gHale

By looking at a piece of material in cross section, it is possible to see how copper sprouts grass-like nanowires that could one day be made into solar cells and also make them more cost-effective.

Washington University in St. Louis assistant professor of materials science and an expert in working with nanomaterials, Parag Banerjee, PhD, and Fei Wu, graduate research assistant, and Yoon Myung, PhD, a postdoctoral research associate, worked with copper foil, a simple material similar to household aluminum foil.

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When most metals heat up, they form a thick metal oxide film. However, a few metals, such as copper, iron and zinc, grow grass-like structures known as nanowires, which are long, cylindrical structures a few hundred nanometers wide by many microns tall. They set out to determine how the nanowires grow.

“Other researchers look at these wires from the top down,” Banerjee said. “We wanted to do something different, so we broke our sample and looked at it from the side view to see if we got different information, and we did.”

The team used Raman spectroscopy, a technique that uses light from a laser beam to interact with molecular vibrations or other movements.

They found an underlying thick film consisting of two different copper oxides (CuO and Cu2O) that had narrow, vertical columns of grains running through them. In between these columns, they found grain boundaries that acted as arteries through which the copper from the underlying layer was pushing through when heat ended up applied, creating the nanowires.

“We’re now playing with this ionic transport mechanism, turning it on and off and seeing if we can get some different forms of wires,” said Banerjee, who runs the Laboratory for Emerging and Applied Nanomaterials (L.E.A.N.).

Like solar cells, the nanowires are single crystal in structure, or a continuous piece of material with no grain boundaries, Banerjee said.

“If we could take these and study some of the basic optical and electronic properties, we could potentially make solar cells,” he said. “In terms of optical properties, copper oxides are well-positioned to become a solar energy harvesting material.”

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