Nanorod Technology Improves New Types of Solar Cells

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The next time you paint your home, you may want to consider a shade made with photovoltaic cells.

Berkeley scientists have developed solar cells that can be painted onto any surface to provide a low-energy source.

"We have made a hybrid cell in which electrons are transported by inorganic nanorods, and holes are transported by a semiconductor polymer," said Paul Alivisatos, a professor of chemistry at UC Berkeley and a member of the Materials Science Division of Lawrence Berkeley National Laboratory.

The photovoltaic cells developed by Alivisatos and his group, which currently produce only .7 volts, are hybrid cells composed of both organic and inorganic material.

Nanorods-microscopically small conducting rods-made of a chemical compound known as cadmium selenide are dispersed in a type of plastic called poly-3-hexythiophene, which is better known as P3HT.

The cadmium selenide and P3HT mixture is then coated onto a transparent electrode, forming a layer 200 nanometers, or 200 billionths of a meter, thick.

Organic solar cell researchers have been challenged by the problem of efficient electron transport. For the solar cell to work, electrons generated by the absorption of light are transported from one region of the cell to another to create electricity.

Fully organic solar cells are engineered to have networked pathways which are used for electron transport.

Unfortunately, problems arise because incomplete pathways are common and cause lower efficiency for the solar cell. Alivisatos solved this problem by using inorganic nanorods, like those used in typical solar cells, as the electron carriers.

"The inorganic rods are small enough that they can still be cast from solution, so in a way we aim to get the best both of both worlds: the superior electrical characteristics of inorganics, and the ease and low cost of processing of organics," Alivisatos said.

Alivisatos' new solar cells are also much more cost-effective than traditional solar cells, as they require less heat energy during production.

"Inorganic solar cells require high-temperature processing at around 2500 degrees Celsius," said Janke Dittmer, a postdoctoral fellow in the group. "They also need a high-powered vacuum for the electrodes, and this makes things awkward for larger areas. Our solar cells are processed at low temperatures, from solution."

The lower temperatures also allow for greater flexibility in the application of the solar cells.

"The high temperatures required for typical inorganic solar cells would melt plastic substrates," Dittmer said. "The hybrid cells can literally be painted or screen-printed onto the substrate."

The research has gained praise from other scientists who see the study as a step toward making solar energy a viable alternative to traditional energy sources.

"There has been much interest recently in the possibility of making cheap, plastic solar cells," said Keith Barnham, a professor of physics at Imperial College in London.

"However, the efficiencies of these polymer-based cells are currently far too low for commercial exploitation. I think this hybrid approach is a most promising way to achieve the efficiencies necessary to make plastic solar cells commercially viable. It would help to make solar electricity competitive with fossil fuels."

However, the research team must still overcome another hurdle before the technology can be used in consumer goods. The scientists must still improve the efficiency of the solar cell so that it meets commercial standards and becomes viable for consumer production.

"10 percent efficiency is when it makes sense to use on a roof," Dittmer said. "We're at 1.7 percent now, and in the next couple of years, we should reach five percent. We're more than one conceptual breakthrough away from 10 percent,, which should be in about a decade."

The research has also sparked interest in nanotechnology companies. Nanosys Incorporated, a firm co-founded by Alivisatos, is interested in developing the technology further.

"Regardless of the success or failure of this particular device,  advances in nanotechnology may well lead to improvements in solar cells in the future." Alivisatos said.


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