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Nanowires boost efficiency of solar cells

ZnO quantum dots could translates fabrication of large-area films making solar panels
Solar cells made from quantum dots could be low-cost, flexible, and easy to make.

But the efficiency with which they convert light into electricity remains too low for practical use.

Capitalising on this, researchers' at the Massachusetts Institute of Technology have combined nanowires with quantum dot solar cells.

The cell's efficiency increases the cells’ efficiency by 35 percent. Quantum dots are semiconductor nanocrystals that absorb different wavelengths of light depending on their size.

Electron micrograph of the nanowire/quantum dot solar cell. (Credit: Joel Jean et al, Bulovic)

Solar cells made from different-sized crystals should absorb light over a much wider range of colours than silicon devices.

What’s more, because quantum dots are made in solution, they could be easily printed or painted onto flexible surfaces. Scientists have calculated that quantum dots could be used to make thin-film solar cells that could convert light to electricity with 15 percent efficiency, the same as commercial silicon devices.

The best-performing quantum dot solar cells consist of a lead sulphide quantum dot layer butted up against a zinc oxide or titanium dioxide layer. The quantum dots absorb light, and electrons created in the process travel to the metal oxide layer to reach the electrical circuit.

The problem is that the quantum dot layer has to be thick enough to absorb light efficiently, but thin enough for the electrons to quickly traverse it.

The MIT researchers, led by electrical engineering and computer science professor Vladimir Bulovic, overcame that trade off by replacing the flat ZnO layer with an array of vertical zinc oxide nanowires.

Nanowires penetrate the quantum dot layer, providing conductive paths for the electrons to follow out to the electrical circuit, says Joel Jean, a graduate student in Bulovic’s group. The researchers published their results in the journal Advanced Materials.  

The researchers start with glass substrates that are coated with indium tin oxide transparent electrodes. They deposit a ZnO layer on top and float the entire susbtrate upside down in an aqueous solution of zinc precursors.

An array of aligned nanowires grows downwards from the ZnO layer. After about an hour, the researchers rinse the substrates. Finally, they deposit PbS quantum dots, which fill up the space between the nanowires, and top it off with a gold electrode.

The nanowires boost the output current of the devices by 50 percent and the efficiency by 35 percent over planar ZnO devices. The overall light-to-electricity conversion efficiency of the new devices is 4.9 percent, among the highest reported for ZnO-based quantum dot solar cells, Jean says.

The researchers believe the efficiency could be further enhanced by using thicker light-absorbing layers and longer nanowires, as well as by controlling the spacing between nanowires to better accommodate quantum dots.

The idea of using ZnO nanowires to increase efficiency in quantum dot solar cells is not new, but this is the first significant implementation of the concept, says Matthew Beard, a senior scientist at the National Renewable Energy Laboratory. “The observed efficiency boost is promising and significant,” he says. “The efficiencies for these types of solar cells are increasing rapidly and this work demonstrates that the improvements in efficiency will continue.”

A key advantage of the nanowire-quantum dot cells, says Jean, is that they could be made on large areas. “One of the main benefits of quantum dots is that they're grown in and deposited from solution,” he adds.

“This translates to fabrication of large-area films, which is necessary for making solar panels. Zinc oxide nanowires are also grown in an aqueous solution process. Scalability should be one of the primary practical advantages of this type of solar cell".

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