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Nanowire’s Novel Properties Could Increase Power Output In Solar Cells

April 5, 2007

Lin-Wang-Wang.jpg

Lead author Lin-Wang Wang

A CRD scientist has designed a nanowire with potential of generating electricity more efficiently than many conventional materials currently used for solar cells.

Using gallium nitride and gallium phosphide, Lin-Wang Wang and two collaborators at the National Renewable Energy Laboratory in Colorado have modeled the nanowire—a coaxial cable—that overcomes several key problems encountered in bulk material solar cells and hydrogen fuel production today.

“Bulk materials have lots of impurities, which compromise the electric current generation in a solar cell application,” Wang said. “The chances of having impurities are less in nanomaterials.”

Silicon crystal, for example, is a common bulk material for thin film solar cell devices, with the film being a few microns thick. Purifying silicon is possible, but that makes the material more expensive for the commercial market.

Wang and his research partners, Yong Zhang and Angelo Mascarenhas, chose gallium nitride and gallium phosphide because the two, when put to work together, yield attractive properties. They also are abundant on earth and resistant to corrosion.

The scientists manipulated the two materials to create a nanowire that could increase power output by making electrons more readily available during electricity production.

Preventing electrons from recombining with “holes” is a key problem for designing better solar cells. Electrons jump to a higher energy state when excited by light (photons), leaving behind holes in the electrons’ former state. The roaming electrons could recombine with the holes and become unavailable as a source of electricity, however.

The 4-nanometer wire, modeled using a NERSC supercomputer, proved to be good at keeping electrons away from the holes and minimize energy loss. Unlike a conventional coaxial cable, which has an insulator to separate the central copper core from the braided copper conductor in the outer shell, the nanowire designed by Wang and his colleagues doesn’t require an insulating material to keep the charges separate between the core and the shell.

Using gallium nitride and gallium phosphide together also creates a structure with a smaller band gap than they would otherwise have separately, leading to better solar cell efficiency. Having a smaller energy gap between the conduction and valence bands means the device can utilize a wider spectrum of the sunlight.

However, since the output voltage also depends on the conduction and valence band gap, there is an optimal band gap for solar cell application. The core-shell structure allows the scientists to manipulate the band gap by changing the thickness ratio between the core and the shell.

The scientists also alternated the use of gallium nitride and gallium phosphide for the core and the shell and found both models can produce similar band gaps, around 1.2 eV, which is similar to the band gap of silicon, and ideal for solar cell application.

Wang’s work was published in the April 5 online edition of Nano Letters by the American Chemical Society.            

The research is far from finished.

“We’ll look for different materials and different geometries, and how we can manipulate them,” Wang said.