Flexible, Nanowire Solar Cells

Exotic materials and cheaper substrates could lead to better photovoltaics.

Nano solar: A side profile of gallium arsenide nanowires growing on a silicon substrate. The nanowires grow upward from the substrate, creating a surface that's able to absorb more sunlight than a flat surface is. Credit: McMaster University

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Researchers at McMaster University, in Ontario, say that they have grown light-absorbing nanowires made of high-performance photovoltaic materials on thin but highly durable carbon-nanotube fabric. They've also harvested similar nanowires from reusable substrates and embedded the tiny particles in flexible polyester film. Both approaches, they argue, could lead to solar cells that are both flexible and cheaper than today's photovoltaics.

Now the researchers' challenge is to improve the efficiency of the cells without increasing cost. The research team, led by Ray LaPierre, a professor in the university's engineering physics department, has been given three years to achieve its goals--backed by about $600,000 from the Ontario government and private-sector research partner Cleanfield Energy, a Toronto-area developer of wind and solar technologies.

LaPierre says that the aim is to produce flexible, affordable solar cells composed of Group III-V nanowires that, within five years, will achieve a conversion efficiency of 20 percent. Longer term, he says, it's theoretically possible to achieve 40 percent efficiency, given the superior ability of such materials to absorb energy from sunlight and the light-trapping nature of nanowire structures. By comparison, current thin-film technologies offer efficiencies of between 6 and 9 percent.

"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."

In addition to reducing costs by using less active material, LaPierre's team can also cut the cost of the substrate that the nanowires are grown on. LaPierre's team doesn't require an expensive Group III-V substrate. It has successfully grown its nanowires on substrates made of more plentiful and relatively cheaper silicon. It's also working on using even lower cost substrates made of glass, which would be ideal for building-integrated PV applications. Flexible substrates such as polymer films and carbon nanotube fabric could be useful for many applications, and could be manufactured with inexpensive roll-to-roll processes.

To further drive down costs, the focus on cheaper substrates will be complemented by an attempt to replace the gold catalysts used to grow the nanowires with aluminum, although more work in this area is needed to achieve the necessary nanowire densities. "We have grown nanowires from aluminum, but gold works much better," says LaPierre.

Charles Lieber, a professor of chemistry at Harvard University who has created single light-harvesting nanowires made of silicon, says that his team is also pursuing the use of other materials for making nanowires. "But there are many challenges in going from nanowire to photovoltaic," says Lieber. He adds that comparison of approaches is difficult without data on the energy-conversion properties of each material.

Nathan Lewis, a professor of chemistry at the California Institute of Technology and an expert on nanowire structures, says that it's too early to say which approach and materials are best. "We know nanowires work in bulk form, but we don't know if you can make high-purity, high-quality nanowires and control all their electrical properties," says Lewis. "There's no theory that one works better than the other. It's just a question of getting any of them to work."

It's still early days for McMaster, which in prototypes has only achieved low efficiencies--"where silicon PV was in the 1950s," says LaPierre. But he's optimistic that the higher-efficiency materials and the approach chosen will get results.

http://www.technologyreview.com/Nanotech/20163/