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."