Light-Trapping Photovoltaics

In 1995, finishing her undergraduate degree in physics, Kylie Catchpole decided to take a risk on a field that was nearly moribund: photovoltaics. "There was a sense that I might have difficulty ever being employed," she recalls. But her gamble paid off. In 2006 Catchpole, then a postdoc, discovered something that opened the door to making thin-film solar cells significantly more efficient at converting light into electricity. It's an advance that could help make solar power more competitive with fossil fuels.

Thin-film solar cells, which are made from semiconductor materials like amorphous silicon or cadmium telluride, are cheaper to produce than conventional solar cells, which are made from relatively thick and expensive crystalline wafers of silicon. But they are also less efficient, because if a cell is thinner than the wavelength of incoming light is long, that light is less likely to be absorbed and converted. At just a few micro­meters thick, thin-film cells only weakly absorb wavelengths in the near-infrared part of the spectrum; that energy is lost. The result is that thin-film photovoltaics convert 8 to 12 percent of incoming light to electricity, versus 14 to 19 percent for crystalline silicon. Thus, larger installations are required in order to produce the same amount of electricity, limiting the number of places the technology can be used.

Catchpole, who is now a research fellow at the Australian National University in Canberra, began work on this problem in 2002 at the University of New South Wales in Sydney. "It was a case of 'start at the beginning: can you think of a completely different way to make a solar cell?' " she says. "One of the things I came across was plasmonics--looking at the strange optical properties of metals."

Plasmons are a type of wave that moves through the electrons at the surface of a metal when they are excited by incident light. Others had tried harnessing plasmonic effects to make conventional silicon photovoltaics more efficient, but no one had tried it with thin-film solar cells. Catchpole found that nanoparticles of silver she deposited on the surface of a thin-film silicon solar cell did not reflect back light that fell directly onto them, as would happen with a mirror. Instead, plasmons that formed at the particles' surface deflected the photons so that they bounced back and forth within the cell, allowing longer wavelengths to be absorbed.

Catchpole's experimental devices produce 30 percent more electrical current than conventional thin-film silicon cells. If Catchpole can integrate her nanoparticle technology with the processes used to mass-produce thin films commercially, it could shift the balance of technology used in solar cells. Thin-film photovoltaics could not only gain market share (they currently have just 30 percent of the market in the United States) but sustain growth in the solar industry overall.

Thus far, silicon has been losing out to cadmium telluride as the material of choice for thin-film solar cells. (First Solar, the market leader, is planning gigawatt-scale solar farms that will use cadmium telluride thin-film technology to deliver as much electricity as conventional power stations.) But tellurium is a rare material, and experts question whether the supply will support such grand ambitions. "There just isn't enough tellurium to make a substantial difference to the way the world's energy is produced," says Catchpole. "Silicon is the way to go."