Nanopatterns Improve Thin-Film Solar Cells

Thin-film silicon solar cells are more efficient with tiny holes in the back electrical contact.

Any given solar-cell technology has drawbacks and advantages. Thin-film solar cells, for instance, require less material than traditional solar cells, and are therefore cheaper, lighter, and flexible. And if those thin films are made with amorphous silicon, the cost is further reduced. The problem, however, is that thin-film solar cells made of amorphous silicon tend to have extremely low efficiencies compared to thicker, crystalline silicon photovoltaics.

Rainbow response: The absorption of light with a wavelength of 660 nanometers is greatest for the spacing and diameter of the holes in the dark-red portion of this graphic. The inset shows the layout of the cell. Credit: Harry Atwater

But now, research from Caltech shows that it's possible to increase the efficiency of thin-film amorphous silicon cells 37 percent--from 4.5 percent efficiency to 6.5 percent, which is still significantly lower than commercial crystalline silicon cells that achieve efficiencies of more than 30 percent--by simply adding a pattern of nanoscale holes to the electrical contact on the back side of cells. Importantly, the research, led by Harry Atwater, professor of applied physics and materials science at Caltech, appears to be practical for scaling up to large-scale production of the cells.

A number of researchers and startups are exploring thin-film solar cells made of nonsilicon materials. But, says Atwater, these materials are relatively rare, and as such, they aren't practical for widespread use. "These represent challenges at extremely large scale," he says.

Silicon has the great advantage of being abundant and of having a long history in the manufacturing of electronics. But as a thin solar cell, silicon is less than ideal. There is a mismatch between the distance it takes for photons to be absorbed in silicon and the distance traveled by electrons that produce electrical current. Essentially, electrons knocked out of position by photons tend to spring back to their spots before they can be collected, resulting in low efficiencies in converting sunlight to electricity. But, if the optical absorption can be improved, then more electrons overall can be collected, increasing efficiencies.

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Researchers and companies are exploring various options for improving efficiencies in such cells. For example, StarSolar, an MIT spinoff in Cambridge, MA, is exploring photonic crystals, structures that reflect light many times within the solar cell to increase the chance it has to produce electrical current. But so far, the approach appears to be difficult to scale up.

Atwater's approach targets the back side of the solar cell, the metal electrical contact that sits below layers of "active" silicon material where photons are absorbed. Instead of using a grating to produce multiple internal reflections, he's using an array of holes 225 nanometers in diameter. When light hits metal with an array of holes at this scale, some interesting physics occurs. The energy from the light is essentially trapped onto the two-dimensional wave on the surface of the metal. The electrons in these surface waves, called plasmons, are easier to use for generating an electrical current than those in silicon, which quickly snap back into place.