A new nanostructure that can absorb light at any polarization and across the entire visible spectrum has been made by physicists in the US. The “plasmonic” structure has been used to convert absorbed light into heat and might be able to improve the efficiency of solar cells.

Solar cells may be a tempting green-energy technology, but they remain much less cost-effective than fossil-fuel energy. Most of the high cost of solar cells resides in the production cost of silicon – the most commonly used semiconductor. For this reason, industry is interested in solar cells made of far thinner films – around 1 µm, rather than 300 µm – so that less material is required.

The problem is that thinner conventional solar cells are not efficient. Light of a longer wavelength disperses its energy over a greater distance; so the thinner the cell, the less light at the red end of the spectrum is absorbed. To combat this, scientists have begun to investigate so-called plasmonic nanostructures, which are excellent at scattering light. By placing such nanostructures on top of a solar cell, light rays travelling downwards can be turned 90° so they travel horizontally. As a result, the entire width of the cell can be used for absorption.

Wavelength and polarization are limited

The plasmonic nanostructures created so far, however, have not fitted the bill because they are able to scatter light only at a narrow range of wavelengths – or for just a single polarization. Now, Harry Atwater and colleagues at the California Institute of Technology have come up with a plasmonic design that can scatter and absorb light independent of polarization and for all wavelengths of visible light.

The design consists of rows of silver trapezoids, each 300 nm long and joined together to look like the teeth of a saw. As each trapezoid has a varying width of 40–120 nm, the absorption works over a range of wavelengths: from blue light at 400 nm to red light at 700 nm. On its own, though, this design would only work for a single polarization; to make it polarization independent, Atwater and colleagues cross the trapezoidal rows with another set of trapezoidal rows at 90°.

At just 260 nm thick, the nanostructure has the potential to be a good plasmonic layer for an ultrathin solar cell. However, this is not possible yet because the metal and dielectric construction converts the absorbed photons to heat. This is in contrast to a semiconductor, which could convert the light into electricity. However, group member Koray Aydin, who is now at Northwestern University, says he and his colleagues have ideas for enhancing light absorption in semiconductors using the design principles they have learned.

Semiconductors are next

“The next step is obviously [to] demonstrate enhancements in semiconductors,” says Aydin. “First looking at how we can increase the light absorption in semiconductors, [and] eventually we [will be] interested in actual solar cells.”

Jeremy Baumberg, a nanophotonics expert at the University of Cambridge in the UK, is sceptical that applications in solar cells will be straightforward. “The designs [would have to be] changed so that the light is then absorbed not in the silver but in the semiconductor,” he says. “[Therefore the] effectiveness remains completely unclear. So, the results take us further in extending the colour range that gets absorbs, but [not] whether it is useful.”

Still, Aydin notes that there could be other applications in addition to solar cells – perhaps simply as a material that is very thin and very good at blocking the transmission of light.

The research was published last week in Nature Communications.