"Our technique allows structures based on amorphous silicon just 70 nm thick to absorb 90% of incoming solar radiation," team leader Linyou Cao of North Carolina State University told nanotechweb.org. “Such high light absorption typically requires amorphous silicon slabs that are more than 300 nm thick."

The Sun is by far the best source of clean, potentially limitless energy. Although great strides are being made in advancing solar-cell technology, efficient and inexpensive solar-energy conversion devices are still lacking for the most part.

A good solar cell must satisfy two important conditions. The first is that the active, light-absorbing, material in the cells must be "optically thick" so that every incoming photon can be used to generate electrons and holes. However, the material must not be too physically thick or these photogenerated carriers will recombine before they have time to be extracted and produce useful current. One way to satisfy both of these criteria is to use relatively pure (and therefore expensive) materials as the absorbing layer, an option that solar cells manufacturers are not too keen on.

However, there may be another way: increasing the amount of light absorbed by a thin layer of conventional photovoltaic material by using light-trapping techniques.

Sandwich structures

The researchers did their experiments on sandwiched nanostructures that consisted of a layer of light-absorbing semiconductor (amorphous silicon) placed between two non-absorbing dielectric layers such as ZnO and Si3N4. The structures act as optical antennas that concentrate light in the silicon layer. Optical antennas are structures that collect and focus light and work by exploiting "plasmonic modes", which increase the coupling between light emitted by neighbouring molecules and the antenna.

According to Cao and colleagues, the sandwich structure is able to absorb more light thanks to so-called leaky resonance modes (LMRs) in the semiconductor part of the device. Another important point is that the dielectric part is antireflective, which means that it reflects less sunlight and so improves the overall efficiency of the system since less light is lost. The researchers fabricated their device in such a way as to optimize the thickness of the dielectric shell so that it was as antireflective as possible. They also made sure that the size ratio of the semiconductor in the semiconductor core-dielectric shell structure was larger than 0.5 to preserve the intrinsic LMRs.

Compatible technology

"Although we focused on amorphous silicon in this work, our technique could be applied to a variety of light-absorbing materials, such as other semiconductors (cadmium telluride and copper indium gallium selenide, for example) and organic materials," said Cao. "What is more, the technology is also compatible with standard, thin-film deposition and patterning techniques routinely employed in the industrial manufacture of solar cells today."

The team, which includes researchers from the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, says that it is continuing to optimize its solar cells in the lab. “We would also be very interested in working with industrial partners to commercialize our technique,” added Cao. So, if there are any companies out there that are interested?

The light-trapping technique is described in Nano Letters.