Hollow spheres of photovoltaic nanocrystalline silicon exploit a peculiar phenomenon usually associated with sound to better trap light and could dramatically reduce materials usage and processing costs for manufacturing PV panels. “Nanocrystalline silicon is a great photovoltaic material,” said Shanhui Fan, a professor of electrical engineering and a member of the Stanford University team that conducted the research. “It has a high electrical efficiency and is durable in the harsh sun. Both have been challenges for other types of thin solar films.” One setback of nanocrystalline silicon, however, is its relatively poor absorption of light, requiring thick layering that takes a long time to manufacture. To overcome this obstacle, the group used a little engineering magic to create hollow spheres called nanoshells. Their work appeared in Nature Communications. The scanning electron microscope image shows a cross section of a layer of hollow nanoshells made of photovoltaic silicon. The thin spherical structure improves light absorption by trapping the light inside the material, creating what are known as optical whispering galleries. (Image: Yan Yao) First, the researchers created tiny balls of silica — the same stuff glass is made of — and coated them with a layer of silicon. Next, they etched away the glass center using hydrofluoric acid that does not affect the silicon, leaving behind the all-important light-sensitive shell. These shells form optical whispering galleries that capture and recirculate the light. A whispering gallery is a curious acoustic phenomenon that allows a person at one end of a half-domed room to hear what a person at the other end of the gallery has whispered, because the sound was whisked around the semicircular perimeter of the room almost without flaw. “The light gets trapped inside the nanoshells,” said Yi Cui, an associate professor of materials science engineering. “It circulates round and round rather than passing through, and this is very desirable for solar applications.” The engineers estimate that light circulates around the circumference of the shells a few times, during which time the energy from the light is gradually absorbed by the silicon. The longer they can keep the light in the material, the better the absorption will be. “This is a new approach to broadband light absorption. The use of whispering-gallery resonant modes inside nanoshells is very exciting,” said Yan Yao, a postdoctoral researcher. “It not only can lead to better solar cells, but it can be applied in other areas where efficient light absorption is important, such as solar fuels and photodetectors.” Through thick and thin By measuring the light absorption in a single layer of nanoshells, the team discovered significantly more absorption over a broader spectrum of light than a flat layer of the silicon deposited side by side with the nanoshells. “The nanometer spherical shells really hit a sweet spot and maximize the absorption efficiency of the film,” said Jie Yao, a postdoctoral researcher. “The shells both allow light to enter the film easily, and they trap it so as to enhance the absorption in a way larger-scale counterparts cannot.” The researchers found that when they deposited two or three layers of nanoshells on top of each other, they were able to get even higher absorption. With a three-layer structure, they achieved total absorption of 75 percent of light in certain important ranges of the solar spectrum. Clever structure After demonstrating improved absorption, the scientists wanted to demonstrate how their structure would pay dividends beyond light trapping. Because the nanoshell structure uses substantially less material — one-twentieth that of solid nanocrystalline silicon — these nanoshells could allow for more cost-effective production of solar cells from rare or expensive materials. In addition, they can be made more quickly. “The solar film in our paper is made of relatively abundant silicon, but down the road, the reduction in materials afforded by nanoshells could prove important to scaling up the manufacturing of many types of thin film cells, such as those which use rarer materials like tellurium and indium,” said Vijay Narasimhan, a doctoral candidate. The scientists also noted that the new structure is relatively indifferent to the angle of incoming light and that the nanoshell’s layers are thin enough that they can bend and twist without damage. These factors could lead to new applications in situations where achieving optimal incoming angle of the sunlight is not always possible. “This new structure is just the beginning and demonstrates some of exciting potentials for using advanced nanophotonic structures to improve solar cell efficiency,” Fan said. For more information, visit: www.stanford.edu