February 9, 2012
By Andrew Myers
Visitors to Statuary Hall in the U.S. Capitol Building may have experienced a curious acoustic feature that allows a person to whisper softly at one side of the cavernous, half-domed room and for another on the other side to hear every syllable. Sound is whisked around the semi-circular perimeter of the room almost without flaw. The phenomenon is known as a "whispering gallery."
In a paper published in Nature Communications, a team of engineers at Stanford describes how it has created tiny hollow spheres of photovoltaic nanocrystalline-silicon and harnessed physics to do for light what circular rooms do for sound. The results, say the engineers, could dramatically reduce materials usage and processing cost.
“Nanocrystalline-silicon is a great photovoltaic material. It has a high electrical efficiency and is durable in the harsh sun,” said Shanhui Fan, a professor of electrical engineering at Stanford and co-author of the paper. “Both have been challenges for other types of thin solar films.”
The downfall of nanocrystalline-silicon, however, has been its relative poor absorption of light, which requires thick layering that takes a long time to manufacture.
The engineers call their spheres nanoshells. Producing the shells takes a bit of engineering magic. The researchers first create tiny balls of silica — the same stuff glass is made of —and coat them with a layer of silicon. They then etch 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.
“The light gets trapped inside the nanoshells,” said Yi Cui, associate professor of materials science engineering at Stanford and a senior author of the paper. “It circulates round and round rather than passing through and this is very desirable for solar applications.” Cui is also a photon-science faculty member at SLAC.
Visit the Stanford School of Engineering online for the full announcement.