St Paul’s inspires Stanford engineers
ENGINEERS at Stanford have created photovoltaic nanoshells that harness a peculiar physical phenomenon to better trap light. The results could dramatically improve the efficiency of thin-film solar cells while reducing their weight and cost. 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 whispering galleries do for sound. The results, say the engineers, could dramatically reduce materials usage and processing cost.
A whispering gallery, such as the one in St. Paul’s Cathedral, is a circular wall, often beneath a dome, that produces an unusual acoustic effect. A person on one side of the cavernous room can whisper and a person on the other side can hear every word clearly. Harnessing physics in a similar way but for light, a team led by Professor Yi Cui at Stanford’s Department of Materials Science and Engineering, has created tiny hollow spheres of photovoltaic nanocrystalline-silicon, which they call nanoshells. Balls of the crystalline material are dipped in silicon, and then hydrofluoric acid etches away the center of the sphere, leaving a path for light to enter. The more often light circulates around the circumference of the shells the more light can be gradually absorbed by the silicon.
“Nanocrystalline-silicon is a great photovoltaic material. It has a high electrical efficiency and is durable in the harsh sun,” said Shanhui Fan, an associate professor of electrical engineering at Stanford and co-author of the paper. “Both have been challenges for other types of thin solar films.”
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The downfall of nanocrystalline-silicon has been its poor absorption of light, which requires thick layering. The engineers call their spheres nanoshells. Producing the shells takes a bit of engineering magic. The researchers first create tiny balls of silica and coat them with a layer of silicon. They then etch away the glass centre 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 researchers estimate that light circulates around the circumference of the shells a few times, during which energy from the light gets absorbed gradually by the silicon. The longer the shells can keep the light in the material, the better the absorption will be. In measuring light absorption in a single layer of nanoshells, the research showed significantly more absorption over a broader spectrum of light than a flat layer of the silicon deposited side-by-side with the nanoshells.
Further, by depositing two or even three layers of nanoshells atop one another, the team teased the absorption higher still. With a three-layer structure, they were able to achieve total absorption of 75 percent of light in certain important ranges of the solar spectrum.
“The solar film in our paper is made of 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 in the Cui Lab and co-author of the paper.
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