Photovoltaic nanoshell "whispering galleries" trap light for more efficient solar cells


February 8, 2012

A scanning electron microscope image of a single layer of the nanocrystalline-silicon nanoshells

A scanning electron microscope image of a single layer of the nanocrystalline-silicon nanoshells

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For those unfamiliar with the term, a "whispering gallery" is a round room designed in such a way that sound is carried around its perimeter - this allows a person standing on one side to hear words whispered by a person on the other. Now, scientists from Stanford University have developed a new type of photovoltaic material, that essentially does for sunlight what whispering galleries do for sound. Not only does the material have a structure that circulates light entering it, but it could also result in cheaper, less fragile, and less angle-sensitive solar panels.

The new material consists of tiny hollow spheres, made out of nanocrystalline-silicon. While nanocrystalline-silicon has good electrical efficiency and is able to stand up to the damaging effects of sunlight, is isn't particularly good at absorbing light - in past attempts at using it for photovoltaics, it has had to be thickly layered, which has in turn resulted in long manufacturing times. That's where the spheres - known as nanoshells - come into play.

To make the nanoshells, the scientists coated individual balls of silica with silicon, then used hydrofluoric acid to etch away the silica in the center. This left them with the hollow transparent nanoshells.

When sunlight enters one of them, instead of passing straight through, it gets trapped and is circulated several times within the nanoshell. This is a good thing, as the longer the light stays in contact with the nanocrystalline-silicon, the more energy the material can absorb.

This image from a computer simulation shows how waves of light (in red/orange, traveling from the top of image to the bottom) strike a layer of nanoshells and how the light resonates within the shell structure (in red)

In a side-by-side comparison with a flat layer of silicon, a layer of the nanoshells showed "significantly more absorption over a broader spectrum of light." When the nanoshells were subsequently stacked three layers deep, that improvement went up to 75 percent for certain important ranges of the solar spectrum.

Not only are they more efficient than nanocrystalline-silicon film, but they are easier to make - according to team member Yan Yao, "A micron-thick flat film of solid nanocrystalline-silicon can take a few hours to deposit, while nanoshells achieving similar light absorption take just minutes." They also require only about one-twentieth the amount of material, which translates into one-twentieth the cost and weight, too. This point could be particularly significant if the technology were used with other, rarer substances, such as tellurium or indium.

Additionally, the efficiency of the nanoshells isn't greatly affected by their angle to the Sun, so they could be used in locations where an optimal angle isn't always possible. Finally, layers of them are thin enough that they can stand up to twisting and bending, so they could possibly even be built into items such as sails or clothing.

A paper on the research was recently published in the journal Nature Communications.

Source: Stanford University

About the Author
Ben Coxworth An experienced freelance writer, videographer and television producer, Ben's interest in all forms of innovation is particularly fanatical when it comes to human-powered transportation, film-making gear, environmentally-friendly technologies and anything that's designed to go underwater. He lives in Edmonton, Alberta, where he spends a lot of time going over the handlebars of his mountain bike, hanging out in off-leash parks, and wishing the Pacific Ocean wasn't so far away. All articles by Ben Coxworth

This sounds really great. I really hope this isn\'t another one of those \"breakthroughs\" that never make it out of the laboratory and into the real world. We\'ve been promised photovoltaics for everyone for four decades now.


I hope you\'re right, too, Gadgetter - however, I first bought a 32w PV panel back in 1980 for 450 bucks ... I recently purchased a Kyocera 135w job for only $405 ... we didn\'t quite waste the 30 years ... I\'m getting impatient, too, at age 73 ...

Jansen Estrup

hydrofluoric acid, Google it! extremely nasty stuff. I\'m sure they have a safe way of working with the stuff.

Denis Klanac

Hector: I\'m sure Gadgeteer meant we have been promised a \"competitive\" priced panel was \"coming soon\" for 40 years. When it comes I would buy an electric car and go off the grid.


Building these panels in the USA and getting them out to homes asap will be a blessing! Fuel prices are so high, I may have to migrate to india to apply for one of our out sourced jobs.


Until more efficient and cheaper panels make it to market it is still cheaper to make your own from readily obtained cells online and the local hardware store. If you can use simple tools and a soldering iron and have help to mount them on the roof you can avoid the huge initial cost and lengthy payback period. The major expense item and least mentioned in a whole house system is the inverter if you feed the grid in the day and drawdown at night and don\'t want a shed full of batteries.

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