This 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. Credit: Yan Yao |
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.
Whispering galleries
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.”
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 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 post-doctoral researcher in the Cui Lab and a co-lead
author of the paper. “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
In
measuring light absorption in a single layer of nanoshells, the team
showed 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. 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. That is the power of
nanotechnology,” said Jie Yao, a post-doctoral researcher in Cui’s lab
and co-lead author of the paper.
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% of light in
certain important ranges of the solar spectrum.
Clever structure
Having
demonstrated improved absorption, the engineers went on to show how
their clever structure will pay dividends beyond the mere trapping of
light.
First,
nanoshells can be made quickly. “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,” said
Yan.
The nanoshell structure likewise uses substantially less material, one-twentieth that of solid nanocrystalline-silicon.
“A
twentieth of the material, of course, costs one-twentieth and weighs
one-twentieth what a solid layer does,” said Jie. “This might allow us
to cost effectively produce better-performing solar cells of rare or
expensive materials.”
“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 in the Cui Lab and
co-author of the paper.
Finally,
the nanoshells are relatively indifferent to the angle of incoming
light and the layers are thin enough that they can bend and twist
without damage. These factors might open up an array of new applications
in situations where achieving optimal incoming angle of the sun light
is not always possible. Imagine solar sails on the high seas or
photovoltaic clothing for mountain climbing.
“This
new structure is just the beginning and demonstrates some of exciting
potentials for using advanced nanophotonic structures to improve solar
cell efficiency,” said Shanhui Fan.
This
work was funded as part of the Center for Nanostructuring for Efficient
Energy Conversion (CNEEC) at Stanford University, and by awards from
the U.S. Department of Energy (DOE). Yi Cui is a joint faculty of
Stanford University and DOE’s SLAC National Accelerator Laboratory.