Technological developments
has to roll out some more efficient solar Center of Excellence for Photovoltaics
cells very soon, as 4.9% efficiency is way Research and Education – headed by
below other solar cells available on the Professor Ajeet Rohatgi of the Georgia
market. Tech School of Electrical and Computer
Engineering – to evaluate the surface
Self-cleaning, low-reflectivity treatment with real solar cells.
surface could improve pV cells However, adoption of the
Using two different types of chemical superhydrophobic surface treatment
etching to create features at both the will ultimately depend on its long-term
micron and nanometer size scales, robustness and cost.
researchers at the Georgia Institute of “Because the structures are so small,
Technology have developed a surface
Image shows silicon pyramid structures etched for
they are fairly fragile,” Hess noted.
treatment that could boost the light
two minutes using a hydrogen fluoride/hydrogen “Mechanical abrasion to the surface can
absorption of silicon photovoltaic cells in peroxide/water solution. The resulting structure destroy the superhydrophobicity. We have
two complementary ways.
has roughness at the micron and nanometer scales.
tried to address that here by creating a
The surface treatment increases
(Source: Georgia Tech)
large superhydrophobic surface area so that
absorption both by trapping light in three- small amounts of damage won’t affect the
dimensional structures and by making hydroxide (KOH) solution to etch the overall surface.”
the surfaces self-cleaning—allowing rain or silicon surface. The solution preferentially Large scale cost estimates haven’t yet
dew to wash away the dust and dirt that removes silicon along crystalline planes, been done, but Hess said the additional
can accumulate on photovoltaic arrays. creating micron-scale pyramid structures in etching and vacuum deposition steps
Because of its ability to make water bead the surface. shouldn’t add dramatically to the already
up and roll off, the surface is classified as An e-beam process is then used to complex manufacturing process used for
superhydrophobic. apply nanometer-scale gold particles to fabricating silicon PV cells.
“The more sunlight that goes into the the pyramid structures. Using a solution In addition to photovoltaic cells,
photovoltaic cells and the less that reflects of hydrogen fluoride (HF) and hydrogen the surface treatment could be used to
back, the higher the efficiency can be,” peroxide (H2O2), a metal-assisted etching create anti-bacterial coatings on medical
said C.P. Wong, Regents’ professor in process – with gold as the catalyst – equipment, micro-electromechanical
Georgia Tech’s School of Materials Science produces the nanometer-scale features. The devices that don’t stick together, and
and Engineering. “Our simulations show feature size is controlled by the diameter improved microfluidic devices.
that we can potentially increase the final of the gold particles and the length of time
efficiency of the cells by as much as two the silicon is exposed to the etching. Sencera demonstrates 8.7%
percent with this surface structure.” Finally, the gold is removed with efficient thin-film silicon solar cell
Supported by the National Science a potassium iodide (KI) solution and Sencera, a manufacturer of thin-film
Foundation (NSF) and the National the surface coated with a fluorocarbon silicon based photovoltaic modules, has
Electric Energy Testing Research and material, perfluorooctyl tricholosilane successfully deposited single-junction
Applications Center (NEETRAC) at (PFOS). silicon solar cells with an initial 8.7%
Georgia Tech, the research was described The combination of increased light sunlight to electricity conversion efficiency
March 24th at the Spring 2009 National absorption from the textured surface and under standard test conditions.
Meeting of the American Chemical Society the self-cleaning ability both help boost Sencera’s solar device efficiency
in Salt Lake City. absorption of sunlight hitting the silicon gain was achieved with process and
The silicon etching treatment mimics surface. hardware enhancements to its Viper™
the superhydrophobic surface of the lotus “A normal silicon surface reflects a lot platform. The Viper is a proprietary, fully
leaf, which uses surface roughness at two of the light that comes in, but by doing automated, plasma enhanced chemical
different size scales to create high contact this texturing, the reflection is reduced to vapor deposition (PECVD) manufacturing
angles that encourage water from rain or less than five percent,” said Dennis Hess, platform developed entirely at Sencera.
condensation to bead up and run off. As a professor in the Georgia Tech School of Recent innovations on The Viper have
the water runs off, it carries with it any Chemical and Biomolecular Engineering. improved cell absorption of both blue and
surface dust or dirt – which also doesn’t “As much as 10 percent of the light that red light sections of the solar spectrum
adhere because of the unique surface hits the cells is scattered because of dust resulting in the conversion of more light to
properties. and dirt of the surface. If you can keep the electricity.
In the silicon surface treatment, the cells clean, in principle you can increase “This efficiency milestone validates
two-tier roughness—created with both the efficiency. Even if you only improve our manufacturing platform, and our cost
micron- and nano-scale structures—works this by a few percent, that could make a big model,” said Dr. Rusty Jewett, Sencera’s
in the same way as the lotus leaf, difference.” CEO.” “We intend to expand our present
minimizing contact between the water or Even in desert areas where constant 1 MW research capacity to 35 MW annual
dust and the surface, Wong noted. “When sunlight provides ideal conditions for capacity over the next two quarters.”
a water droplet reaches the surface, it sits photovoltaic arrays, nighttime dew should After two rounds of equity financing,
on top of this two-tier roughness and only provide enough moisture to keep the cells Sencera has fully funded the first
about three percent of it is in contact with clean, Wong said. manufacturing line without incurring debt.
the silicon,” he explained. The research team, which also included Over 75% of the production equipment
Preparation of the superhydrophobic Yonghao Xiu, Shu Zhang and Yan Liu, is and process required to manufacture solar
surface begins with use of a potassium working with Georgia Tech’s University modules is the company’s proprietary
www.globalsolartechnology.com
Global Solar Technology – May/June 2009 – 21
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