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BEST-L  November 2008

BEST-L November 2008

Subject:

Solar improvement

From:

"Crystal D. Hartman" <[log in to unmask]>

Reply-To:

Crystal D. Hartman

Date:

Sun, 23 Nov 2008 23:28:26 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (107 lines)

http://nanotechwire.com/news.asp?nid=6846


Researchers at Rensselaer Polytechnic Institute have discovered 
and demonstrated a new method for overcoming two major hurdles 
facing solar energy. By developing a new antireflective coating 
that boosts the amount of sunlight captured by solar panels and 
allows those panels to absorb the entire solar spectrum from 
nearly any angle, the research team has moved academia and 
industry closer to realizing high-efficiency, cost-effective solar 
power.

???To get maximum efficiency when converting solar power into 
electricity, you want a solar panel that can absorb nearly every 
single photon of light, regardless of the sun???s position in the 
sky,??? said Shawn-Yu Lin, professor of physics at Rensselaer and 
a member of the university???s Future Chips Constellation, who led 
the research project. ???Our new antireflective coating makes this 
possible.???

Results of the year-long project are explained in the paper 
???Realization of a Near Perfect Antireflection Coating for 
Silicon Solar Energy,??? published this week by the journal Optics 
Letters.

An untreated silicon solar cell only absorbs 67.4 percent of 
sunlight shone upon it ??? meaning that nearly one-third of that 
sunlight is reflected away and thus unharvestable. From an 
economic and efficiency perspective, this unharvested light is 
wasted potential and a major barrier hampering the proliferation 
and widespread adoption of solar power.

After a silicon surface was treated with Lin???s new 
nanoengineered reflective coating, however, the material absorbed 
96.21 percent of sunlight shone upon it ??? meaning that only 3.79 
percent of the sunlight was reflected and unharvested. This huge 
gain in absorption was consistent across the entire spectrum of 
sunlight, from UV to visible light and infrared, and moves solar 
power a significant step forward toward economic viability.

Lin???s new coating also successfully tackles the tricky challenge 
of angles.

Most surfaces and coatings are designed to absorb light ??? i.e., 
be antireflective ??? and transmit light ??? i.e., allow the light 
to pass through it ??? from a specific range of angles. Eyeglass 
lenses, for example, will absorb and transmit quite a bit of light 
from a light source directly in front of them, but those same 
lenses would absorb and transmit considerably less light if the 
light source were off to the side or on the wearer???s periphery.

This same is true of conventional solar panels, which is why some 
industrial solar arrays are mechanized to slowly move throughout 
the day so their panels are perfectly aligned with the sun???s 
position in the sky. Without this automated movement, the panels 
would not be optimally positioned and would therefore absorb less 
sunlight. The tradeoff for this increased efficiency, however, is 
the energy needed to power the automation system, the cost of 
upkeeping this system, and the possibility of errors or 
misalignment.

Lin???s discovery could antiquate these automated solar arrays, as 
his antireflective coating absorbs sunlight evenly and equally 
from all angles. This means that a stationary solar panel treated 
with the coating would absorb 96.21 percent of sunlight no matter 
the position of the sun in the sky. So along with significantly 
better absorption of sunlight, Lin???s discovery could also enable 
a new generation of stationary, more cost-efficient solar arrays.

???At the beginning of the project, we asked ???would it be 
possible to create a single antireflective structure that can work 
from all angles???? Then we attacked the problem from a 
fundamental perspective, tested and fine-tuned our theory, and 
created a working device,??? Lin said. Rensselaer physics graduate 
student Mei-Ling Kuo played a key role in the investigations.

Typical antireflective coatings are engineered to transmit light 
of one particular wavelength. Lin???s new coating stacks seven of 
these layers, one on top of the other, in such a way that each 
layer enhances the antireflective properties of the layer below 
it. These additional layers also help to ???bend??? the flow of 
sunlight to an angle that augments the coating???s antireflective 
properties. This means that each layer not only transmits 
sunlight, it also helps to capture any light that may have 
otherwise been reflected off of the layers below it.

The seven layers, each with a height of 50 nanometers to 100 
nanometers, are made up of silicon dioxide and titanium dioxide 
nanorods positioned at an oblique angle ??? each layer looks and 
functions similar to a dense forest where sunlight is 
???captured??? between the trees. The nanorods were attached to a 
silicon substrate via chemical vapor disposition, and Lin said the 
new coating can be affixed to nearly any photovoltaic materials 
for use in solar cells, including III-V multi-junction and cadmium 
telluride.

Along with Lin and Kuo, co-authors of the paper include E. Fred 
Schubert, Wellfleet Senior Constellation Professor of Future Chips 
at Rensselaer; Research Assistant Professor Jong Kyu Kim; physics 
graduate student David Poxson; and electrical engineering graduate 
student Frank Mont.

Funding for the project was provided by the U.S. Department of 
Energy???s Office of Basic Energy Sciences, as well as the U.S. 
Air Force Office of Scientific Research.

~Crystal D. Hartman

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