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Blu-Ray Solar Cells
December 12, 2014
Perhaps you're an aspiring
artist with a brilliant
idea. You decide to produce a
canvas that's entirely
black. Unfortunately, you find that
this has already been done. Undeterred, you have the idea of a black canvas with your
fingerprint, rendered in
white paint, along an edge. Better yet, this will be a series in which black canvases will display fingerprints of different
colors.
After the paint dries on the first canvas, you're disappointed to see that the black isn't completely black. It seems to be somewhat
gray, so you
curse the paint
manufacturer and try again with a different
brand of paint. Same thing! Disgusted, with your artistic aspirations completely ruined, you decide to follow in your
father's footsteps and become a
physicist.
In your study of
physics, you find that the
culprit in this case wasn't shoddy paint manufacture, it was a basic
law of
optics. That's the principle that there's always
reflection of
light at an interface between two different
dielectric constants. In your case, the difference in dielectric constant between the layer of black paint and air. The surface of any
material with a
refractive index n different from
air (n=1) will necessarily reflect light according to the
reflectance equation,
where
n1 and
n2 are the refractive indices of air and your material (in any order). I wrote about black and white materials, and how reflection is important to the qualities of black and white, in several previous articles (
Whiter Whites, September 5, 2014 ,
Thin and Black, August 2, 2013,
Paint it Black, February 13, 2013,
Very White and Very Black, November 23, 2011, and
White Roofs, March 19, 2012).
Reflectance is a problem for
solar cells, since reflected light is lost light. As I wrote in a
previous article (Light Trap, January 7, 2013), one way to increase solar cell
efficiency is to add a light-trapping layer to the surface. This was done in 2012 by a team of
Electrical engineers at
Princeton University who added a
nanoscale aperture mesh of 175 nm diameter to an
organic photovoltaic cell (see figure).[1-2]
A nanoscale cavity mesh
The holes are 175 nm in diameter, separated by 25 nm, in a 30 nm thick gold film.
(Princeton University Image))
In order to stretch
research funding,
scientists are always looking for inexpensive ways to accomplish
laboratory tasks. One example is the use of
adhesive tape to exfoliate
graphene sheets from
graphite, a simple operation that led to the 2010
Nobel Prize in Physics. One common
commercial article containing
nanoscale holes is the
Blu-ray Disc, designed for
optical media applications with 405
nm light.
The
digital data on a Blu-ray Disc is encoded with 150 nm pits in a
spiral pattern with 320 nm pitch. The data on the disc are
compressed, and there's
a principle that ideally compressed data is indistinguishable from
random noise. Because of the data compression, the pits have a nearly random distribution on the Disc.
Materials scientists,
electrical engineers, and
mechanical engineers from
Northwestern University (Evanston, Illinois) decided to use a Blu-ray Disc to make a light-trapping layer for a solar cell.[3-6] Says
Jiaxing Huang, an
associate professor of
materials science and engineering in Northwestern's
McCormick School of Engineering and Applied Science,
"We had a hunch that Blu-ray discs might work for improving solar cells, and, to our delight, we found the existing patterns are already very good... It's as if electrical engineers and computer scientists developing the Blu-ray technology have been subconsciously doing our jobs, too."[4]
The Northwestern University Blu-Ray solar team. From left to right, Dongning Guo, Cheng Sun, Chen Wang, Alexander Smith and Jiaxing Huang. (Northwestern University photo.)
To produce the light-trapping layer for their solar cell, the research team
delaminated the Blu-Ray Disc to expose the bit pattern, and then created a
negative mold. The mold was used to transfer the pattern to the
polymer photoactive layer by pressing, then an
electrode was applied by
evaporation.[5-6] Originally
experimenting on blank Blu-Ray Discs, which have a
periodic pattern of nanoscale pits, it was found that light-trapping wasn't effective.[4-6] Blu-Ray Discs with
video content, however, demonstrated increased efficiency.
Shaorong Liu, a
database engineer at
IBM, suggested to her
spouse, Jiaxing Huang, that the data compression used in the digital encoding was the likely difference. At that point,
Dongning Guo, an associate professor of
electrical engineering and computer science at Northwestern, joined the team, and he confirmed that the compression results in a quasi-random array of pits and islands with feature sizes between 150 and 525 nanometers. This range works well for light-trapping applications over
solar wavelengths.[2-3,5]
It's the pits!
The pits in a Blu-Ray Disc with compressed digital content are about 75 nm wide and range from 150-525 nm long.
(Illustration by the author, rendered using Inkscape.)
Using the Blu-Ray Disc pattern increased the
solar absorption of their solar cell by 21.8%, as compared to a non-patterned cell.[4,6] Since the polymer photovoltaic used is not that efficient, the overall conversion efficiency was raised to just 12%, but the technique can be applied to other types of solar cells.[3-5] This research was funded by the
National Science Foundation.[4]
References:
- Stephen Y. Chou and Wei Ding, "Ultrathin, high-efficiency, broad-band, omni-acceptance, organic solar cells enhanced by plasmonic cavity with subwavelength hole array," Optics Express, vol. 21, no. S1 (Jan. 14, 2013), pp. A60-A76.
- John Sullivan, "Tiny structure gives big boost to solar power," Princeton University Press Release, December 5, 2012.
- Alexander J. Smith, Chen Wang, Dongning Guo, Cheng Sun, and Jiaxing Huang, "Repurposing Blu-ray movie discs as quasi-random nanoimprinting templates for photon management," Nature Communications, Article No. 6517 (November 25, 2014).
- Megan Fellman, "Blu-Ray Disc Can be Used to Improve Solar Cell Performance," Northwestern University Press Release, November 25, 2014.
- Evan Ackerman, "Blu-ray Discs Spin Their Way Into Making Solar Cells More Efficient," IEEE Spectrum, November 25, 2014.
- Richard Chirgwin, "Boffins find Jackie Chan's SUPERCOP is good for something - Blu-Ray's pit and peak pattern supercharges solar cells," The Register (UK), November 26, 2014.
Permanent Link to this article
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