Seeing Red: New Solar Energy Material Can Utilize Infrared Spectrum
by Matthew McDermott, Brooklyn, NY on 08. 4.08
EM Spectrum chart via Wikipedia
As I’m sure the vast majority of TreeHugger readers know, the garden variety, conventional solar cell works by absorbing energy from solar radiation as it falls on the earth. However, what you may not have ever given a second thought to is the fact that the solar energy that actually is usable by the solar cell comes from the visible spectrum. The ultraviolet and infrared portions of the spectrum mean nothing in terms of energy production.
63% : New Material’s Theoretical Efficiency Limit
A Spanish team of researchers has designed a new solar material which can theoretically harness 63% of the sun’s rays. New Scientist explains:
Semiconductor solar cells [TH note: in other words, your ordinary solar cell] produce electricity when photons carrying just the right energy are absorbed by trapped electrons, boosting them into a higher energy level where they can flow freely.Cells can't make electricity from photons with much more or less energy than the right amount. Cells are tuned to visible light because visible photons vary little in energy, but reach earth in very large numbers. But in 1997 other Spanish researchers came up with a way to increase the spectrum cells could use.
Their idea was to create a kind of energy "stepping stone". Instead of having to jump to the higher energy level in one go, electrons can absorb a low-energy photon and then wait at an intermediate energy level until another arrives to let it complete the trip.
By adding titanium and vanadium into the semiconducting material you can alter the material so that it can use the infrared spectrum of light as well as the visible.
It’s because of the utilization of the infrared spectrum that this material has a theoretical absorption limit of 63%, which compares to about 40% for ordinary solar cells.
Real-world Efficiency Will be Lower Than Theoretical Maximum
Researchers are quick to point out, however, that this figure is a theoretical limit. In practice, real-world efficiency of panels using this materials won’t reach that theoretical maximum, but “if the [theoretical] limit is higher, you can presume that the real figure that you will be able to reach will also be higher.”
Before anyone asks, ‘where can I get one?’, the answer is ‘you can’t’. This material has yet to be made into an actual solar cell, though that is a goal for the near future.
via :: New Scientist
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The idea expressed here doesn't seem particularly new. Multiple photons of lower energy (infrared) exciting one higher-energy electron, or conversely, one higher energy photon (ultraviolet) exciting multiple lower-energy electrons to increase theoretical maximum efficiency are ideas that have been around a while. We discussed them in my intro thermodynamics class last semester, after calculating how for single-junction solar cells that produce at most one excited electron per incoming photon, the maximum possible efficiency is about 31%. Multi-junction cells can get higher, of course, because they allow lower energy light to excite lower energy electrons, whereas higher energy light can excite higher energy electrons. As far as I am aware, multi-junction solar cells are not available commercially, but I could be wrong.
But the multiple-photon excitation of a single electron idea has been done in labs before. I'm certainly glad if this discovery indicates one more step towards commercialization of such a technology, but the central idea isn't new.
Triple junction cells have been out for a few years. Try searching "Unisolar".
Maybe coupled with last week MIT development of an increased power absorption using dyes:
http://web.mit.edu/newsoffice/2008/solarcells-0710.html
this could really make the difference we have been waiting for decades...!
WIth an infinite number of junctions a solar cell can theoretically reach over 80% efficiency (http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=8660&DID=193281&action=detail). Like the intermediate band device presented in this article, that theoretical limit is hard to achieve. Current triple junction devices are getting over 40% efficiency and adding more junctions is the most realistic approach to higher efficiencies. Intermediate band type devices using quantum dots in polymer materials have been demonstrated but at very low efficiencies while intermediate bands in, for example, the gallium arsenide material system are yet to be achieved outside of theory.
The maximum energy intensity coming from the sun is at the crest of the visible spectrum, in the green, in the 5770-4920 angstroms range. Anyone focusing on that bandwidth will logically generate the most efficiency from a light to electricity conversion.