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Research Promises Full-Spectrum Solar Cell
nphillips writes "As is being here reported here, a serendipitous discovery was made that a single system of alloys incorporating indium, gallium, and nitrogen can convert virtually the full spectrum of sunlight -- from the near infrared to the far ultraviolet -- to electrical current. For if solar cells can be made with this alloy, they promise to be rugged, relatively inexpensive -- and the most efficient ever created. Solar cells so efficient and so relatively cheap could revolutionize the use of solar power not just in space but on Earth."
As I understand it, UV light hits the earth at all hours.
UV or IR? I've never head of UV light reaching the dark side of the planet in any quantity (other than whatever light you get from stars & off the moon). IR (mostly thermal energy) is usually quite abundant, though.
Got any references?
=Smidge=
What the article didn't mention is that this material could be the killer app for orbital manufacturing. The value of the cells would justify lofting the raw metals into space to form into enormous panels in the open vacuum, free of contaminants. Solar cells with 50% efficiency would compete economically against fossil fuels.
As I understand it, UV light hits the earth at all hours.
Does anyone know how much UV hits the earth during the night?
Almost none. Virtually all of the light that strikes Earth comes from the sun.
As another poster pointed out, you may be confusing this with the mid-IR glow that warm objects (including the ground and the air) give off. The amounts of energy involved are very low, and room-temperature thermal IR is difficult to convert to electricity efficiently.
Any solar power scheme (and so any photovoltaic scheme) has to have enough storage capacity to power the load overnight. Ideally, it should be able to provide power for several days, in case of cloud cover/rain/whatever. This is why most home-powering schemes involve large battery arrays. A city-powering solar plant would probably use fuel cells (energy density is much higher, and there are off-the-shelf models of power-plant scale already available and in use).
Gallium is currently around $640/Kg
Indium is about $147/Kg
Nitrogen, as far as I know, can be obtained quite cheaply.
For comparison, silicon is about $1/Kg
commodity info
current commericial solar cell designs actually absorb a large frequency range of the sun's EM energy; little is reflected unless the angle of incidence is oblique enough. the issue is that only a certain frequency range can push electrons into the conduction layer of the material (i.e. produce electricity). the remainder of this absorbed energy will mostly increase the amount of heat in the material. this heat will either be conducted away or be re-radiated as longer wavelength photons.
What the article doesn't happen to mention is that InAs (Indium Arsenide) was believed to have a bandgap around 1.6eV (not sure the exact number) and it's now known to be somewhere in the range of ~0.6eV. The article also don't mention phosphide compounds, which are far bigger in research and industry right now.
Fact is, nitrides are bastards to grow. You have to use gas-sources (instead of solid sources that most MBE-ers prefer). There's also no current way to make a nitride-based substrate, which means growing (typically) on sapphire or other lattice mis-matched substrates (GaAs, InP, etc). These lead to HUGE dislocation densities that greatly impact performace.
Now, that doesn't mean this can't be done. And in fact, magic is being done all the time in the world of research. But nitrides aren't going to be realized for some time. Not at least until other technologies pan out first (phosphides and the like). Those are cheaper to grow and allow for much lower defect densities.
Just so you folk's know I'm not just talking out of my ars--do some research and look up some papers. Authors to look for are Steve Ringel (OSU), Gene Fitzgerald (MIT), John Carlin (OSU), Sumitomo (Japan, somewhere), and by-far Yamaguchi (Toyota Technological Institute). Read up on these folks' work and those around them--they know space-based photovoltaics better than most, and very, very, very, very few are working with nitrides right now. Not that it's not going to eventually happen--but until defect densities get low enough, there's simply no way to make a good solar cell (read up on the previous authors' works if you want the theoretical calculations as to why).
Long, cute, or funny Sigs are just another form of over compensation, used by geeks, nerdz, etc.
This is an interesting twist on the subject!
The answer is no, as suggested by the Second Law. To see why, you need an account of how solar cells actually work.
Infalling light is absorbed by the material, by dislodging electrons either from a bound orbit, or a semi-free state like a metallic subtrate or semiconductor. The electron absorbs the photon, re-emitting some of the light as another, lower frequency photon, and taking some of the the momentum and energy with it. The new energy and momentum is sufficient to transport it over a potential barrier to the other side of a layered semiconductor or similar. (All semiconductors are layered -- typically they upper few micrometers are doped, wheras lower down it has a different composition).
The solar cell's composition is such that the electron moves up a potential hill, over the brow, and sits in a dip at the top (on the other side of the solar cell). From there, it can get down the hill again (lose it's energy) either by getting over the brow again, or by travelling down the handy attached wires and charging a battery (say).
Imagine if the back of the cell was transparent, and also exposed to sunlight: Then the sunlight falling on that side would knock electrons down the hill as well! This would actually happen EVEN MORE because the work needed to get over the brow out of the little dip at the top is much smaller than the work needed to get all the way up the hill. The difference is that you can't make the electron do any useful work if it is already at the bottom of the hill.
Now if the solar cell is at the same temperature as the black body radiation, the (usually metal or glass) substrate on which the cell is mounted will emit black body radiation too. By the same argument, the equal amount of infalling light from the back of the cell will result in at least as many electrons being knocked down the hill as are being knocked up the hill by the infalling light you want to do the work.
Upshot: The back of the solar cell must be cooler than the temperature of the infalling radiation for it to work.
To clear up a last couple of points: If the back of the cell is not a black-body emitter, it will either be partially transparent or partially reflective or both. There are no other alternatives (because of the quantum symmetries involved). If it is partially transparent, then the temperature of the objects behind the solar cell becomes relevant. If it is partially reflective, the difference is made up by the reflected heat not absorbed by the solar cell -- including the black body radiation of the cell semiconductor if it is not transparent, or the infalling light if it is.
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