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Caltech Makes Flexible, 86% Efficient Solar Arrays
strredwolf writes "Caltech has released a flexible solar array that converts 95% of single-wavelength incandescent light and 86% of all sunlight into electricity. Instead of being flat-panel, they stand thin silicon wires in a plastic substrate that scatters the light onto them. The total composition is 98% plastic, 2% wire — the amount of silicon used is 1/50th that of ordinary panels. So as soon as they can get these to market, solar could be very viable and cheap to produce." Update: 03/01 21:02 GMT by KD : Reader axelrosen points out evidence that the 80%+ efficiency figure is wrong. MIT's Tech Review, in covering the Caltech announcement, says that the new panel's efficiency is in the 15%-20% range — which is competitive with the current state of the art. And the Caltech panel should be far cheaper to manufacture.
It's not incandescent light, it's incident light. sigh.
There was a bit further down that said conversion to electricity was 90-100% of absorption. That means a worst case efficiency of 77% of incident sunlight, which is still a staggering improvement over standard cells. I for one welcome our new silicon-wire overlords.
From TFA:
The silicon-wire arrays absorb up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight.
http://www.rsc.org/chemistryworld/News/2010/February/14021001.asp
'We have shown the optical absorption efficiency and charge carrier collection efficiency of a silicon wire array cell is comparable to a conventional silicon cell, but a wire array cell uses up to 100 times less silicon due to enhanced light-trapping effects,' says Atwater. Significantly, the wire arrays absorb infrared light more efficiently that conventional silicon surfaces, further improving the performance of the new device.
So the gist is that it's more efficient because it converts infrared, uses some type of clear polymer with alumina "reflector particles" in place of 99% of the expensive (doped) silicon, and is flexible and therefore easier to manufacture.
"I assumed blithely that there were no elves out there in the darkness"
The original article is poorly written. MIT's Technology Review has an article that includes information about efficiency of generating electricity, and it says 15%-20%. http://www.technologyreview.com/energy/24665/?a=f
So the story is really that there might be a way to make cheaper, flexible solar panels by mixing silicon and polymers.
Collection efficiency (which is what TFA is claiming to be 86%) vs. conversion efficiency (that 40% number you remember) is what you're missing, but from other articles on the technology it appears that the conversion efficiency for these cells should be higher than existing designs:
The silicon wire arrays created by Atwater and his colleagues are able to convert between 90 and 100 percent of the photons they absorb into electrons--in technical terms, the wires have a near-perfect internal quantum efficiency. "High absorption plus good conversion makes for a high-quality solar cell," says Atwater. "It's an important advance."'
Which could give them ~78% conversion efficiency, still nearly double over the best cells currently.
This is interesting work, but it is in a very immature stage of development. They seem to be no where near demonstrating a practical solar cell, and speculated conversion efficiency numbers like 86% are laughable. One of the fundamental limitations of a cell based on Si wires is that the higher a photon's energy is over the bandgap of Si, the more energy is lost as heat. I believe the theoretical maximum conversion efficiency for a Si solar cell is around 30%, and commercially viable cells are limited to around 20% because of practical issues in creating solid state cells such as making electrical contacts to the device, the high cost of making higher efficency (20+%) Si cells. This work doesn't begin to address such issues. I think it is unfortunate that over-hype like this can take luster off of progress in photovoltaics that seems less spectacular but is much closer to practical realization.
Yes, just like any other dark panel you leave in the sun. Except not as hot, because some of the energy is being exported as electricity. So unless they're flammable at really low temperature we'll probably be okay.
Here's the actual scientific paper, "Predicted Efficiency of Si Wire Array Solar Cells". That's by the same authors mentioned in the press release. While the thing does trap most of the light hitting it, only a fraction of the energy in that light is converted to electricity. In fact, this thing is currently less efficient than the better commercial solar cells.
From the paper: ... simulated photovoltaic efficency of 14.5%. ... Conclusion: ... "Si wire array solar cells have the potential to reach efficiencies competitive with traditional Si crystalline solar cells."
So, an interesting development, but no big breakthrough. There's a claim that it might be a cheaper way to make solar cells, but everybody who comes up with a new design makes that claim. (Nanosolar comes to mind; their technology is supposed to be cheaper, but so far they've spent half a billion dollars and apparently have only produced sample panels.)
> up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight. It says "up to". Which means that the worst case could actually be zero and the numbers are actually meaningless. Read more carefully before welcoming your new overlords.
"When was the last time you find any plastic that can last 10 years under the sun?"
My plastic garbage bins have spent at least a decade out in the Aussie sun. A lot of plastic that you find in throw away stuff these days has been deliberately engineered to be bio-degradeable due to pollution concerns in the 80's. The older non-biodegradable stuff has formed a large "islands" in the North Pacific and North Atlantic.
And did you exchange a walk on part in the war for a lead role in a cage? - Pink Floyd.
There are many different factors that go into making a high efficiency solar cell. You need a front material that has very high transmittance/low reflectance at incident angles, a high absorption semiconductor, a high photon to carrier generation rate, high/easy carrier collection from the semiconductor, and broad spectrum conversion. These silicon-wire arrays appear to have high absorption and high carrier generation, but thats only part of the story. The other issue is that silicon misses out on a fair chunk of the solar spectrum. Anything after 1100nm is not converted, its simply below the bandgap.
The title of this post and the article is incredibly misleading and very annoying/frustrating to someone who's been working on solar technologies for a while. Don't get me wrong, I think this is a very cool thing, sounds like they have to potential to make very cheap cells, but approach, let alone surpass, current multijunction cells (30-40% eff.) they will not.
Disclaimer: University of Michigan Solar Car Team alum
1 Photon to one electron is only half the story. If the photon has more energy than the electron then there is a loss. The electron has a fixed energy (band gap) and the photons *must* have that much energy or more before it works at all. There are other details too, in silicon its not a direct band gap, so each photon cannot just eject a single electron, it must also emit a phonon (heat). Silicon has a theoretical maximum efficiency (electrical) of about 29-30% IIRC in sunlight (thats at 100% quantum efficiency for all photons at and above the band gap).
The Grey Goo disaster happened 3 billion years ago. This rock is covered in self replicating machines!
Yes and no. Yes, UV is the most energetic light, but there isn't much of it in sunlight at the earth's surface. Most of it has been lost to ozone, Rayleigh scattering, etc. There's enough to give you a sunburn, but no, in terms of the actual amount of power in sunlight at the earth's surface, it's less than 5%. Filtering it to zero to obtain much longer PV panel lifetime is generally a net economic benefit in terms of TCO.
Right you are. Ugh I feel dirty. I thought only morons did that! Maybe it is still true ... maybe I am a half-wit!
Infuriate left and right