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Nanotechnology Boosts Solar Cell Performance
Roland Piquepaille writes "Physicists from the University of Illinois at Urbana-Champaign (UIUC) say they have improved the performance of solar cells by 60 percent. And they obtained this spectacular result by using a very simple trick. They've coated the solar cells with a film of 1-nanometer thick silicon fluorescing nanoparticles. The researchers also said that this process could be easily incorporated into the manufacturing process of solar cells with very little additional cost. Read more for additional references and a photo of a researcher holding a silicon solar cell coated with a film of silicon nanoparticles."
I wish I had access to the slashdot front page for my articles.
The nanoparticles improve efficiency by 60% in the ultraviolet spectrum. The visible light spectrum is only nominally affected.
It's still pretty cool, though.
It only bothers me because he linkjacks it with his blog.
If he was just posting an article, with a link to the EurekAlert post, it'd be all good. Instead, he has to post about his spammy blog, as well as his (paid?) blog on ZDnet.
The ratio of decent links to spam is 1:2 in this article.
Also, he doesn't post the whole story (60% improvement in the UV spectrum) but rather the more sensational version (60% improvement!). That's pretty dishonest.
While at Purdue one of my friends worked on a process to increase solar cell efficiency by etching TiO2 coatings into long, thin whiskers that helped 'whisk' photons down into the surface of the material. It basically doubled the efficiency of a 3% cell in the visible range. Solar hasn't taken off.
_ by_gre_1.php uses lenses to concentrate light onto small, very efficient space-grade solar panels. Each panel (if memory serves) was on the order of 1 sqcm, allowing these very expensive but very efficient (25%+) panels to be used. The overall effect was to to take 1 m2 down to 10 sqcm of chips.. and yet have the power output be about the same. Combine that concentrator technology with higher utilization of UV bands AND ultra-efficient space grade panels and you've got a winner (concentrators work ONLY in direct sun- no clouds).
Glass typically blocks UV. Most glazings contain glass. If this only boosts (and 60%, while a large number, is still a tiny increment in efficiency) the UV efficiency then there may be limited use... unless you count concentrator applications.
The "Sun Cube" (http://www.treehugger.com/files/2007/04/sun_cube
Just some food for thought.
So I RTFA, and here's the bit: "improves power performance by 60 percent in the ultraviolet range of the spectrum" and "in conventional solar cells, ultraviolet light is either filtered out or absorbed by the silicon and converted into potentially damaging heat, not electricity."
So a conventional solar cell gets ~0 energy from this part of the spectrum, but if you coat it with this special coating, it gets 60% more! And how much is that exactly?
Now if you use a different coating (2.85nm), then it improves performance "in the visible part of the spectrum" by 10%. How much energy does a conventional solar cell get from just the visible part of the spectrum? Unspecified!
It's still something, because to knock an electron out, the minimum frequency of the photon has to be at least the difference between the conduction band (where you want that electron) and the lower-energy valence band (where the electron originally is.) So you have a minimum energy cut off point. Exactly where that is, depends on the material, but generally you won't get any power out of the infrared falling on that cell.
However, the downside is that photons with higher energy than that bandgap, well, the extra energy is essentially wasted.
So basically, say, if you used Germanium at 0.67 EV bandgap, you'd catch more photons than with Silicium at 1.11 EV bandgap, but get less useful energy (i.e., electricity as opposed to heat) out of each photon.
And the higher frequency the photon, the more you waste as heat. So you won't waste more in the visible spectrum (well, unless the photon had less energy than the bandgap, in which case it's completely wasted), but in the UV spectrum you waste a lot.
So reducing the waste in the UV spectrum is really where it counts the most. Sure, it would be neat to gain everywhere, but the UV range is where we waste the most.
Their talk about fluorescent particles, makes me think they're essentially converting an UV photon into at least one lower frequency photon. The question is what they do with the extra energy. At the simplest imaginable way, you'd get at least two low energy photons from one UV photon.
On the other hand, it seems to be a bit more than that, from that short summary linked to. From their claim that they improve voltage, not just current, and that something happens at the interface between the particles and the substrate, it sounds like essentially they created a bunch of new junctions there. I.e., that it's a new way to make a multi-junction solar cell.
Multi-junction cells aren't exactly new, but traditionally they've been very expensive so far. If these guys invented a cheap way to make one, kudos to them.
On yet another hand, it will be interesting to see on exactly what existing cells can their film be applied. On silicon or other semiconductors, ok, I can see how it would form an extra junction. Would it also work on, say, Dye-sensitized Solar Cells? There essentially their particles would come on top of the dye, and I'm not sure how well that works. It'll be interesting to find out, eventually.
A polar bear is a cartesian bear after a coordinate transform.
First, we need to be careful here. A 60% improvement in the conversion among UV spectrum does not necessarily equate to a 60% increase in a given PV cell. If the particular cell is more of an infrared or visible light spectrum oriented cell, you'll see a minor, if any, improvement. So before anyone starts grabbing random solar cell outputs and starts applying a 60% increase in power and get modded "insightful" for bad information, let's get that part out there.;)
With the main advantage being in the UV spectrum, it seems to me the best application would be to UV preferential cells in orbit or on Mars, Luna, etc.. Doubly so given the difficulty in shedding excess heat in Space.
My Suburban burns less gasoline than your Prius.
That's a whole 12 characters shorter, and leaves out the important words 'in the ultraviolet spectrum', which changes the meaning completely. Also, those emitted words are 27 characters long, so if they were properly included, his summary is actually more wordy than the original source.
It's almost word for word. And it's wrong.
Oh FFS, what is it with /.ers and their "Why can't I buy it at Walmart yet?" comments?
Have you people never heard of research?
A lot of these stories are of lab demonstrations, or even just theoretical breakthroughs that MAY, one day, be developed to the point that they become useful. Or they may inspire further research that may lead to further research that may eventually be commercialised in a completely different form to how they were first demonstrated.
If you want to read about new ideas and developments that are unlikely to impact your life for several years, you're in the right place. If you want to read about new products that you can buy right now at your local mega-mall, try the junk mail they stuff in your mail box.
In a detector you are correct but in a power device you use the doping gradient because bias voltages leak, defeating the purpose.s -selling-solar.html
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At the end of the blog Roland asked why they didnt use multiple sizes of silicon nanoparticles, this was my long winded reply:
I am a graduate student working on the synthesis of silicon nanoparticles for solar cells and other applications. While silicon nanoparticles have been syntheszed for over 20 years, and their are many ways of synthesizing them, it is still very difficult to control the size of the particles. Unlike CdSe based quantum dots where the size of the particles is determined by how long your let the reaction run for, 1 min for blue 30 min for red, and various time lengths for other colors, silicon nanoparticles are more complex.
Silicon nanopariticles while they are still quantum dots, since the energy levels are somewhat quantized emit very differently as well. Silicon is an indirect band gap semiconductor, however the blue emitting silicon nanoparticles emit light with a direct band gap transisiton, where as they red emitting silicon nanoparticles are controlled by more surface effects and emit in a low energy indirect band gap transition which is slower and allows for more energy loss in other modes.
Anyways, what it comes down to, is it is difficult to make various sizes of silicon nanoparticles. I would also like to add that this technique is not very promissing for several reasons, they epense and other problems with traditional silicon etched solar cells still exist. Cost, lack of flexibility, low effeciency, heavy, glass... This method does not take full advantage of multiple exciton generation which was just proved for silicon nanoparticles in ACS journal of Nano Letters this week. PbSe quamtum dots have shown to generate 7 exciton for just one photon, which in theory could be converted to 7 electrons from 1 photon...someday. But 2.6 excitons from silicon nanoparticles is still pretty good. Especially when I have a way to get the excitons into free electrons And silicon is a non-toxic cheaper alternative to the PbSe quantum dots.
So now I can use my calculator with my black light......groovy!
I've asked that question after every Gerard Depardieu movie.
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The nanoparticles improve efficiency by 60% in the ultraviolet spectrum. The visible light spectrum is only nominally affected.
.90s could be a whole heck of a lot better than even the experimenters were originally chasing. So it's no wonder they published now, with only two sizes of particles tested.
It's still pretty cool, though.
This whole series of "only 60% of the UV part" threads is missing the rest of the article. That was just for ONE size of naonparticle, suitable for converting light to the middle of the visible range. They ran the tests for another size, suitable for converting to visible red, and got a higher conversion result, as expected.
Solar cells completely miss photons below the bandgap energy and only peel off the bandgap energy from those above it. They have a bandgap in the infrared so they get most photons, but only take that first 0.6 electron-volt chunk of their energy and lose the rest as heat. That's great if you have an infrared photon at 0.603 eV, not so hot for visible light photons at 1.8-3.1 eV, and pretty crummy for UV photons at 3.1 to 12 or so eV.
Films of nanoparticles have an interesting property: They absorb photons of various wavelengths and emit photons of particular wavelengths related to their size. But they don't do that in the solar-cell style of chopping the right-sized hunk off a more energetic photon and throwing the rest away. Instead they are able to combine energy from multiple lower-energy photons to generate one of the desired energy, chop several desired energy photons out of a high-energy one (and keep the leftover shavings to combine with others to make more desired-energy photons), and trade energy among their neighboring particles.
So it was expected that a film of nanoparticles on a solar cell would grab the energy from photons all over the spectrum, convert it to the energy characteristic of the nanoparticle size, and re-emit that. The improvement from efficiently salami-slicing and stacking photons should be better than losses from such things as emitting the photon in the wrong direction, giving a big boost to the cell.
And to some extent that was happening: Feed UV photons to nanoparticles that chunk 'em into something in the 3 eV range and you get more out of the UV hitting the cell than you would without the film - without appreciably affecting the output from the visible light. You're averaging about 1 2/3 IR photons worth of energy, instead of 1, for each incoming photon. Feed it to nanoparticles that chunk it up finer, down to 2 eV or so, and you get more out of your UV and also start improving on even visible light.
That's a good sign for doing what you really wanted to do: Use nanoparticles that emit just a tiny squidge above the solar-cell's bandgap, chunking all the photons into the right size for the cell and wasting very little of their energy. (But maybe still losing a bunch by emitting them in the wrong direction. That might be improved by putting the nanoparticles at the bottom of wells in the cell rather than on a flat surface.)
But the experiment produced a surprise: The VOLTAGE went up! WTF?
That means one of two things:
a) The nanoparticles affected the bandgap.
b) The nanoparticles coupled directly into the cell's "circuitry" in some non-obvious way.
b) might lead to something even better: Nanoparticles that capture the photons, chunk and stack them into some desired size (voltage), and deliver them directly to the wiring. That could get virtually ALL the incoming energy into your wires.
A solar cell with efficiencies in the
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