Scientists design solar cells that exceed the conventional light-trapping limit

Jan 20, 2012 by Lisa Zyga feature
Scientists have found that the key to overcoming a light-trapping limit lies in increasing the density of optical states in the absorbing material. The finding could lead to the design of highly efficient solar cells that are also very thin, and therefore inexpensive. Image credit: National Renewable Energy Lab

(PhysOrg.com) -- The best performing solar cells are those that are thick enough to absorb light from the entire solar spectrum, while the cheapest solar cells are thin ones, since they require less, and potentially cheaper, material. In an attempt to combine the best of both worlds, a team of scientists has outlined designs for solar cells that can absorb light from the entire solar spectrum yet are as little as 10 nanometers thick. The new design approach, which could lead to improved low-cost solar cells, requires overcoming a thermodynamic light-trapping limit proposed in the 1980s.

The scientists, Dennis Callahan, Jeremy Munday, and Harry Atwater, of the California Institute of Technology in Pasadena, California, have reported the new method of light trapping beyond the conventional limit in a study published in a recent issue of .

Their work addresses a 1982 study that proposed a thermodynamic limit on how much of the range can be absorbed by homogeneous bulk semiconductor slabs. The limit requires these materials to have a minimum thickness in order to absorb light from the full solar spectrum. As a result, today’s semiconductor are generally designed with thick absorbing layers in order to trap as much sunlight as possible, which can be expensive and complicated to fabricate.

Previous analyses of this light-trapping limit (which is sometimes called the ray optic limit or ergodic light-trapping limit) have shown that some solar cells actually do exceed the limit by taking advantage of wave interactions. Although researchers have theoretically explained how this happens in select cases, there is no general explanation that can be extended to the wide variety of proposed light-trapping schemes that may also be capable of exceeding the limit.

Here, the Caltech scientists have proposed that the key to overcoming the light-trapping limit lies in increasing the density of a semiconductor’s optical states. Because each of these states can accept light of a certain wavelength, having more of them can increase the amount of light a material can absorb.

“It is now clear how to think about and design solar cells that can potentially exceed this previous light-trapping limit,” Callahan told PhysOrg.com. “All you have to do is think of a way to increase the density of optical states, and then populate these states. There are lots of tools and methods that have been designed for increasing the density of optical states for other areas of research, for example optical communication and quantum optics. But now solar cell researchers can take these ideas and put them in the appropriate context for solar cells with the help of our work. Also, if someone is working with a particular type of solar cell, it should now be clear whether it has potential to exceed the previous limit or not.”

The researchers demonstrated that any semiconductor material can exceed the light-trapping limit when the local density of optical states (LDOS) of its absorbing layer exceeds the LDOS of the bulk material. They also show that enhancing the LDOS of the absorber to a level needed to absorb 99.9% of the solar spectrum is feasible even for semiconductors as thin as 10-100 nanometers (compared with micrometer-thick layers used in today’s commercial devices).

“Our results suggest that if you can engineer the electromagnetic environment in the right way it should be possible to go as thin as 10 nm,” Callahan said. “It’s just a matter of how to design it appropriately and without introducing unwanted parasitic losses. This is certainly a challenge, but is something we are currently thinking about. Now, a 10-nm solar cell is likely impractical for other reasons such as the need for multiple layers, surface recombination, potential quantum effects, etc., but is still within the realm of possibility.”

The most important limit to increasing the absorbing layer’s LDOS arises due to the “density of states sum rules,” which say that increasing the LDOS in one region of the spectrum results in a decrease in another region of the spectrum. As the scientists explain, this conservation of LDOS occurs naturally by a process called spectral reweighting, and can also potentially be artificially engineered. Although this rule imposes an upper bound on the absorbance of a solar cell, the researchers explain that it shouldn’t limit solar cell absorbance for practical purposes. This is because LDOS enhancement is only needed in the solar spectrum, while LDOS can be decreased in any region outside of the , a much larger area. For this reason, other physical and practical limits, such as saturation or fabrication challenges, will likely become relevant before a limit is reached for increasing the LDOS.

The scientists also showed that a variety of solar absorber designs can meet the fundamental criteria proposed here for exceeding the conventional light-trapping limit, i.e., exhibiting an LDOS that is higher than that of the bulk material. Some designs include using plasmonic materials, dielectric waveguides, photonic crystals, and other devices.

“We are currently trying now to find ways to engineer and increase the density of optical states as high as we can within a practical solar cell design,” Callahan said. “This is a challenging task for high index materials like silicon, but there are many possibilities which we are currently examining that look promising.”

Explore further: NIST offers electronics industry two ways to snoop on self-organizing molecules

More information: Dennis M. Callahan, et al. “Solar Cell Light Trapping beyond the Ray Optic Limit.” Nano Letters 2012, 12, 214-218. DOI: 10.1021/nl203351k

Journal reference: Nano Letters search and more info website

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antialias_physorg
3.9 / 5 (14) Jan 20, 2012
Another one of those seemingly unbeatable hurdles beaten. This should pve th way for really inexpensive thin film solar 'panels' with high efficiency. Great job!
aennen
4.4 / 5 (16) Jan 20, 2012
We have seen so many of these "hurdles" / "improvements" here that we should in the middle of a major solar cell revolution... Why arent we ???
rawa1
1.3 / 5 (27) Jan 20, 2012
Because solar energy is diluted, unreliable and material hungry solution, the financial effectiveness of which is disputable at the most areas of world. It will remain so, even if the effectiveness of solar cells would raise to 100 percent and it cannot compete with cold fusion in nearly any aspect.
IngDutch
4.5 / 5 (8) Jan 20, 2012
We have seen so many of these "hurdles" / "improvements" here that we should in the middle of a major solar cell revolution... Why arent we ???


Taking into account that solar prices have been dropping to a point that each 5 years or so 50% cost reduction is feasible. It also takes time to implement these improvements. At least 5 years, something that works in a lab does not have to work in practice. This for example is a study which possible could effect the PV market but need to be developed into a pilot project and then integrated into current production methodes. Then because of patents, legal stuff, investors that need to be happy, one cannot simply take a solution from one company to another. Then theres the fact that it's a highly technical solution with input of electricians, physicists, this solution could might have a bad side and other efficiency problems could arise.. and is it then worth it? As you can see one does not simple make a highly technological product.
stealthc
1.7 / 5 (6) Jan 20, 2012
why would potential quantum effects be a bad thing? They are about the only thing that can improve upon this idea and make something better.
GSwift7
3.4 / 5 (15) Jan 20, 2012
We have seen so many of these "hurdles" / "improvements" here that we should in the middle of a major solar cell revolution... Why arent we ???


Because the price of other kinds of energy keeps going down (in adjusted dollars) too. Solar keeps getting better, but not fast enough to outpace improvements in things like the new advanced hybrid gas turbine generators and falling cost of natural gas.

There's also the problem that university X might hold the patent for technology X and company Y holds the patent for technology Y, and by the time they get together to make a product (which might take years), university A comes up with a technology wich makes technology X and/or Y obsolete. Why would an investor take a chance on something that has a high probability of being obsolete before making money? Solyndra, for example, had that happen to them.
GSwift7
3 / 5 (10) Jan 20, 2012
why would potential quantum effects be a bad thing? They are about the only thing that can improve upon this idea and make something better


Effects like electrons tunnelling, or your charge carrier becoming an insulator at small sizes might limit how thin you can make it. Basic properties change when you get really small. They run into some of these problems in computer chip design already, as you can see from the following wiki page.

It has been claimed that transistors cannot be scaled below the size achievable at 16 nm due to quantum tunnelling


http://en.wikiped...anometer
antialias_physorg
4.2 / 5 (6) Jan 20, 2012
We have seen so many of these "hurdles" / "improvements" here that we should in the middle of a major solar cell revolution... Why arent we ???

Time from research to market is measured in years (sometimes a decade). First you have to do the research (which is where this is at).
Then you have to spend a couple of years optimizing it - no sense setting up a factory if the next guy will put you out of business half a year later with a product that is twice as good based on the same principle.
Then you have to see if you can manufacture this cheaply. If you have to invent new manufacturing methods then that's a show stopper in most cases.
Then you have to do the venture capital rigmarole
...

It just takes time. Be patient.
antialias_physorg
4.1 / 5 (9) Jan 20, 2012
things like the new advanced hybrid gas turbine generators and falling cost of natural gas.

Natural gas production has stagnated over the past years and price has risen. Putting your money on a finite source of power that exacerbates global warming isn't an option.

Gas power plants are ideal for bridging the variability of other sources of power production because they are quick to start up (unlike nuclear which can take up to a week to get going). They should not be used as a mainstay of energy production.
Osiris1
1.3 / 5 (12) Jan 20, 2012
Just U watch. Lightspeed is next! Like the idea of nutrino rays goin superliminal on us to make commo to our proposed colonization efforts a LOT more efficient. And the new (and they SAID IT COULDNT BE DONE) collectors in space will make a lot of other thinks go good too. Like to see reflectors ten miles in diameter shinin on a collector array, and the power goin down by microwave beam to a an earthbound RCVR for distribution. Multiply that by thousands and we have a space based solar system that never sees the night and never gets dusty and never rusts..and...and..runs....FOREVER
Osiris1
1.1 / 5 (10) Jan 21, 2012
Several comments here about legal barriers to development. This is all the fault of Al Gore, and THAT is an 'inconvenient truth! It is him that is the deep dyed villain that inflicted all this copy protection, excessive patenting, patenting of intangibles like programs, business methods, and other REAL job killing crap that hobbles the American way of creativity. Lets TRASH ALL of the so called INTELLECTUAL PROPERTY 'LAWS'. And go back to doing things the way we did them in the 1950's when we just...built it! and did not worry about being sued by some blood sucking lawyer that never had a inventive idea in his life but managed to steal some one elses by buying a company...much like Caldera cum SCO did with Unix. Fifteen years is plenty for an inventor to get paid, and no corporation should be allowed to own any patents. Patents should belong to the original inventor and not be transferable. That will take the Greedy Oligarchic Pigs (GOP) out of it! Gore is really a GOP too.
MarkyMark
3.3 / 5 (12) Jan 21, 2012
Just U watch. Lightspeed is next! Like the idea of nutrino rays goin superliminal on us to make commo to our proposed colonization efforts a LOT more efficient. And the new (and they SAID IT COULDNT BE DONE) collectors in space will make a lot of other thinks go good too. Like to see reflectors ten miles in diameter shinin on a collector array, and the power goin down by microwave beam to a an earthbound RCVR for distribution. Multiply that by thousands and we have a space based solar system that never sees the night and never gets dusty and never rusts..and...and..runs....FOREVER

Are you high?
Telekinetic
2.5 / 5 (11) Jan 21, 2012
We have seen so many of these "hurdles" / "improvements" here that we should in the middle of a major solar cell revolution... Why arent we ???

It takes financing at both federal and state levels, to subsidize the manufacturers and the end users, even though there are generous incentives in place to solarize your home.
The country is broke. Until Congress sees the incredible opportunity solar represents for our needs for energy and job creation, the economy will continue to stagnate. The Republican Guard will protect oil industry interests to the death.
Callippo
1.1 / 5 (9) Jan 21, 2012
The Republican Guard will protect oil industry interests to the death
The solar energy is subsidized heavily with Obama's government, yet it fails from apparent reason: the total cost of solar energy production and ownership is higher, than at the case of fossil fuel sources. In particular, solar plants doesn't work at morning and evening, where the consumption of energy is highest and above latitude of California they're not effective at all.
Until Congress sees the incredible opportunity solar represents for our needs for energy and job creation, the economy will continue to stagnate.
The solar energy is not so lucrative with respect to job creation at all, as most of investments end at China and East Asia countries, which can produce the solar panels in much cheaper way, than the USA. http://www.physor...rgy.html Just face it: solar energy is another hype and it exhausts material sources.
Telekinetic
2.5 / 5 (8) Jan 21, 2012
Even in its primitive state, it is possible to be "off-the-grid" with solar. If you were to investigate the political and corporate machinations of a couple of years ago when solar was considered viable, you will find some nasty behind-the-scenes interference from lobbyists, legislators and industries with the most to lose. Remember BP promoting itself as "green" with a solar carport shading its electric recharging station? Have we as a nation fallen so quickly into an apathetic state to have let the ball drop on solar without so much as a squeak? This is what oil companies do best- wait.
Callippo
1.4 / 5 (9) Jan 21, 2012
Even in its primitive state, it is possible to be "off-the-grid" with solar.
It indeed is, but you'll need a batteries and many other technologies, the total price of which will make this solution economically less advantageous, than the fossil fuel based solution (not to say about cold fusion). And you can forget to powering of your car with solar energy, until you're not an owner of huge piece of desert. Apparently, the solar is feasible solution for Brangelina or diCaprio, but it's not solution for masses at their contemporary level of consumption. My problem with solar energy is, it drains the money from cold fusion research, which the proponents of "green technologies" are ignoring as obstinately, as the proponents of fossil fuel lobby from apparent reason: both groups just want to make money on their technologies, they don't care about destiny of civilization as a whole.
Estevan57
2.4 / 5 (25) Jan 21, 2012
Every advance in technology will have its detractors and naysayers. I find it sad that advances in solar cell technology and hot fusion have the same guys hounding the technologies with the "only cold fusion works and should be exclusively promoted and funded" dogma. I live in Oregon, 45th parallel, and heat my water with solar energy. Where can I buy a cold fusion cell that someone other than rawa1 and Callipo have seen work? I enjoy the progress made in all technologies, and would appreciate a little less "only cold fusion" talk and a little more "advances made in cold fusion".
Callippo
1.1 / 5 (7) Jan 22, 2012
I enjoy the progress made in all technologies, and would appreciate a little less "only cold fusion" talk and a little more "advances made in cold fusion".
These advances indeed exists (despite the cold fusion research is still underground science, just because of people like you), but the solar energy has its apparent limits, which are related to the price of installation of solar panels and the price of energy accumulation for dark time. The price of two items is already higher, than the price of fossil fuels sources and we still don't talk about effectiveness of solar cell itself (BTW I've solar cells at my house too, but it doesn't change the fact, it's my hobby project - not the energetic solution affordable).
kevinrtrs
Jan 22, 2012
This comment has been removed by a moderator.
bewertow
2.6 / 5 (5) Jan 22, 2012
FYI Callippo and Rawa1 are the same account. He's a loser who made a sockpuppet account so that it would look like people agree with him.
bewertow
2.6 / 5 (5) Jan 22, 2012
Because solar energy is diluted, unreliable and material hungry solution, the financial effectiveness of which is disputable at the most areas of world. It will remain so, even if the effectiveness of solar cells would raise to 100 percent and it cannot compete with cold fusion in nearly any aspect.

Strange that all plants then seem to enjoy making use of solar power, doesn't it? As far as I can see, humans are simply getting closer to the same solution of using entangled photonic states to transfer energy. The Creator has solved this little conundrum long, long ago.


Why do you have to bring fairy tales about sky wizards into every discussion?
Callippo
1.3 / 5 (4) Jan 22, 2012
Strange that all plants then seem to enjoy making use of solar power, doesn't it?
They don't use cars for their travel. Anyway, it has no meaning to solve it by now. The commercial success of cold fusion will decide about effectiveness of energy technology. Without cold fusion we would face the global war, which will reduce the number of people bellow one billion, which is natural process with respect of sustainability of organic life at this planet. The contemporary financial crisis is apparently not sufficient for convicting of people, who just hope, their problems will solve someone else.
who made a sockpuppet account so that it would look like people agree with him
I'm not using any of my accounts for voting here and your voting preferences are not my concern.
Telekinetic
1.8 / 5 (8) Jan 22, 2012
I can't help but relate to Callippo's impatience for the announcement of availability of the cold fusion electric generator from Andrea Rossi's company. Like a Black Friday sale on flat screen TV's, I'd be the first to elbow my way through to get at one of them. How long would it take to distribute enough of them is the question, since they would be sold to private individuals. How willing or prepared would the government be to scale it up and switch over to a completely new technology. I hope they would. But the numbers involved in the energy of the sun as it hits earth are compelling also. I say develop both, and let market forces decide which technology stays for the long term.
physhack
5 / 5 (1) Jan 22, 2012
Strange that all plants then seem to enjoy making use of solar power, doesn't it? As far as I can see, humans are simply getting closer to the same solution of using entangled photonic states to transfer energy. The Creator has solved this little conundrum long, long ago.

Plants don't have to run refrigerators, furnace blowers and air-conditioners - they can afford to be inefficient.
antialias_physorg
3.9 / 5 (7) Jan 23, 2012
Strange that all plants then seem to enjoy making use of solar power, doesn't it?

Plants are woefully inefficient (about 3%). If that is supposed to be an argument for a creator then he's a bungling idiot.

The reason WHY they are so inefficient is evolution: Plants don't have to be perfect. They just have to be better than the plant next to them.
rawa1
1 / 5 (6) Jan 23, 2012
..relate to Callippo's impatience for the announcement of availability of the cold fusion electric generator from Andrea Rossi's company...
The cold fusion of hydrogen at nickel has been announced before twenty years already, A. Rossi is just latest hype on it. We have full rights to be impatient, because the number of evidence increases every day. http://technology...enr.html
antialias_physorg
3.7 / 5 (6) Jan 23, 2012
This is not an article about cold fusion.

When an article about cold fusion comes along you and all your schizophrenic sockpuppets can post and discuss it amongst yourself (yourselves). Until then: keep the cold fusion out of these threads where people would like to discuss the article at hand.

Thank you.

(This message has been brought to you by the 99 percent of physorg posters fed up with you)
GSwift7
3 / 5 (10) Jan 23, 2012
I agree. It's like a toy doll with a string on the back that says "cold fusion" every time you pull it, only he pulls his own string.

The article is about a theoretical limit to the thickness of thin film solar cell design, despite the misleading title of the article that claims they "designed a solar cell". They actually didn't design anything, they just did theoretical research to figure out the theoretical limit on thickness. They even said that their limit isn't the real limit, because other factors might preclude their limit before you got to it. Still, it does add to the knowledge bank, so it is usefull work for people who do want to design an actual solar panel.