New coating could enable major boost in solar-cell efficiency

Apr 18, 2013 by David Chandler
Credit: Christine Daniloff/MIT

Throughout decades of research on solar cells, one formula has been considered an absolute limit to the efficiency of such devices in converting sunlight into electricity: Called the Shockley-Queisser efficiency limit, it posits that the ultimate conversion efficiency can never exceed 34 percent for a single optimized semiconductor junction.

Now, researchers at MIT have shown that there is a way to blow past that limit as easily as today's jet fighters zoom through the —which was also once seen as an ultimate limit.

Their work appears this week in a report in the journal Science, co-authored by graduate students including Daniel Congreve, Nicholas Thompson, Eric Hontz and Shane Yost, alumna Jiye Lee '12, and professors Marc Baldo and Troy Van Voorhis.

The principle behind the barrier-busting technique has been known theoretically since the 1960s, says Baldo, a professor of electrical engineering at MIT. But it was a somewhat obscure idea that nobody had succeeded in putting into practice. The MIT team was able, for the first time, to perform a successful "proof of principle" of the idea, which is known as singlet exciton . (An exciton is the of a molecule after absorbing energy from a photon.)

In a standard (PV) cell, each photon knocks loose exactly one electron inside the PV material. That loose electron then can be harnessed through wires to provide an electrical current.

But in the new technique, each photon can instead knock two electrons loose. This makes the process much more efficient: In a standard cell, any carried by a photon is wasted as heat, whereas in the new system the extra energy goes into producing two electrons instead of one.

While others have previously "split" a photon's energy, they have done so using ultraviolet light, a relatively minor component of sunlight at Earth's surface. The new work represents the first time this feat has been accomplished with visible light, laying a pathway for practical applications in solar PV panels.

This was accomplished using an organic compound called pentacene in an organic solar cell. While that material's ability to produce two excitons from one photon had been known, nobody had previously been able to incorporate it within a PV device that generated more than one electron per photon.

"Our whole project was directed at showing that this splitting process was effective," says Baldo, who is also the director of the Center for Excitonics, sponsored by the U.S. Department of Energy. "We showed that we could get through that barrier."

The theoretical basis for this work was laid long ago, says Congreve, but nobody had been able to realize it in a real, functioning system. "In this system," he says, "everyone knew you could, they were just waiting for someone to do it."

Since this was just a first proof of principle, the team has not yet optimized the energy- of the system, which remains less than 2 percent. But ratcheting up that efficiency through further optimization should be a straightforward process, the researchers say. "There appears to be no fundamental barrier," Thompson says.

While today's commercial solar panels typically have an efficiency of at most 25 percent, a silicon solar cell harnessing singlet fission should make it feasible to achieve of more than 30 percent, Baldo says—a huge leap in a field typically marked by slow, incremental progress. In solar cell research, he notes, people are striving "for an increase of a tenth of a percent."

Solar panel efficiencies can also be improved by stacking different solar cells together, but combining solar cells is expensive with conventional solar-cell materials. The new technology instead promises to work as an inexpensive coating on .

The work made use of a known material, but the team is now exploring new materials that might perform the same trick even better. "The field is working on materials that were chanced upon," Baldo says—but now that the principles are better understood, researchers can begin exploring possible alternatives in a more systematic way.

Explore further: Going nuts? Turkey looks to pistachios to heat new eco-city

More information: "External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission–Based Organic Photovoltaic Cell," by D.N. Congreve, Science, 2013.

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User comments : 15

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2.7 / 5 (14) Apr 18, 2013
another breakthrough, aren't we up to about 600% efficency now?
3.3 / 5 (7) Apr 18, 2013
for once I agree -- for once I woulld like to see a solar cell make of all the coating that we have heard of in the last 4 years be made into a solar cell and an effeincy number given -- it is always this could work and scientists say improving it should be very straight forward -- it almost never works and if it does improving is never straight forward
2.8 / 5 (5) Apr 18, 2013
If they can harness 2 electrons per photon then why wouldn't they be able to double the efficiency? An improved efficiency of 5% seems underrated for this development.
2.6 / 5 (7) Apr 18, 2013
Solar energy is a great field to find examples of how lab experiments look great but in practice it doesn't hold up. I imagine various reasons can be cited, like unable to bring costs down when scaling up production. Unable to maintain efficiency (materials degrade) for an acceptable length of time.

And my *FAVORITE* reason: "Oh, we sold the process/patent to a major maker of panels that needed to squash it lest their capitol investments in old technology has to be written off as a loss.
4 / 5 (2) Apr 18, 2013
There's a little too much pessimism on this thread. In practice, PV have steadily been getting cheaper and more efficient. I fully expect to install solar panels on my roof within the next five years, and my first panels may very well incorporate this 'excitonic" coating.
1.8 / 5 (5) Apr 18, 2013
I guess most folks here haven't the foggiest idea what advances in practical manufacturing and assembly will need to be fool proofed before these advanced materials coating and entirely new and cost effective assemblies reach the consumer/commerical markets.

Remember, in a world of today that expects 40% profit margins to not be ridiculed and buried by hedge fund managers some serious decisions will need to be made to insulate against such ignorance.
Whydening Gyre
1.8 / 5 (5) Apr 18, 2013
Well, they say they busted the theoretical limit of 34% (blow past it like a jet thru the sound-barrier, I believe it was worded) but he summated that with their new technique it was conceivable to achieve 30% with panels using it.
Am I missing something here?
Matt Holmes
3.5 / 5 (4) Apr 18, 2013
Wayyy too much cynicism here. What, researching into renewables is worthless because of the challenges? Name me something we perfected on the first try. Of course there's going to be a lot of baby steps, of course a lot of things aren't going to pan out, of course some things that look great on paper don't end up working. This is true of every single field of research, don't nit-pick on solar. Keep in mind, we weren't interested in longevity, maintenance costs or materials needed, efficiency ratings and so on, when designing other power sources, only the power output for the cost. We are, essentially, trying to make the perfect solar cell, without the generations of products being replaced. We don't want to go from vinyls to 8 tracks to cassettes to CDs to MP3, we just want the best cell we can make, BEFORE we invest millions or billions installing it around the world. Be glad we're trying so hard, and that such research is being done!
Whydening Gyre
1 / 5 (6) Apr 18, 2013
Well, this is fun. I seem to have corralled a new "1" stalker - open.
5 / 5 (3) Apr 18, 2013
If they can harness 2 electrons per photon then why wouldn't they be able to double the efficiency? An improved efficiency of 5% seems underrated for this development.

Because the kinetic energy of each electron is lower than in single photon systems. Basically that means that the voltage output of the solar cell will be somewhat lower, partially offsetting the increase in current and preventing the power from doubling.
2.3 / 5 (3) Apr 18, 2013
I thought the same of 'lite' until I noticed they seemed to make all sides of various debates down no matter what; assertions or evidenced facts and reasons etc.

'open' seems to be like 'lite'; purposely running around marking virtually everyone 1. Occasionally they appear to randomly mark up a nonsense post. More a troll lurker than a stalker.
1 / 5 (2) Apr 19, 2013
They should have taken the best currently available solar cell and use their pentacene power-doubler coating on that. Using a crappy organic solar cell made of fullerene and poly(3-hexylthiophene), their efficiency numbers are less than impressive.
2.6 / 5 (5) Apr 21, 2013
And while these trolls keeps spitting FUD on anything that doesn't stink of oil, solar energy has reached, in some country, cost competitiveness with fossil fuels (see I hope they are paid for this.
4.2 / 5 (5) Apr 21, 2013
If they can harness 2 electrons per photon then why wouldn't they be able to double the efficiency?

The energy you can get per photon is detemined by the bandgap of your material. The bandgap is fixed, so the closer the photon's energy is to the bandgap the better (because any excess energy in the photon will just get lost).

Now here's the difficulty:
You can choose a material with a bandgap that is large which lets you catch high energy photons. The downside is that there are fewer high energy photons in sunlight than low energy photons.
Or you can choose a bandgap material with low bandgap and collet the infrared - but you'll lose all these juicy high energy photons.

What they are doing here is splitting the high energy photons into two low energy photons and then having two photons impact on a low bandgap material.

In effect you make better use of the full energy of all the photons and lowering the waste factor.
1 / 5 (1) Apr 23, 2013
If they can harness 2 electrons per photon then why wouldn't they be able to double the efficiency? An improved efficiency of 5% seems underrated for this development.

Because you didn't read the article

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