Claim: solar cells that produce two electrons for every photon, resulting in over 100% quantum efficiency

Solar Panels

Solar Panels (Photo credit: Living Off Grid)

From the I’ll believe it when I see it department comes this claim.

From R&D magazine:

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% for a single optimized semiconductor junction.

Now, researchers at Massachusetts Institute of Technology (MIT) have shown that there is a way to blow past that limit as easily as today’s jet fighters zoom through the sound barrier—which was also once seen as an ultimate limit.

They have published a compelling case that the key to greater solar efficiency might be an organic dye called pentacene. More from R&D:

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 fission. (An exciton is the excited state of a molecule after absorbing energy from a photon.)

In a standard photovoltaic (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 excess energy 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 today’s commercial solar panels typically have an efficiency of at most 25%, a silicon solar cell harnessing singlet fission should make it feasible to achieve efficiency of more than 30%, 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.”

Full story here

The paper:

External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission–Based Organic Photovoltaic Cell

Daniel N. Congreve*, Jiye Lee*, Nicholas J. Thompson*, Eric Hontz, Shane R. Yost, Philip D. Reusswig, Matthias E. Bahlke, Sebastian Reineke, Troy Van Voorhis, Marc A. Baldo
Science 19 April 2013:  Vol. 340 no. 6130 pp. 334-337  DOI: 10.1126/science.1232994

Abstract

Singlet exciton fission transforms a molecular singlet excited state into two triplet states, each with half the energy of the original singlet. In solar cells, it could potentially double the photocurrent from high-energy photons. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in a portion of the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, we show a peak external quantum efficiency of (109 ± 1)% at wavelength λ = 670 nanometers for a 15-nanometer-thick pentacene film. The corresponding internal quantum efficiency is (160 ± 10)%. Analysis of the magnetic field effect on photocurrent suggests that the triplet yield approaches 200% for pentacene films thicker than 5 nanometers.

Some figures:

Solar_double_F1_medium

Figure 1 Singlet fission dynamics in pentacene.
Calculations of singlet and triplet excitons and charge transfer states at the pentacene/fullerene interface are shown, with the purple (orange) density indicating where less (more) electron density is found in the excited state. The delocalized singlet exciton and two localized triplet excitons are circled in red. The loss pathway for singlet excitons is direct dissociation into charge before singlet exciton fission.

Solar_double_F2_medium

Fig. 2 Device architecture and EQE of a pentacene solar cell.
(A) Chemical structures and architecture of the solar cell with the thickness of each layer in nanometers and energy levels of the lowest unoccupied and highest occupied molecular orbitals in electron volts (12, 18, 20, 2931). The anode is composed of indium tin oxide (ITO) and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). The cathode employs bathocuproine (BCP) and a silver cap. (B) External quantum efficiency of devices without optical trapping (blue line), and device measured with light incident at 10° from normal with an external mirror reflecting the residual pump light (red line). Optical fits from IQE modeling are shown with dashed lines: modeled pentacene EQE (blue dashes), modeled P3HT EQE (purple dashes), and modeled device EQE (black dashes) for comparison to the measured device efficiency without optical trapping.
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Jimmy Haigh

The greens will not like this…

davidmhoffer

The headline makes no sense. That’s not the claim the paper is making. The paper is claiming that energy that otherwise would have gone into heat, is instead put into moving two electrons instead of one. If it work exactly as claimed, then the efficiency would still be well below 100%.
REPLY: Look at the figure 2 graph, over 100% (I’ve updated the headline and added a section from the R&D article to make the distinction of quantum efficiency and overall efficiency clearer) – Anthony

StuartMcL

Over 100% efficiency? Wonderful!
Use the first 100% of to power a light above the cell to generate yet more electricity and use the rest for something else.
Once you have it started, you’ve got an endless supply of free energy. .
What could possibly be wrong with that idea.

Judging by the context,what they mean by “quantum efficiency” isn’t the ratio of electric energy out to photon energy in but the ratio of electrons out to photons in.

William Sears

There is always hype in research and as you say we will just have to wait and see. Just in case there is some confusion, I will point out that quantum efficiency is a much different thing then thermodynamic efficiency. This is made clear in the linked article where it is also stated that the thermodynamic (overall) efficiency is only 2% in the device at present. Quantum efficiencies greater than one (100%) are not unusual. What the authors claim is new is the extension of this from the ultraviolet into the visible. They may, of course, be underestimating the problems yet to be surmounted, but such is the game of research.

No Name Guy

“Singlet exciton fission transforms a molecular singlet excited state into two triplet states, each with half the energy of the original singlet”…. Not sure of the physics in this, but it sounds like all they’re saying is that this process yields more electrons at the same (or closer to the) total energy of the photon, whereas the previous process only could yield a single electron at a certain energy level, with excess energy from the photon being lost as heat. The above sounds like there’s no claim of violation of the 2nd law (e.g. more energy out than in). Also, doesn’t a high energy gamma (nothing more than a very energetic photon) produce thousands of free / knocked loose electrons as part of the ionization process? Each one takes some of the energy from the gamma…..no violation of 2nd law there even though it frees up thousands of electrons per photon. Of course, I do agree with the sentiment of show me. And detail how you did it so it can be independently replicated (e.g. real science), don’t pull the whole “peer review” pseudo-science.

Ray

OK we’re talking about converting Radiant Energy (Photons) to Electrical Energy (Electrons). So how many Photons = 1 Electron, or is it as I suspect more complicated than that?

Paul Adomshick

Sorry, but your headline is completely misleading. 100% efficiency for a solar cell would mean that all of the incoming solar radiation is converted to electricity. That is not what the article claims. The science may or may not be legitimate, but your headline is 100% wrong.

William Sears:
You beat me – and I suspect several others – to it.
Well done!
Richard

Steve Richards

The writers talk of taking PV panel efficiency from 25% to 30%, not a bad improvement.
I do not know where the 100% cam from.
REPLY: it is in the title of their paper:
External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission–Based Organic Photovoltaic Cell
– Anthony

David Thomas Bronzich

This reopens a ‘discussion’ into Einstein’s view on light propagation and photons. Also the differences between emitted and absorbed photons.

commieBob

100% efficiency would be 1 watt out for every watt in. In theory there should be nothing to stop one sufficiently energetic photon from knocking loose many electrons, if the electrons are sufficiently loosely bound. Of course, in practice …

Is what this is really doing is [potentially] boosting the Shockley-Queisser efficiency limit to 68% at certain wavelengths? This effect would be wavelength dependent? Am I understanding correctly that 670nm seems to be a critical wavelength and less energetic photons won’t have enough energy to eject two photons? Thus what would the real efficiency gain be assuming normal sunlight? Of course if you are also reducing thermal gain to the cell you might reduce loss of efficiency to increasing cell temperature as well.
It will be interesting to see what comes of this. Potentially a game changer.

Paul Adomshick

OK, I see that your headline has been updated. Disregard my previous comment.
REPLY: yes there’s a difference between overall efficiency and quantum efficiency, and it is important to make that clear – Anthony

Jim A

Brilliant! Pentacene + Dilithium Crystal – Add a mirror backing and a one way mirror, atop, and start it with a laser…
“Look out, Jim.. she’s gonna blow!”
Or is it really an ambient temp superconductor…
“The Power Companies hates this simple trick! Click here!”
“Well hell yeah, why not.. I was playing pool last night and pocketed 2 balls on break, twice, and 3 one time”
It’s April 28, fucrineouloud.

Q. Daniels

I keep seeing confusion about the First and Second Laws of Thermodynamics….
Converting local heat into electricity is a Second Law violation. It does not imply a First Law violation. It doesn’t mean you can use a unit to power itself and produce even more energy. That would be a First Law violation, and something completely different.
At most, this line leads to a situation where the conversion of phonons into electron/hole pairs is statistically more probable than the reverse. That isn’t anything like trying to convert previously non-existent energy into electron/hole pairs.

Choey Tuqiri

If the cell was 100% efficient you would not be able to see it since all the light striking it would be converted to electricity.

RS

Longevity may be an issue here, pentacene is sensitive to moisture and oxygen at the molecular level, The resulting solar panel will have to be perfectly sealed.

Bloke down the pub

The solar panels on my roof are reckoned to last about twenty five years before the output starts to drop off significantly. Just guessing, but if each photon starts knocking off twice as many electrons, doesn’t that imply that the panels will last half as long?

Steve

No, the electrons are circulated through the materlal and the external circuit. You don’t “run out of” electrons.

Choey Tuqiri

If the cells were 100% efficient you wouldn’t be able to see them since all the light striking them would be converted to electricity.

Typical thin film efficiencies aren’t much beyond 10%. Crystalline are around 16 to 18% as
I recall. Whether the greater efficiency is valuable depends on the added cost to achieve same.
My calculations show 80,000 acres of thin film commercial panels needed to produce the same amount of power as a modern nuclear reactor plant, although the power is uncontrollable and is
thus not worth anywhere near as much. If they manage to double efficiency of thin film commercial panels, then “only” 40,000 acres would be needed to match the bulk output of a nuclear
plant. Whoopee.

EW3

Reminds me of the math used to calculate GDP in Q1 2013.
They managed to show growth of 2.5% rather then a loss of .5%
😉

Yea I would be careful about this one. It looks like the quantum efficiency is increased not so much by this effect but by the mirroring of the light. Also, there is a huge gap between what looks good in the lab and what works in the manufacturing environment.

In the typical photovoltaic cell, a cubic crystalline Silicon base, with four outer shell electrons, is irradiated with Boron having five and Phosphorus having three outer shell electrons. Sunlight ‘excites’ the one, least stable, outer shell electron, which ‘flows’ thru the Phosphorus electron ‘hole’, exiting the cell as one-way, direct current. This is molecular erosion that never returns the mining, refining, manufacturing, transporting, installing and in less than 20 years, disposing of this solar Ouija board parlor trick. Forcing an extra electron out of one of these three grid materials would make only a marginal difference in this electron erosion process. More on this in, “The Green Prince of Darkness”….
One Wonders Where the World Would Be….Without these Wicked Weather Wiccans….
or the banking Ponzi scheme that has finally gotten so bad, even the greennies have noticed….
http://www.rollingstone.com/politics/news/everything-is-rigged-the-biggest-finacial-scandal-yet-20130425
[the green meanie thingy is rigged]

It still doesn’t address the problem of what to do when the sun goes down. What we need in panels that work off starlight. Until then, I’m planning on setting up a humongous flywheel in my basement. A thousand tons sounds right. During daylight energy would go into getting it spinning, and then you could draw energy from it at night.
(Of course, if everyone copies me it will likely have some strange, unintended consequence. Something I never considered about torque, or some such thing.) (Might be a good plot for a science fiction thriller……”Soon coming to a theater near you, ‘The day Earth stopped spinning.'”)

David L. Hagen

Encouraging insights.
Re: “commercial solar panels typically have an efficiency of at most 25%,”
This reflects the challenge of apples/oranges comparison differences in “efficiency”.
i.e., monochromatic photon to electron efficiency
broadband multispectral sunlight to electricity efficiency
Sunlight to delivered AC electricity etc.
See NREL’s graph of Best research-cell efficiencies
See: “Award-Winning PV Cell Pushes Efficiency Higher”

The three-layered cell, SJ3, converted 43.5% of the energy in sunlight into electrical energy — a rate that has stimulated demand for the cell to be used in concentrator photovoltaic (CPV) arrays for utility-scale energy production.
Last month, that record of 43.5% efficiency at 415 suns was eclipsed with a 44% efficiency at 947 suns. Both records were verified by NREL.

Rectenna potential > 70% conversion
The greater potential is to develop a commercial rectenna capable of directly receiving solar energy and converting it to DC electricity.
Solar Rectenna articles.
Efficiency limits of rectenna solar cells Saumil Joshi and Garret Moddel, APPLIED PHYSICS LETTERS
102, 083901 (2013)

We developed the calculations based on the theory of PAT and showed that, for diodes that are optimally matched to antennas, monochromatic power conversion efficiencies approach 100% and the multispectral efficiency is limited to 44% at solar terrestrial intensities. Spectral splitting can improve the efficiency, and can be accomplished simply by setting the operating voltage for each rectenna to rectify the desired spectral range.

UConn Professor’s Patented Technique Key to New Solar Power Technology

For years, scientists have studied the potential benefits of a new branch of solar energy technology that relies on incredibly small nanosized antenna arrays that are theoretically capable of harvesting more than 70 percent of the sun’s electromagnetic radiation and simultaneously converting it into usable electric power.

e.g. see Solar rectenna images.
PS Q. Daniels – cf “local heat into electricity is a Second Law violation”. Please rephrase/clarify. Every engine-generator or turbine-generator converts local heat into electricity while obeying the 2nd Law – i.e., with less than the Carnot efficiency.

I write for a sister site: http://www.ecnmag.com/tags/Blogs/M-Simon/

Gary

Cold frisson?

Catcracking

Anthony,
Given the huge number of exaggerated, false claims coming out of the Universities and the renewable energy business, I share the skepticism of each claim until the process has been demonstrated to be commercially viable and fully vetted by competent engineers and scientists. This is based on experience over many years since I have supported, via Engineering, numerous R&D projects that have not been commercially viable for a variety of reasons.
Given the above, I support fundamental research projects to find more efficient or new energy sources, but despise the rush to build commercial plants even demonstration plants with tax payer dollars before the proper research is completed. The DOE are incompetent to decide where to spread tax dollars around and actually hurt development by pushing losers. Recently I have seen, from the inside, biofuel projects that have utterly failed because the work process is totally broken, and the shortcut is to get government funding for expensive demonstration scale plants before the technology is ready from the bench scale research/development.
While I am skeptical, I am hopeful that someday there will be a breakthrough.
If this is real and can be commercially applied, think how stupid our governments have been to force commercialization of useless solar facilities that were not actually commercially ready for implementation; and now we have numerous white elephants on the ground at huge taxpayer expense while connected friends of the government have enriched themselves by selling extravagantly priced electricity which forces rate payers into near poverty and kills industry. .

KevinM

Choey Tuqiri says:
Yes, it’s true. That would be an awesome stealth tech. But I fear that you would have to approach zero Kelvin to achieve that. The pilot would need big mittens to fly the jet.

This is a fascinating post. Of course solar cells still have to be put into perspective. Ozzie Zehner is the ridiculously anti-consumption author of Green Illusions, yet he still does the best takedown of PV solar I’ve ever seen:

” Choey Tuqiri says:
April 28, 2013 at 10:47 am
If the cell was 100% efficient you would not be able to see it since all the light striking it would be converted to electricity.”
It works by “PFM”, Chooey. A nebulous ill-defined phenomenon. Kind of like Quanom Physics

Q. Daniels

David L. Hagen wrote:

PS Q. Daniels – cf “local heat into electricity is a Second Law violation”. Please rephrase/clarify. Every engine-generator or turbine-generator converts local heat into electricity while obeying the 2nd Law – i.e., with less than the Carnot efficiency.

If the energy of an incoming photon generates carrier pairs with more energy (pair-count times bandgap) than the photon, the extra energy would come from local thermal energy. In effect, the carrier pair energy would be “topped up” by local thermal energy. That would be a local violation of the Second Law, which in itself is not very interesting. At that scale, they happen constantly.
If the average extracted energy exceeded the energy of all incoming photons for an entire device, the device as a whole would be converting its internal thermal energy into externally available power. It would get cold when exposed to light. That would violate the Second Law. It would not, however, violate Conservation of Energy (the First Law).

NikFromNYC

Pentacene is one of the nicely tough molecules we have actual AFM microscope images of, including the hydrogens:
http://www.newscientist.com/article/dn17699-microscopes-zoom-in-on-molecules-at-last.html

jim2

From the paper:
“Since this was just a first proof of principle, the team has not yet optimized the energy-conversion efficiency of the system, which remains less than 2%. 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.”
Of course, there is no fundamental barrier to fusion energy, either. It’s always the perennial 50 years in the future.

OldWeirdHarold

Next question: how much does pentacene cost?

Kasuha

I don’t see any particular problem with efficiency over 100% as long as 100% efficiency is defined as ratio of number of incoming photons and number of electrons transferred over cell’s energy barrier. Crossing that barrier requires certain energy but incoming photons may carry more than that energy. If they managed to transfer two electrons before the excess energy is dissipated as heat, no physical principles got violated.

Joseph A Olson – I’m interested by your comment, and just want to make sure I have understood it correctly. You say “This is molecular erosion that never returns the mining, refining, manufacturing, transporting, installing and in less than 20 years, disposing of this solar Ouija board parlor trick. Forcing an extra electron out of one of these three grid materials would make only a marginal difference in this electron erosion process“. Are you saying that :
– The process removes electrons from the material.
– Those electrons are not replaced.
– In less than 20 years, the material is so depleted of electrons (“molecular erosion“) that it has to be replaced.
– The energy cost (“mining, refining” etc) of making and installing the material is greater than the energy generated during its lifetime.
– Knocking out 2 electrons at a time instead of 1 doesn’t change the basic equation.
Is there anywhere we can verify what you say, eg. by accessing figures for total energy costs and output?

Werner Brozek

Q. Daniels says:
April 28, 2013 at 12:13 pm
If the average extracted energy exceeded the energy of all incoming photons for an entire device, the device as a whole would be converting its internal thermal energy into externally available power. It would get cold when exposed to light. That would violate the Second Law.
I could certainly be wrong here, but in a different context, is it not possible for infrared light to shine on the surface of the water in such a way that the energy is just a bit less than the energy required to evaporate a water molecule? However due to the Boltzmann distribution, is it not possible for a more energetic water molecule to be struck and have it evaporate thereby lowering the temperature of the remaining water without violating any laws?

otsar

Pentacene is a good material for making sensitive X ,Y array photon detectors when combined with PbS, P and N nanoparticles, however, it degrades at the high power levels needed for power generation.

Photons come in various frequencies which we interpret, in the visual range, as colors. The different frequencies correspond to different energies E = hv where h is a constant and v is the frequency.
The energy you get from an electron depends on the voltage you move it across. That is, the unit of energy is electron-volt.
So what’s going on here is that they’re getting two electrons to move over a lower voltage while previously technology had one electron moving over a higher voltage. Whether this is better or worse than before depends on the ratio of the voltages.
But generally, this is a well known result in the solid state physics community. It’s outside my area but I vaguely remember going to a talk that mentioned the subject this past semester.
It’s really no big deal, just steady attempts at improving efficiency of solar cells. The journalists end up mangling some of the message.

About “greater than 100%”: This device does that: http://www.hidroonline.com/ [An otherwise reputable Australian university was going to install and demonstrate a very expensive large-scale version of this machine. I wrote to them, explaining the physics and the exact way in which it could never be a net generator of energy, and I urged them not to go ahead with it. They did. A few decades ago, any university’s BS antennae would have instantly picked that there was a problem.]
Still on “greater than 100%” : I can theoretically get a greater than 100% return on the electricity that runs my heat pump. But that is because some of the delivered energy comes from underground (which is left cooler) not directly from the electricity input. In the case of the photovoltaic material, I suppose it would be theoretically possible to get above “100% efficiency” if the material cooled. Doesn’t sound even remotely possible in practice, though.

otsar

Pentacene is only one of the important ingredients. There are other ingredients and the matter of the entire electron transport structure. Sensors made with these technologies come close to what night vision goggles can do. Imagine your smart phone enabling you to see at very low light levels. The problem is however is that your face is lit up by the phone screen.

kadaka (KD Knoebel)

Thin flexible solar cells are a proven technology, companies like United Solar Ovonic were making sheets that could cover a football field which were cut into individual products, like their popular peel & stick “roofing” rolls. December 2012, Stanford announced flexible cells that could be stickers. January 2013, Swiss researchers announced a record-breaking 20.4% energy conversion efficiency, with a flexible cell on a polymer film substrate.
Basically, they can print cells. It’s proven, they do it regularly.
So what will be an upcoming major solar panel development, that might actually be worth something?
The dimple panel. Why print only flat? Have a “dimpled” surface, with a pattern of recesses, squares would be good, perhaps 2 to 3 inches on a side. That increases the surface area of cells without increasing the panel footprint, with increased output. With encapsulated panels, the cells are coated rather than protected with a flat sheet of glass, the horrible ugly glare from solar panels is practically non-existent.
This would be far easier than you may think, as the “print head” would have to move in another direction (towards & away from the surface) at high speed, but synchronized 3-axis motion is relatively ancient technology in machining and other manufacturing.
Thus there shall be thin dimple panels, with some competition between different makers as to superiority of their own pattern, between “dimples” and “bumps”, etc. Cheap, basically we already have the technology. They will also have some built-in flexibility, you can screw them down directly onto an uneven or somewhat-curved surface, and not be as concerned with flatness as with current rigid panels.
Heck, you won’t even need the usual panel mounting track system. Just spacers to keep them off the roofing, and long lag bolts, screw them right to the rafters.
Increasing panel efficiency by material technology is very difficult, to do so in a commercially-viable manner is the Holy Grail of solar power research. Much money has been spent pursuing the goal, including much taxpayer money “for our own good”.
But a quick increase in output of perhaps 50%, using existing technology, would be so cheap and easy to accomplish, manufacturers would be dead stupid to not try it.
Wait for it, it will happen.

otsar

After all is said and done in photon energy capture, it goes without saying, there is no free lunch. There are rare situations in PIN diodes, when the photon counts are high enough, that two photons arrive in coincidence and produce a sum peak, in other words the number of electron hole pairs are those that would have been produced by a photon of twice the energy of an identifiable energy peak of incoming photons. After all is said and done E=H nu.
A lot of these materials can also act as photon emitters. Some losses in these materials are also due to electron recombination that lead to fluorescence.
Squeezing 1% more efficiency out of PV devices should not be trivialized, it is just as important as squeezing 1% more out of an oil field, or a power plant.

A Crooks

I sure hope it works but ….
How much energy does it take to make pentacene? And is it used up during this process?

Rob Dawg

So. It appears God, indeed, does not play dice with the universe. He plays billiards.

Q. Daniels
Thanks for clarifying what you said on local energy and the 2nd Law.
You noted: ” It would get cold when exposed to light. That would violate the Second Law.”
However, it is possible for selection of electrons can cause cooling. e.g. see Introducing the quantum refrigerator.
Do first principles necessarily forbid cooling by photon conversion to electricity?

Chuck Nolan

RobRoy says:
April 28, 2013 at 11:46 am
” Choey Tuqiri says:
April 28, 2013 at 10:47 am
If the cell was 100% efficient you would not be able to see it since all the light striking it would be converted to electricity.”
It works by “PFM”, Chooey. A nebulous ill-defined phenomenon. Kind of like Quanom Physics
——————————
That would be Photon Frequency Modulation.
I think I’ve read about that.
cn