Scientists report first solar cell producing more electrons in photocurrent than solar photons entering cell

December 15, 2011, National Renewable Energy Laboratory

(PhysOrg.com) -- Researchers from the National Renewable Energy Laboratory (NREL) have reported the first solar cell that produces a photocurrent that has an external quantum efficiency greater than 100 percent when photoexcited with photons from the high energy region of the solar spectrum. 

The external for photocurrent, usually expressed as a percentage, is the number of electrons flowing per second in the external circuit of a solar cell divided by the number of photons per second of a specific energy (or wavelength) that enter the solar cell. None of the solar cells to date exhibit external photocurrent quantum efficiencies above 100 percent at any wavelength in the solar spectrum.

The external quantum efficiency reached a peak value of 114 percent. The newly reported work marks a promising step toward developing Next Generation Solar Cells for both solar electricity and solar fuels that will be competitive with, or perhaps less costly than, energy from fossil or nuclear fuels.

Multiple Exciton Generation is key to making it possible

A paper on the breakthrough appears in the Dec. 16 issue of Science Magazine. Titled “Peak External Photocurrent Quantum Efficiency Exceeding 100 percent via MEG in a Quantum Dot Solar Cell,” it is co-authored by NREL scientists Octavi E. Semonin, Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, Arthur J. Nozikand Matthew C. Beard. The research was supported by the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the DOE Office of Science, Office of Basic Energy Sciences. Semonin and Nozik are also affiliated with the University of Colorado at Boulder.

The mechanism for producing a quantum efficiency above 100 percent with solar photons is based on a process called Multiple Exciton Generation (MEG), whereby a single absorbed photon of appropriately high energy can produce more than one electron-hole pair per absorbed photon.

NREL scientist Arthur J. Nozik first predicted in a 2001 publication that MEG would be more efficient in semiconductor quantum dots than in bulk semiconductors. Quantum dots are tiny crystals of semiconductor, with sizes in the nanometer (nm) range of 1-20 nm, where 1 nm equals one-billionth of a meter. At this small size, semiconductors exhibit dramatic effects because of quantum physics, such as:

• rapidly increasing bandgap with decreasing quantum dot size,
• formation of correlated electron-hole pairs (called excitons) at room temperature,
• enhanced coupling of electronic particles (electrons and positive holes) through Coulombic forces,
• and enhancement of the MEG process.
Quantum dots confine the charges and harvest excess energy

Quantum dots, by confining charge carriers within their tiny volumes, can harvest excess energy that otherwise would be lost as heat – and therefore greatly increase the efficiency of converting photons into usable free energy.

The researchers achieved the 114 percent external quantum efficiency with a layered cell consisting of antireflection-coated glass with a thin layer of a transparent conductor, a nanostructured zinc oxide layer, a quantum dot layer of lead selenide treated with ethanedithol and hydrazine, and a thin layer of gold for the top electrode.

In a 2006 publication, NREL scientists Mark Hanna and Arthur J. Nozik showed that ideal MEG in solar cells based on quantum dots could increase the theoretical thermodynamic power conversion efficiency of solar cells by about 35 percent relative to today’s conventional solar cells. Furthermore, the fabrication of Quantum Dot Solar Cells is also amenable to inexpensive, high-throughput roll-to-roll manufacturing.

Such potentially highly efficient cells, coupled with their low cost per unit area, are called Third (or Next) Generation Solar Cells. Present day commercial photovoltaic solar cells are based on bulk semiconductors, such as silicon, cadmium telluride, or copper indium gallium (di)selenide; or on multi-junction tandem cells drawn from the third and fifth (and also in some cases fourth) columns of the Periodic Table of Elements. All of these cells are referred to as First- or Second-Generation Solar Cells.

First experiment to show 100-percent-plus in operating solar cells

MEG, also referred to as Carrier Multiplication (CM), was first demonstrated experimentally in colloidal solutions of quantum dots in 2004 by Richard Schaller and Victor Klimov of the DOE’s Los Alamos National Laboratory. Since then, many researchers around the world, including teams at NREL, have confirmed MEG in many different semiconductor quantum dots. However, nearly all of these positive MEG results, with a few exceptions, were based on ultrafast time-resolved spectroscopic measurements of isolated quantum dots dispersed as particles in liquid colloidal solutions.

The new results published in Science by the NREL research team is the first report of MEG manifested as an external photocurrent quantum yield greater than 100 percent measured in operating quantum dot solar cells at low light intensity; these cells showed significant power conversion efficiencies (defined as the total power generated divided by the input power) as high as 4.5 percent with simulated sunlight. While these are un-optimized and thus exhibit relatively low power conversion efficiency (which is a product of the photocurrent and photovoltage), the demonstration of MEG in the photocurrent of a solar cell has important implications because it opens new and unexplored approaches to improve solar cell efficiencies.

Another important aspect of the new results is that they agree with the previous time-resolved spectroscopic measurements of MEG and hence validate these earlier MEG results. Excellent agreement follows when the external quantum efficiency is corrected for the number of photons that are actually absorbed in the photoactive regions of the cell. In this case, the determined quantum yield is called the internal quantum efficiency. The internal quantum efficiency is greater than the external quantum efficiency because a significant fraction of the incident are lost through reflection and absorption in non-photocurrent producing regions of the cell. A peak internal quantum yield of 130% was found taking these reflection and absorption losses into account.

Explore further: Study predicts nanoscience will greatly increase efficiency of next-generation solar cells

More information: Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell, Science 16 December 2011: Vol. 334 no. 6062 pp. 1530-1533. DOI: 10.1126/science.1209845

ABSTRACT
Multiple exciton generation (MEG) is a process that can occur in semiconductor nanocrystals, or quantum dots (QDs), whereby absorption of a photon bearing at least twice the bandgap energy produces two or more electron-hole pairs. Here, we report on photocurrent enhancement arising from MEG in lead selenide (PbSe) QD-based solar cells, as manifested by an external quantum efficiency (the spectrally resolved ratio of collected charge carriers to incident photons) that peaked at 114 ± 1% in the best device measured. The associated internal quantum efficiency (corrected for reflection and absorption losses) was 130%. We compare our results with transient absorption measurements of MEG in isolated PbSe QDs and find reasonable agreement. Our findings demonstrate that MEG charge carriers can be collected in suitably designed QD solar cells, providing ample incentive to better understand MEG within isolated and coupled QDs as a research path to enhancing the efficiency of solar light harvesting technologies.

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33 comments

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Osiris1
2.7 / 5 (10) Dec 15, 2011
I LIKE IT....do it again.....I LIKE IT! Pave the southwest with it like the Arabs are gonna pave the Sahara. Then both Europe and America have plenty of energy and no waste heat or CO2!
Nanobanano
3 / 5 (10) Dec 15, 2011
I LIKE IT....do it again.....I LIKE IT! Pave the southwest with it like the Arabs are gonna pave the Sahara. Then both Europe and America have plenty of energy and no waste heat or CO2!


Solar boilers are actually cheaper and more efficient for large scale operations for commercial and industrial power.

This invention is probably best used for small scale stuff where land area is at a minimum, like the power supply of a space probe, or an individual residence.
astro_optics
1.1 / 5 (13) Dec 15, 2011
Dude...where do you think that the energy released from the "Eco-Friendly Solar Generated" electrical power will go to?
astro_optics
1.3 / 5 (16) Dec 15, 2011
I'll answer it for you ...back into the atmosphere! The renewables will not eliminate the Global Warming, and just might do the opposite ironically!
chip_engineer
3.1 / 5 (8) Dec 15, 2011
" or perhaps less costly than, energy from fossil or nuclear fuels."

While very interesting I doubt the numbers will work out.

As an example the Long Island solar plant that just went up claims 32MW to power 4500 homes. With a COP of 7, it is really 4.5MW continuous and actually cost $298M so $65/W. It is the tombstone for the Shoreham plant shuttered by greenpeace in the 70s.

With this tech, it could "perhaps" make several times the power, the panels would likely be far more expensive than run of the mill.

The latest small nuclear story on nextbigthing suggests plants can be built for $4.2/W at the 300MW range for mass produced small reactors that can be transported on trains using Henry Ford style production rather than the individual custom designs we have had so far.

Of course nothing beats fossil when you don't have to pay for the carbon. If you could do coal with no CO2 emissions, it would produce no energy in the first place unless there is a magic catalyst out there.
chip_engineer
2.1 / 5 (9) Dec 15, 2011
astro optics

"The renewables will not eliminate the Global Warming, and just might do the opposite ironically!"

Exactly, renewables are land use and capital challenged esp solar and produce hugely overstated amounts of power. Its like they always work 24/7 at the nameplate power. The Treehuggers don't understand the COP factor that kills the numbers. There is also the little issue of indium and other rare earths needed for these.

Does anyone have figures for other pure solar or wind plants that produce actual continuous power numbers and total costs. The Long Island example seems representative to me, it would be much cheaper in the south though or even just put small PV installs on roofs.
chip_engineer
2.5 / 5 (8) Dec 15, 2011
TSI

Take a look at "flowcharts llnl gov".

The USA uses up around 100Quads or 1e20J annually mostly thermal. That is equiv to 3200GW thermal or 1000GW as electric power.

Assume 130M dwellings and you get 8kW/house or 3.3kW/person which doesn't jive with your typical 1kw home rate. This is the actual footprint of your share of US consumption as electrical (thermal is 3x that). I figure the panels would cover 4 typical roofs per person. Your body needs 10MJ or 100W continuous thermal from food, which is about 1% of your true energy footprint.

If the USA built 1000GW or 1M 1MW solar plants the cost would be $55 trillion using Long Island example with COP of 6. The acreage would be >40M acres. We don't really want it all as electric so we need some proportion as fuels.

If every single roof had continuous 1kW of PV then it gives us maybe 130GW and a grid that would fall apart. We still gonna need nuclear and fossils (with CO2) for heat and other electric or start cutting back.
chip_engineer
3.7 / 5 (6) Dec 15, 2011
VD said

Every watt generated by solar is a watt not generated by coal, oil, natural gas etc.

That is true but...

It is also more expensive than they would ever want us to know about, took some searching to find the actual price of the Long Island Tombstone plant that replaced a perfectly good nuclear plant that was built for 1% of the equiv energy price even with todays dollar.

Shoreham 540MW for $75M in 1973 (today about $350M) vs 4.5MW for $300M. One was stable baseload on a small piece of land, the other is erratic on a large piece of land. Newer and much better passive safe nuclear would be closer to $4/W vs Tombstone $65/W.

Lets do that 220,000 more times to cover the USA demand for 100 Quads of equiv electrical energy.
Osiris1
3.3 / 5 (7) Dec 16, 2011
I can cover my bus with about 2.5kW of panels, keep the buffer batteries charged, and actually sell power back to SCE after subtracting home use most of the times of the day except heavy AC use or cooking. At nite its another story, but then stored power in the batteries comes back on line. There are three persons and a little doggie in our house and we do not overuse power. All lites are CF, and screens are LCD...no CRTs. There really is a case for household solar to stop power plant greed, especially in Southern California where there is LOTS of sun and usage unfriendly avaricious five tier rates that eat your lunch in the fifth 'tier'...tear!
chip_engineer
1.7 / 5 (6) Dec 16, 2011
VD

You are nuts if you really believe that solar/wind is cheaper than nuclear per MW. Of course the greens have made nuclear artificially higher priced than it would have been, didn't help that the plants were not standardized and mass produced either. Most of the worlds plants were designed before computers were everywhere.

Without the 100Quads/yr, life will roll back to the way it was a few hundred years ago, lots of extinction along the way, it will be violent for sure. Humans will survive, few conveniences will work, some PV may work for awhile. Coal, biomass will be the new home fuel and the cycle repeats again.

We have basically climbed a tree and now looking down can't see a way to climb down.

Soylent movie, yummy.
chip_engineer
3.7 / 5 (3) Dec 16, 2011
VD

Bushies war could pay for alot of things so you could pick any power source you want for trillions.

PV panels don't last 55 years, well maybe 30 years.

Same wind turbines, the towers fatigue, need to be replaced.

Also PV and wind are intermittent, you know that, they contribute to the grid when ever they feel like, while something else keeps the grid up like fossils or nuclear. Could change if Isentropic or other huge storage comes along.

So why not explain how COP works then, do you really think the plant produces 32M avg because if it did it would power 32000 homes and then some. Try asking a solar installer for a quote for say 700W continuous. I was quoted $25k although subsidies would hide half of that. So 32MW is $1.1B at a residential rate.

If they really got 32MW continuous for $300M, they would need to get 224MW of nameplate panels for just $1.34/W and include frames, inverters, and all the rest of the systems inside that price. I don't buy that.
chip_engineer
2.8 / 5 (4) Dec 16, 2011
VD

"But in reality you only have to get down to an 80% to 90% reduction in CO2 emissions, and solar is only part of the solution. Efficiency improvements for example can get America most of the way to to sustainability, so solar need be implemented to only half of the extent estimated above. And if you include ##geothermal##, and wind and ######nuclear###### in equal proportion then you are down to 1/4 of that again. So say an initial investment in solar Cont."

WOW
We could actually call it even right there. So you will accept nuclear in the mix, because I can accept cheap solar on my roof where it belongs if I can just get the 50c/W panels to pass code (not gonna happen). So now we argue over practical proportions.

I think your ambitions for energy savings only go so far, say the European level, maybe the 60s with internet, really good insulation, heat pumps, denser population, public transport, less consumption so on, maybe we could make it.

Another time...

goodnight
Skepticus
3 / 5 (8) Dec 16, 2011
I see a bunch of timid old men who are afraid of the new world. The Chinese are laughing and building solar tech like there is no tomorrow, and cheaply too. That's why the US is accusing China of dumping, because all they do is sitting around arguing on Physorg and lament the costs, the disadvantages, blah blah blah instead of research and development. Tragic.
Husky
5 / 5 (5) Dec 16, 2011
Skeptic, and the chinese are building reactors like no tomorrow also, they go all out on the whole energy mix

I applaud any efforts to make energy more affordable/greener, including wind/solar but watching how the tech is trending I have higher hopes for smaller mass produced molten salt reactors and thorium based travelling wave reactors. Old style Nuclear has its own problems, mainly with their pressured water cooker nature and mechanical pumps being the common point of failure, but removing the pressure and the pumps using molten salts and thermo/gravitational convection flow combined with high burnup rates of the fuel that should take safety/green/efficiency to the next level in a way that would be hard to match with wind/solar
antialias_physorg
5 / 5 (3) Dec 16, 2011
Every watt generated by solar is a watt not generated by coal, oil, natural gas etc.

While I'm all for solar (the more the better) this is not entirely true.

Due to the short term variability of solar some reserve capacity of other power plants has to be kept going for a stable grid. This usually means that some conventional powerplant has to be kept running to pick up any unexpected slack (though that powerpant need not run on a 1:1 basis - it can run at somewhat lower power). The problem is one shared by wind energy.

This is why we absolutely need a buffer system (hydrodynamic, battery, compressed air, hydrogen, whatever) in the long run if the changeover is ever to be complete.

Alternatively we can switch over to powerplants that are online at a moment's notice (gas, pump storage hydro powerplants, etc. ). But coal - and especially nuclear - have far to long run up times to be shut down entirely when solar produces enough energy.
Husky
not rated yet Dec 16, 2011
shalegas is the new carbonblack
vlaaing peerd
3.3 / 5 (3) Dec 16, 2011
@chipengineer. cheaper...more expensive, unconstructive thinking in contempary terms as costs, the money put into it goes to paying people that build the thing and they put that same money back into society. The world only loses money by interest, not by investing it. The only thing is that it might not end up in your pocket.

It also blocks you in seeing any other advantage of renewable energy?

If your country can spend 1000.0000.0000.000 on a war, it sure doesn't seem unreasonable to invest in oil-independency. Think of it...never having to buy oil again. Might save some of that precious money on future wars over it as well.

The disaster scenario you describe is also a tat oversimplistic. Would be better if you had some facts to back such claims.
_nigmatic10
3 / 5 (5) Dec 16, 2011
@vlaaing peerd.

As long as there are machines with moving parts, the need for oil will never be eliminated, and the only reason "our country" isn't oil independent is due to contracts, not lack of resource.
rawa1
1 / 5 (5) Dec 16, 2011
Theoretically it could be possible to drain electrons just from external heat. The panel would could itself bellow environmental temperature, which makes no problem, as there is lotta environmental heat available.
antialias_physorg
5 / 5 (4) Dec 16, 2011
The panel would could itself bellow environmental temperature

An you keep the panel below ambient temperature exactly how...? Magic?
rawa1
1 / 5 (5) Dec 16, 2011
With draining electricity from it. Quantum mechanics allows temporal violation of 2nd law of thermodynamics.
antialias_physorg
5 / 5 (3) Dec 16, 2011
You should patent your Maxwell's Demon and perpetuum mobile machine then.
Because that is what you have just proposed.
AWaB
2.5 / 5 (4) Dec 16, 2011
I'll answer it for you ...back into the atmosphere! The renewables will not eliminate the Global Warming, and just might do the opposite ironically!


Unfortunately you have a point. What's been seen from the current solar farms is that they bake the ground underneath them causing everything to die. Since this is occurring, I think you're probably right about excess heat being returned to the atmosphere, much like concrete does. Food for thought, anyway!
rawa1
1 / 5 (4) Dec 16, 2011
You should patent your Maxwell's Demon and perpetuum mobile machine then. Because that is what you have just proposed.
These devices exist already and it has no meaning to speculate about it. The quantum statistics differs from this classical one http://arxiv.org/.../0311104 The amount of energy extractable in such way is definitely desperately limited, but it's still worth of further research.
eigenbasis
5 / 5 (1) Dec 16, 2011
"these cells showed significant power conversion efficiencies (defined as the total power generated divided by the input power) as high as 4.5 percent with simulated sunlight"

Read thoroughly, the quantum efficiency of the cell stimulated over 100% of photons into conduction electrons. This is not a power efficiency, no free energy machines yet!
dnatwork
3 / 5 (2) Dec 16, 2011
Every time, I hear "current typical use in USA." The current level of energy consumption is hugely wasteful. Better insulation would eliminate most of the need for heat. Higher efficiency devices and appliances would eliminate most of the need for electricity. Cut our consumption by half or three-quarters, then redo the math. Nothing is going to happen overnight anyway, so attack the problem from both sides.
Skepticus
3 / 5 (2) Dec 17, 2011
my co-worker's ex-wife makes $69/hr on the computer. She has been unemployed for 7 months but last month her income was $7180 just working on the computer for a few hours. Read more here... MakeCash7.com

Yeah sure. Must be wanking 24/7 in front of a webcam to make that sort of money at that rate!
kochevnik
1 / 5 (1) Dec 17, 2011
Small quantities of ZPE can be extracted over long periods of time but the energy is conserved in that the system can not be reset to a higher energy level without pumping the energy you originally got out back into the system.
This is correct. Energy is completely conserved and no ZPE machine can function...in four dimensions. In 3D we can violate it freely because energy is NOT conserved. I'd go so far as to say that much of the world around us is due to violating energy conservation laws. Time itself may be nature's way of accounting for our cheating so that balance persists in 4D.
kochevnik
not rated yet Dec 18, 2011
Looks like the positivist/neocon rank-terrorist Noumenon slept though his relativity classes. Failed basic spinor [electron] theory.
despinos
not rated yet Dec 19, 2011
As usual with this type of articles, (most) people just argue about the "economics" of photovoltaics vs. nuclear power and the cost of the Irak war and bad-boy-Bush...

Is no one paying attention to the interesting part of the article, multiple exciton generation (MEG)?
bluehigh
1 / 5 (4) Dec 19, 2011
Failed basic spinor [electron] theory.
- kochevnik

Are they still teaching that crap in schools?

Electrons are a spherical shell of zero thickness surrounding the nucleus that in free space would consist of a two dimensional disk of spinning charge.
jumojack
not rated yet Dec 22, 2011
I'll answer it for you ...back into the atmosphere! The renewables will not eliminate the Global Warming, and just might do the opposite ironically!

Not at all! Fossil fuels bring in the present era CO2 "stolen" from the carbon-cycle of dozens of milions of year ago and trapped into oil and carbon.
Solar panel just turn into electricity solar energy which would ANYWAY come to Earth, without adding anything to the global system.
jumojack
not rated yet Dec 22, 2011
BTW, which are the figures for "Photocurrent Quantum Efficiency" of CURRENT solar panels?!? This "110%" means nothing to me if I don't have a comparable value!

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