May 24, 2005
Quantum Dots May Boost Photovoltaic Efficiency To 65%

Photovoltaic solar cell efficiency might be boosted to 65% by use of quantum dots.

Golden, Colo. — Researchers at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have shown that nanotechnology may greatly increase the amount of electricity produced by solar cells.

In a paper published in a May issue of the American Chemical Society's Nano Letters journal, an NREL team found that tiny "nanocrystals," also known as "quantum dots," produce as many as three electrons from one high energy photon of sunlight. When today's photovoltaic solar cells absorb a photon of sunlight, the energy gets converted to at most one electron, and the rest is lost as heat.

The research demonstrates the potential for solar, or photovoltaic, cells that reduce wasteful heat and maximize the amount of the sun's energy that is converted to electricity—a key step toward making solar energy more cost-competitive with conventional power sources.

The NREL research team, led by Arthur Nozik, included Randy Ellingson, Matt Beard, Justin Johnson, Pingrong Yu, and Olga Micic, and worked in collaboration with theorists Alexander Efros and Andrew Shabaev of the Naval Research Laboratory (NRL) in Washington, D.C.

The findings are further confirmation of pioneering work by Nozik, who in 2000 predicted that quantum dots could increase the efficiency of solar cells, through a process now termed "multiple exciton generation," or "MEG". Last year, Richard Schaller and Victor Klimov of Los Alamos National Laboratory in New Mexico were the first to demonstrate the electron multiplication phenomenon predicted by Nozik, using quantum dots made from lead selenide.

They say the existing solar cells go as high as 33% efficiency of conversion. But production solar cells installed on roofs are typically much lower efficiency than that. So if this new approach could be manufactured cheaply it would be at least 3 times higher in efficiency than existing manufactured solar cells.

"We have shown that solar cells based on quantum dots theoretically could convert more than 65 percent of the sun's energy into electricity, approximately doubling the efficiency of solar cells," Nozik said. The best cells today convert about 33 percent of the sun's energy into electricity.

The NREL and NRL researchers' paper also describes a new theoretical foundation for the multiple exciton generation process that is based on certain unique aspects of quantum theory.

The recent work demonstrates MEG in quantum dots of a second semiconductor material, lead sulfide.

The NREL/NRL work not only shows higher overall efficiency for multiple exciton generation,

it also establishes that the process occurs with lower photon energies, meaning it could make use of an even greater portion of the sun's light spectrum.

I am just guessing but this approach might be cheap to manufacture. Lead and sulfur are cheap. Selenium is also used (see below). Is selenium expensive?

Note above the reference to work by Richard Schaller and Victor Klimov that provided experimental evidence that led to this work. See my post from April 26, 2004 "Nanocrystal Photovoltaics May Achieve 60% Conversion Efficiency" for more on Schaller and Klimov's work.

From the abstract.

We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap.

Thanks to Dave Gobel for the tip.

Share |      Randall Parker, 2005 May 24 10:15 AM  Energy Solar

anonymous said at May 24, 2005 10:47 AM:

these are the kind of breakthroughs that can change energy economics

the raw elements have very little to do with costs.... for example silicon wafers are expensive, but Si is the most abundant element in earth's crust. the high cost has everything to do with processing. i am glad it does rely on tantalum and/or scandium though

Joseph said at May 24, 2005 4:49 PM:

Okay missing something here. I see the higher effeciency for the dots by better utulization of the photons but is this only in certain frequencies or across a wide band? Not being picky just uncertain. I saw the origional post related to a broad spectrum capture potential but didn't see that speciefically addressed with the quantum dots.

Rob McMillin said at May 24, 2005 6:28 PM:

Selenium has been cheap in the past, but its price has skyrocketed recently. Wikipedia reports it is a byproduct of copper mining, but also a good deal of Central Valley (California) farmland is contaminated by selenium salts from selenium-laden irrigation water, which would tend to indicate selenium is fairly common in nature. However -- all that said -- Yukon Zinc, a Canadian company purporting to potentially hold 15% of known world selenium reserves, notes (PDF) that price has increased 1,466% over the last 18 months (from $3.75/lb to $55/lb) principally because of sudden demand from China. Orders outstrip supply at the moment. However, the market for selenium is highly volatile.

Braddock said at May 25, 2005 5:39 AM:

High efficiency is only helpful when paired with low cost. Otherwise simply installing numerous cheap and less efficient units will work as well when producing bulk electrons. Economics kills solar time and again.
The problem with solar is low availability (few hours a day), high cost, poor load matching (no output in early evening), highly seasonal (poor winter production). High efficiency does not solve those problems.
Leave the efficiency the same but lower the cost plus improve energy storage. Then you will see an impact.

Engineer-Poet said at May 25, 2005 6:33 AM:

The biggest single electrical load in most of the USA is air conditioning.  We've got a very simple and cheap storage scheme for this; it's called ice.

Joseph:  The conversion improvement appears to be broadband.  Photons with 2x the band-gap energy can create two charge carriers, photons 3x can make 3, etc.  Excess energy that's not sufficient to make another carrier pair winds up as heat, but that's a lot better than ALL the energy greater than the band gap (0.7 eV in the case of silicon) going to heat.  A red photon at 8000 Å has 1.55 eV of energy; a blue photon at 4000 Å has twice that.

Zaiden said at May 25, 2005 9:19 AM:

Hmmm, I'd heard that biological organisms' famed photosynthesis process only harnesses 1% of received solar energy, is this true? If so... Would this mean that an artificial replicator(using this) would have 6500% higher energy acquisition capability compared to a natural one, substantially freeing it from energetic constraints?

Joseph said at May 25, 2005 1:22 PM:


Ahh okay, my memory retention of simple physics is atrociouse after all these years.

This does have considerable potential even if obcenely expensive. As an example I found a reference to using such cells on hybrid/FC vehicles with the author estimating a 5 year payback in normal sunlight conditions.

I do find the hype a bit annoying at times though. By hype I mean there's announcements then...nothing. The paint on solar cell is a prime example. It's been advertised before as being on the verge of production yet I still don't see anything on the market. It's hard to really make an estimate under these conditions.

aluxeterna said at May 27, 2005 3:31 PM:

So, if this technology ever saw widespread use, could we expect an over-all cooling of the earth? Could really effective solar-energy panels actually present a danger to the ecosystem?

Tj Green said at May 29, 2005 4:15 PM:

We could see a cooling of the earth because quantum dots can also be used in solid state lighting(LED)which would cut global energy requirements dramatically.A reduction in CO2 would cause our planet to cool,which would cause more wind,which would cause sand from the deserts to be deposited on the oceans,which would cause the algae to grow faster,which would remove more CO2,which would cause more cooling. Which means we can all stop worrying about global warming and start worrying about a new ice age.

Tobias said at June 2, 2005 7:40 AM:

Zaiden: I've also heard that the photosynthesis is around 1% efficient in converting suns energy to chemical energy in biomass. However, natures own self-assembling and self-replicating nanomachines beat any artificial system in cost and robustness as they are independent of rare raw materials and adaptive to varying operating conditions.

Aluxeterna: You are joking, right? Widespread use of solar panels has a potential to cut CO2 emissions but that would hardly cause cooling, just slow down warming. Whether or not we use solar panels have no effect on influx of solar radiation. This planet has got along fine for billions of years without human input of atmospheric gases and would still do better without.

Tj Green: What a load of BS! In a cooler planetary atmospheric and oceanic system there's _less_ energy to cause winds. Sand in oceans has hardly an effect on algae growth.

Tj Green said at June 6, 2005 11:05 AM:

Tobias I disagree. Life reduces entropy. Life blocks energy from the sun with dimethyl sulphide(DMS). If you get an over production of DMS you get a runaway effect to an ice age. Our weather is caused by our movement around the sun,and biological systems,in an elaborate feedback loop. An excellent book on this subject is "Deep simplicity-chaos,complexity,and the emergence of life" by John Gribben.

R. Mastromatteo said at June 10, 2005 2:38 PM:

Lets everyone rain on the parade eh!
The first eco friendly good news in the energy field in decades and all the nay sayers crawl out of the woodwork!
I for one welcome the prospect of quantum dot plastifilm photovoltaics, the implications for the future are staggering!
1. Use of the technology to convert water to hydrogen (for liquification)for replacement of fossil fuels, or stored for use during none daylight or cloudy periods.
2. Use of photovoltaics to convert sea water to fresh water for water starved coastal areas.
3. Direct home power for use in air conditioning independent of the power grid!
These three areas alone, scream for engineers to begin mass production.

Christine said at June 14, 2005 11:44 AM:

Ok guys. A few corrections/ points:

The figure for efficiency of photosynthesis being 1% has some conditions that were not announced, if it is a legitimate figure. Once a photon is captured by the complex of proteins, the efficiency of the the conversion of the energy in the photon to chemical energy is around 99%. (My credentials: I researched photosynthetic proteins for about 1.5 years.) If you look at all light incident on the earth, I could believe that life on earth converts about 1% of all light on earth to chemical energy. (But I would want to see a citation and a justification first.) So energy efficiency is: energy in/energy out. You have to say what you are calling the "energy in" and what you are calling the "energy out." Without knowing that, it is meaningless to talk about an efficiency.

The most relevant efficiency for photocells is probably the one where "energy in" = incident light energy on the photocell and "energy out" is the usable electricity from the photocell. Just because you can convert photons into some other form of energy does not mean you can use the energy like you would use electricity. Notably, with photocells, the most light is available during the day (close to noon,) but household energy use is highest in the morning and in the evening. So you need energy storage. Solar cells alone can't provide all of the energy you need in a society because available energy is time dependent. Same thing for wind power. Of course, the easiest thing to do is reduce demand for energy. But a realistic solution to our energy problems must be a combination of things.

And, guys, the second law of thermodynamics is a wonderful thing. And it is a law. In the existence of science as we know it, no one has ever seen evidence to the contrary. What it says is that entropy (of a closed system) increases with time. Life does not decrease entropy overall. The earth is not a closed system. I strongly encourage those of you who are interested to look up a good thermodynamics book. (I recommend Thermodynamics and an Introduction to Thermostatics by Herbert Callen, if you have the time to sit down and read the whole thing. Any undergraduate textbook on thermodynamics would be ok too, or if your math background is weak, pick up a basic physics textbook.) If you're going to get into extended discussions on energy, I recommend sucking it up and learning some basic thermodynamics well. (Now, you don't have to believe in the second law of thermodynamics -- no one will beat you senseless until you do, but if you don't, you're out of the realm of science.)

And with my majors in physics, biochemistry, and physical science and minors in chemistry and math and my two years of graduate study in physics, I have never heard of life blocking energy with dimethyl sulfide. (Though this contribution gets funnier if you look up the MSDS for dimethyl sulfide.) Something got lost somewhere in there. There might have been a grain of truth in there -- I don't know -- but someone threw it out the window. (Kinda like with efficiencies -- you have to know what you're saying before you say it.) There are a lot of chemicals in the atmosphere which block light. Ozone (O3) is the biggest contributer, but there's a complex mix.

And anyone who tells you they can predict exactly what will happen in the atmosphere in response to changes of the conditions is either a liar or a theologian, not a scientist. Come on, look at how bad we are at predicting tomorrow's (or even today's) weather! There are too many variables and we don't have a good enough theoretical understanding of how the atmosphere works. We do know human activities are changing the climate and it's probably mostly because of CO2. And we are pretty sure increasing the amount of CO2 in the atmosphere will increase the temperature of the planet more. But saying solar cells will lower the temperature of the earth? Um, I'm not going to respond to that the way I want to because what would come out would not be nice. We can pretty confidently say that solar cells are not as bad for the environment as fossil fuels. (They still wear out and eventually need to be disposed of. This and probably other things mean they are not harmless to the environment.)

And with hydrogen: before we can practically use hydrogen as a fuel we need to improve our fuel cells and figure out how to store large amounts of compressed gas in small spaces (which means high pressure) without blowing ourselves up. It's not insurmountable, but we're years away from technology that would be usable for large, high demand applications (like cars.)

This is very promising and exciting, but it appears to still inapplicable to life outside of the lab so far. There are a lot of things which need to be studied and engineered before more efficient photocells will hit the shelves. Oh yes and most crystal growth is expensive and takes energy so it might not be economically feasible. (And if it takes more energy to make the really efficient photocells than you would save by using them, you have not saved anything.) If you really want to be kind to the environment, get a bike and leave the car at home, turn off the AC, and turn of the TV (and maybe even the computer!) Don't buy things that are excessively packaged and bring your own bags to the grocery store. These are proven, feasible things that we can do today and they don't cost much money.

Sorry for the rant, and I hope you will forgive the frustrated scientist.

Paul said at December 29, 2006 5:33 PM:

As for the hydrogen gas storage issue...

GM is definately behind the times in trying to store it under pressure, and I don't care how thick they make the tanks. If you are driving a PEM fuel cell car next to me that stores compressed hydrogen, I'm going to back waaaaaaaaaaaaaaay off.

Other technologies are far better, already largely developed, and are certainly viable. Sintered powdered metal alloy storage tanks with obscene amounts of internal surface area have been developed that don't require pressurization at all and can be produced inexpensively and use temperature variations that don't exceed 400 degrees. If punctured, they won't explode because the hydrogen they store is not a free floating gas.

Also, the worlds largest known reserves of borax are here in the United States. Borohydrate storage of hydrogen gives better hydrogen storage density that the cryogenic hydrogen slushes NASA wanted to use on the Venture Star. And borohydrates are far less hazardous in a crash than today's gasoline tanks. Even if you used a borohydrate tank for a vehicle bumper, it'd be safer than today's ICE vehicle gasoline storage. What's more, borohydrates can use almost the same distribution infrastructure already used for gasoline. They just need to solve the problem of being able to efficiently charge the borohydrates with hydrogen without excessive polution emissions at the processing plants... a task that shouldn't be insurmountable.

If you're curious about safe hydrogen storage via borohydrates, check out the Chrysler Natrium Minivan (which stores its borohydrate borne hydrogen fuel in an unpressurized tank that gives the van a 300 mi operational radius and mileage comparable in mpg to a gasoline powed ICE based vehicle).

So, as for being a scientist frustrated with the problem of hydrogen fuel storage, you frustration is perhaps not warranted, unless it is solely due to impatience for hydrogen safety to hit the ecconomic big time.

mike jones said at January 24, 2007 11:10 PM:

Paul Ive seen gas powered cars and gas stations and motorcycles all blow up and burn people alive, gas rains
down like napalm and burns close to the surface, hydrogen goes straight up and mushroom clouds because its lighter than air,Personally Id rather be in the hydrogen fire if its a choice. I saw a small propane tank blow in a house,
it produced a mushroom cloud high above the house and lit it on fire and it burned to the ground. everyone in the house made it out safely.the only danger was the house on fire. the gas station down the street burned for a hours
in a fiery hell. We`ve got a nuclear reactor under us and a nuclear reactor above us and we`re completely
dependent on burning fuels like our caveman ancestors burned logs in their fires;wich is probably the biggest joke
on mr. scientist

Lance Ventura said at June 6, 2008 12:59 AM:

I'm glad I read all this forum, it is usually more enlightening and wholly educationally for me to read various opinions. It makes me think of things I hadn't. And, I got a laugh from the google advertisements because, at the end of mike jones' comment, there was an ad for "clean coal, americas energy solution"

Chris said at August 10, 2008 3:00 PM:

This is so cool! The discussions here over the past years running are well worth the read. And to the frustrated scientist, it is good to see such a dissertation of thought and mindset, thank you.

GM behind the 8 ball? When are we all going to wake up to the fact that big business or the "corporations" respond to only one thing, their bottom line. They listen to Marketing not science, never have and never will. Marketing told them years ago that yes we are running out of fuels, but the American public wants SUVs and trucks. This renaissance with big oil and poor gas mileage is America's last gasp at high polluting, wasteful, unsustainable blah, blah, blah energy consumption. Conservation is not the answer here, the American or World public for that matter will only do a small part of conservation. To get real buy in we need technologies like this and other breakthroughs like lithium iron batteries which cannot burn or explode. I don't blame GM and their "VOLT" being in a 10 year design cycle if the battery technology is hazardous; however, that argument is over, using the latest in Lithium ion technology. And given the explosion of patents in this area, I foresee battery technology getting much better over the next 10 years. The time is now, our economic forecast of prosperity is seriously in danger of collapsing like the Japanese markets did in the 1980’s. We have to get back into a manufacturing mindset in the country, the failed service sector economics have failed.

The time is now to adopt everything we can do in sustainable, non polluting energy technologies. The "Quantum Nanodot Crystals" technology shows immense promise and probably is the next breakthrough technology that I hope will finally start to put the naysayers out of business once and for all. If 65% efficiency can be attained, and in a format that allows it to be sprayed, I could care less if it is $1000 a gallon, it is sustainable and that is what we all should be discussing. Oil and natural gas are finite and unsustainable.

For California's Slim Pickens crowd, his technology is all fine and good for the wind farms but the fact of the matter is the natural gas angle of his proposition is simply more of the same. Slim Pickens by his own admission is an oil and gas man. Want to line his pockets with billions in natural gas profits?

shipi arya said at August 17, 2008 5:38 AM:

Christine ur comment is ultimate man. i wish to see a book written by u on the topic. keep it up man.

Warren said at September 3, 2008 9:40 AM:

It shouldn't be too expensive to manufacture and make the DSSC quantum dot (nanocrystal) cells. With self-assembling colloidal manufacturing, the quantum dots are made easily in lab ( Also, with DSSC, you don't really need to worry about the weather as much as you do with traditional cells, or second generation thin films since they operate on a different mechanism than that of the semi-conducters ( Again, the most important thing will be the cost to the industry and end user from raw materials, as cheap ways to produce somewhat efficient cells have already started to become a reality. I'm sure with more research, the efficiencies will be realized eventually, and the cost will remain to be seen, depending on world economics. They even have some DIY kits for the DSSCs, although they're not "doped" with quantum dots.

Zeev said at January 14, 2010 5:21 PM:

Extremely novel idea, but what I am looking forward to is how to incorporate quantum dots with conducting polymer solar cells so as to increase both the quantum yield and efficiency of bulk heterojunction organic solar cell. I found several research papers on this idea, but most of them are lacking in physics of charge transport between the quantum dots and polymer interface. I would highly appreciate if anyone could direct me to these idea.

Joshua said at September 12, 2011 3:04 AM:

First off the posters on this thread seem very smart haha so I wanted to post a question. Is a space based solar power system feasible? I know that in space you would virtually have 24/7 sunlight, the light is about 33% stronger, no weather damage and the cold temperatures are idea but as far as beaming this energy back to earth via a microwave or tethering a huge wire from space does anyone know if this is possible and will we see this technology in the future?

Smart_ass said at October 11, 2012 4:35 PM:

Christine: Will you marry me? :)

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