A Polymer Solar Cell with Near-Perfect Internal Efficiency

June 17, 2009 by Laura Mgrdichian, Phys.org feature

An international group of scientists has developed a polymer-based solar cell with an ability not yet seen in similar cells: almost every single photon it absorbs is converted into a pair of electric-charge carriers, and every one of those pairs is collected at the cell's electrodes.

The overall efficiency of the cell is six percent, meaning a total of six percent of the absorbed energy is converted into usable electricity when illuminated in the lab with similated solar light. This may seem low, but polymer solar cells to date have not yielded efficiencies better than five percent.

“These characteristics make our polymer solar cell the best of its kind produced so far,” said the study's corresponding scientist, Alan Heeger of the University of California at Santa Barbara and the Heeger Center for Advanced Materials at the Gwangju Institute of Science and Technology, in South Korea.

Heeger collaborated with colleagues from UC Santa Barbara, the Heeger Center, and the University of Laval in Quebec, Canada.

The group's work is a good sign that it is possible to produce polymer solar cells with efficiencies good enough for commercial production. As alternative-energy media, polymer solar cells are already promising because they would be much cheaper to produce and far more lightweight than conventional solar cells or cells made using other materials. They would also be highly portable and physically flexible, making it possible to place them in locations that standard solar cells cannot go.

The solar cell is made of a “copolymer,” a polymer consisting of two different alternating polymer chains. Its role is to release when hit by sunlight; the electrons are accepted by a fullerene derivative, a material based on a form of carbon that tends to form large spherical molecules known as fullerenes. When the two materials are combined into a composite “active layer,” regions form that separating the positive and negative charge - the positively charged “holes” left by electrons as they leave the copolymer and, of course, the electrons themselves. The regions are known as bulk heterojunctions, or BHJs.

Historically, increasing the photocurrent produced in BHJ solar cells has proven difficult. Simply increasing the thickness of the copolymer-fullerene layer so that it absorbs more light and thus releases more charge carriers doesn't work because charge carriers don't travel far within the material.

Heeger and his colleagues tried an approach that would retain a typical active layer thickness, about 80 nanometers, yet maximize the photocurrent. They added another layer to the cell, a sheet of titanium-oxide sandwiched between the copolymer and the top electrode, which has two roles. First, it redirects the intensity of the light such that it is maximized in the active layer. With higher intensity light reaching the active layer, the photocurrent increases. Second, it acts as a “hole blocker,” helping to keep the photo-generated electrons from recombining with holes.

The group discovered that at a copolymer-to-fullerene ratio of 1:4, the internal quantum efficiency, the number of electrons produced per absorbed photon, is remarkably high - close to 100 percent for light with a wavelength of 450 nanometers (violet-blue light) and above 90 percent for all other wavelengths in the absorbed spectrum.

When illuminated by monochromatic green light, a wavelength of 532 nanometers, the group measured an overall efficiency - the efficiency that measures how much usable current is produced - of 17 percent. This is very high for a solar cell.

“Although, in practice, a solar cell would never be used under a light source that emitted only green light, this shows that it should be possible to achieve efficiencies of 10 to 15 percent in bulk heterojunction ,” says Heeger.

More information: Sung Heum Park, Anshuman Roy, Serge Beaupre, Shinuk Cho, Nelson Coates, Ji Sun Moon, Daniel Moses, Mario Leclerc, Kwanghee Lee and Alan J. Heeger, Nature Photonics, advance online publication, DOI: 10.1038/nphoton.2009.69

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8 comments

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PinkElephant
5 / 5 (3) Jun 17, 2009
How long will these polymers last when exposed to sunlight and the elements in the real world?

It wouldn't be much of a cost savings if the solar cells have to be replaced every couple of years...
Birger
5 / 5 (2) Jun 18, 2009
Apart from life span, production cost is an issue. What would be the cost per watt for bulk production? Polymers should be substantially cheaper than solar cells that require high-quality silica raw materials.
Ethelred
3 / 5 (2) Jun 23, 2009
You must have the lowest average rating on your statements in the history of ratings.


Fraid not. He is very low though.

Lowest five I can find.

5. Weir 2.1

4. HeyZeuss a 2.0

3. Alexa has a 2.0

2. NeilFarbstein has a 1.9

1. Alizee has a 1.7

Some others are lower but they have very few posts.

Ethelred

QubitTamer

Quantum Physicist, torturer of AGW religious zealots like Ethelred because i laugh at his hysterics.


QubitTroll will be released from my sig at the end of June.

N_O_M
3 / 5 (2) Jun 23, 2009
Lowest five I can find...

Lowest I can find is JukriS with an underwhelming 1.1
PaulLove
5 / 5 (1) Jun 25, 2009
I sense a truely interesting artical regarding the behavior of time as NOM future stalks NeilFarbstein through the boards.

My initial hypothisis is that by sampling a few particals of Neil and focusing the electrical energy of these polymer cells Nom forces the quantum entanglement of a few particles of Neil with his own. Then at some future date NOM jumps into the Hadron collider with a backpack of dark mater using the entangled particals to control his backflow through the timestream to futurestalk Neil.

Sure the science is bad but the story is good. tune in next article for the next installment of Mystery Physorg 3000 theater.
Ethelred
3 / 5 (2) Jun 26, 2009
What I wonder about this little war is if Neil will try to take revenge.

Ethelred

QubitTamer

Quantum Physicist, torturer of AGW religious zealots like Ethelred because i laugh at his hysterics.


QubitTroll will be released from my sig at the end of June.
Ethelred
3.7 / 5 (3) Jun 27, 2009
Whitepaw:

People like to argue.

Neil keeps spamming the site with links to his business. He constantly claims to have already done what is in the article.

He has ONE patent on the aforementioned website.
For an adhesive.
For mucus membranes.
With nano-tubes in the adhesive.

That is so going to go over with a public that is aware of the problems with asbestos. Especially since he has absolutely no sign on his site that the stuff works or is safe. Or exists except as a fantasy in the patent. He appears to extra full of it even by the standards of this site.

This is NOT a great site. Wish it was but the better science sites don't allow the discussions. I suspect that if Science News began allowing comments this site would quickly become empty.

Churches have nothing real to say so bickering is all that is available. Often the articles say nothing to convince anyone that the article should have wasted one TCP/IP packet. Perhaps this lack of substance is the cause of the similarity in behavior.

I suspect the lack of replies to your posts is due to the simple fact that they are pretty much spot on. So there is little to argue about. Without a source of conflict discussion tends to be:

Yeah

Right

Me too

Me too too.

Me three

Verily you have it.

Yeah team

By aetherwave this is obvious.

According to the Plasma Universe you are all full of crap.

My company worked on this 6 months ago and we have some patents pending.

Neil-quadrotricale.com

Neil

Get the idea? Say something that actually invites comments. Double space between paragraphs and keep them short. Cultivate some enemies. Annoy the Global Warming Deniers. Post on other discussions. You have three posts and all are here.

Oh yes. It the material can be made cheap enough it doesn't matter if it doesn't last long. Just recycle it. Silicon chips cost way too much for use except in satellites. Amorphous silicon costs less but its efficiency is to poor for heavy duty use. So if someone can produce this stuff cheap enough it might be useful.

Ethelred

QubitTamer

Quantum Physicist, torturer of AGW religious zealots like Ethelred because i laugh at his hysterics.


QubitTroll will be released from my sig at the end of June.
PinkElephant
not rated yet Jun 28, 2009
@Ethelred

"Amorphous silicon costs less but its efficiency is to poor for heavy duty use. So if someone can produce this stuff cheap enough it might be useful."

By way of response, I'll take an opportunity here to shamelessly plug a solar company in which I hold some stock:

http://www.evergreensolar.com

Some high-level details on the technology in question here:

http://www.evergreensolar.com/app/en/technology/item/48

They claim that not only is their process cheaper (particularly when it gets scaled up some more), but that it is also quite energy-efficient. They say that in terms of total energy pay-off, over the whole product lifecycle, they have the fastest return on investment of any silicon-based solar cell maker. Oh, and their panel efficiencies are around 15-17%, with credible process improvements in the pipeline to lift the efficiency up to ~20% or so in a couple of years.

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