A Polymer Solar Cell with Near-Perfect Internal Efficiency

Jun 17, 2009 by Laura Mgrdichian 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

Copyright 2009 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

Explore further: Germanium tin could mean better and cheaper infrared cameras in smartphones

add to favorites email to friend print save as pdf

Related Stories

Higher efficiency organic solar cell created

Jul 12, 2007

Using plastics to harvest the energy of the sun just got a significant boost in efficiency thanks to a discovery made at the Center for Polymers and Organic Solids at the University of California, Santa Barbara.

Special Coating Greatly Improves Solar Cell Performance

Feb 22, 2008

The energy from sunlight falling on only 9 percent of California’s Mojave Desert could power all of the United States’ electricity needs if the energy could be efficiently harvested, according to some estimates. Unfortunately, ...

Quantum Dots May Lead to Rainbow Solar Cell

Mar 07, 2008

For the first time, researchers have created solar cells made of different-sized quantum dots, each tuned to a specific wavelength of light. By arranging these quantum dots in an ordered pattern, the scientists ...

Recommended for you

User comments : 13

Adjust slider to filter visible comments by rank

Display comments: newest first

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.
jonnyboy
2.3 / 5 (3) Jun 22, 2009
Neil - go away, we are all tired of your ridiculous stances on all of the issues. You must have the lowest average rating on your statements in the history of ratings.
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
whitepaw
5 / 5 (1) Jun 24, 2009
Hi: My experience with plastic of many kinds under sun exposure is that they all will deteriorate over time. Plastic containers, plastics of many kinds all seem to break down under solar exposure. Maybe there are more kinds which do not.
whitepaw
not rated yet Jun 24, 2009
Where the sun has almost no clouds to protect the exposure is in some parts of southern Arizona. In that area I have notice that the interior plastics of the cars and trucks will fall apart a lot sooner than in the area we live in in Illinois. They also have almost no rust damage due to salt or chemicals mixed with water exposure. In the center part of the USA, we have a lot of salt and chemical damage to the auto and truck bodies due to the melt chemicals.
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.
whitepaw
not rated yet Jun 27, 2009
My comment on your arguments about other posters on this board.
I think you guys are like some spoiled kids, they continually fight over little things.
You have a great site here on science. Make it count, not comments on one another. Also sounds like some churches I have been to, always commenting and bickering over little things. Come on now, be men, and not brats.

I have posted some of my comments on solar collection and the building of solar panels. I would like to see some comments on what I have stated, not some of the other authors of posts.
thanks,
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.
whitepaw
not rated yet Jun 28, 2009
"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"
Ethelred:
Sure we can recycle.

But in the real world it is not done to the extent that it could be.
I have been around recycling for over 40 years, and have been involved in it as well. I know what it is, but most people who I have met simply do not care, nor do they take the time in such places as I have lived and been, such as Chicago, to recycle anything. They will not take the time to separate the items into 3, 4 or 5 categories.

Recycling of computers and related items is what I have been doing for the last 20 years. This kind of recycling has many categories. There is the "third world" market for used modules and chips. This market exists because people are making money at this business. In some countries computer equipment has been bought from USA by the millions of tons, and other places to large centers where many people are disassembling the equipment. Many of the parts and modules are then reused and sold on the world market.

In the USA there is a market for old, and used computer equipment of almost every kind. Some of this is going to collectors, most to those to scrap for the recovery of gold and other metals.

Older computer equipment has more value, as far as scrap value than most of the newer equipment, because of what they are made of.

Profit is a great motivator, and it is not in many of the items that we would like to see recycled.

In the USA, we have a great need to find a way to recycle with a profit items such as: paper, plastics of all kinds, glass (including TV, monitors, colored glass, auto glass, and other kinds of glass), all plastics (including computer cases, printer cases, monitor and tv cases, and many other kinds of plastics), all other metals (at present most metals can be recycled at a profit) and building materials such as used and removed drywall materials. Also there are many other kinds of daily-use items which could be recycled.
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.