Scientists Study How to Stack the Deck for Organic Solar Power

July 28, 2009,
In this cross-section of an organic photovoltaic cell, light passes through the upper layers (from top down, glass, indium tin dioxide, and thermoplastic) and generates a photocurrent in the polymer-fullerene layer. Channels formed by polymers (tan) and fullerenes (dark blue) allow electric current to flow into the electrode at bottom. NIST research has revealed new information about how the channels form, potentially improving cell performance. Credit: NIST

( -- A new class of economically viable solar power cells--cheap, flexible and easy to make--has come a step closer to reality as a result of recent work at the National Institute of Standards and Technology, where scientists have deepened their understanding of the complex organic films at the heart of the devices.

Organic , which rely on to capture sunlight and convert it into electricity, are a hot research area because in principle they have significant advantages over traditional rigid silicon cells. Organic photovoltaics start out as a kind of ink that can be applied to flexible surfaces to create solar cell modules that can be spread over large areas as easily as unrolling a carpet. They’d be much cheaper to make and easier to adapt to a wide variety of power applications, but their market share will be limited until the technology improves. Even the best organic photovoltaics convert less than 6 percent of light into electricity and last only a few thousand hours.

“The industry believes that if these cells can exceed 10 percent efficiency and 10,000 hours of life, technology adoption will really accelerate,” says NIST’s David Germack. “But to improve them, there is critical need to identify what’s happening in the material, and at this point, we’re only at the beginning.”

The NIST team has advanced that understanding with their latest effort, which provides a powerful new measurement strategy for organic photovoltaics that reveals ways to control how they form. In the most common class of organic photovoltaics, the “ink” is a blend of a polymer that absorbs sunlight, enabling it to give up its electrons, and ball-shaped carbon molecules called fullerenes that collect electrons. When the ink is applied to a surface, the blend hardens into a film that contains a haphazard network of polymers intermixed with fullerene channels. In conventional devices, the polymer network should ideally all reach the bottom of the film while the fullerene channels should ideally all reach the top, so that electricity can flow in the correct direction out of the device. However, if barriers of fullerenes form between the polymers and the bottom edge of the film, the cell’s efficiency will be reduced.

By applying X-ray absorption measurements to the film interfaces, the team discovered that by changing the nature of the electrode surface, it will repulse fullerenes (like oil repulses water) while attracting the polymer. The electrical properties of the interface also change dramatically. The resultant structure gives the light-generated photocurrent more opportunities to reach the proper electrodes and reduces the accumulation of fullerenes at the film bottom, both of which could improve the photovoltaic’s efficiency or lifetime.

“We’ve identified some key parameters needed to optimize what happens at both edges of the film, which means the industry will have a strategy to optimize the cell’s overall performance,” Germack says. “Right now, we’re building on what we’ve learned about the edges to identify what happens throughout the film. This knowledge is really important to help industry figure out how organic cells perform and age so that their life spans will be extended.”

More information: D.S. Germack, C.K. Chan, B.H. Hamadani, L.J. Richter, D.A. Fischer, D.J. Gundlach and D.M. DeLongchamp. Substrate-dependent interface composition and charge transport in films for organic photovoltaics. Applied Physics Letters, 94, 233303 (2009), DOI: 10.1063/1.3149706 .

Provided by National Institute of Standards and Technology (news : web)

Explore further: Bottoms up: Better organic semiconductors for printable electronics

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not rated yet Jul 28, 2009
All the whilst trying desperately to schieve 4 or 6 kiloWatt-hours per square meter per 24 hour day.

The Solar Constant is 1350 Watts per square meter.
not rated yet Jul 28, 2009
All the whilst trying desperately to schieve 4 or 6 kiloWatt-hours per square meter per 24 hour day.

The Solar Constant is 1350 Watts per square meter.


I notice that on many articles related to solar energy you quote the Solar Constant, (which defines the total amount of electromagnetic radiation reaching the top of the earth's atmosphere,) whether or not the article is about spaced-based vs. ground-based energy production. You are absolutely correct that the Solar Constant is roughly 1350 W/M2, and I also understand your comment of trying to achieve 6 KW-H/M2/Day, which would require solar collectors with 100% efficiency. But, because of the attenuation of the solar radiation by the earth's atmosphere, isn't the correct number closer to 1000 W/M2 (at the equator at high noon on a clear day)? I have always believed that that number, or 250 W/M2 averaged over a day, (250x24 to get your 6 KW), to be more useful when speaking of ground-based systems and their potential. Am I mistaken? It HAS been known to happen....
not rated yet Jul 29, 2009
200-300 W/m^2 is available in a band along the equator between approximately 40 degrees north and 40 degrees south.

Above and below that it drops of rapidly. In Stockholm it's about 100 W/m^2.
not rated yet Jul 29, 2009
Regardless of the solar constant, organic photovoltaic cells have a way to go...

However, the research effort needed to reach the 10% mark and the 10,000 hours life span will likely dwarf the extra effort of making them effective enough to be cost-effective outside the 40 degree band.

Once the cells reach the first performance mark, they will soon reach the other.

Other power sources will still be needed to supplement solar power during cloudy days and the evenings, but this is probably a big step in the right direction.
not rated yet Jul 29, 2009
Would someone explain the importance of a technology (whatever its efficiency) that will only last a little over a year?
(10,000hrs/24hrs)/365days=1.14 years.
not rated yet Jul 30, 2009
The automobile is a peice of "technology", how many go longer than a year off the lot without requiring maintenance or repair? How important are automobiles....or "golfzilla" the Taylor made R9 quad driver was obsolete 5 months after it came out...

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