Tandem polymer solar cells that set record for energy-conversion

February 13, 2012 By Wileen Wong Kromhout, University of California - Los Angeles

Tandem solar cell
(PhysOrg.com) -- In the effort to convert sunlight into electricity, photovoltaic solar cells that use conductive organic polymers for light absorption and conversion have shown great potential. Organic polymers can be produced in high volumes at low cost, resulting in photovoltaic devices that are cheap, lightweight and flexible.

In the last few years, much work has been done to improve the efficiency with which these devices convert sunlight into power, including the development of , device structures and processing techniques.

In a new study, available online this week in the journal , researchers at the UCLA Henry Samueli School of Engineering and Applied Science and UCLA's California Nanosystems Institute (CNSI) report that they have significantly enhanced polymer solar cells' performance by building a device with a new "tandem" structure that combines multiple cells with different absorption bands. The device had a certified power-conversion efficiency of 8.62 percent and set a world record in July 2011.

Further, after the researchers incorporated a new infrared-absorbing provided by Sumitomo Chemical of Japan into the device, the device's architecture proved to be widely applicable and the power-conversion efficiency jumped to 10.6 percent — a new record — as certified by the U.S. Department of Energy's National Renewable Energy Laboratory.

By using cells with different absorption bands, tandem provide an effective way to harvest a broader spectrum of solar radiation. However, the efficiency doesn't automatically increase by simply combining two cells. The materials for the tandem cells have to be compatible with each other for efficient light harvesting, the researchers said.

Until now, the performance of tandem devices lagged behind single-layer solar cells, mainly due to this lack of suitable polymer materials. UCLA Engineering researchers have demonstrated highly efficient single-layer and tandem polymer solar cells featuring a low-band-gap–conjugated polymer specially designed for the tandem structure. The band gap determines the portion of the solar spectrum a polymer absorbs.

"Envision a double-decker bus," said Yang Yang, a professor of materials science and engineering at UCLA Engineering and principal investigator on the research. "The bus can carry a certain number of passengers on one deck, but if you were to add a second deck, you could hold many more people for the same amount of space. That's what we've done here with the tandem polymer solar cell."

To use solar radiation more effectively, Yang's team stacked, in series, multiple photoactive layers with complementary absorption spectra to construct a tandem polymer solar cell. Their tandem structure consists of a front cell with a larger (or high) band gap material and a rear cell with a smaller (or low) band gap polymer, connected by a designed interlayer.

When compared to a single-layer device, the tandem device is more efficient in utilizing solar energy, particularly by minimizing other energy losses. By using more than one absorption material, each capturing a different part of the solar spectrum, the tandem cell is able to maintain the current and increase the output voltage. These factors enable the increase in efficiency, the researchers said.

"The solar spectra is very broad and covers the visible as well as the invisible, the infrared and the UV," said Shuji Doi, research group manager for Sumitomo Chemical. "We are very excited that Sumitomo's low–band gap polymer has contributed to the new record efficiency."

"We have been doing research in tandem solar cells for a much shorter length of time than in the single-junction devices," said Gang Li, a member of the research faculty at UCLA Engineering and a co-author of the paper. "For us to achieve such success in improving the efficiency in this short time period truly demonstrates the great potential of tandem solar cell technology."

"Everything is done by a very low-cost wet-coating process," Yang said. "As this process is compatible with current manufacturing, I anticipate this technology will become commercially viable in the near future."

This study opens up a new direction for polymer chemists to pursue designs of new materials for tandem polymer solar cells. Furthermore, it indicates an important step towards the commercialization of polymer solar cells. Yang said his team hopes to reach 15 percent efficiency in the next few years.

Yang, who holds UCLA's Carol and Lawrence E. Tannas Jr. Endowed Chair in Engineering, is also faculty director of the Nano Renewable Energy Center at the California Institute at UCLA.

The study was supported by the National Science Foundation, the U.S Air Force Office of Scientific Research, the U.S. Office of Naval Research and the U.S. Department of Energy, together with the National Renewable Energy Laboratory.

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not rated yet Feb 13, 2012
not rated yet Feb 13, 2012
5 / 5 (1) Feb 13, 2012
It has a 10.6% conversion rate (4th paragraph). So around 106 W/m^2
1 / 5 (2) Feb 14, 2012
Such polymer cells may be good for powering of some cheap gadgets of limited lifetime and/or for production of wearable light accumulating fabric (tents in particular). If the polymer cells cannot survive ten years at the direct sun, then the cost of installation and maintenance will supersede the savings of the price of material.
2 / 5 (20) Feb 15, 2012
Good to see constant advancement in solar cell technology on so many fronts. Perhaps this could lead to cheap flexible high efficiency panels in the future.

By "suitable polymers", I would assume that longevity in direct sunlight would be a primary consideration. Perhaps a cheap gadget could be a house or factory!

If the process is not as labor intensive as other technologies, perhaps if could be made in America too.
not rated yet Feb 15, 2012
"watts/m2?" - Scottingham

1366 (approx) * 8.6 percent = 117 watts / m**2

1366 W/m^2 is sunlight at the top of the atmosphere, at daytime! That's completely irrelevant.

Here on earth the year average irradiance on a horizontal plate is ~100-300 W/m^2(With Sweden being representative of the low end, world average being around 200 W/m^2 world average and 300 W/m^2 being equatorial deserts) in the inhabited world. At 8.6% conversion efficiency that's ~9-26 W/m^2.

Tack on a few tens of percents extra for optimal fixed angle south-facing plate.

If you're worrying about land area and not collector area, then cut that number in half or so(access roads, spacing to prevent self-shading at lower angles).

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