By Gail Gallessich

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 UC Santa Barbara.

Nobel laureate Alan Heeger, professor of physics, worked with Kwanghee Lee of Korea and a team of other scientists to create a new “tandem” organic solar cell with increased efficiency. The discovery, which was published recently in the journal Science, marks a step forward in materials science.

Tandem cells are comprised of two multilayered parts that work together to gather a wider range than usual of the spectrum of solar radiation––at both shorter and longer wavelengths. “The result is 6.5 percent efficiency,” said Heeger. “This is the highest level achieved for solar cells made from organic materials. I am confident that we can make additional improvements that will yield efficiencies sufficiently high for commercial products.”

Heeger and Lee have collaborated for years on developing solar cells. The new tandem architecture that they discovered both improves light harvesting and promises to be less expensive to produce. In their paper, the authors explain that the cells “… can be fabricated to extend over large areas by means of low-cost printing and coating technologies that can simultaneously pattern the active materials on lightweight, flexible substrates.”

The multilayered device is the equivalent of two cells in series, said Heeger. The deposition of each layer of the multilayer structure by processing the materials from solution is what promises to make the solar cells less expensive to produce. He expects this technology to be on the market in about three years.

The cells are separated and connected by a transparent titanium oxide (TiOx), which allows for the higher-level efficiencies. TiOx transports electrons and acts as a collecting layer for the first cell. In addition, it provides a foundation that allows the fabrication of the second cell.

The work described in the Science article was performed at UCSB’s Center for Polymers and Organic Solids. Lee, a professor in the Department of Materials Science and Engineering at the Gwangju Institute of Science and Technology, in Gwangju, Korea, made fundamentally important contributions, along with first author Jin Young Kim, a postdoctoral fellow who is also from Korea.

Other collaborators from UCSB’s center include Nelson E. Coates, Mark Dante, Daniel Moses, and Thuc-Quyen Thai Nguyen, assistant professor of chemistry and biochemistry.

Heeger shared the Nobel Prize in Chemistry in the year 2000, with Alan MacDiarmid and Hideki Shirakawa, for the “discovery and development of conducting polymers.” The tandem solar cells reported in the Science article uses semiconducting polymers from the class of materials that were recognized by the award of the Nobel.

Heeger recently was presented with the Italian Prize for Energy and the Environment (Eni Italgas Prize) for his discoveries and research accomplishments in the field of “plastic” solar cells. The Italian agency cited Heeger “for research that will begin to contribute to the energy needs of our planet in the near future.”

With Howard Berke, Heeger in 2000 co-founded Konarka Technologies.