Nobel laureate Alan Heeger, Professor of physics at UC Santa Barbara, worked with Kwanghee Lee of Korea and a team of other scientists to create a new ‘tandem’ organic solar cell with increased efficiency.

Tandem cells comprise two multilayered parts that work together to gather a wider range of the spectrum of solar radiation - at both shorter and longer wavelengths.

“The result is six and a half per cent efficiency,” said Professor 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.”

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This technology is expected to be on the market in about three years.

The team has collaborated for many 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 Professor 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.

“Tandem solar cells, in which two solar cells with different absorption characteristics are linked to use a wider range of the solar spectrum, were fabricated with each layer processed from solution with the use of bulk hetero-junction materials comprising semi-conducting polymers and fullerene derivatives,” wrote the authors.

The cells are separated and connected by the material TiOx, a transparent titanium oxide. This is the key to the multilayer system that allows for the higher-level efficiencies. TiOx transports electrons and is a collecting layer for the first cell. In addition, it acts as a stable foundation that allows the fabrication of the second cell, thus completing the tandem cell architecture.

An exciting aspect of this discovery is that it is expected to contribute to the use of technologies such as laptop computers in areas of developing countries that are off the electricity grid.