Organic solar cells that convert 6.5 percent of incoming light into usable power have been created with a design approach that incorporates two multilayered parts that act together to boost output.

Although organic solar cells have been around for years, their relatively low conversion efficiencies — in the neighborhood of 5 percent — have been an obstacle to widespread commercial use.

Researchers from the University of California, Santa Barbara, and from Gwangju Institute of Science and Technology in South Korea, created the more efficient cells by processing each layer from solution with the use of bulk heterojunction materials comprising semiconducting polymers and fullerene derivatives. The tandem design essentially conjoins two distinct photovoltaic cells — each with different absorption characteristics — allowing light to be absorbed and converted into energy from a broader range of the solar spectrum, said lead investigator Kwanghee Lee of Gwangju Institute. The front cell has a low bandgap, whereas the back cell has a higher one. The research was reported in the July 13 issue of Science.

A key design element was the transparent titanium oxide layer that separated and connected the cells and that broke the symmetry of the front cell to create an open-circuit voltage, functioning as an electron transport layer.

The team fabricated 20 tandem cells with efficiencies above 6.2 percent and as high as 6.5 percent. Dozens of design architectures were built and tested before the final design was chosen.

The research team, which also included Alan J. Heeger, a professor at the University of California and a winner of the 2000 Nobel Prize in chemistry, reported that further research with the organic solar cell’s design likely will result in still higher efficiency levels. It also expects that the technology could be ready for commercial use within three years.