Solar Material Produces ‘Twin’ Charges on Single Molecules

A new polymer solar cell seeks to achieve lower thermal loss by producing two charge carriers per molecule rather than one.

The materials were designed and synthesized by a Columbia University research team, then analyzed using time-resolved optical spectroscopy at Brookhaven National Laboratory. The research team said devices based on this charge multiplication concept — called singlet fission — have the potential to break through the upper limit on the efficiency of single-junction solar cells, which is around 34 percent.

“Even though we have demonstrated the concept of multiplication in single molecules, the next challenge is to show we can harness the extra excitations in an operating device,” said Brookhaven staff scientist Dr. Matthew Sfeir. “This may be in conventional bulk-type solar cells, or in third-generation concepts based on other inorganic (non-carbon) nanomaterials. The dream is to build hot-carrier solar cells that could be fully assembled using solution processing of our organic singlet fission materials.”

Postdoctoral fellow Erik Busby and staff scientist Dr. Matthew Sfeir with optical equipment they used to study charge carrier production in organic photovoltaic polymers. Courtesy of Brookhaven National Laboratory.

Most singlet fission materials explored so far result in twin charge carriers being produced on separate molecules. These only work well when the material is in a crystalline film with long-range order, where strong coupling results in an additional charge being produced on a neighboring molecule. Producing such high-quality crystalline films and integrating them with solar cell manufacturing complicates the process.

Having the two charges on the same molecule means the light-absorbing, energy-producing materials don’t have to be arrayed as perfect crystals. Instead, the self-contained materials work efficiently when dissolved in liquids, which opens the way for a range of industrial-scale manufacturing processes, including printing.

“We expect a significant leap in the development of third-generation, hot-carrier solar cells,” said Columbia professor Dr. Luis Campos. “This approach is especially promising because the materials’ design is modular and amenable to current synthetic strategies that are being explored in second-generation organic solar cells.”

Funding came from the U.S. Department of Energy Office of Science, the National Science Foundation and a 3M Non-Tenured Faculty Award.

The research was published in Nature Materials (doi: 10.1038/nmat4175).

For more information, visit www.bnl.gov.