Two for the Price of One: Singlet Fission and Improved Solar Cells

New research by investigators at the National Renewable Energy Laboratory suggest an interesting way of improving the output of solar photovoltaic cells. Singlet fission is a process in which the absorption of a single photon can produce two electron-hole pairs, thereby potentially doubling the output of a solar cell.

The research, published in the Journal of the American Chemical Society, involved the molecule 1,3-diphenylisobenzofuran. In an e-mail, one of the investigators told me that singlet fission is closely related to another process called multiple exciton generation, previously indicated as another possibility for improving solar PV output. MEG occurs in quantom dots, though, compared to the chromophore molecules used in singlet fission, according to Dr. Justin Johnson, of the NREL.

"There is a lot of fundamental research yet to be done to understand singlet fission before it could be useful in solar energy," Johnson told me. He guessed that commercial-scale use of the idea in solar technology is at least five years off, but added that the idea could be applied right away for two types of solar cell: "One is an organic photovoltaic designs (e.g., pentacene/fullerene systems) and the other is dye-sensitized solar cells (i.e., a molecular dye attached to nanocrystalline titanium dioxide)." Johnson continued:

"The efficiency gain arises from the fact that without singlet fission these configurations waste energy to heat during the steps after light absorption and before charge collection. In many cases, the two excitons produced by singlet fission can both be harvested in the same way that one exciton is harvested in a conventional device but without the lost heat, leading to a factor of two increase in efficiency. This is just one scenario, and the exact details of how the device is configured must be taken into account to determine the actual expected enhancement."

He also said that though the molecules used in the recently published work can be difficult to work with, his group is now starting to use easier materials that could be mass produced and would remain stable for at least 10 years, meaning the eventual cost of such solar cells wouldn't be much higher than currently available PV technology.

(Image via NREL)

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