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How energy flows in organic solar cells

Reducing the energy loss using different donor-acceptor materials

15.06.2022 - New approach to exploit the full potential of organic solar cells.

Efficient and environ­mentally friendly solar cells are required for a transition to a fossil-free energy supply. Researchers at Linköping University have mapped how energy flows in organic solar cells, something that previously had been unknown. The results can contribute to more efficient solar cells. “To enable the full potential of organic solar cells to be exploited, there is a need for a clear picture of how they work. We have now obtained that picture. This provides a better under­standing of how to create new efficient and sustainable solar cell materials,” says Mats Fahlman from the Labora­tory of Organic Electronics at Linköping Univer­sity. 

Today, solar energy meets around two percent of the world's energy needs. But its potential is far in excess of that. The energy contained in the sun's rays is more than enough to meet our needs today and in the future. Solar cells that are cheap and environ­mentally friendly to manu­facture are needed to be successful. In addition, they need to be efficient at absorbing a large proportion of the sun's rays and converting to electrical energy. Organic solar cells based on organic semi­conductors are increasingly emerging as a sustainable option. But until just a few years ago they could not stand comparison with traditional silicon-based solar cells for effi­ciency. This was due to energy loss in charge separation, which was thought to be unavoidable.

But in 2016, a research team at Linköping Univer­sity together with colleagues in Hong Kong were able to show that it was possible to avoid the energy loss using different donor-accep­tor materials that help the electron to escape from its hole more easily. Energy loss then decreased and effi­ciency increased. The problem was that no one knew exactly how it happened. It was possible to see that it worked, but not why.

Some of the same research team at Linköping University have now solved the mystery that had led to disagreement in this field of research. The researchers have identified what energy levels are required to minimise energy losses. “To find out how the energy flows, we laid nano­meter-thick organic semi­conduction films in several layers one on top of the other, rather like a strawberry and cream cake. After that we measured the energy required to separate the electrons from their holes in each indi­vidual layer,” says PhD student Xian’e Li. The researchers were then able to map the mechanism behind the energy-effi­cient charge separation. This sys­tematic mapping points a new way forward for the develop­ment of organic solar cells. (Source: Linköping U.)

Reference: X. Li et al.: Mapping the energy level alignment at donor/acceptor interfaces in non-fullerene organic solar cells, Nat. Commun. 13, 2046 (2022); DOI: 10.1038/s41467-022-29702-w

Link: Laboratory of Organic Electronics, Dept. of Science and Technology (ITN), Linköping University, Norrköping, Sweden

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