"Science" publishes article on organic photovoltaics

Oldenburg. Organic solar cells could become one of the key technologies of our century. They convert light into electricity with high efficiency. However, the processes taking place inside the solar cell are so complex that they have so far eluded direct scientific observation. Now, scientists led by Oldenburg physicist Prof Dr Christoph Lienau have succeeded for the first time in filming the light-to-current conversion in an organic solar cell in real time. In the report published today by the renowned scientific journal "Science", the research team from Oldenburg, Milan and Modena (Italy) explains in detail how the light-current conversion, a light-induced electron transfer, takes place in an organic solar cell. They show: The quantum mechanics of this process - specifically the wave-like nature of the electrons and their coupling to the surrounding nuclei - is of crucial importance.

Organic solar cells utilise nanomaterials made of polymers and fullerenes to convert sunlight into electricity. They are inexpensive, lightweight, flexible and can be customised in any colour. The conjugated polymers are long chains of carbon atoms and act as light absorbers during the light-to-current conversion. Until now, all experiments had indicated that the current is generated by electron particles jumping from the polymers to the surrounding fullerenes - spherical molecules made of carbon atoms.

"Our initial experiments really surprised us," says Lienau. The Oldenburg physicist Sarah Falke, together with cooperation partners led by Prof Dr Giulio Cerullo from Milan, used extremely short light pulses in the femtosecond range - i.e. with a duration of a few billionths of a millionth of a second - to illuminate the polymer layer in an organic cell. They discovered that the light pulses not only stimulate the atomic nuclei to oscillate, but also cause the electrons to behave like waves that oscillate back and forth between the polymer and the fullerene. Lienau: "We had not expected this, because in organic cells the interface between polymers and fullerenes is extremely complex, and the two components are not connected by an atomic bond."

The scientists asked their long-standing research partners Prof Dr Elisa Molinari and Dr Carlo Andrea Rozzi from the Istituto Nanoscienze of the National Research Council CNR and the University of Modena and Reggio Emilia for support. They succeeded in filming the temporal evolution of the electrons and the atomic nuclei - in other words, mapping the system that is responsible for the experimentally detected oscillations. "Our calculations show that the concerted movement of the atomic nuclei is very important for efficient charge transfer," explains Molinari. "They have to wobble for the current to flow."

The scientists are not yet able to predict whether the new results will quickly lead to improved solar cells. "But the results provide impressive new insights into one of the fundamental processes of organic photovoltaics. We were able to show that phenomena that nature has produced in photosynthesis also appear to occur in organic cells," says Lienau.

Recent studies have suggested that quantum coherence may play a central role in photosynthesis. The research results of the German and Italian scientists now provide evidence for similar phenomena in the function of photovoltaic systems: "A conceptual advance that will be incorporated into the design of future artificial light-harvesting systems and solar cells," Lienau is certain.

To be read at:

Sarah Maria Falke, Carlo Andrea Rozzi, Daniele Brida, Margherita Maiuri, Michele Amato, Ephraim Sommer, Antonietta De Sio, Angel Rubio, Giulio Cerullo, Elisa Molinari and Christoph Lienau (2014) "Coherent ultrafast charge transfer in an organic photovoltaic blend", Science 344, No. 6187, pp. 1001-1005