The exact mechanism explaining how incident light is converted into free charges in organic bulk-heterojunction films is still under debate. Studies on biological systems have recently highlighted the role of quantum coherence in the separation of charges in natural photosynthetic processes. Now, Sarah Maria Falke and colleagues observe that coherent oscillations between the donor and the acceptor in organic blends are also important for the efficiency of light conversion in solar cells. They studied the ultrafast response to optical stimulation of films of a conjugated donor polymer, of a fullerene-derivative acceptor and of a mixture of the two materials. Only in the latter sample, did they observe the onset of an additional oscillation, revealing the coupling of vibrational modes between the polymer and the fullerene. A simplified model of this system confirms that the energetic levels of the donor and acceptor coherently oscillate on the same femtosecond timescale of these vibrational modes, periodically increasing the charge-transfer probability when the levels resonate at the same energy.
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Martiradonna, L. The importance of coherence. Nature Mater 13, 664 (2014). https://doi.org/10.1038/nmat4026
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DOI: https://doi.org/10.1038/nmat4026
