In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination1. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor–acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors2. The nanoscale morphology and high charge densities give a high rate of electron–hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes3. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency4. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer–fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet (1CT) and spin-triplet (3CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of 3CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of 3CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron–hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.
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We thank N. Greenham for discussions. A.R. thanks Corpus Christi College, Cambridge for a Research Fellowship. S.G. thanks Fonds Québécois de Recherche sur la Nature et les Technologies for funding. This work is supported by the EPSRC and the Winton Programme for the Physics of Sustainability. C.W.S. was supported by the National Science Foundation (DMR-1215753). D.S.G., C.-Z.L., H.-L.Y. and A.K.-Y.J. acknowledge support from the Office of Naval Research (N00014-11-1-0300). Some of the work was done at the UW NanoTech User Facility, a member of the NSF National Nanotechnology Infrastructure Network. We thank J. Richards and D. Pozzo for performing grazing-incidence small-angle X-ray scattering measurements, S. Williams for transmission electron microscopy and G. Shao for help with atomic force microscopy measurements.
The authors declare no competing financial interests.
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Rao, A., Chow, P., Gélinas, S. et al. The role of spin in the kinetic control of recombination in organic photovoltaics. Nature 500, 435–439 (2013). https://doi.org/10.1038/nature12339
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