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Singlet fission in pentacene through multi-exciton quantum states

Abstract

Multi-exciton generation—the creation of multiple charge carrier pairs from a single photon—has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multi-exciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through rapid internal conversion of the photoexcited state into a dark state of multi-exciton character that efficiently splits into two triplets. We show that singlet fission to produce a pair of triplet excitons must involve an intermediate state that (i) has a multi-exciton character, (ii) is energetically accessible from the optically allowed excited state, and (iii) efficiently dissociates into multiple electron–hole pairs. The rational design of photovoltaic materials that make use of singlet fission will require similar ab initio analysis of multi-exciton states such as the dark state studied here.

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Figure 1: Proposed mechanism of singlet fission in crystalline pentacene.
Figure 2: Molecular and localized representations of the four important pentacene monomer π orbitals involved in the singlet fission process.
Figure 3: Energies of excited states of a parallel pentacene dimer as a function of separation distance.

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Acknowledgements

The authors would like to thank the National Nanotechnology Infrastructure Network (NNIN) for providing computational time for this project. Z.Z. acknowledges partial support from NNIN.

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P.M.Z. and Z.Z. completed the computations and analysis. All authors contributed to writing the manuscript.

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Correspondence to Paul M. Zimmerman, Zhiyong Zhang or Charles B. Musgrave.

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Zimmerman, P., Zhang, Z. & Musgrave, C. Singlet fission in pentacene through multi-exciton quantum states. Nature Chem 2, 648–652 (2010). https://doi.org/10.1038/nchem.694

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