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The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain

Nature Chemistry volume 4, pages 840–845 (2012)Cite this article

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Abstract

One strategy to improve solar-cell efficiency is to generate two excited electrons from just one photon through singlet fission, which is the conversion of a singlet (S1) into two triplet (T1) excitons. For efficient singlet fission it is believed that the cumulative energy of the triplet states should be no more than that of S1. However, molecular analogues that satisfy this energetic requirement do not show appreciable singlet fission, whereas crystalline tetracene displays endothermic singlet fission with near-unity quantum yield. Here we probe singlet fission in tetracene by directly following the intermediate multiexciton (ME) state. The ME state is isoenergetic with 2 × T1, but fission is not activated thermally. Rather, an S1 ⇔ ME superposition formed through a quantum-coherent process allows access to the higher-energy ME. We attribute entropic gain in crystalline tetracene as the driving force for the subsequent decay of S1 ⇔ ME into 2 × T1, which leads to a high singlet-fission yield.

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Figure 1: TR-2PPE spectra reveal the near-concurrent rise in singlet and ME populations.
Figure 2: TR-2PPE spectra at a pump photon energy of 2.32 eV for pump-probe delay times up to 250 ps.
Figure 3: TR-2PPE spectra reveal the coherent coupling between a hot ME state and a hot singlet state.
Figure 4: The absence of singlet-fission dynamics on sample temperatures between 176 and 248 K.

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Acknowledgements

This work was supported until 30 June 2011 by the US National Science Foundation under grant DMR-0946346; all experimental measurements were carried out during this time period. The work was continued after 30 June 2011 based on work supported as part of the program ‘Center for Re-Defining Photovoltaic Efficiency Through Molecule Scale Control’, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001085. All theoretical developments were achieved during the period after 30 June 2011. M.L. acknowledges SFB616 and the Leopoldina Fellowship Program LPDS 2009-41 that made his visit to the Zhu group at the University of Texas at Austin possible. We thank R. Wyatt, F. Spano, Al. Efros and A. Shabaev for fruitful discussions.

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  1. Manuel Ligges

    Present address: Present address: University of Duisburg-Essen, Physics Department, Lotharstrasse 1, 47048 Duisburg, Germany,

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  1. Department of Chemistry and Biochemistry, University of Texas, Austin, 78712, TX, USA

    Wai-Lun Chan, Manuel Ligges & X-Y. Zhu

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W-L.C. and X-Y.Z. conceived and designed the experiments, W-L.C. and M.L. performed the experiments and W-L. C. and X-Y.Z. analysed the data and co-wrote the paper.

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Correspondence to X-Y. Zhu.

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Chan, WL., Ligges, M. & Zhu, XY. The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain. Nature Chem 4, 840–845 (2012). https://doi.org/10.1038/nchem.1436

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