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Resolving ultrafast exciton migration in organic solids at the nanoscale

Nature Materials volume 16, pages 1136–1141 (2017)Cite this article

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Abstract

Effectiveness of molecular-based light harvesting relies on transport of excitons to charge-transfer sites. Measuring exciton migration, however, has been challenging because of the mismatch between nanoscale migration lengths and the diffraction limit. Instead of using bulk substrate quenching methods, here we define quenching boundaries all-optically with sub-diffraction resolution, thus characterizing spatiotemporal exciton migration on its native nanometre and picosecond scales. By transforming stimulated emission depletion microscopy into a time-resolved ultrafast approach, we measure a 16-nm migration length in poly(2,5-di(hexyloxy)cyanoterephthalylidene) conjugated polymer films. Combined with Monte Carlo exciton hopping simulations, we show that migration in these films is essentially diffusive because intrinsic chromophore energetic disorder is comparable to chromophore inhomogeneous broadening. Our approach will enable previously unattainable correlation of local material structure to exciton migration character, applicable not only to photovoltaic or display-destined organic semiconductors but also to explaining the quintessential exciton migration exhibited in photosynthesis.

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Figure 1: TRUSTED implementation.
Figure 2: Using TRUSTED to capture the fraction of remaining excitations versus time to obtain Ld.
Figure 3: Mapping the site distinguishability—downhill bias phase space to reveal the character of exciton migration.

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Acknowledgements

This work was supported by a David and Lucile Packard Fellowship for Science and Engineering to N.S.G., by The Dow Chemical Company under contract #244699, and by STROBE, A National Science Foundation Science and Technology Center under Grant No. DMR 1548924. Instrument development was supported by the Director, Office of Science, Chemical Sciences, Geosciences, and Biosciences Division, of the US Department of Energy under Contract No. DEAC02-05CH11231. We thank A. Tosi and M. Buttafava of SPAD lab, Politecnico di Milano, for discussions and the generous trial of the fast-gated SPAD and N. Bertone and PicoQuant GmbH for providing a demo of the HydraHarp400 photon counting apparatus. We thank D. M. Neumark for the use of a grating stretcher. S.B.P. acknowledges a Department of Energy Graduate Research Fellowship (contract no. DE-AC05-060R23100), R.N. thanks the Philomathia Foundation for postdoctoral support, and N.S.G. acknowledges an Alfred P. Sloan Research Fellowship and the Camille and Henry Dreyfus Teacher-Scholar Program.

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Author notes

  1. Samuel B. Penwell

    Present address: James Franck Institute, University of Chicago, Chicago, Illinois, 60637, USA

  2. Rodrigo Noriega

    Present address: Department of Chemistry, University of Utah, Salt Lake City, Utah, 84112, USA

Authors and Affiliations

  1. Department of Chemistry, University of California, Berkeley, California, 94720, USA

    Samuel B. Penwell, Lucas D. S. Ginsberg, Rodrigo Noriega & Naomi S. Ginsberg

  2. Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, California, 94720, USA

    Naomi S. Ginsberg

  3. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, 94720, USA

    Naomi S. Ginsberg

  4. Kavli Energy NanoScience Institute, Berkeley, California, 94720, USA

    Naomi S. Ginsberg

  5. Department of Physics, University of California, Berkeley, California, 94720, USA

    Naomi S. Ginsberg

Authors
  1. Samuel B. Penwell
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Contributions

S.B.P., L.D.S.G. and N.S.G. designed the research. S.B.P. and L.D.S.G. constructed the apparatus and performed the experiments. L.D.S.G. prepared the samples. S.B.P. performed and analysed the simulations. R.N. aided in the design and interpretation of the simulations. N.S.G. supervised the project. S.B.P. and N.S.G. wrote the manuscript and all authors revised and approved the manuscript.

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Correspondence to Naomi S. Ginsberg.

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Penwell, S., Ginsberg, L., Noriega, R. et al. Resolving ultrafast exciton migration in organic solids at the nanoscale. Nature Mater 16, 1136–1141 (2017). https://doi.org/10.1038/nmat4975

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