Article | Published:

Resolving ultrafast exciton migration in organic solids at the nanoscale

Nature Materials volume 16, pages 11361141 (2017) | Download Citation

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.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.

from $8.99

All prices are NET prices.

References

  1. 1.

    & Excitons in nanoscale systems. Nat. Mater. 5, 683–696 (2006).

  2. 2.

    , , & Charge-transfer and energy-transfer processes in π-conjugated oligomers and polymers: a molecular picture. Chem. Rev. 104, 4971–5004 (2004).

  3. 3.

    , , , & Excited state and charge photogeneration dynamics in conjugated polymers. J. Phys. Chem. B 111, 6303–6321 (2007).

  4. 4.

    & Exciton diffusion in organic photovoltaic cells. Energy Environ. Sci. 7, 499–512 (2014).

  5. 5.

    & Electronic Processes in Organic Semiconductors: An Introduction (John Wiley, 2015).

  6. 6.

    , , & Spectrally resolved dynamics of energy transfer in quantum-dot assemblies: towards engineered energy flows in artificial materials. Phys. Rev. Lett. 89, 186802 (2002).

  7. 7.

    et al. Excitation energy transfer and charge separation in photosystem II membranes revisited. Biophys. J. 91, 3776–3786 (2006).

  8. 8.

    , & A structure-based model of energy transfer reveals the principles of light harvesting in photosystem II supercomplexes. J. Am. Chem. Soc. 135, 9164–9173 (2013).

  9. 9.

    et al. Hot charge-transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics. Nat. Mater. 12, 66–73 (2013).

  10. 10.

    , & Exciton diffusion in organic semiconductors. Energy Environ. Sci. 8, 1867–1888 (2015).

  11. 11.

    et al. Systematic study of exciton diffusion length in organic semiconductors by six experimental methods. Mater. Horiz. 1, 280–285 (2014).

  12. 12.

    et al. Real-time tracking of singlet exciton diffusion in organic semiconductors. Phys. Rev. Lett. 116, 057402 (2016).

  13. 13.

    & Anomalous exciton diffusion in the conjugated polymer MEH-PPV measured using a three-pulse pump–dump–probe anisotropy experiment. J. Phys. Chem. A 108, 10801–10806 (2004).

  14. 14.

    , , , & Accurate measurement of the exciton diffusion length in a conjugated polymer using a heterostructure with a side-chain cross-linked fullerene layer. J. Phys. Chem. A 109, 5266–5274 (2005).

  15. 15.

    , , , & Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching. J. Appl. Phys. 105, 053711 (2009).

  16. 16.

    , & Heterogeneity in polymer solar cells: local morphology and performance in organic photovoltaics studied with scanning probe microscopy. Acc. Chem. Res. 43, 612–620 (2010).

  17. 17.

    , , & Role of intermolecular coupling in the photophysics of disordered organic semiconductors: aggregate emission in regioregular polythiophene. Phys. Rev. Lett. 98, 206406 (2007).

  18. 18.

    et al. A general relationship between disorder, aggregation and charge transport in conjugated polymers. Nat. Mater. 12, 1038–1044 (2013).

  19. 19.

    et al. Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells. Nat. Commun. 5, 5293 (2014).

  20. 20.

    , , & Controlling interchain interactions in conjugated polymers: the effects of chain morphology on exciton–exciton annihilation and aggregation in MEH–PPV films. J. Phys. Chem. B 104, 237–255 (2000).

  21. 21.

    , & Ultrafast spatial imaging of charge dynamics in heterogeneous polymer blends. J. Phys. Chem. Lett. 3, 879–884 (2012).

  22. 22.

    & Effects of optical interference and energy transfer on exciton diffusion length measurements in organic semiconductors. J. Appl. Phys. 100, 034907 (2006).

  23. 23.

    et al. Visualization of exciton transport in ordered and disordered molecular solids. Nat. Commun. 5, 3646 (2014).

  24. 24.

    et al. Subdiffusive exciton transport in quantum dot solids. Nano Lett. 14, 3556–3562 (2014).

  25. 25.

    , & Bringing far-field subdiffraction optical imaging to electronically coupled optoelectronic molecular materials using their endogenous chromophores. J. Phys. Chem. Lett. 6, 2767–2772 (2015).

  26. 26.

    & Breaking the diffraction resolution limit by stimulated emission: stimulated-emission-depletion fluorescence microscopy. Opt. Lett. 19, 780–782 (1994).

  27. 27.

    , & Efficient interchain photoluminescence in a high-electron-affinity conjugated polymer. Phys. Rev. B 52, R11573–R11576 (1995).

  28. 28.

    , , , & Time-gated single-photon detection module with 110 ps transition time and up to 80 MHz repetition rate. Rev. Sci. Instrum. 85, 083114 (2014).

  29. 29.

    et al. Sharper low-power STED nanoscopy by time gating. Nat. Methods 8, 571–573 (2011).

  30. 30.

    , & Can disorder enhance incoherent exciton diffusion? J. Phys. Chem. B 119, 9501–9509 (2015).

  31. 31.

    & Local exciton ground states in disordered polymers. Phys. Rev. B 81, 165201 (2010).

  32. 32.

    Energy transport: singlet to triplet and back again. Nat. Chem. 7, 764–765 (2015).

  33. 33.

    et al. Site-selective fluorescence studies of poly(p-phenylene vinylene) and its derivatives. Phys. Rev. B 53, 15815–15822 (1996).

  34. 34.

    & Transport energy for hopping in a Gaussian density of states distribution. J. Non-Cryst. Solids 190, 112–116 (1995).

  35. 35.

    et al. Singlet exciton diffusion in MEH-PPV films studied by exciton–exciton annihilation. Org. Electron. 7, 452–456 (2006).

  36. 36.

    et al. Singlet exciton lifetimes in conjugated polymer films for organic solar cells. Polymers 8, 14 (2016).

  37. 37.

    et al. Ultrafast intramolecular exciton splitting dynamics in isolated low-band-gap polymers and their implications in photovoltaic materials design. J. Am. Chem. Soc. 134, 4142–4152 (2012).

  38. 38.

    et al. Single-junction polymer solar cells with high efficiency and photovoltage. Nat. Photon. 9, 174–179 (2015).

  39. 39.

    , , & Exciton diffusion in conjugated polymers: from fundamental understanding to improvement in photovoltaic conversion efficiency. J. Phys. Chem. Lett. 6, 3417–3428 (2015).

Download references

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.

Author information

Author notes

    • Samuel B. Penwell
    •  & Rodrigo Noriega

    Present addresses: James Franck Institute, University of Chicago, Chicago, Illinois 60637, USA (S.B.P.); Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, USA (R.N.).

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. Search for Samuel B. Penwell in:

  2. Search for Lucas D. S. Ginsberg in:

  3. Search for Rodrigo Noriega in:

  4. Search for Naomi S. Ginsberg in:

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.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Naomi S. Ginsberg.

Supplementary information

PDF files

  1. 1.

    Supplementary Information

    Supplementary Information

About this article

Publication history

Received

Accepted

Published

DOI

https://doi.org/10.1038/nmat4975