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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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.
The authors declare no competing financial interests.
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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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