  The effectiveness of molecular-based light harvesting relies on transport of
optical excitations, excitons, to charg-transfer sites. Measuring exciton
migration has, however, been challenging because of the mismatch between
nanoscale migration lengths and the diffraction limit. In organic
semiconductors, common bulk methods employ a series of films terminated at
quenching substrates, altering the spatioenergetic landscape for migration.
Here we instead define quenching boundaries all-optically with sub-diffraction
resolution, thus characterizing spatiotemporal exciton migration on its native
nanometer and picosecond scales without disturbing morphology. By transforming
stimulated emission depletion microscopy into a time-resolved ultrafast
approach, we measure a 16-nm migration length in CN-PPV conjugated polymer
films. Combining these experiments with Monte Carlo exciton hopping simulations
shows that migration in CN-PPV films is essentially diffusive because intrinsic
chromophore energetic disorder is comparable to inhomogeneous broadening among
chromophores. This framework also illustrates general trends across materials.
Our new approach's sub-diffraction resolution will enable previously
unattainable correlations of local material structure to the nature of exciton
migration, applicable not only to photovoltaic or display-destined organic
semiconductors but also to explaining the quintessential exciton migration
exhibited in photosynthesis.
