Resolving ultrafast exciton migration in organic solids at the nanoscale

TL;DR

This paper introduces a novel all-optical, sub-diffraction method to measure ultrafast exciton migration in organic solids at the nanoscale, revealing diffusive behavior and enabling detailed structure-migration correlations.

Contribution

It develops a time-resolved stimulated emission depletion microscopy technique to measure exciton migration with nanometer and picosecond resolution without disturbing material morphology.

Findings

Measured a 16-nm exciton migration length in CN-PPV films.

Demonstrated that exciton migration is diffusive due to energetic disorder.

Framework applicable to organic semiconductors and photosynthesis studies.

Abstract

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.