  Efficient energy transport is highly desirable for organic semiconductor
(OSC) devices such as photovoltaics, photodetectors, and photocatalytic
systems. However, photo-generated excitons in OSC films mostly occupy highly
localized states over their lifetime. Energy transport is hence thought to be
mainly mediated by the site-to-site hopping of localized excitons, limiting
exciton diffusion coefficients to below ~10^{-2} cm^2/s with corresponding
diffusion lengths below ~50 nm. Here, using ultrafast optical microscopy
combined with non-adiabatic molecular dynamics simulations, we present evidence
for a new highly-efficient energy transport regime: transient exciton
delocalization, where energy exchange with vibrational modes allows excitons to
temporarily re-access spatially extended states under equilibrium conditions.
In films of highly-ordered poly(3-hexylthiophene) nanofibers, prepared using
living crystallization-driven self-assembly, we show that this enables exciton
diffusion constants up to 1.1+-0.1 cm^2/s and diffusion lengths of 300+-50 nm.
Our results reveal the dynamic interplay between localized and delocalized
exciton configurations at equilibrium conditions, calling for a re-evaluation
of the basic picture of exciton dynamics. This establishes new design rules to
engineer efficient energy transport in OSC films, which will enable new devices
architectures not based on restrictive bulk heterojunctions.
