Efficient Energy Transport in an Organic Semiconductor Mediated by Transient Exciton Delocalization

TL;DR

This paper uncovers a transient exciton delocalization mechanism in organic semiconductors that significantly enhances energy transport efficiency, challenging traditional localized exciton models and enabling longer diffusion lengths.

Contribution

It demonstrates the existence of transient exciton delocalization in OSCs, leading to higher diffusion constants and longer diffusion lengths than previously thought possible.

Findings

Exciton diffusion constants up to 1.1 cm^2/s in ordered poly(3-hexylthiophene) nanofibers.

Diffusion lengths of approximately 300 nm achieved.

Evidence for a new energy transport regime mediated by transient delocalization.

Abstract

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.