Long-Range Electrostatics Supercharge Exciton Transport
Alexander Sneyd a, Tomoya Fukui b, David Beljonne c, Ian Manners d, Richard Friend a, Akshay Rao a, Suryoday Prodhan c
a Cavendish Laboratory, Department of Physics, University of Cambridge, UK, JJ Thomson Avenue, Cambridge, United Kingdom
b Tokyo Institute of Technology, 2-12-1 Oookayama, Meguro-ku, Tokyo, Japan
c Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc 20, B-7000 Mons, Belgium
d University of Victoria, Canada, Engineering Office Wing, Room 448 Victoria BC Canada, Canada
International Conference on Hybrid and Organic Photovoltaics
Proceedings of 13th Conference on Hybrid and Organic Photovoltaics (HOPV21)
Online, Spain, 2021 May 24th - 28th
Organizers: Marina Freitag, Feng Gao and Sam Stranks
Oral, Alexander Sneyd, presentation 046
Publication date: 11th May 2021

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 cm2/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. In this regime, long-range electrostatic interactions enable the presence of low-lying spatially-extended states which excitons can temporarily re-access via energy exchange with vibrational modes 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 cm2/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 based on the power of long-range electrostatics to engineer efficient energy transport in OSC films, which will enable new devices architectures not based on restrictive bulk heterojunctions.

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