Exciton Diffusion in Highly-Ordered One Dimensional Conjugated Polymers: Effects of Back-Bone Torsion, Electronic Symmetry, Phonons and Annihilation

Raj Pandya,Akshay Rao,Antonios M Alvertis,Christoph Schnedermann,Laurent Legrand,Alex W Chin,Jooyoung Sung,Thierry Barisien,Qifei Gu,David Kréher

doi:10.1021/acs.jpclett.1c00193

Abstract

Many optoelectronic devices based on organic materials require rapid and long-range singlet exciton transport. Key factors controlling exciton transport include material structure, exciton–phonon coupling and electronic state symmetry. Here, we employ femtosecond transient absorption microscopy to study the influence of these parameters on exciton transport in one-dimensional conjugated polymers. We find that excitons with 21Ag– symmetry and a planar backbone exhibit a significantly higher diffusion coefficient (34 ± 10 cm2 s–1) compared to excitons with 11Bu+ symmetry (7 ± 6 cm2 s–1) with a twisted backbone. We also find that exciton transport in the 21Ag– state occurs without exciton–exciton annihilation. Both 21Ag– and 11Bu+ states are found to exhibit subdiffusive behavior. Ab initio GW-BSE calculations reveal that this is due to the comparable strengths of the exciton–phonon interaction and exciton coupling. Our results demonstrate the link between electronic state symmetry, backbone torsion and phonons in exciton transport in π-conjugated polymers.

Highlights

Organic optoelectronic devices ranging from light-emitting diodes to photovoltaic cells and transistors[1−3] are frequently based on linear polymers with a C2h point group symmetry
Our results demonstrate the link between electronic state symmetry, backbone torsion and phonons in exciton transport in π-conjugated polymers
Crystals were masked under a polarized optical microscope prior to measurement (SI Section S1) to ensure excitation was performed away from grain boundaries which can be well resolved in the materials. As such the energetic disorder is larger than kBT in the materials our results suggests that trap states are not significant in our observations or responsible for the difference in transport between “blue” and Despite the high diffusion coefficients obtained at room temperature in these systems (Figure 2), which would be suggestive of a strong tendency of excitons to delocalize, we observe subdiffusive exciton transport in both 21Ag− and 11Bu+ states, which is typical of strong exciton−phonon interactions[52] that lead to localized excitons

Summary

Introduction

Organic optoelectronic devices ranging from light-emitting diodes to photovoltaic cells and transistors[1−3] are frequently based on linear polymers with a C2h point group symmetry. The electronic ground state (S0) exhibits 11Ag+ symmetry. Depending on the polymer conjugation length and backbone geometry the first (S1) and second (S2) excited electronic states are either of 21Ag− or 11Bu+ symmetry.[4,5] Irrespective of the exact state ordering, photoexcitations in conjugated polymers rapidly form excitons in the lowest energy excited. When the S1 state is of 21Ag− symmetry the materials are typically nonfluorescent, pair character 1(TT)[6−8] the and. S1 state supports some frequently shows a tripletshort electronic lifetime. On-the-other hand polymers with an S1 state of 11Bu+ character are often luminescent with long electronic lifetimes.[9,10]

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Results

Discussion

Conclusion