Ultrafast Exciton Dynamics in Two Dimensional Covalent Organic Frameworks Reveals Size Dependence to Exciton Diffusion

03 April 2020, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Large singlet exciton diffusion lengths are a hallmark of high performance in organic based devices such as photovoltaics, chemical sensors, and photodetectors. In this study, exciton dynamics of a two-dimensional covalent organic framework, COF- 5, is investigated using ultrafast spectroscopic techniques. Following photoexcitation, the COF-5 exciton decays via three pathways: 1) excimer formation (4 ± 2 ps), 2) excimer relaxation (160 ± 40 ps), and 3) excimer decay (>3 ns). Excitation fluence-dependent transient absorption studies suggest that COF-5 has a relatively large diffusion coefficient (0.08 cm2/s). Furthermore, exciton-exciton annihilation processes are characterized as a function of COF-5 crystallite domain size in four different samples, which reveal domain- size dependent exciton diffusion kinetics. These results reveal that exciton diffusion in COF-5 is constrained by its crystalline domain size. These insights indicate the outstanding promise of delocalized excitonic processes available in 2D COFs, which motivate their continued design and implementation into optoelectronic devices.

Keywords

covalent organic frameworks
excitons
optoelectronics
aromatic stacking
two-dimensional polymers

Supplementary materials

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2020 03 ChemRxiv Flanders TOC
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COF5excitondynSI
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