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

preprint
submitted on 01.04.2020 and posted on 03.04.2020 by Nathan C. Flanders, Matthew S. Kirschner, Pyosang Kim, Thomas Fauvell, Austin Evans, Waleed Helweh, Austin P. Spencer, Richard D. Schaller, William Dichtel, Lin X. Chen

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.

Funding

ARO W911NF-15-1-0447

DOE DE-AC0206CH11357

NSF DGE-1324585

NSF ECCS-1542205

NSF DMR-1720139

ARO W911NF-18-1- 0359

History

Email Address of Submitting Author

wdichtel@northwestern.edu

Institution

Northwestern University

Country

USA

ORCID For Submitting Author

0000-0002-3635-6119

Declaration of Conflict of Interest

The authors declare no conflicts of interest.

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