Large Exciton Diffusion Coefficients in Two-Dimensional Covalent Organic Frameworks with Different Domain Sizes Revealed by Ultrafast Exciton Dynamics

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, 2D COF-5, is investigated using ultrafast spectroscopic techniques. After 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.

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

“…It can be attributed to an increased conjugation length with layer thickness and/or the J-type aggregation with the chromophores between adjacent layers. 44,45 The UV-vis reflectance of solid TP-TTA/SiO 2 -x showed the similar tendency Page 8 of 25 CCS Chemistry (Supporting Information Figure S11). Calculated by Tauc plots, 46,47 the optical band gaps of TP-TTA/SiO 2 -x varied from 2.28 eV to 2.41 eV, showing a slight increase of band gap with TP-TTA layer decreasing (Supporting Information Figure S11).…”

Topology-Mediated Enhanced Polaron Coherence in Covalent Organic Frameworks

“…We believe that these results are particularly important in the context of recent experimental efforts aimed at synthesizing COFs with a high degree of crystallinity [44][45][46][47][48][49][50] or larger domains. 51,52 While such efforts are certainly important for achieving higher hole mobility, our analyses demonstrate that, independently of the degree of crystallinity and/or domain sizes, if the counter anions residing inside the COF pores trap the free charge carriers, the conductivity of COFs will always be lower 18,43,[53][54][55][56][57][58] than that measured for doped conjugated polymers. 25,59,60 Our results thus suggest that it would be important to explore the potential use of other dopants and doping techniques in order to achieve higher conductivities in COF films.…”

Unraveling the effect of defects, domain size, and chemical doping on photophysics and charge transport in covalent organic frameworks

“…Therefore, polaron absorption from COFs having different domain sizes will be different unlike in the case of excitons for which it has recently been shown that the absorption remains the same while the emission changes drastically. 15 Peak B in the IR spectrum in any direction (x, y, z) is an electronic transition from the polaron ground state to the next higher electronic state in the same direction. We emphasize that peak A is an electronic transition and is distinct from the superimposed narrow resonances also observed in the experimental measurements which are instead due to IRAV modes.…”