Abstract
Covalent organic frameworks (COFs) have emerged as a promising light-harvesting module for artificial photosynthesis and photovoltaics. For efficient generation of free charge carriers, the donor–acceptor (D-A) conjugation has been adopted for two-dimensional (2D) COFs recently. In the 2D D-A COFs, photoexcitation would generate a polaron pair, which is a precursor to free charge carriers and has lower binding energy than an exciton. Although the character of the primary excitation species is a key factor in determining optoelectronic properties of a material, excited-state dynamics leading to the creation of a polaron pair have not been investigated yet. Here, we investigate the dynamics of photogenerated charge carriers in 2D D-A COFs by combining femtosecond optical spectroscopy and non-adiabatic molecular dynamics simulation. From this investigation, we elucidate that the polaron pair is formed through ultrafast intra-layer hole transfer coupled with coherent vibrations of the 2D lattice, suggesting a mechanism of phonon-assisted charge transfer.
Highlights
Covalent organic frameworks (COFs) have emerged as a promising light-harvesting module for artificial photosynthesis and photovoltaics
The 2D D-A COFs were synthesized by direct polycondensation of the building blocks and the details for the synthesis and characterization of the COFs are described in Methods, Supplementary Notes [1,2,3], and Supplementary Figs. 1–5
The electronic interaction between perylenetetracarboxylic acid diimide (PDI) and free-base porphyrin units is manifested as spectral red shift and broadening of the absorption spectrum of COFs compared with the absorption spectrum of free-base porphyrin, as shown in Fig. 1b and Supplementary Fig. 6
Summary
Covalent organic frameworks (COFs) have emerged as a promising light-harvesting module for artificial photosynthesis and photovoltaics. Electronic interaction among the building blocks of a 2D COF and inter-layer π–π interaction among the stacked layers of COFs provide efficient pathways for photogenerated charge carriers Due to their robust structure and high charge mobility through the framework, the COFs have the potential to serve as a primary light-harvesting module in photovoltaic and optoelectronic devices[2,3,4,5,7,8,9,10]. According to previous studies of photoinduced charge carrier dynamics in D-A COFs21,22, it was suggested that light absorption induces charge transfer between each donor and acceptor units within 1.8 ps and the charges stay separated for up to 10 μs due to charge delocalization in two separate donor and acceptor arrays aligned perpendicular to stacked 2D layers This result shows a unique molecular configuration of D-A COFs where the D-A interfaces serve as a well-ordered columnar heterojunction throughout the three-dimensional framework
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