Fluorinating pi-Extended Molecular Acceptors Yields Highly Connected Crystal Structures and Low Reorganization Energies for Efficient Solar Cells
ADVANCED ENERGY MATERIALS(2020)
摘要
The synthesis and characterization of new semiconducting materials is essential for developing high-efficiency organic solar cells. Here, the synthesis, physiochemical properties, thin film morphology, and photovoltaic response of ITN-F4 and ITzN-F4, the first indacenodithienothiophene nonfullerene acceptors that combine pi-extension and fluorination, are reported. The neat acceptors and bulk-heterojunction blend films with fluorinated donor polymer poly{[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b ']-dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c ']dithiophene-1,3-diyl]]} (PBDB-TF, also known as PM6) are investigated using a battery of techniques, including single crystal X-ray diffraction, fs transient absorption spectroscopy (fsTA), photovoltaic response, space-charge-limited current transport, impedance spectroscopy, grazing incidence wide angle X-ray scattering, and density functional theory level computation. ITN-F4 and ITzN-F4 are found to provide power conversion efficiencies greater and internal reorganization energies less than their non-pi-extended and nonfluorinated counterparts when paired with PBDB-TF. Additionally, ITN-F4 and ITzN-F4 exhibit favorable bulk-heterojunction relevant single crystal packing architectures. fsTA reveals that both ITN-F4 and ITzN-F4 undergo ultrafast hole transfer (<300 fs) in films with PBDB-TF, despite excimer state formation in both the neat and blend films. Taken together and in comparison to related structures, these results demonstrate that combined fluorination and pi-extension synergistically promote crystallographic pi-face-to-face packing, increase crystallinity, reduce internal reorganization energies, increase interplanar pi-pi electronic coupling, and increase power conversion efficiency.
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关键词
computational chemistry,crystal structures,femtosecond transient absorption spectroscopy,impedance spectroscopy,organic solar cells
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