Fine‐Tuning the Energy Levels of a Nonfullerene Small‐Molecule Acceptor to Achieve a High Short‐Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells |
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Authors: | Bin Kan Jiangbin Zhang Feng Liu Xiangjian Wan Chenxi Li Xin Ke Yunchuang Wang Huanran Feng Yamin Zhang Guankui Long Richard H. Friend Artem A. Bakulin Yongsheng Chen |
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Affiliation: | 1. State Key Laboratory and Institute of Elemento‐Organic Chemistry, The Centre of Nanoscale Science and Technology and Key Laboratory of Functional Polymer Materials, College of Chemistry, Nankai University, Tianjin, China;2. Department of Chemistry, Imperial College London, London, UK;3. Cavendish Laboratory, University of Cambridge, Cambridge, UK;4. Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai, China |
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Abstract: | Organic solar cell optimization requires careful balancing of current–voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor–donor–acceptor (A–D–A)‐type small‐molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near‐IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the Voc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT‐ and NFBDT‐based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge‐pair states is observed, followed by effective charge extraction. As a result, the PBDB‐T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%—among the best for solution‐processed organic solar cells. |
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Keywords: | charge separation high‐performance organic solar cells low bandgap small‐molecule nonfullerene acceptors |
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