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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
Author(s) -
Kan Bin,
Zhang Jiangbin,
Liu Feng,
Wan Xiangjian,
Li Chenxi,
Ke Xin,
Wang Yunchuang,
Feng Huanran,
Zhang Yamin,
Long Guankui,
Friend Richard H.,
Bakulin Artem A.,
Chen Yongsheng
Publication year - 2018
Publication title -
advanced materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.707
H-Index - 527
eISSN - 1521-4095
pISSN - 0935-9648
DOI - 10.1002/adma.201704904
Subject(s) - homo/lumo , materials science , organic solar cell , acceptor , energy conversion efficiency , band gap , optoelectronics , molecule , photochemistry , chemistry , organic chemistry , physics , polymer , composite material , condensed matter physics
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 V oc 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.