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

• Published in: Advanced materials (Weinheim) Vol. 30; no. 3
• Main Authors: 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
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.01.2018; Wiley
• Subjects: charge separation; Chemistry; Energy conversion efficiency; Energy levels; Fluorination; high‐performance organic solar cells; low bandgap; Materials Science; Molecular orbitals; Optimization; Photovoltaic cells; Physics; Science & Technology - Other Topics; Short circuits; small‐molecule nonfullerene acceptors; Solar cells; charge separation; low bandgap; high-performance organic solar cells; small-molecule nonfullerene acceptors
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.201704904

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. An acceptor‐donor‐acceptor nonfullerene acceptor NCBDT is reported. NCBDT exhibits a low optical bandgap of 1.45 eV and broadened absorption range. The PBDB‐T:NCBDT‐based device achieves an impressive PCE of 12.12% and Jsc over 20 mA cm‐2—one of the best results for solution‐processed OSCs. Further photophysical study reveals slow (≈400 ps) yet efficient free charge generation.