Systematic Merging of Nonfullerene Acceptor π-Extension and Tetrafluorination Strategies Affords Polymer Solar Cells with >16% Efficiency

Guoping Li, Xiaohua Zhang, Leighton O. Jones, Joaquin M. Alzola, Subhrangsu Mukherjee, Liang Wen Feng, Weigang Zhu, Charlotte L. Stern, Wei Huang, Junsheng Yu, Vinod K. Sangwan, Dean M. Delongchamp, Kevin L. Kohlstedt*, Michael R. Wasielewski, Mark C. Hersam, George C. Schatz, Antonio Facchetti, Tobin J. Marks

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

42 Scopus citations


The end-capping group (EG) is the essential electron-withdrawing component of nonfullerene acceptors (NFAs) in bulk heterojunction (BHJ) organic solar cells (OSCs). To systematically probe the impact of two frequent EG functionalization strategies, π-extension and halogenation, in A-DAD-A type NFAs, we synthesized and characterized four such NFAs: BT-BIC, LIC, L4F, and BO-L4F. To assess the relative importance of these strategies, we contrast these NFAs with the baseline acceptors, Y5 and Y6. Up to 16.6% power conversion efficiency (PCE) in binary inverted OSCs with BT-BO-L4F combining π-extension and halogenation was achieved. When these two factors are combined, the effect on optical absorption is cumulative. Single-crystal π-πstacking distances are similar for the EG strategies of π-extension. Increasing the alkyl substituent length from BT-L4F to BT-BO-L4F significantly alters the packing motif and eliminates the EG core interactions of BT-L4F. Electronic structure computations reveal some of the largest NFA π-πelectronic couplings observed to date, 103.8 meV in BT-L4F and 47.5 meV in BT-BO-L4F. Computed electronic reorganization energies, 132 and 133 meV for BT-L4F and BT-BO-L4F, respectively, are also lower than Y6 (150 meV). BHJ blends show preferential π-face-on orientation, and both fluorination and π-extension increase NFA crystallinity. Femto/nanosecond transient absorption spectroscopy (fs/nsTA) and integrated photocurrent device analysis (IPDA) indicate that π-extension modifies the phase separation to enhance film ordering and carrier mobility, while fluorination suppresses unimolecular recombination. This systematic study highlights the synergistic effects of NFA π-extension and fluorination in affording efficient OSCs and provides insights into designing next-generation materials.

Original languageEnglish (US)
Pages (from-to)6123-6139
Number of pages17
JournalJournal of the American Chemical Society
Issue number16
StatePublished - Apr 28 2021

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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