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



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


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–π stacking 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–π electronic 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.
Journal of American Chemical Society


Mukherjee, S. , DeLongchamp, D. , Li, G. , Zhang, X. , Jones, L. , Alzola, J. , Feng, L. , Zhu, W. , Stern, C. , Yu, J. , Sangwan, V. , Kohlstedt, K. , Wasielewski, M. , Hersam, M. , Schatz, G. , Facchetti, A. and Marks, T. (2021), Systematic Merging of Nonfullerene Acceptor π-Extension and Tetrafluorination Strategies Affords Polymer Solar Cells with >16% Efficiency, Journal of American Chemical Society, [online], (Accessed December 4, 2023)
Created April 13, 2021, Updated March 5, 2023