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

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, pi-extension and halogenation, in ADAD-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 pi-extension and halogenation was achieved. When these two factors are combined, the effect on optical absorption is cumulative. Single-crystal pi-pi stacking distances are similar for the EG strategies of pi-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 pi-pi 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 pi-face-on orientation, and both fluorination and pi-extension increase NFA crystallinity. Femto/nanosecond transient absorption spectroscopy (fs/nsTA) and integrated photocurrent device analysis (IPDA) indicate that pi-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 pi-extension and fluorination in affording efficient OSCs and provides insights into designing next-generation materials.

Automated summary

The study found that combining the π-extension and halogenation strategies of end-capping groups (EG) in organic solar cells significantly impacts optical absorption, leading to an increase in power conversion efficiency (PCE).