What is the role of non-fullerene acceptor symmetry in polymer solar cell efficiency?

The molecular-level design of non-fullerene acceptors (NFAs) is crucial for enhancing the efficiency of polymer solar cells (PSCs). Here, we investigate the influence of NFA symmetry on bulk-heterojunction (BHJ) PSC performance. Our study introduces a series of closely related asymmetric NFA architectures (A-D-D type and A-DA'D-D type) and compares them with traditional symmetric NFAs. We find that BHJ PSCs using symmetric NFAs outperform those with asymmetric NFAs, achieving power conversion efficiencies (PCEs) of 10%–17% compared with 0.1%–3% for asymmetric NFAs. Analysis reveals that the lower performance of asymmetric NFAs results from disrupted end group-end group stacking in the NFA crystal network, which further causes reduced electron mobility, weaker crystallinity, slower hole transfer, and higher rates of exciton recombination in BHJ blends. These findings provide valuable insights for designing high-performance NFAs in the future.