Impact of meta- and para-Direction External Side Chains in Y-Series Acceptors on the Molecular Packing and Charge Carrier Dynamics of Organic Photovoltaics

The side-chain directions in nonfullerene acceptors (NFAs) strongly influence the intermolecular interactions in NFAs; however, the influence of these side chains on the morphologies and charge carrier dynamics of Y6-based acceptors remains underexplored. In this study, we synthesize four distinct Y6-based acceptors, i.e., meta -HOP–Y6-F ( m F), meta -HOP–Y6-Cl ( m Cl), para -HOP–Y6-F ( p F), and para -HOP–Y6-Cl ( p Cl), with outer side chains of alkoxy-2-ethylhexyl attached at the meta or para positions. Devices containing the meta -position acceptors blended with the polymer donor PM6 achieve power conversion efficiencies (PCEs) at least 1.27-fold higher than those of devices containing para -position acceptors. The enhanced performance can be attributed to the formation of donor–acceptor domains that are advantageous for charge carrier generation, transport, and collection. This is due to variations in phase aggregation that result from steric hindrance effects at the meta - and para -position acceptors. As a result, meta -position acceptors with lower steric hindrance improved π–π and lamellar stacking, whereas the para -position acceptors encountered excessive steric hindrance, reducing their photovoltaic efficiencies. Additionally, the meta -position acceptors demonstrate long charge carrier lifetimes, which suppress recombination in the charge transfer state and promote efficient charge separation. These results underline the critical role of side-chain direction in optimizing Y6-based acceptors for improving photovoltaic performance.