Impact of <i>meta</i>- and <i>para</i>-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., <i>meta</i>-HOP-Y6-F (<i>m</i>F), <i>meta</i>-HOP-Y6-Cl (<i>m</i>Cl), <i>para</i>-HOP-Y6-F (<i>p</i>F), and <i>para</i>-HOP-Y6-Cl (<i>p</i>Cl), with outer side chains of alkoxy-2-ethylhexyl attached at the <i>meta</i> or <i>para</i> positions. Devices containing the <i>meta</i>-position acceptors blended with the polymer donor PM6 achieve power conversion efficiencies (PCEs) at least 1.27-fold higher than those of devices containing <i>para</i>-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 <i>meta</i>- and <i>para</i>-position acceptors. As a result, <i>meta</i>-position acceptors with lower steric hindrance improved π-π and lamellar stacking, whereas the <i>para</i>-position acceptors encountered excessive steric hindrance, reducing their photovoltaic efficiencies. Additionally, the <i>meta</i>-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.