Impact of <i>meta</i>- and <i>para</i>-Direction External Side\nChains in Y‑Series Acceptors on the\nMolecular Packing and Charge Carrier Dynamics of Organic Photovoltaics

The side-chain directions in nonfullerene acceptors (NFAs)\nstrongly\ninfluence the intermolecular interactions in NFAs; however, the influence\nof these side chains on the morphologies and charge carrier dynamics\nof Y6-based acceptors remains underexplored. In this study, we synthesize\nfour distinct Y6-based acceptors, i.e., <i>meta</i>-HOP–Y6-F\n(<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\nthe <i>meta</i>-position acceptors blended with the polymer\ndonor PM6 achieve power conversion efficiencies (PCEs) at least 1.27-fold\nhigher than those of devices containing <i>para</i>-position\nacceptors. The enhanced performance can be attributed to the formation\nof donor–acceptor domains that are advantageous for charge\ncarrier generation, transport, and collection. This is due to variations\nin phase aggregation that result from steric hindrance effects at\nthe <i>meta</i>- and <i>para</i>-position acceptors.\nAs a result, <i>meta</i>-position acceptors with lower steric\nhindrance improved π–π and lamellar stacking, whereas\nthe <i>para</i>-position acceptors encountered excessive\nsteric hindrance, reducing their photovoltaic efficiencies. Additionally,\nthe <i>meta</i>-position acceptors demonstrate long charge\ncarrier lifetimes, which suppress recombination in the charge transfer\nstate and promote efficient charge separation. These results underline\nthe critical role of side-chain direction in optimizing Y6-based acceptors\nfor improving photovoltaic performance.