Regulating π-Linker Side Chains to Control Molecular Conformation and Aggregation of Non-Fused-Ring Acceptors

Nonfused ring electron acceptors (NFREAs) are promising for practical organic solar cells (OSCs), yet rational side-chain design rules─particularly for fully conjugated NFREAs without noncovalent conformational locks─remain unclear. Here, we develop three NFREAs (BTPOSi-EHPh, BTPOSi-U, and BTPOSi-BO) featuring a bulky 4,5-bis(triisopropylsilyloxy)benzo[2,1-b:3,4-b′]dithiophene core and thieno[3,2-b]thiophene π-bridges with systematically varied side chains. By correlating molecular conformation with aggregation, donor–acceptor miscibility, and solid-state packing in PM6 blend films, we show that the π-bridge side-chain architecture governs a trade-off between backbone planarity and excessive aggregation. Among these NFREAs, BTPOSi-BO with 2-butyloctyl substituents achieves moderate planarity and controlled aggregation, leading to improved miscibility with PM6, optimized nanoscale phase separation, enhanced face-on packing with tighter π–π stacking, more balanced charge transport, and suppressed bimolecular recombination. Consequently, PM6:BTPOSi-BO OSCs deliver an efficiency of 10.51% (Jsc = 16.21 mA cm–2, Voc = 0.988 V, FF = 65.62%) and improved thermal stability (T80 = 2250 h at 65 °C). These results highlight bulky silyloxy-substituted BTP cores combined with π-bridge side-chain engineering as an effective strategy to design efficient NFREAs without noncovalent conformational locks.