Conformationally Locked Steric Regulation Enables a Y‐Series Medium‐Bandgap Acceptor Achieving 20.10% Efficiency in Organic Photovoltaics

Organic photovoltaics (OPVs) are promising candidates for sustainable energy applications, yet further efficiency improvements remain necessary. Medium‐bandgap acceptors have proven effective for improving device performance, but their development still requires effective molecular design strategies. Here, a Y‐series medium‐bandgap acceptor, X3, was developed through side‐chain engineering without altering the conjugated backbone by replacing the outer alkyl side chain with a 2,4,6‐triisopropylphenyl (Tip) substituent. Tip functioned as a conformationally locked steric unit rather than merely as a bulkier substituent, thereby more effectively suppressing excessive intermolecular aggregation while only modestly perturbing the intrinsic electronic structure. As a result, X3 exhibited an optical bandgap of 1.50 eV. Single‐crystal analysis further showed that Tip participated in multiple weak intermolecular interactions, giving rise to diverse packing motifs that consisted of localized π–π contacts and non‐stacking interactions. Binary devices based on D18‐Cl:X3 delivered a high open‐circuit voltage ( V OC ) of 1.026 V, while the D18‐Cl:N3:X3 ternary device achieved a power conversion efficiency (PCE) of 20.10%. These results highlight conformationally locked steric regulation as an effective side‐chain design mode for pushing a Y‐series scaffold toward the medium‐bandgap regime and provide structure–property insight into side‐chain‐engineered acceptors for OPVs.