Non‑Fused Conjugation Extension for Suppressed Energy Loss in Organic Solar Cells

ABSTRACT Organic solar cells (OSCs) suffer from substantial energy losses arising from strong electron‐phonon coupling and structural relaxation, which limit their open‐circuit voltage. Conjugation extension has recently been employed to rigidify molecular backbones and suppress non‑radiative recombination; however, conventional fused‑ring extension often induces excessive aggregation that compromises the short‑circuit current density and fill factor. Here, we report a non‑fused conjugation extension strategy that simultaneously enhances molecular rigidity and optimizes intermolecular packing in small molecular acceptors. By extending the central core with a single thiophene group and introducing an intramolecular F···S conformational lock, the resulting acceptor C5Qx‐FTh achieves a nearly coplanar conformation with a dihedral angle below 1°. This rigidified framework suppresses the electron‐phonon coupling and reduces the excitonic static disorder, while also suppressing molecular chain dynamics and enhancing device stability. Theoretical simulations and morphological characterizations reveal that this non‑fused conjugation extension enlarges the intermolecular interaction area (from 302 to 342 Å 2 ) and promotes the transition from A‐to‐A to A‐to‐D packing motifs, leading to tighter molecular stacking with shortened π‐π and lamellar distances. Consequently, the PM6:C5Qx‐FTh binary device achieves a remarkable PCE of 20.1% with a low nonradiative energy loss of 0.18 eV.