The Demonstration ofa Multiply Noncovalently AssembledDimeric Acceptor Enabling Organic Solar Cells Approaching 21% Efficiency

Organic solar cells (OSCs) have achieved power conversion efficiencies exceeding 20%; yet, further progress is hindered by limited molecular ordering and high exciton binding energies. Here, by tuning the number of sulfur atoms within small molecule acceptors (SMAs), we designed and synthesized four acceptors CHSMe, CHSPh, CH2SPh, and CHDF, where the strong intermolecular C-H···S and C═S···S noncovalent interactions lead to clear bimolecular structure units. Single-crystal X-ray diffraction analysis of CHDF reveals that the sulfur atoms within its DTF unit participate in intermolecular noncovalent interactions with neighboring molecules. These include a dual hydrogen-bonding interaction (C-H···S) with a benzene ring and a dual C═S···S interaction with a thiophene unit. Collectively, these multiple interactions drive the formation of a nearly coplanar dimer between two adjacent CHDF molecules─representing the first observation of such a bimolecular self-assembly in single crystals of SMAs. This behavior is further confirmed across a series of derivatives with different side chains and end groups. Thus, the resulting CHDF exhibits enhanced crystallinity, extensive π-π stacking, and a markedly reduced exciton binding energy of 150 meV. When paired with PM6, CHDF-based devices achieve a champion efficiency of 20.69%, ranking among the highest for binary OSCs.