Synergistic Steric Hindrance and Chlorination Enable Efficient Binary Organic Solar Cells with Low Energy Loss

We report a series of small-molecule acceptors Z1-Z4 that reconcile low non-radiative energy loss with efficient charge transport by combining a nitrogen-containing core with out-of-plane steric-hindrance/chlorination-regulated packing. The dibenzo[b,f]azepine unit enables low non-radiative energy losses below 0.20 eV, while progressive backbone unlocking and targeted chlorination reshape intermolecular packing by suppressing unfavorable core-centered aggregation and promoting multidimensional electronic coupling. Single-crystal analysis, GIWAXS, and electronic-structure calculations reveal enhanced packing coherence and stronger coupling pathways from Z1 to Z4. Transient absorption spectroscopy and kinetic analysis further show that improved exciton delocalization accelerates interfacial exciton dissociation and suppresses monomolecular recombination. Photo-induced force microscopy confirms more continuous acceptor-rich fibrils in Z3- and Z4-based blends. Consequently, the optimized Z4-based binary device achieves a PCE of 20.28% with a high open-circuit voltage of 0.95 V, demonstrating an effective molecular strategy for low energy-loss high-performance organic solar cells.