Prolonged exciton lifetime <i>via</i> conjugation-length engineering in M-series acceptors for 19.39% efficiency polymer solar cells

Developing non-fullerene acceptors (NFAs) that combine high device efficiency with superior stability remains a significant challenge. Based on M-series acceptors featuring an acceptor-donor-acceptor (A-D-A)-type framework, we report two dimerized NFAs (DM-8F and DM-8Cl) containing different halogen atoms in their terminal groups. Compared to the small-molecule acceptor M68, both dimerized acceptors exhibit increased glass transition temperatures and enlarged dielectric constants. The choice of halogen atoms in the terminal groups significantly affects their π-π-packing distances, exciton diffusion lengths, and ultimately, photovoltaic performance. Owing to enhanced charge transport, reduced exciton binding energy, and extended exciton diffusion length, DM-8F achieves an efficiency of 19.39% (certified at 19.20%) in small-area polymer solar cells (PSCs) and 15.72% in minimodules with an effective area of 11.09 cm². These efficiencies are the highest reported to date among all A-D-A-type NFAs. Moreover, DM-8F-based devices exhibit significantly improved thermal and photostability compared to those based on M68.