Prolonged exciton lifetime via 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.