Regulating Intermolecular Interactions and Film Formation Kinetics for Record Efficiency in Difluorobenzothiadizole‐Based Organic Solar Cells

Abstract The difluorobenzothiadizole (ffBT) unit is one of the most classic electron‐accepting building blocks used to construct D‐A copolymers for applications in organic solar cells (OSCs). Historically, ffBT‐based polymers have achieved record power conversion efficiencies (PCEs) in fullerene‐based OSCs owing to their strong temperature‐dependent aggregation (TDA) characteristics. However, their excessive miscibility and rapid aggregation kinetics during film formation have hindered their performance with state‐of‐the‐art non‐fullerene acceptors (NFAs). Herein, we synthesized two ffBT‐based copolymers, PffBT‐2T and PffBT‐4T, incorporating different π‐bridges to modulate intermolecular interactions and aggregation tendencies. Experimental and theoretical studies revealed that PffBT‐4T exhibits reduced electrostatic potential differences and miscibility with L8‐BO compared to PffBT‐2T. This facilitates improved phase separation in the active layer, leading to enhanced molecular packing and optimized morphology. Moreover, PffBT‐4T demonstrated a prolonged nucleation and crystal growth process, leading to enhanced molecular packing and optimized morphology. Consequently, PffBT‐4T‐based devices achieved a remarkable PCE of 17.5 %, setting a new record for ffBT‐based photovoltaic polymers. Our findings underscore the importance of conjugate backbone modulation in controlling aggregation behavior and film formation kinetics, providing valuable insights for the design of high‐performance polymer donors in organic photovoltaics.