Asymmetric Π‐Bridge Engineering Enables High‐Permittivity Benzo[1,2‐B:4,5‐b′]Difuran‐Conjugated Polymer for Efficient Organic Solar Cells

Organic solar cells (OSCs) exhibit complex charge dynamics, which are closely correlated with the dielectric constant (ɛr) of photovoltaic materials. In this work, a series of novel conjugated copolymers based on benzo[1,2‐b:4,5‐b′]difuran (BDF) and benzotriazole (BTz) is designed and synthesized, which differ by the nature of π‐bridge from one another. The PBDF‐TF‐BTz with asymmetric furan and thiophene π‐bridge demonstrates a larger ɛr of 4.22 than PBDF‐dT‐BTz with symmetric thiophene π‐bridge (3.15) and PBDF‐dF‐BTz with symmetric furan π‐bridge (3.90). The PBDF‐TF‐BTz also offers more favorable molecular packing and appropriate miscibility with non‐fullerene acceptor Y6 than its counterparts. The corresponding PBDF‐TF‐BTz:Y6 OSCs display efficient exciton dissociation, fast charge transport and collection, and reduced charge recombination, eventually leading to a power conversion efficiency of 17.01%. When introducing a fullerene derivative (PCBO‐12) as a third component, the PBDF‐TF‐BTz:Y6:PCBO‐12 OSCs yield a remarkable FF of 80.11% with a high efficiency of 18.10%, the highest value among all reported BDF‐polymer‐based OSCs. This work provides an effective approach to developing high‐permittivity photovoltaic materials, showcasing PBDF‐TF‐BTz as a promising polymer donor for constructing high‐performance OSCs.