Non‐fullerene acceptor aggregation regulation via additives dipole moment leads to 19.9% power conversion efficiency in bilayer organic solar cells

The aggregation behavior of the non‐fullerene acceptor (NFA) directly influences the carrier dynamics and overall performance in bilayer organic solar cells. However, the typically weak intermolecular interactions in many NFAs limit the formation of well‐ordered aggregates, severely restricting efficient charge transport. Herein, we systematically investigate the regulatory mechanism of isomeric halogenated additives on the aggregation behavior of L8‐BO acceptors within the PM6/L8‐BO system. The results demonstrate that these additives interact with both the end groups and the benzothiadiazole‐fused thieno[3,2‐b]thiophene (BTP)core of L8‐BO, effectively enhancing the degree of ordered aggregation by slowing the thin‐film drying kinetics. The performance enhancement is attributed to a synergistic improvement in charge dynamics: the additives promote the molecular ordering of the acceptors, which facilitates efficient charge transport, prolongs carrier lifetime, and effectively suppresses bimolecular recombination. Notably, devices processed with the 2‐chloro‐1,3‐dibromobenzene treatment, which has a dipole moment of 2.56 D, achieved a remarkable power conversion efficiency of 19.9%. Our findings shed light on the critical role of positional isomeric additives in regulating the aggregation behavior of acceptors, offering a design strategy for future additive development in bilayer organic photovoltaics.