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

Abstract

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.