Computational and Terahertz Spectroscopic Synergy to Elucidate the Mechanism of Solid Additives in Organic Solar Cells

ABSTRACT The key characteristics and selection criteria for solid additives in organic solar cells (OSCs) continue to be debated, posing challenges in establishing reliable descriptors to predict the power conversion efficiency (PCE) of OSCs treated with such additives. In this study, the D18:Y6 blend films processed is used with three solid additives—1,4‐diiodobenzene (DIB), 1‐bromo‐4‐iodobenzene (BIB), and 1,4‐dibromobenzene (DBB)—as a case study. By integrating density functional theory (DFT) calculations, transient absorption (TA), photovoltaic characterization and optical pump–terahertz probe spectroscopy (OPTP), it is demonstrated that these additives preferentially bind to the Y6 end groups. This interaction promotes the formation of “Endgroup‐Endgroup (E‐E)” and “Endgroup‐Additive‐Endgroup (E‐A‐E)” stacking motifs with strong electronic coupling, which facilitates exciton dissociation via hole transfer and enhances overall charge transport. As a result, OSC devices processed with DIB achieved a PCE of 19.29%, ranking among the highest reported for binary D18:Y6‐based systems, and the universal regulatory efficacy of DIB is further validated in the D18:L8‐BO system, yielding a remarkable PCE of 20.09%. This work thus provides a multidimensional dataset to aid in screening potential solid additives for high‐performance OSCs.