In Situ Removable Additive Assisted Organic Solar Cells Achieving Efficiency over 19% and Fill Factor Exceeding 81%

Abstract Additive engineering can precisely regulate the bulk‐heterojunction active layer morphology with ideal domain size and purity, playing a critical role in development of organic solar cells (OSCs). Herein, two solid additives, 1,4‐dichlorobenzene (DCB) and 1‐chloro‐4‐iodobenzene (CIB), with low melting point (mp.) of ≈52 °C, are investigated comprehensively with comparison to 1,4‐diiodobenzene (DIB, mp. 131 °C). After spin‐coating, DIB residue is found in the as‐cast PM6:BTP‐eC9 based blend film, whereas the DCB and CIB are completely removed during the spin‐coating, showing in situ removable properties that enable convenient processing. In OSCs, the DCB‐ and CIB‐processed active layers afford power‐conversion efficiencies (PCEs) of 18.2% and 18.4%, respectively, all higher than that of 17.8% for DIB. Among the three solid additives, the CIB is most effective in enhancements of absorption coefficients of the donor and acceptor, affording fast and more balanced carrier transports, and suppressing recombination. Of particular note, the CIB can provide some universality as an in situ removable solid additive, based on its elevations of PCEs for several binary and PM6:D18‐Cl:L8‐BO ternary active layers. Impressively, a prominent PCE of 19.1% with a remarkable fill factor of 81.1% is achieved for the CIB‐processed ternary active layer. This work demonstrates the potential of in situ removable solid additive engineering in high‐efficiency OSCs.