Strengthened Interfacial Coupling Between Self-Assembled Monolayers and Bulk Heterojunctions Enables Thermally Stable Organic Solar Cells.

Self-assembled monolayers (SAMs) have emerged as an effective interfacial strategy for improving charge extraction and interfacial energetics in organic solar cells (OSCs); however, limited operational stability, particularly under prolonged high-temperature conditions, remains a critical challenge for practical deployment. Here, we systematically engineer SAM terminal groups to elucidate how interfacial molecular interactions between the SAM and the bulk-heterojunction active layer govern device efficiency and thermal stability. Expanding the aromatic ring size of the SAM pendant groups enhances π-π and van der Waals interactions, leading to stronger molecular coupling and a more intact and robust interfacial structure at both the electrode/SAM and SAM/active-layer interfaces. In particular, SAMs incorporating naphthalene pendant groups exhibit significantly strengthened intermolecular interactions, effectively suppressing thermally induced morphological degradation under elevated temperatures. As a result, PM6:BTP-eC9-based binary and ternary organic solar cells achieve power conversion efficiencies of 19.73% and 20.15%, respectively. Notably, devices employing this interfacial molecular locking strategy deliver a T