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 T₉₀ operational lifetime of 150 h under maximum power point tracking at 85°C, representing an order-of-magnitude improvement compared to SAMs without pendant groups. These findings establish aromatic terminal group expansion as an effective molecular design strategy for simultaneously enhancing efficiency and thermal stability in organic solar cells.