Toward a Robust ZnO Interface via Fullerene-Based SAMs: Defect Passivation and Compatibility Tuning for High-Performance Inverted Organic Solar Cells.

The performance and operational stability of inverted organic solar cells (OSCs) are often limited by charge recombination and interfacial instability at the electron transport layer (ETL). To address this, we designed two fullerene-based self-assembled monolayers (SAMs)-C2-PA and 4EG-PA-as interfacial modifiers for zinc oxide (ZnO). Systematic comparisons reveal that the tetra(ethylene glycol) linker in 4EG-PA induces a denser and more uniform SAM morphology than the alkyl chain in C2-PA, which more effectively passivates the polar ZnO surface. This superior molecular packing translates into a champion power conversion efficiency of 19.46%. More critically, transient absorption spectroscopy (TAS) provides direct evidence that the 4EG-PA-modified interface facilitates the formation of a favorable charge-transfer state, which not only promotes electron extraction but also enhances hole transfer efficiency from the acceptor to the donor, thereby suppressing non-geminate recombination. Concurrently, the dense SAM acts as a robust buffer, improving the thermodynamic compatibility with the active layer and inhibiting its deleterious reaggregation. This dual mechanism-enhanced charge extraction and optimized interfacial morphology-underpins the exceptional operational stability, with devices retaining 84% of their initial performance after 2000 h. Our work elucidates the critical link between SAM molecular structure, interfacial properties, and device longevity, providing a strategic blueprint for future interfacial material design.