Buried Interface Passivation with Sulfonic Acid–Based Co-Assembled Monolayer Enables High-Performance Inverted Perovskite Solar Cells

Interfacial engineering, particularly at the buried interface between the hole-transporting layer (HTL) and the perovskite layer, is pivotal for attaining high-performance and long-term stable perovskite solar cells (PSCs). Self-assembled monolayers (SAM) have garnered significant attention as a promising hole-transporting material for improving the performance of perovskite solar cells. Nevertheless, interfacial recombination losses induced by inadequate wetting properties and molecular aggregation of SAMs pose a major challenge, limiting further advances in both power conversion efficiency (PCE) and device stability. This work introduces a novel synergistic coself-assembled monolayer (Co-SAM) strategy by incorporating 4-Fluorobenzenesulfonic Acid (4FBSA), a molecule featuring sulfonic acid and fluorine functional groups, with [4-(3,6-dimethyl-9<i>H</i>-carbazol-9yl)butyl]phosphonic acid (Me-4PACz) on a NiO_<i>x</i> substrate. The 4FBSA not only suppresses the self-aggregation of Me-4PACz, leading to a more uniform and compact monolayer, but its sulfonic acid group also effectively passivates undercoordinated Pb²⁺ defects at the buried interface via Lewis acid-base interaction (Pb-O coordination). Concurrently, the sulfonic acid and fluorine groups work synergistically to mitigate halide vacancy defects. This multifunctional modification promotes charge transport, optimizes energy level alignment at the interface, and consequently minimizes nonradiative recombination losses. This approach enabled the fabricated devices to attain a champion PCE of 24.37%. Furthermore, even without encapsulation, they retained 83.1% of their initial efficiency after 400 h under continuous operation in ambient air at 65 ± 5% relative humidity.