Dynamic Redox-Active Self-Assembled Monolayers Enable Robust Inverted Tin-Lead Perovskite Solar Cells

Tin-lead (Sn-Pb) perovskites have quickly emerged as essential absorbers for narrow-bandgap (NBG) perovskite solar cells (PSCs). However, their development is severely constrained by interfacial energy-level misalignment and chemical instability at the buried interface. This mismatch induces charge extraction barriers, while the rapid oxidation of Sn^{2 +} creates deep trap states and detrimental p-type self-doping. Here, we propose novel redox-active self-assembled monolayers (SAMs) as functional hole-transport layers (HTLs) using ferrocene (FC) derivatives, ferrocene acetic acid (FCAA) and ferrocene carboxylic acid (FCCA), showing better interfacial energetics and eliminating chemical defects via redox mediation. We found that the energy levels of FC-based SAMs align more closely with the Sn-Pb perovskite, promoting efficient hole extraction. Moreover, the reversible FC/FC⁺ redox process establishes a dynamic cycle, effectively suppressing the undesired oxygen- and light-inducing metallic Pb⁰ and oxidized Sn⁴⁺. FCAA, with a longer alkyl chain that enables stronger electron-donating and redox properties, shows superior to FCCA, achieving a champion power conversion efficiency (PCE) of 23.8%, and retains 95.8% after 2000 hours of storage with significantly inhibited oxidized Sn species. This study proposes a novel functional HTL for Sn-Pb PSCs, providing a promising way for high-performance and stable all-perovskite tandem photovoltaics.