Phenothiazine‐Based Self‐Assembled Monolayer with Thiophene Head Groups Minimizes Buried Interface Losses in Tin Perovskite Solar Cells

Self‐assembled monolayers (SAMs) have revolutionized the fabrication of lead‐based perovskite solar cells, but they still remain underexplored in tin perovskite systems. To date, PEDOT remains the most effective hole‐selective layer in tin perovskite solar cells (TPSCs), yet it presents challenges for both performance and stability. MeO‐2PACz, the only SAM reported for tin perovskites consistently underperforms when compared to PEDOT. In this work, it is identified that MeO‐2PACz's limitations stem from excessively strong interactions with the perovskite surface and poor lattice matching, which leads to inferior interface quality. To address these issues, a novel SAM‐forming molecule called Th‐2EPT is designed, synthesized, and characterized. Density functional theory (DFT) is used to evaluate coordination strength and lattice compatibility, complemented by electro‐optical characterisation techniques that show significantly reduced interfacial recombination and improve material crystallinity in Th‐2EPT/Perovskite films. With Th‐2EPT, the first SAM‐based tin perovskite solar cells that outperform PEDOT‐based devices, delivering a power conversion efficiency (PCE) of 8.2% with a DMSO‐free solvent system, are demonstrated.