Catechol‐Functionalized Self‐Assembled Monolayer for Highly Efficient Sn─Pb and All‐Perovskite Tandem Solar Cells

The performance of all-perovskite tandem solar cells is strongly dependent on the properties of Sn-Pb narrow-bandgap subcells. Self-assembled monolayers (SAMs) such as 2PACz have shown great success in lead-based perovskite solar cells (PSCs), yet their application in Sn─Pb PSCs remains limited, largely due to suboptimal carrier-extraction capability and insufficient interfacial stability. Here, we design and introduce a new SAM, 9-(3,4-dihydroxyphenyl)-9H-carbazole (DOPhCz), featuring a conjugated catechol anchoring group, into Sn─Pb PSCs for the first time. The well-formed and uniform DOPhCz SAM layer promotes more homogeneous perovskite nucleation and improved crystallinity, lowering interfacial defect density and enhancing device performance compared with phosphonic-acid-based [2-(9Hcarbazol-9-yl)ethyl]phosphonic acid (2PACz)M. In addition, DOPhCz accelerates hole extraction and reduces chemical perturbation during device operation. As a result, the DOPhCz-based Sn─Pb PSC achieves a power conversion efficiency of 24.17%, outperforming its 2PACz-based counterpart (22.87%) and exhibiting significantly improved operational stability. When applied in all-perovskite tandem solar cells, a high efficiency of 28.30% is obtained. This work establishes a coherent mechanism for SAM-induced interfacial stabilization and provides valuable insights for molecular engineering of high-performance all-perovskite tandem devices.