Bilayer Co‐Ligand Strategy Enables Grain Boundary and Surface Regulation for Wide‐Bandgap Perovskite and Perovskite/Silicon Tandem Solar Cells

ABSTRACT The performance of wide‐bandgap (WBG) perovskite sub‐cells in tandem solar cells is largely constrained by grain boundary and surface defects, along with unfavorable interfacial energy level alignment. To address these issues, we propose a bilayer co‐ligand post‐treatment strategy, initially employing 3‐(methylthio)propylamine hydroiodide (3MTPAI) and 1, 3‐propanediamine dihydroiodide (PDAI 2 ), followed by a secondary piperazinium iodide (PI) layer. In this design, 3MTPAI penetrates the grain boundaries to passivate both grain boundary and surface defects, thereby suppressing non‐radiative recombination, while PDAI 2 forms an initial surface dipole layer to enhance the built‐in electric field. The subsequent PI layer not only passivates residual defects but also generates a second dipole layer that acts synergistically with the first, facilitating carrier transport at the interface. Consequently, the optimized WBG (1.68 eV) single‐junction and tandem devices achieve power conversion efficiencies (PCEs) of 23.55% and 32.33%, respectively, and show no efficiency loss after 500 h of operation. This work demonstrates that the bilayer co‐ligand strategy offers valuable insights for developing efficient and stable single‐junction and tandem solar cells.