Piperazine‐Assisted Construction of 2D/3D Wide‐Bandgap Perovskite for Realizing High‐Efficiency Perovskite/Organic Tandem Solar Cells

Comprehensive Summary Monolithic perovskite/organic tandem solar cells (TSCs) have gained significant attention due to their easy device integration and the potential to surpass the Shockley–Queisser limit of single‐junction solar cells. However, the surfaces of wide‐bandgap perovskite films are densely populated with defects, leading to severe non‐radiative recombination and energy loss. As a consequence, the power conversion efficiency (PCE) of perovskite/organic TSCs lags behind that of other TSC counterparts. To address these issues, we designed a functional ammonium salt, 4‐(2‐hydroxyethyl)piperazin‐1‐ium iodide (PZOI), comprising a piperazine iodide and a terminated hydroxyl group, which was applied for post‐treating the perovskite surface. Our findings reveal that PZOI reacts with and consumes residual PbX 2 (X: I or Br) to form a 2D perovskite component, thereby eliminating Pb 0 defects, while the terminated hydroxyl group in PZOI can also passivate uncoordinated Pb 2+ . Consequently, the shallow/deep‐level defect densities of the 2D/3D perovskite film were significantly reduced, leading to an enhanced PCE of single‐junction 2D/3D wide‐bandgap perovskite solar cells to 18.18% with a reduced energy loss of 40 meV. Importantly, the corresponding perovskite/organic TSCs achieved a remarkable PCE of 24.05% with enhanced operational stability ( T 90 ~500 h).