Molten Phosphonoalkanoic Acid‐Driven Grain Boundary Passivation in Evaporated/Blade‐Coated 1.68‐eV Perovskite Solar Cells

ABSTRACT Non‐radiative recombination induced by grain boundary defects remains a critical challenge limiting the performance of perovskite solar cells (PSCs). This work proposes a dynamic passivation mechanism using molten 4‐phosphonobutyric acid (4‐PBA) during perovskite crystallization. Unlike solid‐state passivators, molten phosphonoalkanoic acid molecules penetrate deeply into bulk regions and access hidden defect sites that are otherwise unreachable. In the molten state, the phosphonic (─PO 3 H 2 ) and carboxylic (─COOH) groups exhibit high conformational freedom, enabling spontaneous rotation and optimal spatial alignment to match the atomic arrangement of perovskite defects. The butane chain in 4‐PBA creates an ideal separation of 5.4–7.7 Å between its ─PO 3 H 2 and ─COOH groups. This allows it to bridge the dual defects in adjacent [PbI 6 ] 4− octahedra, enabling cooperative dual‐site passivation. As a result, 1.68 eV wide‐bandgap PSCs employing this bulk‐doping strategy with the evaporation‐coating hybrid method achieve a power conversion efficiency (PCE) of 22.53% (certified 21.95%) and show minimal efficiency degradation over time, demonstrating robust operational stability.