Reactive Monomer‐Driven Crystallization Control and Polymeric Grain‐Boundary Networks for Efficient Wide‐Bandgap and All‐Perovskite Tandems

Wide‐bandgap (WBG) halide perovskites are crucial for high‐efficiency two‐terminal all‐perovskite tandem solar cells, yet their performance is limited by crystallization‐induced defects, halide segregation, and non‐radiative recombination. Conventional additives typically target either early‐stage crystallization or post‐growth passivation, leaving defect mitigation spatially imprecise and chemically static. Here, we introduce a reactive monomer strategy using allylamine hydrochloride (AACl), which undergoes a programmed chemical transformation to perform distinct functions during film formation. In situ spectroscopic and structural characterization reveals a complex five‐stage crystallization pathway in WBG films. The ammonium terminal groups of AACl coordinate with perovskite constituents, suppressing undesirable intermediates and streamlining crystallization into three dominant stages with improved phase purity and grain uniformity. Upon thermal annealing, the vinyl groups polymerize in situ, forming a poly(AACl) network confined to grain boundaries and buried interfaces. This network anchors halide ions, passivates undercoordinated lead sites, and reduces non‐radiative recombination. As a result, 1.77 eV WBG single‐junction cells achieve a steady‐state power conversion efficiency of 20.10%, while two‐terminal all‐perovskite tandems reach a steady‐state efficiency of 28.87%. This work demonstrates a chemically programmed additive approach, where reactive monomers dynamically evolve to coordinate crystallization and grain‐boundary passivation, providing a general strategy to improve efficiency and stability in hybrid perovskites.