Mechanically Adaptive Viscoelastic Grain-Boundary Regulation in Blade-Coated Tin–Lead Perovskites

Nonequilibrium crystallization during scalable coating processes strongly influences the nanoscale structure of perovskite films, often inducing coupled mechanical stress and defect accumulation at grain boundaries. Controlling this stress-defect interplay remains a major challenge for achieving high-performance perovskite materials and their scalable integration. Here, we present a mechanically adaptive viscoelastic grain-boundary regulation strategy enabled by an in situ phase transition of polyacrylonitrile. Upon thermal activation, the polymer forms a transient viscoelastic state within the crystallization window, acting as a deformable intergranular medium. This viscoelastic phase accommodates crystallization-induced stress through elastic deformation and viscous flow, while interacting with grain-boundary regions to tune local interfacial configurations. As a result, the perovskite films exhibit reduced residual stress, suppressed defect density, and enhanced structural coherence. Blade-coated tin-lead perovskite solar cells achieve a power conversion efficiency of 21.02%, and monolithic two-terminal all-perovskite tandem devices reach 26.94%. These results demonstrate an effective strategy for grain-boundary regulation, offering a practical pathway toward scalable, high-performance perovskite tandem photovoltaics.