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.