Dual-Layer Grain-Boundary In Situ Polymerization Modulates Elastic Modulus for Mechanically Stable Flexible All-Perovskite Tandem Solar Cells

Flexible all-perovskite tandem solar cells (PTSCs) are promising for lightweight photovoltaics, yet their mechanical degradation under bending remains poorly understood. Although bending instability has often been attributed to brittle fractures, halide perovskites are mechanically soft materials. Here, we demonstrate that bending-induced degradation in flexible PTSCs primarily associated with strain-induced evolution of residual-stress accumulation prior to crack formation, while elastic-modulus engineering through grain-boundary polymerization mitigates this mechanically induced degradation. Grazing-incidence X-ray diffraction analyses reveal that repeated bending accumulates residual stress of both wide-bandgap and narrow-bandgap perovskite layers in PTSCs, leading to fatigue-associated degradation and enhanced nonradiative recombination even in the absence of visible cracks. To address this intrinsic limitation, a dual-layer grain-boundary in situ polymerization strategy is introduced for both perovskite layers, enabling elastic relaxation and suppressing residual-stress accumulation. As a result, flexible PTSCs achieve a power conversion efficiency of 24.97% and retain over 90% of their initial efficiency after 5000 bending cycles at a radius of 5 mm. This work establishes elastic-modulus engineering as a key design principle for mechanically robust flexible perovskite tandem solar cells.