Redox Cascade‐Driven Structural Degradation Under Reverse Bias in All‐Perovskite Tandem Solar Cells

Reverse bias, originating from current mismatch or shading, has emerged as a hidden yet fatal instability in all-perovskite tandem solar cells. Here we reveal that sustained reverse bias triggers a redox-coupled lattice collapse within the narrow-bandgap subcells. Under electrical stress, hole injection oxidizes iodide ions into neutral iodine species, which subsequently oxidize tin cations and drive field-directed ion migration. This redox cascade propagates vertically through the lattice, coupling ionic transport with structural reconstruction and interfacial corrosion, ultimately leading to irreversible performance loss. These findings expose all-perovskite tandems as chemomechanical systems in which electrical stress is converted into internal electrochemical damage. To suppress this feedback loop, we introduce a "multideck-fence" interfacial design that integrates a nanometric oxide diffusion barrier with a chemically stable dual-metal electrode. This strategy halts the redox-driven degradation, yielding a power conversion efficiency of 29.03% and more than 30-fold enhancement in reverse-bias endurance. Our results redefine reverse bias as a chemomechanically driven failure mode and establish a mechanistic framework for bias-resilient perovskite architectures.