Dynamic Isomers Fortify Perovskite Grain Boundaries Under Cyclic Illumination
Perovskite solar cells (PSCs) are a promising photovoltaic technology, yet their commercialization is hindered by insufficient long-term operational stability under real-world conditions. While progress has improved resilience against static stressors (constant heat, electrical bias, humidity), durability under dynamic outdoor environments—particularly diurnal light cycling and temperature fluctuations—remains inadequately understood. Cyclic illumination induces interfacial expansion and contraction, leading to mechanical stress accumulation and grain-boundary (GB) fragmentation, ultimately decomposing the perovskite into Pb2+ and inactive phases. To address this, Zhang et al. propose a dynamic GB-resilience strategy integrating a photoswitchable isomeric compound, 4-(phenylazo) benzoic acid (Ca-Abz), into triple-cation lead-based perovskite grain boundaries. In the dark, Ca-Abz adopts a planar trans (E) configuration, anchoring via carboxylic acid-Pb2+ coordination. Under UV illumination, electronic excitation lowers the rotational barrier of the azo bond, triggering a transition to a sterically twisted cis (Z) configuration. This reversible molecular switching dynamically accommodates periodic lattice expansion and contraction, dissipating intergranular stress and regulating local strain. Density functional theory and first-principles molecular dynamics simulations show the Z isomer binds slightly stronger to the perovskite surface (−11.01 eV) than the E isomer (−10.99 eV), indicating illumination actively strengthens passivation. This approach represents a shift from static passivators to stimuli-responsive interfacial regulators that utilize UV energy to stabilize grain boundaries during lattice expansion, offering a promising route to enhance PSC operational stability under cyclic illumination.