Abstract
<jats:p>Hybrid perovskite composition controls ion transport and environmental tolerance, but its role in residual-stress relaxation and implications for ambient fabrication remain unclear. We establish a composition-dependent stress–diffusion framework linking cation/halide chemistry, grain-boundary halide transport, and moisture uptake. In situ curvature and grazing-incidence X-ray diffraction measurements show that tensile stress relaxes by diffusion-mediated inelastic deformation across single- and mixed-composition perovskites. Temperature-dependent relaxation yields activation energies consistent with vacancy-mediated grain-boundary diffusion in methylammonium-based films and lower-barrier grain-boundary and triple-junction transport in formamidinium-based films. Humidity accelerates relaxation through mobile moisture adsorbed near grain boundaries and trapped moisture incorporated into the lattice. Stress pre-relaxation reduces moisture uptake by up to an order of magnitude, extending ambient storage stability within a universal processing window. In devices, moderate humidity up to 50% relative humidity passivates shallow traps and improves average performance without compromising 300-hour operational stability, establishing a stress-diffusion framework for ambient perovskite device manufacturing.</jats:p>