Long-term (∼3000 h) in situ XRD investigation of environmental stabilization in hybrid perovskites via trace 2D loading

The poor environmental stability of three-dimensional (3D) halide perovskites remains a critical barrier to their commercialization. Mixed-dimensional systems incorporating two-dimensional (2D) perovskites have shown improved stability, yet high 2D fractions often compromise optoelectronic performance. Here, we demonstrate that introducing only 0.02 (2%) 2D perovskite into methylammonium lead iodide (MAPI) films - both undoped and with 15% nominal bromine substitution - yields a pronounced enhancement in intrinsic environmental stability. Long-term (∼3000 h) in situ X-ray diffraction measurements on non-encapsulated films exposed to ambient air (∼40% relative humidity) reveal a two-to four-fold reduction in perovskite decomposition and PbI 2 formation rate, with a significant fraction of the perovskite phase remaining intact. This stabilization arises from the preferential localization of the 2D phase at film surfaces and grain boundaries, where it encapsulates 3D grains and kinetically suppresses diffusion of reactive species such as moisture and oxygen. Bromine incorporation provides an additional, though smaller, stabilizing effect. These findings identify low-fraction 2D incorporation as a highly efficient and scalable strategy for enhancing the environmental stability of hybrid perovskites.