Is Lead-Free Cs₂AgBiBr₆ Truly Stable? Surface Degradation of Porous Single Crystals in Ambient and Inert Environments

Lead-free halide double perovskites, such as Cs₂AgBiBr₆, have gained attention as environmentally benign alternatives to lead-based perovskites for optoelectronic and photovoltaic applications. Despite their chemical robustness and ambient phase stability, detailed insights into surface degradation mechanisms remain scarce. Here, we systematically investigate the degradation behavior of porous Cs₂AgBiBr₆ single crystals, composed of faceted nanoplatelets, under high relative humidity (RH of ∼70-90%) and inert argon conditions. Field-emission scanning electron microscopy reveals progressive surface degradation in ambient air, manifested by facet coalescence and nanoscale pinhole formation, while the degradation rate is significantly reduced under an inert atmosphere. Interestingly, gold thin films intended for surface protection undergo nanoparticle segregation and fail to prevent degradation. Bulk structural (X-ray diffraction pattern and Raman spectroscopy) and optical (photoluminescence and ultraviolet-visible absorption spectroscopy) properties remain largely unchanged, indicating that degradation is primarily surface-triggered morphological changes. These findings uncover critical pathways of environmental degradation and highlight the need for effective passivation strategies to ensure the long-term viability of Cs₂AgBiBr₆ in energy-relevant applications.