Defect‐Assisted Self‐Trapped Exciton Emission in Zero‐Dimensional K ₃ InCl ₆ Perovskite: Reversible Hydration Switching Toward Multimodal Anti‐Counterfeiting

ABSTRACT The luminescence origin and underlying mechanism in 0D halide perovskites remain subjects of ongoing debate. In this study, we report on K 3 InCl 6 perovskite that exhibits green emission under specific synthesis conditions. Our investigations reveal that the luminescence in K 3 InCl 6 originates from defect‐assisted electronic transition processes, wherein cation vacancies and anti‐site defects form within the crystal lattice. These defects introduce sub‐band exciton absorption and facilitate the formation of self‐trapped excitons (STE), ultimately leading to radiative recombination and green luminescence emergence. Furthermore, K 3 InCl 6 demonstrates reversible phase transformations upon atmospheric exposure and thermal treatment due to hydration and dehydration processes, enabling spectral and color tunability. This distinctive characteristic positions emissive K 3 InCl 6 as a promising candidate for multimodal anti‐counterfeiting applications. Our findings provide significant insights into elucidating the luminescence origination and mechanism in 0D halide perovskite materials, offering a valuable avenue for future research in this field.