Simultaneous optimization of film morphology and structure dimensionality in Cs₃Sb₂I₉ through butylamine gas...

Metal halide perovskite derivatives (A₃B₂X₉) are attractive alternatives to toxic and unstable lead-based perovskites, but their optoelectronic performance is often limited by poor film morphology and zero-dimensional (0D) crystal structures. Here, we report an amine gas post-treatment strategy that simultaneously improves film uniformity and transforms Cs₃Sb₂I₉ from a 0D phase into a two-dimensional (2D) layered phase. The transformation is driven by the reversible adsorption and desorption of amine molecules, which induce coordination-mediated liquefaction and subsequent recrystallization of the material. Among the amines investigated, the liquefaction capability increases with alkyl-chain length, leading to progressive improvements in film morphology and 2D phase purity. Consequently, butylamine (BA⁰)-treated Cs₃Sb₂I₉ solar cells exhibit nearly an order of magnitude improvements in both short-circuit current density and power conversion efficiency compared to untreated reference devices. This work demonstrates the broad applicability of amine-gas treatment to all-inorganic perovskites and their derivatives for optoelectronic applications.