Controlled Surface Growth of Cs2SnI6 for 0D/3D All-Inorganic Perovskite Bilayers with Enhanced Environmental Stability.

CsSnI3 is a promising all-inorganic metal halide perovskite because of its 3D connectivity and optoelectronic properties, but its use is limited by degradation in the ambient environment and structural transformations to wider-band gap phases. Vacancy-ordered double perovskite phases can have higher environmental stability than their analogous 3D phase, suggesting the potential for use as stabilizing layers in heterostructures. To examine the ability of 0D vacancy-ordered phases to stabilize 3D perovskites, a solution-processed 0D/3D all-inorganic bilayer heterostructure was formed through growth of Cs2SnI6 atop thin films of CsSnI3. Spin-coating a solution of SnI4 in ethanol onto CsSnI3 films produced a capping layer of Cs2SnI6 without dissolving the underlying perovskite. X-ray diffraction revealed that Cs2SnI6 grew with a preferred orientation templated by the CsSnI3 underlayer, while X-ray photoelectron spectroscopy and incidence-angle-dependent grazing-incidence wide-angle X-ray scattering confirmed formation of a thin capping layer. The conversion is self-limiting and preserves the morphology of the original CsSnI3 film. Time-resolved photoluminescence revealed substantial emission quenching, consistent with interfacial charge transfer across a type-II heterojunction. The Cs2SnI6 layer strongly suppressed degradation of CsSnI3 under exposure to the ambient. These results establish Cs2SnI6 as an effective protective and charge-extracting capping layer for Sn-based perovskites and demonstrate a simple route toward stable lead-free perovskite heterostructures.