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Organic–inorganic hybrid perovskite solar cells have demonstrated unprecedented high power conversion efficiencies in the past few years. Now, the universal instability of the perovskites has become the main barrier for this kind of solar cells to realize commercialization. This situation can be even worse for those tin-based perovskites, especially for CsSnI₃, because upon exposure to ambient atmosphere the desired black orthorhombic phase CsSnI₃ would promptly lose single crystallinity and degrade to the inactive yellow phase, followed by irreversible oxidation into metallic Cs₂SnI₆. By alloying CsSnI₃ with CsPbI₃, we herein report the synthesis of alloyed perovskite quantum dot (QD), CsSn_1–xPb_xI₃, which not only can be phase-stable for months in purified colloidal solution but also remains intact even directly exposed to ambient air, far superior to both of its parent CsSnI₃ and CsPbI₃ QDs. Ultrafast …