Circularly polarized light (CPL) is essential for optoelectronic and chiro-spintronic applications. Hybrid perovskites, as star optoelectronic materials, have demonstrated CPL activity, which is, however, mostly limited to chiral perovskites. Here, we develop a simple, general, and efficient strategy to stimulate CPL activity in achiral perovskites, which possess rich species, efficient luminescence, and tunable bandgaps. With the formation of van der Waals heterojunctions between chiral and achiral perovskites, a nonequilibrium spin population and thus CPL activity are realized in achiral perovskites by receiving spinpolarized electrons from chiral perovskites. The polarization degree of room-temperature CPL in achiral perovskites is at least one order of magnitude higher than in chiral ones. The CPL polarization degree and emission wavelengths of achiral perovskites can be flexibly designed by tuning chemical compositions, operating temperature, or excitation wavelengths. We anticipate that unlimited types of achiral perovskites can be endowed with CPL activity, benefiting their applications in integrated CPL sources and detectors.

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The liquid‐air interface offers a platform for the in‐plane growth of free‐standing materials. However, it is rarely used for inorganic perovskites and ultrathin non‐layered perovskites. Herein the liquid‐air interfacial synthesis of inorganic perovskite nanosheets (Cs3Bi2I9, Cs3Sb2I9) is achieved simply by drop‐casting the precursor solution with only the addition of iodine. The products are inaccessible without iodine addition. The thickness and lateral size of these nanosheets can be adjusted through the iodine concentration. The high volatility of the iodine spontaneously drives precursors that normally stay in the liquid to the liquid‐air interface. The iodine also repairs in situ iodine vacancies during perovskite growth, giving enhanced optical and optoelectronic properties. The liquid‐air interfacial growth of ultrathin perovskites provides multi‐degree‐of‐freedom for constructing perovskite‐based heterostructures and devices at atomic scale.