LAUSR

Synthesis of 2D perovskites often demands long and bulky organic spacer cations, but they hamper optoelectronic properties of the resulting 2D perovskites. Novel low‐dimensional single‐cation perovskites with a general formula of FAm+2PbmBr3m+2 are prepared by using a quenching‐assisted solution process, which leads to the wide dimensional control over 1D FA3PbBr5 (m = 1), 2D FAm+2PbmBr3m+2 (m ≥ 2), and 3D FAPbX3 (m = ∞) perovskites simply by changing the composition of precursors. In this case, formamidinium (FA) acts as both an A‐site cation and spacer cation in FAm+2PbmBr3m+2. Unlike conventional 2D perovskites, FAm+2PbmBr3m+2 perovskites have (110) orientation. PVDF (poly(vinylidene fluoride)) preferentially stabilizes the low‐dimensional FAm+2PbmBr3m+2 phases, which is utilized to fabricate the stable FAm+2PbmBr3m+2‐PVDF composite films. The phase transitions from 1D and 2D to 3D are investigated in response to various stimuli, including humidity, ultraviolet, oxygen, and solvents, are exploited for rewritable phase‐change memory films.