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α‐phase formamidine lead iodine (α‐FAPbI3) as a competitive candidate to pursue high‐performance perovskite solar cells (PSCs) suffers from the weakness of random nucleation and disordered growth in chasing high‐grade polycrystalline films. Here, single‐crystal seeds crystallization (SCSC) method is proposed to obtain high‐quality α‐FAPbI3 perovskite films for the first time. Six single‐crystals seeds (CsPbBr3, CsPbI3, FAPbBr3, FAPbI3, MAPbBr3, and MAPbI3) are employed to regulate α‐FAPbI3 perovskite crystallization. These seeds act as nucleation sites and allow the perovskite to grow directly. The rapid crystallization significantly improves the crystallinity of perovskite films with uniform morphology, large grain size, and less defects. Of particular note, FAPbBr3 seed prefers to grow α‐FAPbI3 epitaxially from FAPbBr3@PbI2 heterostructure through a halogen anion exchange process. The average grain size of FAPbBr3‐perovskite film is boosted to 2.6 µm, and the trap density reduced by 11 times. The resultant FAPbBr3‐PSCs achieve a maximum power conversion efficiency (PCE) of 23.84% with a high open‐circuit voltage of 1.18 V. The unencapsulated PSC retains 91% of the initial PCE after 3000 h of storage at ≈30–50% relative humidity and preserves 98.5% of the original value after 155 h illumination with maximum power point tracking. [ABSTRACT FROM AUTHOR]