Although the fabrication of two-dimensional (2D)/three-dimensional (3D) halide perovskite heterostructures has been employed to balance the long-term stability and high efficiency of perovskite solar cells, the formation of metastable quasi-2D perovskites remains the most serious challenge. Here, we demonstrate that large lattice mismatch derived from halide and cation differences between 2D and 3D perovskites are key to avoiding the formation of unintended 2D phases in the preparation of 2D/3D bulk heterostructure because the phase transformation becomes less thermodynamically favorable. Specifically, by employing chloride 2D perovskite (PYA)(2) PbCl4 (PYA = propargylammonium) crystals into a 3D precursor solution, we achieve a phase-pure 2D/3D heterojunction with clean type-I band alignment, which exhibits greatly reduced charge recombination. Furthermore, the incorporation of alkyne perovskites is also shown to suppress iodine diffusion and formation due to their exceptional iodine capture capacity. The resultant 2D/3D heterostructured devices exhibited enhanced efficiencies and stabilities compared with their 3D counterparts.