Three-dimensional (3D) metal-halide perovskites (MHPs) show great potential for next-generation photovoltaics, but low-temperature solution-processed 3D MHP thin films exhibit a high density of defects at the top and bottom interfaces, inducing nonradiative charge carrier recombination and acting as degradation centers under external stress, compromising device performance and operational stability. While two-dimensional (2D) MHPs are widely used to passivate the top surface of 3D MHPs, the buried bottom interface is largely overlooked, owing to the difficulty of avoiding solvent dissolution of the pre-deposited 2D layer during subsequent 3D MHP deposition. In this work, we systematically investigated solvent properties and developed a low-solvation, highly volatile mixed solvent system to enable the deposition of 3D formamidinium lead iodide (FAPbI3) perovskite atop 2D Dion-Jacobson perovskite (DJP) films, forming 3D-on-2D (3D/2D) MHP bilayer heterostructures while preserving the integrity of the underlying DJP, as confirmed by GIWAXS and UV–Vis absorption spectroscopy. Such 3D/2D MHP heterostructure exhibited enhanced crystallinity, suppressed nonradiative charge carrier recombination, and prolonged carrier lifetimes, suggesting that the underlying DJP effectively passivates the buried bottom interface. We further integrated the resulting 3D/2D bilayer stack into p-i-n solar cells, which exhibited improved open-circuit voltage (Voc), fill factor (FF), and even short-circuit current density (Jsc), achieving a champion power conversion efficiency (PCE) of 24.04% and enhanced operational stability compared to 3D-only counterparts. In contrast to existing strategies, our approach provides a straightforward and generalizable route for precise control over the phase, orientation, and energy landscape of the underlying 2D MHP layer in the 3D/2D heterostructures. This study further offers guidance for the design and fabrication of various advanced MHP architectures with tailored energy landscapes—such as double-sided 2D/3D heterostructures, 3D/3D heterostructures, and even quantum-well-like structures with alternating 2D/3D/2D/3D stacking.