Defects at the top and bottom interfaces of three-dimensional (3D) perovskite photo-absorbers diminish the performance and operational stability of perovskite solar cells (PSCs) due to charge recombination, ion migration, and electric-field inhomogeneities.1-5 Here, we demonstrate that long alkyl-amine ligands can generate near-phase pure two-dimensional (2D) perovskites at the top and bottom 3D perovskite interfaces and effectively resolves these issues. At the rear-contact side, we find that the employed alkyl-amine ligand strengthens the interactions with the substrate through acid-base reactions with the phosphonic acid group from the employed organic hole-transporting self-assembled monolayer molecule, thus regulating the 2D perovskite formation. With this, inverted PSCs with double-side 2D/3D heterojunctions achieved a power conversion efficiency (PCE) of 25.6% (certified 25.0%), retaining 95% of their initial PCE after 1000 hours of 1-sun illumination at 85 degrees Celsius in air.