All-perovskite tandem solar cells, composed of both wide-bandgap and narrow-bandgap subcells, have demonstrated efficiencies surpassing those of single-junction perovskite solar cells. However, wide bandgap perovskites are particularly affected by light-induced halide segregation. To mitigate this problem, a popular strategy involves the creation of 2D/3D heterostructures to improve the contact interfaces. However, this approach is challenging to apply to methylammonium-free and cesium/bromide-enriched wide-bandgap perovskites, which have different surface chemistry and higher activation energy requirements for surface reconstruction growth. Hairen Tan and colleagues present a generalizable method to obtain 2D/3D perovskite heterostructures involving a 3D-to-2D perovskite conversion. To obtain the desired final heterostructure, a MAPbI3 layer is deposited using a two-step method, first by evaporating lead iodide and then by spin-coating methylammonium iodide onto the substrate. This layer is subsequently converted into a 2D structure by spin-coating phenylethylammonium iodide on top. The resulting tandem solar cell presents a power conversion efficiency of 28.1%, stable at 90% after 855 hours of operation at its maximum power point.