Phase‐Pure 2D/3D Perovskite Heterojunctions Enabled by n ‐Specific Nanocrystal Templates for High‐Efficiency and Stable Solar Cells

Tailoring phase purity in 2D/3D perovskites is essential for balanced charge transport and long‐term stability. Here, we develop a nanocrystal‐templated epitaxial strategy to construct phase‐pure 2D/3D perovskite heterojunctions. Phase‐pure 2D perovskite nanocrystals with tunable quantum‐well thickness ( n = 1–4) are used as templates to direct lattice‐matched epitaxial growth of 3D perovskites, yielding coherent 2D/3D interfaces. By precisely controlling the 2D quantum‐well thickness, we eliminate the phase‐mixing issues of conventional methods and ensure high interfacial phase purity. A series of well‐defined 2D/3D heterojunctions ( n = 1–4) exhibits balanced charge transport and markedly reduced recombination losses. The optimized 2D ( n = 4)/3D perovskite solar cell delivers a power conversion efficiency of 26.48% and retains over 90% of its initial performance after 1 600 h, demonstrating enhanced efficiency and long‐term stability. Our work provides a versatile route for constructing phase‐pure 2D/3D perovskite heterojunctions, enabling optoelectronic devices with balanced charge transport and long‐term stability.