The Electronic Properties of a 2D Ruddlesden‐Popper Perovskite and its Energy Level Alignment with a 3D Perovskite Enable Interfacial Energy Transfer

Abstract The success of using 2D Ruddlesden‐Popper metal halide perovskites (MHPs) in optoelectronic devices has ignited great interest as means for energy level tuning at the interface with 3D MHPs. Inter alia, the application of 2D phenylethylammonium lead quaternary iodide (PEA 2 PbI 4 )/3D MHPs interfaces has improved various optoelectronic devices, where a staggered type‐II energy level alignment is often assumed. However, a type‐II heterojunction seems to contradict the enhanced photoluminescence observed for 2D PEA 2 PbI 4 /3D MHP interfaces, which raises fundamental questions about the electronic properties of such junctions. In this study, using direct and inverse photoelectron spectroscopy, it is revealed that a straddling type‐I energy level alignment is present at 2D PEA 2 PbI 4 /3D methylammonium lead triiodide (MAPbI 3 ) interfaces, thus explaining that the photoluminescence enhancement of the 3D perovskite is induced by energy transfer from the 2D perovskite. These results provide a reliable fundamental understanding of the electronic properties at the investigated 2D/3D MHP interfaces and suggest careful (re)consideration of the electronic properties of other 2D/3D MHP heterostructures.