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

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 PEA2PbI4 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 PEA2PbI4 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 PEA2PbI4 3D methylammonium lead triiodide MAPbI3 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 heterostructure