Spontaneous Formation of 2D/3D Heterostructures on the Edges of 2D Ruddlesden-Popper Hybrid Perovskite Crystals

The observation of low-energy edge photoluminescence and its beneficial effect on the solar cell efficiency of Ruddlesden-Popper perovskites has unleashed an intensive research effort to reveal its origin. This effort, however, has been met with more challenges as the underlying material structure has still not been identified; new modelings and observations also do not seem to converge. Using two-dimensional (2D) (BA)(2)(MA)(2)Pb3Br10 as an example, we show that three-dimensional (3D) MAPbBr(3) is formed due to the loss of BA on the edge. This self-formed MAPbBr(3) can explain the reported edge emission under various conditions, while the reported intriguing optoelectronic properties such as fast exciton trapping from the interior 2D perovskite, rapid exciton dissociation, and long carrier lifetime can be understood via the self-formed 2D/3D lateral perovskite heterostructure. The 3D perovskite is identified by submicron infrared spectroscopy, the emergence of X-ray diffraction (XRD) signature from freezer-milled nanometer-sized 2D perovskite, and its photoluminescence response to external hydrostatic pressure. The revelation of this edge emission mystery and the identification of a self-formed 2D/3D heterostructure provide a new approach to engineering 2D perovskites for high-performance optoelectronic devices.