  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 modellings and observations also do not seem
to converge. Using 2D (BA)2(MA)2Pb3Br10 as an example, we show that 3D MAPbBr3
is formed due to the loss of BA on the edge. This self-formed MAPbBr3 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 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.
