Why Do We Have Dual Emission in Two-Dimensional Hybrid Perovskites?

Two-dimensional halide perovskites have been found to have unusual optoelectronic properties. The members which have the organic molecule sandwiched between single inorganic layers are found to exhibit a dual excitonic feature, with the one associated with the surface at higher energy than the feature emerging from the interior of the crystal. Surface calculations do not capture this effect. In this work, we show that neutral defects involving a molecule-halogen unit have a small formation energy and hence form easily at the edges/surface. This is a consequence of the weak interactions between the organic cations and the anions. The states introduced by these defects are responsible for the higher band gap state associated with the dual emission that has been observed in experiments. In contrast, these defects are difficult to form in three-dimensional (3D) perovskites, with their formation energy following the molecule-inorganic layer interaction strength. The stronger interactions in MAPbI3 lead to the larger formation energy for the defect, while the weaker interactions in FAPbI3 reduce the formation energy of these defects. Apart from providing the microscopic origin of the dual emission, the present study also provides a route to reduce the concentration of these defects.