Quasi-2D Ruddlesden-Popper Lead Halide Perovskites: How Edge Matters

A band-mapping technique is introduced to investigate the formation of low-energy edge states in quasi-2D Ruddlesden-Popper (RP) perovskites, (BA)₂(MA)PbI₃, through a localized mode of measurement, namely, scanning tunneling spectroscopy. The local band structures measured at different points reveal the formation of 3D CH₃NH₃PbI₃ (MAPbI₃) at the edges of the perovskite nanosheets; for thin films, the 3D phase (n = ∞) could be seen to form at grain boundaries. The presence of MAPbI₃ at the edges or grain boundaries of the perovskites has led to self-forming type-II band alignment in BA₂MA₂Pb₃I₁₀ (n = 3). The rationale behind achieving a high-efficiency solar cell based on the material, which has a large exciton binding energy, has been inferred. Kelvin probe force microscopy studies under illumination have yielded a higher surface photovoltage at the edges compared to the interior and supported the inference of exciton dissociation due to internal type-II band alignment in the quasi-2D RP perovskites.