Anisotropic Cation Migration Drives the Formation and Stability of Lateral 3D/2D Perovskite Heterostructures

Lateral 3D/2D perovskite heterostructures, which combine the high efficiency of 3D phases with the stability of 2D structures, have shown great potential in energy and optoelectronic applications. However, the ion migration dynamics driving their formation and stability remain unclear. Here, we use fluorescence microscopy to investigate the anisotropic migration of A-site cation methylammonium (MA+) and formamidinium (FA+) in phenylethylammonium lead bromide (PEA2PbBr4) nanoplatelets. We demonstrate that edge-initiated, preferential in-plane migration of these cations leads to the lateral growth of 3D domains at the edge of nanoplates, forming lateral 3D/2D perovskite heterostructures. FA+ migrates more slowly than MA+ due to the relatively large steric hindrance, suppressing the formation of transient quasi-2D phases and enhancing the phase purity and structural stability of the heterostructures. These results highlight the importance of cation selection and interface design in optimizing ion migration dynamics, offering key insights into improving the stability and performance of 3D/2D perovskite-based optoelectronic devices.