Kinetics of the Cation Migration between 2D BA₂PbBr₄ and 3D MAPbBr₃ Perovskite Nanoparticles in Solution

Understanding the kinetics of cation migration in hybrid perovskite materials is critical for enhancing their stability and performance in optoelectronic applications. This study investigates postsynthesis cation migration dynamics between 2D BA2PbBr4 and 3D MAPbBr3 perovskite nanoparticles (NPs) in solution. Using in situ temperature-dependent photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM), we observed the formation of a quasi-2D intermediate phase (BA2MAPb2Br7) resulting from interparticle cation exchange. The activation energy of the cation migration is measured to be 57 kJ/mol, and we associated this with the migration of methylammonium (MA+) ions, which migrate faster compared to the larger butylammonium (BA+) ions. This faster migration of MA+ is responsible for the formation of the quasi-2D phase, while BA+ ions migrate to the surface of 3D MAPbBr3 NPs and efficiently passivate the surface defects, significantly enhancing the PL emission intensity and lifetime of the 3D MAPbBr3 NPs. Furthermore, our study reveals that the reaction conditions, including temperature and the relative concentration of the 2D and 3D perovskite NPs, strongly influence the extent and dynamics of cation migration. Higher temperatures accelerate the migration process, as evidenced by the increasing intensity of the quasi-2D emission peak at 436 nm, while a higher concentration of BA2PbBr4 promotes the formation of the quasi-2D phase. TEM analysis confirms that the migration of MA+ ions into the 2D BA2PbBr4 lattice drives the formation of the quasi-2D structure, while BA+ ions predominantly passivate the 3D MAPbBr3 surface. These findings provide mechanistic insights into interparticle cation migration and its impact on the optoelectronic properties of perovskite nanostructures, advancing their potential for stable device applications.