Influence of Intermolecular Interactions on Ion Migration at 2D/3D Perovskite Interfaces^†

Passivating the surface defects of three-dimensional (3D) perovskite layers with two-dimensional (2D) perovskites is a critical strategy for achieving high efficiency and stability in perovskite solar cells. However, the dynamic evolution of 2D/3D interfaces under external stimuli such as thermal stress and long-term illumination significantly impacts device performance. In this study, we systematically investigate the role of intermolecular interactions in governing ion migration at 2D/3D interfaces by physically stacking three 2D perovskite—(BA)₂PbI₄ (BA=butylammonium), (PEA)₂PbI₄ (PEA=phenethylammonium), and (BDA)PbI₄ (BDA=1,4-butane diammonium)—with 3D perovskite-MAPbI₃ (MA=methylammonium), followed by thermal annealing; and subsequent characterization was carried out using ultraviolet-visible (UV-Vis) absorption and femtosecond-transient absorption (fs-TA) spectra. Our findings reveal that small MA⁺ ions migrate from the 3D perovskite into the 2D perovskites, forming quasi-2D perovskites and introducing new decay pathways, while BA⁺ and PEA⁺ ions back-incorporate into the 3D lattice, causing a slight blue shift of 2−3 nm in exciton peaks. Notably, no significant ion migration is observed at the (BDA)PbI₄/MAPbI₃ interface due to strong hydrogen bonds, demonstrating superior robustness against ion movement. Further analysis indicates that the stability of 2D/3D interface is governed by intermolecular interactions, following the order: hydrogen bonds > π-π stacking > van der Waals forces. These findings highlight the pivotal role of molecular interactions in modulating ion migration at 2D/3D interfaces and provide a clear design principle for constructing stable 2D/3D heterojunctions by selecting diammonium cations with robust hydrogen bonds, offering key insights for the rational design of stable perovskite interfaces.