Myth behind Metastable and Stable n-Hexylammonium Bromide-Based Low-Dimensional Perovskites

In perovskite solar cells, passivating the surface or interface that contains a high concentration of defects, specifically deep-level defects, is one of the most important topics to substantially enhance the power conversion efficiency and stability of the devices. Long-chain alkylammonium bromides have been widely and commonly adapted for passivation treatment. However, the mechanism behind is still not well explored as the formation route and the exact structure of these alkylammonium bromide-based low-dimensional perovskites are unclear. Herein, we investigate the physical and chemical properties of an n-hexylammonium bromide (HABr)-based low-dimensional perovskite including both thin films and single crystals. First of all, the HA₂PbBr₄ perovskite film and aged single crystal demonstrate different X-ray diffraction patterns from those of the fresh as-prepared single crystal. We found that the fresh HA₂PbBr₄ single crystal exhibits a metastable phase as its structure changes with aging due to the relaxation of crystal lattice strains, whereas the HA₂PbBr₄ perovskite film is pretty stable as the aged single crystal. Upon reacting with FAPbI₃, HABr can be intercalated into the FAPbI₃ lattice to form a mixed-cation perovskite of HAFAPbI₃Br, which is in a dynamic equilibrium of decomposition and formation. In contrast, the reaction of HABr with excess PbI₂ forms a stable HA₂PbI₂Br₂ perovskite. Based on such findings, we rationally develop a HA₂PbI₂Br₂-passivated FACs-based perovskite by reacting HABr with excess PbI₂, the photovoltaics based on which are more stable and efficient than those passivated by the HAFAPbI₃Br perovskite. Our discovery paves way for a more in-depth study of bromide-containing low-dimensional perovskites and their optoelectronic applications.