Effect of Organic-Cation Exchange Reaction of Perovskites in Water: H-Bond Assisted Self-Assembly, Black Phase Stabilization, and Single-Particle Imaging

Despite outstanding performance of organic-inorganic lead halide perovskite (OILHP)-based solar cells and light-emitting devices, they are still unusable for practical applications due to instability of OILHP in humid conditions. The extreme moisture sensitivity of these materials restricts all kinds of study in water media such as organic-cation exchange reaction, H-bonding assisted self-assembly, and phase stabilization in water. In order to address this issue, we report the full mechanistic details of nontemplate aqueous-phase synthesis and band-gap engineering of formamidinium (FA) lead halide (FAPbX₃; X = Cl/Br/I) as well as its conversion to fluorescent MAPbX₃ (MA = methylammonium) in water. Our study is the first example of an organic cation exchange reaction of hybrid perovskites family in water. By this aqueous ion-exchange synthetic approach, the yellow phase of FAPbI₃ converts into black phase of MAPbI₃ and a unique tube-structure of MAPbBr_3-xI_x self-assembles through hydrogen bonding, which is a well-known factor for degradation of OILHPs. The as-synthesized tube-shaped perovskites monitored at the single-particle level exhibit shape-correlated fluorescence images. Our mechanistic synthetic details for ion-exchange reaction in water open an exciting path for synthesizing mixed organic cation perovskites in water for diverse applications such as solar-cells, light-emitting devices, and single-particle imaging/tracking.