General Synthesis Principles for Ruddlesden-Popper Hybrid Perovskite Halides from a Dynamic Equilibrium

Two-dimensional (2D) Ruddlesden-Popper hybrid perovskites are a homologous series of compounds with the formula A'(2)A(n-1)B(n)X(3n+1) (A' = bulky organic cation; A = small organic cation; B = Pb2+ or Sn2+, X = Cl-, Br-, I-; n is an integer) composed of inorganic octahedral layers separated by organic spacer cations. The octahedral layer thickness is modified by the stoichiometry of the A-site cation, but limited methods exist for controlled and discriminating synthesis for all compositions. We report a general synthesis method and its principles that yield phase-pure 2D hybrid perovskites; the chemistry operates within a dynamic equilibrium established by the molar solubility of the compounds within the homologous series. A solvent-antisolvent (HI-acetic acid) pair and the common-ion effect provide selective control over the molar solubility to precipitate phase-pure compounds, as is supported by simple and predictive calculations. Here, this approach is demonstrated in detail with A' = n-butylammonium, A = methylammonium, and n <= 3 and is applied to the synthesis and discovery of materials with other bulky ammonium cations (e.g., iso-butylammonium and benzylammonium).