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′2An–1BnX3n+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).