H4rdr4d4 Media - Chemistry

 Halide perovskites are soft, ionic semiconductors with exceptional optoelectronic properties. They typically have direct bandgaps in the visible spectrum that can be tuned by composition. For example, MAPbI₃ has a direct bandgap of about 1.5–1.6 eV, while replacing I⁻ by Br⁻ or Cl⁻ raises the gap to approximately 2.3 eV (green emission) or about 3.0 eV (blue) respectively. Depending on the halide used, the band gap can be tuned from around 1.6 eV (I) to approximately 3.2 eV (Cl). Mixed-halide perovskites allow continuous tuning across this range.

  For example, MAPbI₃ crystallizes in a tetragonal perovskite structure at room temperature, consisting of PbI₆ octahedra sharing corners, with MA⁺ cations occupying the interstitial spaces.  Variants of the perovskite structure also exist: for instance, layered (2D) perovskites have alternating organic and inorganic sheets (formula A₂BX₄), and “double” perovskites (e.g. A₂BB′X₆) replace Pb²⁺ by a combination of a monovalent and trivalent cation to eliminate lead. However, the defining feature of halide perovskites is the 3D corner-sharing BX₆ framework with monovalent A ions in the cavities

  Ideal halide perovskites adopt a cubic ABX₃ lattice (space group  Pm3̅m ), but most undergo distortions (to tetragonal or orthorhombic symmetry) as a function of temperature, composition and ionic sizes.  In the cubic form the B cation is octahedrally coordinated (6-fold) by X, and the A cation sits in a 12-fold cuboctahedral site formed by the vertices of eight adjacent BX₆ octahedra.  (This can be quantified by the Goldschmidt tolerance factor  t  = (r_A + r_X) / √2(r_B + r_X), which must be ≈0.8–1 for a stable perovskite.)

  Halide perovskites are crystalline semiconductors of the general formula  ABX₃ , where  A  is a monovalent organic or inorganic cation (e.g. methylammonium CH₃NH₃⁺ (MA), formamidinium HC(NH₂)₂⁺ (FA), Cs⁺),  B  is a divalent metal cation (most commonly Pb²⁺ or Sn²⁺), and  X  is a halide anion (Cl⁻, Br⁻, I⁻). In these materials the B-site cation is surrounded by six halide anions to form a BX₆ octahedron, and the A cations occupy the cavities between the corner-sharing octahedral  This ABX₃ motif is derived from the same perovskite structure as CaTiO₃, but with oxygen replaced by a halide. For example, CH₃NH₃PbI₃ (MAPbI₃) consists of a 3D network of PbI₆ octahedra with CH₃NH₃⁺ in the interstitial cuboctahedral sites.  Common A‐site ions include MA⁺, FA⁺ or Cs⁺; B‐site is typically Pb²⁺ or Sn²⁺; and X is I⁻, Br⁻ or Cl⁻.