Halide perovskite materials and devices
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Introduction The name “perovskite” honors Russian mineralogist L.A. Perovski, who first characterized the structure of the mineral calcium titanium oxide (CaTiO3). A perovskite material has the chemical formula of ABX3, where A is a cation with a large ionic radius, B is a metal cation, and X is an anion; its crystallographic structure is the same as that of CaTiO3. The easiest way to describe the perovskite structure is to display it as an AX12 cuboctahedron that shares its edges with a BX6 octahedron, as shown in Figure 1.1 This oxide perovskite structure is energetically stable and is flexible enough to structurally accommodate a variety of elements, which facilitates adjustment and choice of materials to achieve the desired properties. Through successful application as dielectric and piezoelectric materials, the excellent structural properties that are achieved can be used in almost any application, such as superconductors, multiferroics, batteries, fuel cells, photovoltaic electrodes, catalysts, resistive switches, and sensing materials.2–4 Halide perovskites have the same perovskite structure as oxide perovskites, but monovalent halide ions occupy the sites populated by divalent oxygen ions in an oxide perovskite. Therefore, halide perovskites can only host inorganic metal cations, such as Pb2+, Sn2+, and Ge2+, with a valence state of 2+ to satisfy charge neutrality. This means that diversity in terms of composition can be greatly limited compared to that of oxide perovskites which can also accept trivalent and tetravalent cations at the B site. This limitation, however, is compensated by the possibility of lower-temperature synthesis of
halide perovskites compared to oxide perovskites. This allows organic cations to be accommodated in the halide perovskite and has led to the development of a new class of materials called inorganic–organic hybrid halide perovskites. Inorganic–organic hybrid perovskites are well known for their success in solar power generation. In 1978, Weber5 had already studied the basic crystallographic properties. The acceptance of organic cations in the A position of the perovskite structure could yield new properties that conventional oxide perovskites do not have, which presents a new avenue in materials science. Moreover, organic cations allow easy conversion of a three-dimensional (3D) perovskite to a two-dimensional (2D) structure depending on the size of the ions, and new categories of material groups are expected to be created because a combination of 3D and 2D structures is possible. For example, higher dielectric and piezoelectric properties than those of oxide perovskites BaTiO3 or Pb(Zr,Ti) O3 were observed in the 2D perovskite ferroelectric (4-amino tetrahydropyran)2PbBr4.6 As shown in Figure 1, applications using halide perovskites continue to expand to optoelectronic devices, such as solar cells and light-emitting diodes (LEDs); nanoelectronic devices such as thermoelectric and memory devices; and artificial synapses.7–9 Inorganic–organic hybrid perovskite materials such a
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