Organometal Halide Perovskites: Bulk Low-Dimension Materials and Nanoparticles

Abstract

Organometal halide perovskites (hybrid perovskites) contain an anionic metal-halogen-semiconducting framework and charge-compensating organic cations. As hybrid materials, they combine useful properties of both organic and inorganic materials, such as plastic mechanical properties and good electronic mobility related to organic and inorganic material, respectively. They are prepared from abundant and low cost starting compounds. The perovskite stoichiometry is associated with the dimensionality of its inorganic framework, which can vary from three to zero, 3D consisting of corner-sharing MX 6 octahedra, and 0D consisting of isolated octahedra. Small-sized organic cations can fi t into the MX 6 octahedra of the 3D framework and in all dimensions organic cations surround the inorganic framework. Regarding the low dimensionality in the material, this refers to at least one of its dimensions being shorter than approximately 100 nanometers. These materials should be considered as genuine nanomaterials or as bulk materials depending on whether they have three or less than three dimensions on the nanoscale, respectively. In principle, hybrid perovskite nanoparticles can be prepared with different shapes and with inorganic framework dimensionalities varying from 0D to 3D, and this also applies to the bulk material. This report is mainly focused on the unique properties of organometal halide perovskite nanoparticles.

Colloidal lead halide perovskite nanocrystals: synthesis, optical properties and applications

“…This also constituted the first report on the preparation of luminescent perovskite NCs. The first solution-based synthesis of colloidal MAPbBr 3 NCs was carried out by the Pérez-Prieto and colleagues, 30 who employed medium-length alkyl chain organic ammonium cations, octylammonium bromide and octadecylammonium bromide as capping ligands to obtain luminescent NCs via the solvent-induced reprecipitation method. The role of these ligands was to provide a self-termination of the crystallization, leading to the formation of discrete nanoparticles in solution.…”

“…They claimed that the excitonic-like features previously attributed to quantum confinement in spherical MAPbBr 3 nanoparticles were in fact a property of the nanoplatelets with decremental thickness, down to a single unit cell. They found that the absorption peak of their perovskite NC dispersion was blue-shifted by 0.5 eV with respect to the absorption onset of the bulk MAPbBr 3 . The fact that absorption and PL emission maxima were separated by 100 nm indicated that the dispersions were not phase pure, but rather consisted of nanoplatelets of varying thickness, where near-field energy transfer and reabsorption led to this large energetic separation.…”

“…However, an indepth study of the mechanism behind the size/shape-controlled growth of perovskite NCs would be highly valuable. Tyagi et al 46 employed a similar approach, synthesizing colloidal nanoplatelets of MAPbBr 3 . They claimed that the excitonic-like features previously attributed to quantum confinement in spherical MAPbBr 3 nanoparticles were in fact a property of the nanoplatelets with decremental thickness, down to a single unit cell.…”

“…It was assumed that n-octylamine controls the kinetics of crystallization and mainly contributes to the size control of NCs formed, whereas oleic acid suppresses aggregation effects and ensures their colloidal stability. From transmission electron microscope (TEM) studies, MAPbBr 3 NCs were quasi-spherical and had an average diameter of 3.3 ± 0.7 nm (Figure 3b). The authors reasoned that the increased PLQY of the NCs with respect to micrometer-sized bulk perovskite particles resulted from higher excitonic binding energies of up to nearly 400 meV, when the size of the NCs approached the Bohr radius of the respective perovskite, on the order of 13-30 Å, depending on the material.…”

“…20 Since then, many studies have contributed toward the fundamental understanding of this fascinating material and to its various technological applications as well. Halide perovskites have a general formula of ABX 3 , where A and B are monovalent and divalent cations, respectively, and X is a monovalent halide (Cl, Br, I) anion. The three-dimensional (3D) crystal structure of lead halide perovskites is shown in Figure 1a, where the B cation, commonly Pb, but also Sn, is coordinated to six halide ions in an octahedral configuration.…”