Metal Halide Hybrid Perovskites

*Fency Sunny, Linda Maria Varghese, Nandakumar Kalarikkal and Kurukkal Balakrishnan Subila*

## **Abstract**

Halide Perovskites have gained much attention in the past decade owing to their impressive optical and electrical properties like direct tunable bandgaps, strong light absorption, high photoluminescence quantum yield, and defect resistance shown by them. These materials find application in numerous fields including photovoltaics, optoelectronics, catalysis, and lasing applications. Multidimensional hybrid perovskites have been extensively researched as these structures lead to superior results. They combine the properties of three-dimensional variant along with the stability of the twodimensional perovskite. This chapter focuses on the unique properties of metal halide perovskites including the crystal structure, optical, electronic, and electrical properties. The different techniques followed for the synthesis of metal-halide nanostructures and 2D/3D hybrids are also included focusing on the changes in physical properties and the structure of these materials.

**Keywords:** metal halide perovskites, hybrid organic–inorganic perovskites, 2D/3D perovskites, structure, property enhancement

### **1. Introduction**

Perovskite, commonly referring to as a calcium titanium oxide mineral, with the chemical formula of CaTiO3, was discovered by Gustav Rose in the Ural Mountains of Russia in 1839 and is named after Russian mineralogist Lev Alexeievitch Perovski. Later on, the name perovskite structure has been given to any material, which has the same crystallographic structure as calcium titanium oxide (CaTiO3). The general chemical formula of perovskite compound is ABX3, where "A" and "B" are cations in which An atom is bigger in size than B atom, and "X" is an anion that binds to both cations and can be either halide [halide perovskite] or oxygen [oxide perovskite]. The ideal cubic symmetric perovskite structure has the B cation in 6-fold coordination, situated in the center surrounded by an octahedron of anion, the halogen atoms are in the faces center, and the A cation in 12-fold cubo octahedral coordination [1]. The A and B sites, both can accommodate inorganic cations resulting in the formation of inorganic perovskite. In the same way, if we replace inorganic cation A with small organic cations that leads to organic–inorganic hybrid materials [2].

Many physical properties of perovskite materials particularly electronic, magnetic, and dielectric properties depend on the crystal structure of perovskite, and the possibility for cations is limited by the stability of the structure, which can also produce

several distorted structures. These distorted structures as the result of varying cations can be used to tune the properties of perovskite materials. Electro neutrality and ionic radii are the two important factors that determine the stability of perovskite materials. The stability of the perovskite can be estimated by the Goldschmidt tolerance factor and an octahedral factor. Even though these two factors predict the structure of perovskite, it is not easy to predict the type of distortion occurring in the structure, due to both the octahedral and Goldschmidt factors do not account for the various molecular interactions in the compound including ionic, covalent, or hydrogen bonding. Although the chemical formula and coordination number remain the same, these distortions reduce the symmetry of the perovskite to lower symmetry crystal structures like orthorhombic, rhombohedral, hexagonal, and tetragonal forms [3]. Different phases of the perovskite materials mainly depend on the temperature changes, that is at 100 K the perovskite shows a stable orthorhombic phase, but at 160 K, the tetragonal phase replaces the orthorhombic phase. At a higher temperature of about 330 K, the most stable cubic phase starts to appear and replace the tetragonal phase [4].

Compared with any other semiconductor materials, perovskite materials maintain a high crystallinity, and this enables the formation of versatile forms of perovskite materials from nanocrystals to macroscopic single crystals [5]. And one of the amazing facts about the perovskite material is the simplicity of its preparations; however, often simple methods create interesting chemistry and mechanisms that give the material unique properties and applications [3]. Different synthetic methods have their own special features; and hence, it is very important to choose the correct method based on the targeted compound and applications. Although perovskite materials are seen to have excellent properties and applications, there are still many properties that need to be explored. In this chapter, we discuss the unique properties of perovskite materials and their synthesis methods along with the effect of varying doping materials.
