**2. Types of perovskite halides**

#### **2.1 Inorganic perovskite**

Material having the general formula ABX3 is called perovskite material. Inorganic halide perovskite consisting of inorganic A site cations such as Cs<sup>+</sup> , Rb+ , and B site with metal ion and X with halogen ion, have been demonstrated with improved stability toward moisture, light, and heat as compared to organic–inorganic hybrid perovskite and embrace unique structural features. These unique properties of inorganic halide perovskite are used in optical and photovoltaic applications. Meanwhile, inorganic halide perovskite shows dramatic change in phase transition temperature creating stability issues, and causing serious deformation, since most of the properties depend on chemical composition and crystal structure. According to environmental temperature, inorganic perovskite shows rapid phase changes, the favorable phase transition can be achieved by partial substitution of different halogen in inorganic halide perovskite. Incorporation of bromine to iodine, the visible light absorption range decreases and increases the band gap. The tin-based perovskite, CsSnX3, shows a red shift in optical spectra compared to inorganic lead perovskite and CsGeX3 shows similar optical spectra as CsPbX3. In addition to the 3D inorganic halide perovskite ABX3, heterovalent perovskites with +4 oxidation state in the B cation with chemical formula A2BX6 are also synthesized. Cs2PdBr6 synthesized via solution process shows excellent stability to moisture and suitable for optoelectronic

application. Cs3Sb2I9 is another inorganic halide perovskite posse's similar property to hybrid halide perovskite. The lead-free germanium-based perovskite CsGeX3 posseses properties including high dielectric constants, photoabsorption coefficients, effective masses of charge carriers, exciton binding energies, and electronic band structures. Along with Cs ion, Rb ion can also be used as A-site cation in inorganic perovskite. Some of the Rb halide perovskites are reported. Among the A2BX6perovskite, Cs2PdBr6, Cs2SnI6, Cs2TiBr6 have been utilized in photovoltaic devices [6].

Compared with hybrid halide perovskite, inorganic halides show much more stability, therefore, a lot of studies have been focused on the synthesis of inorganic halide perovskite nanostructures using different strategies. Inorganic halide perovskite in nano size have shown superior optical, electrical, and optoelectronic properties which offer excellent platforms for distinct fundamental research and further development for future applications [7].

#### **2.2 Organic inorganic perovskite**

In the organic−inorganic hybrid perovskite, at least one of the A or B sites are occupied with organic ions, typically A sites are occupied with organic ion, B site with metal ion and X site with halogen ion. Methylammonium [MA] and formamidium [FA] with chemical formula CH3NH3 and CH (NH2)2, respectively are the most common organic ion occupied in A site of hybrid perovskite material. The possibilities of the formation of various perovskite materials are determined by various stability parameters, which determine whether a set of A, B or X ion may adopt the perovskite structure. In the case of organic ions, it is not easy to assign the ionic size, especially for nonspherically symmetric and charged complexes. Hence by considering the assumption of the molecule being free to rotate about its center of mass, the effective radii for organic cations are determined. The size restrictions, as outlined by the tolerance factor in the 3D structure can be lifted by slicing the perovskite structure. In the case of 2D layered derivative of perovskite structure, there is no restriction in the A cation length, and in the case of zero-dimension, size restrictions are not even considered as limitation in synthesis. This structural flexibility of structure in lower dimension structure provides a platform for preparation of varying structural material and tunable applications. When A site occupied with organic cation, it is necessary that it must contain a terminal functional group that interact with inorganic material, but the remaining part should not interfere with B or X components. Mono or diammonium cations are important factor in 2D layered perovskite structure, for example (RNH3)2BX4 or (NH3RNH3)BX4, in which R group denote organic functional group. The main advantage of this ammonium cation is that they form H-bonding with inorganic ions which give stability as well as the orientation of organic cations. The metal ion in site B can also influence the organic cation and H-bonding, especially when they show some kind of distortion in structure. Along with size and fit, the charge balance requirement is also important in respect to inorganic cation. In short both organic and inorganic cations are related to each other, allowing certain degree of influence of structure and properties of perovskite material.

The choice of organic cation and stoichiometry are two important factors on the orientation of resultant inorganic framework. In addition to the structural flexibility, the properties of both organic and inorganic cation combined together to give the structural properties of hybrid material. The energy transfer between organic and inorganic ions are studied and reported, include naphtelene and pyrene ion in Pb halide framework, azobenzene in PbBr4, 3–2-(aminoethyl) indole in CuCl4 are some of the examples, in which electronic tunability of both ions to create unique features.

In addition to the monomeric cation, incorporation of polymerizable moieties is also reported. The polyacetylene derivatives on lead bromide framework that have been reported is an example of polymerized structure. The structure shows enhanced air and moisture stability obtained from the protective polymer structure. Generally longer alkyl chains provide more degrees of freedom resulting more structural phase transition. Ferroelectric transitions of hybrid halide perovskite are reported; providing interesting possibilities for material design to ferroelectric random access memory and magnetic data storage. The hybrid perovskites can overcome some of the limitations of organic and inorganic quantum dot LED, including the issues of high cost, poor color purity, and high ionization energy. In total inorganic–organic hybrid halide perovskite found application in optical, electrical, and magnetic fields.

Even though many studies on hybrid perovskite are going on, the continued study of structure–property relationships in hybrid organic–inorganic halide perovskite is an important step for bringing reproducibility and predictability to the diverse and interdisciplinary fields [8].

### **2.3 2D/3D hybrid Heterojunction perovskite**

Perovskite solar cells based on 2D/3D heterostructure have attracted researcher's attention in the past few years due to their promising photovoltaic application, and their amazing properties such as high absorption coefficient, tunable direct band gap and long exciton diffusion, and stability. The 3D halide perovskite attains a power conversion efficiency of 25%, which makes them attractive materials in the field of photovoltaic. Despite this efficiency, perovskite suffers stability issues including phase conversion of perovskite film, degradation in moisture, heat and irradiation, hinders the further development. Researchers have made so much effort to enhance the stability of 3D perovskite, among them introduction of 2D perovskite is one of the potential strategies for the stability improvement. The structural features especially the hydrophobic nature of the spacing as well as packing crystal structure in the 2D perovskite material makes them stable in the ambient temperature by protecting perovskite from the direct contact of moisture. However pure 2D crystals are not desirable, because of wide band gap and non-preferred crystallographic orientation [9]. Cation exchange has been used as a plausible way for transformation between 3D and 2D phases of halide perovskites [10]. Building on the combined benefit of both 2D and 3D perovskite material a new type of 2D/3D heterostructure is synthesized in the field of solar cell, in which 2D perovskite were incorporated onto the surface of 3D component as capping layer to increase the stability of the 3D perovskite phase without changing the device performance. That is in 3D\2D heterojunction architecture only the surface of the light absorbing perovskite layer is altered while the optoelectronic properties of the bulk film are remain intact. The 2D/3D hierarchical structural was first constructed for 3D MAPbI3, the structure is (PEA)2(MA)4Pb5I16/ MAPbI3 (PEA = phenylethylammonium). This 2D/3D structure used in solar cell exhibit a strong moisture hindrance. Another structure (BA)2PbI4/MAPbI3 (BA = butylammonium) shows much enhanced thermal stability and high photo conversion efficiency of 19.8%. Recently the 2D/3D FA (FA = formamidinium) structure also synthesized. For neat 2D perovskite, much study has been takes place and achieved in determining the nature of films, including thickness distribution, and charge transport. But to obtain a clear-cut understanding of the interfacial mechanism at the 2D/3D heterojunction, we need more information about the ligand dependence of 2D/3D heterojunction and its influence on charge collection. The ligand chemistry is very importance in knowing the thickness distribution and orientation of 2D perovskite, which is expected to play a major

role in charge transfer at the heterojunction and solar cell. The ambient phase stability was studied for all the films and found that 2D/3D heterojunction assisted phase shows excellent stability especially for 3D FAPbI3perovskite, particularly FPEA-–based film maintains 84% of its value after ambient exposure for long time [11].

#### **2.4 Double perovskite**

In order to overcome the structural instabilities of halide perovskite especially lead halide perovskite, and the hybrid perovskite, researchers has sought in search of alternate crystal structure, found a promising material called double perovskite. ABX3 is the general formula for an optimal-perovskite materials in which A and B are cations of different size and X is an anion similar in size to B cation. But in the case of double perovskite either A or B site can be occupied by two different types of cations, giving the formula A′A″B2X6 or A2B′B″X6. Among these two structures, the double B site perovskite is the preferred one because the physical properties of perovskite mainly depend on B site cation. The crystal structure of A2B′B″X6 depends on the arrangement of B′ and B″ cations in the sub lattices, which are mainly focusing on reducing the madelung [strain] energy of double perovskite due to the charge difference between these two B cations. Based on the charge difference between B′ and B″ there are three kinds of B cation sub lattices, called random, rock salt and layered structure [12]. The compound having random type sub lattices generally possess cubic or orthorhombic unit cell, and rock salt type sub lattices usually crystallize in cubic or monoclinic unit cell. When the B′ and B″ cations can alternate in one direction, the layered type is formed and possess a monoclinic unit cell [13]. The introduction of vacancy in the B site of A2B′B″X6 creates another type of double perovskite called vacancy ordered double perovskite. Cs2SnI6, Cs3Bi2Br9, and Cs2TeI6 belong to this group which is also called as a defect tolerant semiconductor. **Figure 1** represents the crystal structure of double perovskite [14].

The experimental synthesis of Cs2BiAgCl6 and Cs2BiAgBr6 are the first documented double perovskite in the halide double perovskite avenue. Both these perovskites are crystallized in face centred cubic double structure, called elpasolite. The two B cations need a convenient oxidation state to form the perovskite phase and provide a combined charge of 4+; this charge can be equally or unequally divided. In the unequal distribution, the replacement of divalent cation by a pair of monovalenttrivalent cations maintains the charge neutrality. In search of trivalent cation, researchers reached the nitrogen family and found bismuth and antimony as the most suitable ones. And for monovalent cations, noble metals like copper, silver, or gold the ones with excellent electrical conductivity and optical properties are chosen. So far, among the many reported double perovskites, Cs2BiAgBr6 is the only double perovskite found application in any active devices [15]. Also, Cs2BiAgBr6 was successfully applied for X-ray detection and is of great importance in medical diagnosis, industrial application, and scientific researche. Along with hybrid double perovskite, rare earth metal containing double halide perovskite like MA2KYCl6, MA2KGdCl6 synthesized via solution-evaporation method have also found considerable importance [16]. The Cs2AgBiBr6nanocrystals exhibited impressive photo conversion of CO2 into solar fuels, along with Cs2AgBiBr6, Cs2AgBiCl6, Cs2AgSbBr6, Cs2AgInCl6 found application in water splitting. The hybrid halide double perovskite is also synthesized, following are some of the reported perovskite MA2KBiCl6, MA2TlBiBr6, MA2AgBiBrl6. MA2KBiCl6 is the first synthesized hybrid double perovskite shows high resistivity and superior magnetic properties. Properties of doped double perovskite are also a

**Figure 1.**

*Schematic representation of 2D/3D hierarchical structure: (a) fabrication of 2D/3D heterojunction; (b) spacers with different chemical structure and compositional engineering; (c) schematic model of 2D/3D hierarchical structure. (Source: Niu et al. [11]. Copyright 2019 American Chemical Society. Reprinted with permission).*

topic discussion and Bi doped Cs2SnCl6 showed great potential as blue phosphor and LED exhibit white light emission. Recently, Cs2AgInCl6 doped with Na<sup>+</sup> cation also reported with efficient white emission via radiative recombination.

Compared to halide perovskite, the main issue associated with double perovskite is the much lower defect tolerance which sometime reduces the efficiency of perovskite materials. Based on the recent report, the double perovskite Cs2AgInCl6 shows much less PL efficiency than corresponding bulk structure. However, Cs2AgInCl6 could be used in LED, UV photo detector and scintillators [17]. There is plenty of double halides that can be synthesized according to the theoretical assumption. However, so far only few double perovskites have been synthesized. In order to produce more stable halide double perovskite, one should know some important points such as suitable composition, the properties of the material to be used, and the limitation to the synthetic procedure. Due to the low decomposition temperature of organic starting materials, the hybrid halide double perovskite is somewhat difficult to synthesis [18]. The major challenge in the synthesis of halide double perovskite is the high temperature requirement. The most studied double perovskite Cs2AgBiBr6 require high annealing temperature up to 285°C for obtaining high quality thin films. To some extent, this high temperature requirement puts limits on the synthesis and application of double halide perovskite [14].
