Metal Halide Hybrid Perovskites

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 two-dimensional 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.

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.

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⁺, 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, CsSnX_3, 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, Cs₂PdBr_6, Cs₂SnI_6, 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, Cs₃Bi₂Br_9, 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].

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).

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 monovalent-trivalent 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 topic discussion and Bi doped Cs₂SnCl₆ showed great potential as blue phosphor and LED exhibit white light emission. Recently, Cs₂AgInCl₆ doped with Na⁺ 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].

3. Properties

3.1 Crystal structure

Perovskites are one of the oldest families of materials. Perovskites are normally formed by three molecules in the stoichiometric form ABX₃. The B site occupant is a divalent cation like lead, bismuth, tin, germanium, etc. and X is halide ion. In organic –inorganic perovskites, the A site is occupied by a monovalent cation such as methyl ammonium, or formamidinium. Inorganic perovskite halides have cesium, rubidium etc. The stability of perovskite structure is determined by the Goldschmidt tolerance factor, t, which is given by the equation t = $\frac{r_A+r_X}{\surd 2\left(r_B+r_X\right)}$ based on the ionic radii of the component ions [19]. The different cations that can occupy the place in a stable perovskite should have ionic radii satisfying t with values between 0.76−1.13 constraining the options of possible cations [20]. In HOIPs, a notable number of tolerance factors are found to lie in the range of ~0.8–1.0 for stable perovskite structure. In the ideal case, perovskites take the cubic space group Pm$\acute{3}$m with no variable parameters in the structure [20].

Apart from the original ABX₃ phase, hybrid perovskites also stabilize in a variety of structural phases. Hybrid organic−inorganic perovskites (HOIP) can be categorized from a structural perspective as ABX₃perovskites, A₂BB′X₆ double perovskites and A₃BX antiperovskite subclasses [20]. Halide double perovskites changes the simple perovskite ABX₃ structure to a 2 × 2 × 2 supercell, with the general formula A₂B^IB^IIIX₆. Here, the two bivalent cations B²⁺ are exchanged by a combination of one monovalent cation B⁺ (e.g., Au⁺, Cu⁺, Ag⁺, In⁺) and one trivalent cation B³⁺ (e.g., Bi³⁺, Sb³⁺) [21].

The lower-dimensional 2D perovskites is composed of alternating layers of organic and inorganic phases called Ruddlesden–Popper (RP) phases having general formula A₂A′_n − 1B_nX_3n + 1. The size of the organic cation controls the 3D to 2D structural transition, in particular when it exceeds the critical size of Goldschmidt’s tolerance factor, [22]. The 2D perovskites thus generated have alternating organic and inorganic sheets that are along [001] direction. This leads to the formation of layered structures. Though these phase results in excellent properties, it is affected by stability issues. In order to enhance the stability a combination of 3D and 2D phases are synthesized. The layered structure encompasses alternate 3D and 2D phases connected by the organic cation spacer (Figure 2) [22].

Figure 2.

Schematic representation of different metal halide structures: (a) cubic-phase ABX₃ (3D); (b) pseudocubic ABX₃ (3D); (c) A₄BX₆ (0D); (d) AB₂X₅ (2D); (e) A₂BX₄ (2D); (f) A₂BX₆ (0D); (g) A₂B⁺ B³⁺X₆ (3D); and (h and i) A₃B₂X₉ (2D). (source: Shamsi et al. [23]. Copyright 2019 American Chemical Society. Reprinted with permission.)

3.2 Electronic properties

Halide perovskites find numerous applications in myriad of fields owing to the exceptional electronic properties shown. The bandgaps of these materials are mainly determined by the halide ion due to the strong contribution of the 2p orbital of the halide ion. The bandgap can be tuned by changing the B-site ion as well [20].

HOIP are direct-bandgap semiconductors, with experimentally observed bandgaps ranging from ~1.2 to 2.8 eV. In HOIPs, the A-site cation also affects the band gap as amine cations could distort the anionic framework through hydrogen bonding and van der Waals interactions upon thermal and pressure perturbation. The electronic structure has been studied using computational methods. For a common HOIP structure like that of MAPbI₃, the conduction band minimum originates mainly from the 6p states of Pb, hybridized with a small amount of the 5p states of I, while the valence band maximum is mainly formed from the 5p states of I, mixed with a certain percentage of the 6 s states of Pb [20].

Double perovskites are seen to be materials with predominantly indirect band gaps. Cs₂AgIn_xBi_1−xCl₆ NCs are the first double perovskite nanocrystals (NC) to have direct bad gap. Alloying the ratio of In to Br resulted in the shift of the band gap from indirect to direct. The direct band gap product showed better photoluminescence quantum yield (PLQY) [24]. Vacancy ordered double perovskites have been observed to show direct bandgaps. Hybrid double perovskites that are isoelectronic to common HOIP such as CH₃NH₃PbBr₃ was synthesized to obtain direct bandgap materials [25].

3.3 Optical properties

HOIPs have been studied to show efficient broadband tunable optical properties. In the bulk form, HOIP-based LEDs are constructed as simple multilayered devices. Depending on the halide composition, these were found to be near-infrared, green and red-light emitters at room temperature with bright electroluminescence owing to the efficient radiative recombination of injected electrons and holes. However, the poor morphology of HOIP thin layers leads to lower efficiencies of the HOIP LEDs compared to conventional organic and quantum-dot LEDs [20].

The absorption measurements of Cs₂AgBiX₆ NCs with different halide compositions revealed the tunable exciton peaks ranging from 367 to 500 nm with the corresponding PL peaks varying from 395 to 575 nm [24]. The study of optical properties in double perovskites has shown that ligand passivation of surface defects can increase the PLQY (a 100-time increase). The colloidal NCs of Cs₂AgBiBr₆ were observed to exhibit dual absorption peak at 427 nm and 380 nm, the former from direct Bi s–p transition, while the latter may be assigned to the isolated octahedral BiBr₆³⁺ complex in the colloidal solution. The thin films of the double perovskites have shown stable single peak emissions with better PLQY [24].

In case of 2D/3D hybrid perovskites, the Ruddlesden–Popper structure of layered perovskites results in the variation of charge carrier dynamics and the optoelectronic properties with a varying value of n. As the value of n increases the optoelectronic properties tend to be that of the bulk material with broadened transient absorption features. For low values of n, on the other hand, the excitonic peak is enhanced and the material shows an increased monomolecular recombination rate [26].

3.4 Electrical properties

Metal halide perovskites show great carrier transport properties combined with long carrier lifetimes results in them being effectively used in photovoltaic applications. The long transport length helps in the perovskite thin films being used effectively for long distance transport of charges. This is achieved by the initial photoexcited carriers recombining away from the excitation spot which results in regeneration of photons that is reabsorbed. This forms charge carriers at significant distances away from the initial excitation point [20].

For efficient charge–carrier separation, the exciton binding energy (EB), which defines the lowest energy required to dissociate an exciton (electron–hole pair), must be small. In HOIPs, it has been shown experimentally that photo excitations directly generate free electrons and holes, rather than bound excitons. This is due to the fact that the EB is low enough (≤25 meV) to allow charge separation at room temperature [20].

The combination of 2D and 3D perovskite structures in 2D–3D hybrid structures has yielded solar cells achieving high PCEs and excellent stability over thousands of hours. Engineering of the 2D/3D heterostructures has helped in achieving better performing solar cells [26].

3.5 Ferroelectric properties

Study of the existence of ferroelectric domains in perovskite materials are of great interest as they will enhance the electron–hole separation in the materials, resulting in better photovoltaic performance. Halide HOIPs have cations such as FA or MA that are of polar nature and shows order–disorder transitions across phase transitions. MAPbI₃ was the first to show potential ferroelectric behavior with the observation of a hysteresis in the current voltage plot. Further experiments using piezoelectric force microscopy have since proved that MAPbI₃ is indeed ferroelectric. Density functional theory (DFT) calculations along with symmetry mode analysis have shown that the origin of the spontaneous polarization as a combined effect of the relative movement of MA and the relaxation of the framework, which are coupled through hydrogen bonding [20].

3.6 Other properties

In HIOP the framework stiffness is seen to be proportional to the Pb–X bond strength, tolerance factor, as well as the electronegativity of the halogen atoms. The Young’s moduli have been measured to be about ~10–20 GPa. This value correlates well with chemical and structural differences. The hardness properties show an inverted trend with respect to the halide ion (I > Br > Cl). The least rigid system was seen to be MAPbI₃ which also exhibited the highest resistance to plastic deformation. The thermal expansion behavior of these halide HOIPs had significant thermoelastic response (~30–40 × 10⁻⁶ K⁻¹), due to octahedral flexing. This could contribute to self-helaing property of possible point defects in device applications. At the same time, relatively low hardness properties indicate ease of plastic deformation, which could affect the cyclability of flexible cells and devices [20].

4. Synthesis methods

4.1 Synthesis of nanostructures

Nanostructures of halide perovskites have attracted great deal of attention due to the interesting properties shown in the nano scale. Various methods are followed for the synthesis of halide perovskites.

4.1.1 Ligand assisted synthesis

Colloidal halide perovskite nanostructures are synthesized using ligand assisted method. Long chain amines and a combination of short chain and long chain amines have been used as ligands for the synthesis of stable halide perovskites [27, 28, 29]. The surface adsorption and desorption of the ligands leads to the formation of stable nanostructures at relatively low temperature [29]. This helps in the more efficient application-based synthesis of perovskites. Quantum dots, nanocrystals (NCs), nanosheets (NS), and nanorods of halide perovskites have been synthesized using the colloidal method.

The colloidal hot-injection method has been used to successfully synthesize lead based, lead-free pure inorganic and organic−inorganic hybrid perovskite NPs having excellent structural and optical properties [30]. The ligands and the solvents used and the ratio of the ligand to precursors affect the morphology of the synthesized materials. In a typical method, the synthesis of CsPbX₃ (X = Cl, Br, I) is achieved by “controlled arrested precipitation of Cs⁺, Pb²⁺, and X⁻ ions into CsPbX₃ NCs is obtained by reacting Cs-oleate with a Pb(II)-halide in a high boiling solvent (octadecene) at 140−200°C” [31]. Hot injection method in the original and modified form has been used to synthesize double perovskites. Cs₂AgBiBr₆ double perovskite NCs with high crystallinity were synthesized via hot-injection approach. These NCs were capable of maintaining their structural stability in a set of varied environments such as low polarity solutions, 55% relative humidity and temperature of 100°C [24].

Colloidal synthesis of HOIP was first done using medium-length alkyl chain organic ammonium cations (bromides of octylammonium and octadecylammonium) as capping ligands to obtain luminescent NCs via the solvent-induced reprecipitation method. The change in the organic cation involved in the process led to particles with better emission and properties (Figure 3) [32, 33].

Figure 3.

A schematic of synthesis of different dimensional perovskite nanostructures using different organic ligands: Acetate acid and dodecylamine for nanorods; hexanoic acid and octylamine for spherical quantum dots; oleic acid and octylamine for few-unit-cell-thick nanoplatelets; oleic acid and dodecylamine for nanocubes. Reprinted with permission from [32]. Copyright 2016 American Chemical Society.

Ligand assisted reprecipitation (LARP) method is one of the common low temperature methods widely carried out to produce highly crystalline HOIPs with different morphologies. The nature of the capping ligand and the ratio of precursors to solvent are the controlling factors determining the shape, size and the formation mechanism of the HOIPs in this method. The commonly used solvents are Octylamine (OTAm, C₈H₁₇NH₂), methylamine (MA, CH₃NH₂), toluene (C₆H₅CH₃), N,N′-dimethyl formamide (DMF), γ-butyrolactone, oleylamine (OLAm), oleic acid (OLA), 1-octadecene (1-ODE). It is the interaction between the ligand and the added antisolvent (e.g., Toluene) that results in the formation of crystalline or amorphous products [30]. Dual-ligand-assisted re-precipitation method under the synergistic effect of n-octanoic acid (OTAC), oleylamine (OLA), and (3-aminopropyl) triethoxysilane (APTES) was used for the synthesis of enhanced α-CsPbI₃ NCs [34].

4.1.2 Heating-up synthesis

Direct heating of the precursors in a suitable solvent (octadecene) has been followed for the easier synthesis of high-quality perovskite nanocrystals. The one pot approach results in a relatively simple, reproducible, easily scalable and tunable method [35]. The method has been extensively used in the synthesis of lead-free inorganic halide perovskites, wherein the precursors are added together into the solvent with or without the ligands and heated to a definite temperature. The reaction is allowed to take place for a specified time after which it is cooled to room temperature naturally. On centrifuging and washing the nanocrystals are obtained. A combination of hot injection and heating up method has been used for the production of double perovskite such as Cs₂AgBi(Br/I)₆ exhibiting better performance [36].

4.1.3 Hydrothermal synthesis

Hydrothermal method proves to be an efficient, simple and straight forward method for the synthesis of inorganic halide perovskites. In the method the precursors dissolved in halide acids or DMF are placed in Teflon liner inside an autoclave for the corresponding time [37, 38]. The reaction is allowed to take place at a constant temperature for time as low as 30 minutes. Double perovskites have been studied to be extremely sensitive to impurities. Therefore, the cleaning of the Teflon liner is especially important in this type of synthesis. Alloyed double perovskite materials such as Cs₂AgInCl₆ have been synthesized by the hydrothermal method. 10 M HCl solution was used as the solvent in this case. The product obtained was seen to be extremely stable with efficient white light emission on alloying sodium [38].

4.2 Thin film synthesis

Thin film preparation is done basically by spin coating the precursor solution dissolved in the required molar quantity. The coating is done usually onto substrates fit for the proposed application. Single crystal thin films are fabricated aimed at better efficiency and performance.

Thin films of 2D/3D hybrid structure of BA_x(FA_0.83Cs_0.17)_1−xPb(I_0.6Br_0.4)₃ were prepared by the spin-coating of a blend of FA_0.83Cs_0.17Pb(I_0.6Br_0.4)₃ and BAPb(I_0.6Br_0.4)₃ in N,N-dimethylformamide, DMF. The coated film is then dried at 70°C for 60 s in nitrogen filled atmosphere followed by transfer into an oven where they were annealed in air at 175°C for 80 min to obtain the smooth films with the 2D crystallites at the boundaries of 3D grains [26]. Hybrid double perovskite has also been synthesized easily using the method of spin coating. Two step method has been followed for the synthesis of 2D/3D double perovskite wherein the organic cation such as PEA is spin coated onto the annealed films of the double perovskite [39]. Thin films of inorganic and double perovskites are easily synthesized normally by spin coating solution of the precursors in solvents such as DMF or dimethyl sulfoxide (DMSO) onto the suitable substrate for the appropriate applications in photovoltaics and optoelectronics [25].

4.3 Synthesis of single crystals

Single crystal nanostructures of halide perovskites are synthesized for improved application as semiconductors in electronics, optoelectronics, and photovoltaics. These structures provide improved photophysical properties and are therefore carry much importance [40].

The anisotropic growth rates of the crystals which depend on the feed ratios of the precursors, mineralizers, and solvents helps in the fabrication of the single-crystal thin films. Inverse temperature, temperature lowering, solvent evaporation and anti-solvent assisted crystallization methods are utilized for the process. Vapor phase epitaxial growth of thin films has been extensively used in inorganic halide perovskite monocrystal fabrication. Single crystals of HOIPs such as FAPbI₃ and MAPbI₃ have been reportedly sliced into wafers from their bulk counterpart in a top-down strategy for single crystal thin film strategy [41]. Dissolution-recrystallization pathway in solution synthesis from lead iodide (or lead acetate) films coated on substrates was used to grow single crystal nanowires, nanorods, and nanoplates of methylammonium lead halide perovskites (CH₃NH₃PbI₃ and CH₃NH₃PbBr₃). These single crystals showed increased photoluminescence and long carrier lifetimes [40].

HOIP single crystals have also been synthesized using bottom seeded solution growth and top seeded solution growth. The former method employs seed crystals that are fixed at the middle of a designated tray are rotated by the electric motor and the saturates solution was cooled to obtain the single crystals. The latter method employs seed crystals placed on a silicon substrate on top of a solution which facilitates the dissolution of the lower crystals due to the temperature discrepancy between the bottom and top of the solution that induced super-saturation to form the single crystals. 2D perovskite single crystals have been synthesized by a combination of vapor phase and solution processes. The process involves spin casting one of the precursors at an elevated temperature followed by chemical vapor deposition of the other one [42].

5. Property enhancement by doping

Doping, in general is an effective approach to enhance the properties of semiconductor materials by intentionally introducing impurity or heteroatom into the target lattices. The doping with anions and cations of different charge is also known to modify their micro structural, electrical, and magnetic properties. Considering the fact that halide materials possess interesting properties, researchers recognized that introduction of other materials into the halide perovskite could potentially lead to relevant discoveries and applications, with this aim, over the past few years, different kind of dopant materials are introduced. Among them most relevant ones are discussed here [43].

5.1 Metal ion

Due to the high abundance in the earth’s crust, varying oxidation state and mostly nontoxic nature, doping by transition metals are the best choice to enhance the properties of perovskite materials. There are two methods to doping by metal ions in perovskite materials, alloying to partially replace the metal frame and inserting a small amount of transition metal in the lattices. Among these two insertions of small amount of transition metal ion into the perovskite is the preferred one, cause the energy transfer and charge carrier transfer processes between the dopant and perovskite crystals [44].

So far a large number of reports were being available on CsPbX₃ perovskite nanocrystals doped with divalent transition metals like Mn²⁺, Co²⁺, Cu²⁺ and Zn²⁺. These metal ions play a role in eliminating the defects and distortions of perovskite crystals and exhibit dual color emission and efficient charge transfer. Among various methods available incorporating metal ion into perovskite material through hot injection is the feasible method, moreover, transition metals are economical and eco-friendly compared to other toxic materials and shows excellent properties without destroying the crystal structure [45]. Among these transition metals, Mn²⁺ need special attention due to its excellent properties. Mn-doping in nanocrystals increased the exciton luminescence and attribute the efficient energy transfer between exciton and host. Thus, for Mn doped CsPbX₃ achieved up to 60% luminescence quantum yield [16]. MAPbBr₃ the one of the most studied organic−inorganic perovskites show structural instabilities, which can be improved by doping with Zn metal ions. The Zn doped MAPbBr₃ reveals excellent optoelectronic properties and environmental stability due to increased lattice strain which leads to the improved interaction between bonds [46]. Cs₂SbAgCl₆ and its Cu²⁺ doped perovskite shows well-ordered double perovskite cubic structure with excellent conductivity. The researchers proposed that Cu²⁺ doping creates cation defect, which leads to increased conductivity of double perovskite. In conclusion, the antimony−silver based double perovskite doped with copper ion exhibit desirable properties comparison to the bare perovskite in greater bandgap tunability and stability and has impact on their morphological, optical, electronic behaviors [47].

In addition to the transition metals, alkali and alkaline earth metals, other group metals are also used as dopant in perovskite materials. The possibility of doping with alkali metal ion like Rb was explored and the result shows that Rb ion doping suppress the forming of impurity phases, and also increase the lifetime of perovskite materials [43]. In search of other metal doping, it has been found that Na⁺, in bulk, Cs₂SbInCl₆ bring an increase in PL emission by three order magnitude compared to pure double halide material with an optimum Na content of 40%. This is where explained as the result of improved crystal quality and increased rate of radiative recombination [17]. There is also a reported result of increased hole concentration and mobility by Na doping and promoted electron injection in devices by Li doping in solar cell application [43]. Alkaline earth metals, Ca, Sr., and Ba, are proposed to be able to enhance the properties of perovskite with doping and has remarkable impact on their morphological, optical, and electronic behaviors. Besides alkali and alkaline earth metals, metals like Al³⁺ can provide tremendous morphological control to improve the properties of halide perovskite. The incorporation of indium (In3+) has also been reported to influence morphology by facilitating preferential growth of grains in several orientations [48]. In addition to that, Bi-doped bulk crystals underwent a significant band gap narrowing and shows improved stability and excellent optoelectronic and magnetic properties.

5.2 Lanthanides

To date, several successful doping of inorganic or hybrid perovskite by metal ions have been reported. However, the emissions for transition metal ions are broad band and confined to specific wavelength region limiting their application to limited energy structure. Hence lanthanide ions or called rare earth elements would be the most suitable candidate for energy and optical applications, because they possess rich and unique optical properties and emissions are in wide range with sharp line from UV to infrared region. In addition to that moving from Ce to Lu, a gradual decrease in ionic radii of lanthanides provide varying electrical, magnetic and chemical features provide the opportunities to study the changes of doping. Various lanthanide doped halide perovskite are studied and successful doping of Ce³⁺,Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Er³⁺ and Yb³⁺ into the CsPbCl₃ perovskite through hot injection methods are reported. Lanthanide doped double halide perovskite of the type Cs₂AgInCl₆ are also reported. For the lanthanide doped perovskite nanocrystals stable and tunable multi-color emission from visible to NIR regions are obtained [23]. However, few RE metals have been reported so far make it an attractive field. Eu³⁺ is reported to stabilize CsPbI₃ thin films.

Various materials like main group metals, transition metals and rare earth metals have been successfully doped into perovskite nanocrystals, single crystal, and polycrystalline film, giving rise to enhanced properties of perovskite materials. The various properties obtained through doping technology including improved stability, improved quality of thin films with reduced defect and enlarged grain size found application in optoelectronic devices including LEDs and solar cells. It is evident that dopant engineering has emerged as one of the excellent tools to enhance the properties of perovskite. Despite the fact that doping improves the properties our understanding about the mechanism related to doped halide perovskite is still limited. And number of scientific issues in terms of synthesis, doping methods, and structure and property relation are remaining unsolved. To fully exploit the possibilities of doping, there are certain questions that are needed to be answered, such as the role of metal ions in crystallization, their doping capability, the true position of metal ions, and how these differ with altered perovskite compositions. Although 3D halide perovskite has been the prime target of doping, lower dimensional perovskite including 2D halide perovskite could be the next frontier for doping study, since they provide a rich space to study the interaction of electrically-, optically-, or magnetically-active do pants with quantum confinement effects [43].

5.3 Small molecules

Generally, in semiconductor devices, small molecules are used as dopant to enhance the properties of materials. The mechanism behind these doping is surface and interface charge transfer pathway. The small molecules like HATCN (Hexaazatriphenylenehexacarbonitrile), F₄TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) are used as dopant in low concentration in 2D perovskite materials. Also, molecules like cobaltocene and zethrene was used in inorganic perovskite materials and shows strong charge transfer from dopant to perovskite [44]. Even though doping with small molecule provide enhancement in property, formation of composite with small molecule is considered as more effective way to improve the properties of perovskite materials. The development of low cost, largest surface to volume ratio, low resistivity and high sensitivity along with ecofriendly perovskite-graphene based composite attain widespread attention in electrochemistry, sensing and optoelectrical applications. For example, CsPbBr₃ QDs/GO composite, that is cesium perovskite quantum dots and graphene oxide composite were used as photo catalyst for artificial CO₂ reduction. Quantum dot photo catalyst found wide attention, because of their large surface area and charge transfer mechanism. Moreover, the quantum confinement effect causes the shift in band position provides sufficient energy for photochemical reaction [49]. A perovskite and dual additive composite are reported in recent years, the PEO (polyethylene oxide) and TPBi (2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) blended perovskite act as the barrier for the excitonic dissociation at interface, which contribute to the increase in PL intensity. The possible reason for the change in PL is the smaller grain and higher surface coverage provided by the dual additive perovskite material, which can be possibly used as LEDs with high luminance and current efficiency [50].

5.4 Polymer

Perovskite-polymer composite received great attention in recent years due to their combination of properties from polymers and perovskite. The technique involves the formation of perovskite and polymer matrix in one pot reaction, to avoid the complexity of separate preparation. Compared with other technique like coating and ligand cross linking, forming composite with polymers is relatively easy to handle. In addition to the stability, polymers provide other advantages like convenient device fabrication mechanical performance, and enhanced luminescent properties. Recently poly (methyl methacrylate) (PMMA) and perovskite composite based solar cell provide a power conversion efficiency of 22.1%. Along with this, a lot of composites with excellent stability is reported. Even though excellent applications are reported, some serious issues are also found with polymers blending with perovskite. The blending of perovskite with polymers may result into serious aggregation. The large polarity difference between polymers and perovskite material results the aggregation of perovskite material, which reduce the efficiency of perovskite material. Another issue is the stability, the preparation and storage of perovskite material required high attention, to optimize the property of composite material. The time-consuming blending process, especially with high molecular weight polymer, result the decomposition of perovskite by moisture. The organic solvent used for dissolve polymers should be anhydrous to avoid the aggregation, and these solvents are not environment friendly and of high cost. Also needed to consider the fact that physical properties of polymers are different, hence it is necessary to know the characters of polymers before using as a composite with perovskite material. Commercially available polymers like PMMA, polystyrene (PS), polyethylenimine (PEI), polyvinylpyrrolidone (PVP), and poly (butyl methacrylate) (PBMA) are used with CSPbBr₃ perovskite material to form composite. The one pot synthesis of perovskite-polymer composite is illustrated in Figure 4 [51]. The PMMA and perovskite material have difference in polarity make them less efficient, hence butyl methylacrylate with long alkyl chain of similar polarities was selected to prepare CsPbBr₃-PBMA shows deep green color than those of PMMA, indicating more stability. The organic halide perovskite-polymer composite of MAPbBr₃-PMMA could also synthesize via injection of precursor solution into the bulk MMA. The PL intensity of both CsPbBr₃-PMMA, CsPbBr₃-PBMA could remain high as 70% and 78% for month indicating stability. A white LED device was prepared based on the green emissive composite with phosphor of red emission. Also, the diverse selection of monomer provides controlled mechanical properties and flexibility to the composite also enables the preparation of device in large area. Recently, CsPbBr₃-PMMA was successfully prepared without using organic solvent provides a new direction to the perovskite-polymer composite synthesis and will broaden the use of polymer in perovskite science [51].

Figure 4.

Illustration of one-pot strategy to prepare perovskite-polymer composites (CsPbBr₃-polymer or CH₃NH₃PbBr₃-polymer). a) Formation of perovskite crystals in bulk monomers. The photo taken under room light is for the emissive bulk styrene after adding precursors. b) the UV- or thermal- polymerized perovskite-polymer composites. Representative disks (under room and UV light) are shown in photos reprinted with permission from [51]. Copyright 2018 American Chemical Society.

6. Conclusion

Metal halide hybrid perovskites are one of the most researched materials in the recent years owing to the fascinating properties they exhibit. These materials, therefore, find application in various fields. Inorganic, hybrid, organic–inorganic, 2D/3D mixed dimensional are all different types of halide perovskites that find applications in optoelectronics, photovoltaics, catalysis, etc. today. These variations come from the changes in the compositional elements in the basic structure family. The different phases exhibit peculiar properties that make them multifunctional. The crystal structure variation itself leads to changes in the properties. The changes in the electronic, optical, and electrical properties of the different types of metal halides are explored in the chapter. Ferroelectric property is mainly shown by the organic−inorganic counterparts owing to the polar nature of the organic cation. The synthesis techniques followed for the nanostructure, thin film and single crystals are varied and important as the growth of the materials and enhancement of its properties can be achieved by making changes in the synthesis procedures. The property enhancement of these materials is easily achievable by the addition of dopants or by fabricating polymer composites. These methods also help in enhancing the stability along with the properties making the extant of application of perovskite materials much wider.