**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 = ( ) + √ + 2 *A X B X r r r r* 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 ´ 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 A2BI BIIIX6. 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 − 1BnX3n + 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].

#### **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. Cs2AgInxBi1−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

#### **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.)*

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

#### *Metal Halide Hybrid Perovskites DOI: http://dx.doi.org/10.5772/intechopen.106410*

BiBr6 3+ 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 selfhelaing 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 [27–29]. 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 applicationbased 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].

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

#### **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.*

(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 BAx(FA0.83Cs0.17)1−xPb(I0.6Br0.4)3 were prepared by the spin-coating of a blend of FA0.83Cs0.17Pb(I0.6Br0.4)3 and BAPb(I0.6Br0.4)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].
