Halide Perovskite Materials, Structural Dimensionality ,and Synthesis

Three-Dimensional and Low-Dimensional
Semiconductors: Organic-Inorganic Perovskites

Structures and designs of semiconductor materials have been made to fit specific needs, with a focus on incorporating three-dimensional (3D) or low-dimensional (LD) systems into semiconductor solids. Quantum confinement effects happen because the electronic and visual features of these nanoparticles are very different from those of ordinary materials. When it comes to semiconductor thin films, an electron or hole is thought of as a classical particle when its effective de Broglie wavelength (𝼆D) is much shorter than the characteristic semiconductor size L, that is, when 𝼆D ≥ L. It’s important to remember that the electron is wavelike, so when 𝼆D and L become similar, the electron is contained.

Molecular beam epitaxy (MBE), metal–organic chemical vapour deposition (MOCVD), and electron beam lithography are some of the technologies that have made it possible to tweak these kinds of designs. However, it is still hard to make well-defined shapes on the nanometre scale because of the unavoidable changes in the monolayer.

Organic–inorganic hybrids are another type of natural or self-assembling quantum well structure. Their parts have specific chemical interactions that help or force more perfect ordering and have clear organic and inorganic properties that can be combined to make new materials with multiple uses. Metal halide perovskites are a very important group of hybrid structures. They naturally make 3D and LD structures made up of corner-sharing networks of [MX6]4− (M, divalent metal; X, halogen) octahedral units split by organic cations.

In the past few years, research into perovskite solar cells (PSCs) has made great strides, which has sparked an unusual interest. Quantum confinement effects in 2D perovskites have revealed the presence of excitons that are very steady and have very high binding energies and oscillator strengths. The idea behind this will make it possible for these materials to be used in optics and other fields.

Perovskite-Type Metal Halide Compounds

The perfect 3D organic–inorganic perovskite has a cubic unit cell with the chemical formula AMX3. The M2+ cation, which is usually a metal, is in the body centre, and the X− anions, which are mostly Cl−, Br−, or I−, are at the face centres, making the [MX6]4− octahedra. The structure stays stable thanks to electrostatic interactions and hydrogen bonds between the A+ cations and halogen X− anions. Using small A+ cations like methylammonium (CH3NH3+) and formamidinium (NH2CH=NH2+), [MX6]4− octahedra can share their corners. This is because the A+ cations fit into the tight and small spaces made by the 12 nearest neighbour X atoms. Caesium can also be added to the empty spaces at the A-site to make the 3D shape more stable.

A rough idea of how 3D perovskite structures form comes from the Goldschmidt tolerance factor, t, which is given in terms of the ionic radii rA, rM, and rX. It has been observed that 3D perovskite structures form when the t number is between 0.8 and 1.0. A lot of research has been done on 3D perovskites with the general formula AMX3 as light harvesters in PSCs because they have great features like high suppression coefficients, middling bandgaps, small exciton binding energies, and long exciton and charge diffusion lengths.

When bigger alkylammonium or alkyldiammonium cations are used as the organic cations, the 3D structures look like they’ve been “peeled off.” This means that the compounds naturally form layered structures with 2D [MX4]2− sheets separated by layers of organic ammonium cations. These stacked structures self-organise through hydrogen bonds between the halogens in the artificial sheet and the ammonium groups in the organic cation, as well as van der Waals interactions between organic tails that are next to each other. These layers have a much wider effective bandgap than the artificial [PbX4]2− layers when M = Pb and longer-chain alkylammonium A cations are present. This difference is at least 3 eV.

When systems use both single-valent and double-valent organic A cations, they make different complex structures. For the single-valent cations, the A2PbX4 structures are made up of solid sheets that are divided by two layers of these cations. When two organic cations come together, they make APbX4 structures, and each one links to two artificial layers next to it. The quantum confinement effect depends on how wide the barrier layer is in these two-dimensional systems.

Like exactly 2D perovskites, you can also make compounds that are similar and can be thought of as quasi-2D perovskites. These have the general formula A2Bm−1MmX3m+1, where A = CH3(CH2)nNH3+, C6H5(CH2)nNH3+, B = CH3NH3+, M = Pb2+, Sn2+, and X = Cl−, Br−, or I− [17, 23, 25–27]. Multilayered CH3NH3MX3 perovskite sheets are spread between organic ammonium layers in these materials, which are thought of as a Ruddlesden–Popper series. The number m represents the number of artificial monolayer sheets. As m goes up (for example, m = 1, 2, and 3), the artificial layer goes from being a single layer to two layers to three layers, and it gets closer to the 3D structure (m = ∞). Besides the Ruddlesden–Popper series, other series can also be thought of and planned.

In conclusion, 2D perovskite structures have special electrical and optical qualities because of quantum confinement effects and better moisture stability because they contain organic cations that don’t like water. They can also be used for light-emitting, PSC light harvesters, and other optoelectronic tasks.

Preparation of Two- to Three-Dimensional Lead
Halide-Based Perovskite Compounds

Organic-inorganic perovskite compounds are usually made as crystals or thin films. Because the 2D and 3D compounds have many structure phase transition points, it is best to make the crystals at a uniform temperature. For making crystals, the isothermal solvent evaporation, poor solvent diffusion, and gel processes are often used. The halogen and organic amine species used affect how easy it is to make crystals. One way to make CH3NH3PbBr3 is to use a saturated N,N-dimethylformamide (DMF) solution and evaporate the solvent. But the CH3NH3PbI3 predecessors CH3NH3I and PbI2 prefer to settle out from a similar DMF system, which makes it hard to quickly control the size and shape. For crystal growth, a solution of hydroiodic acid in water is often used to heat and slowly cool CH3NH3I and PbI2. In 2015, a cool inverse temperature crystallisation method was shown to make high-quality single crystals of both CH3NH3PbBr3 and CH3NH3PbI3. This method used the materials’ inverse temperature or backward solubility behaviour in certain solvents. But the perovskite crystals that are made are usually only a few millimetres across, which makes it hard to get high-quality crystals in the right shape for use. Simple ways can be used to get high-quality thin film samples, and the film form factor is usually good for integrating devices.

Spin-Coating Method for Synthesis

The easiest way to make perovskite thin films is to use the spin-coating method. A mixture of organic ammonium hydrohalide (RNH3X) and lead halide (PbX2) is spin-coated on a base. The molar ratios of the two chemicals are about 1:1 (3D perovskite) or 2:1 (2D perovskite). Depending on the halogen species, the best fluid for spin coating will vary. For lead iodide, acetone works well, while DMF works best for lead bromide and lead chloride. Dimethylsulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), GBL, and 2-methoxyethanol are some other popular solvents or cosolvents for halide perovskites.

A 2D perovskite (C6H13NH3)2PbX4 spin-coated film’s optical absorption spectra show strong and sharp absorption at 330 nm (X = Cl), 395 nm (X = Br), or 512 nm (X = I). These are caused by an exciton in the [PbX4]2− layers, which are sand-like and sit between the two layers of organic cations. The UV–Vis absorption spectrum of (C6H13NH3)2PbX4 spin-coated films goes red because of an exciton peak as the halogen changes from Cl to Br to I. This is because the energy gap between the artificial perovskite layers gets smaller.

Using X-rays to look at a (C6H13NH3)2PbBr4 spin-coated film and microcrystals (made by dropping a pre-prepared perovskite solution into a poor solvent) shows that they have a strong preference for one direction over another. This is because the hybrid compound is two-dimensional, with perovskite layers that are parallel to the substrate surface. The 2D perovskite substance has layers of organic ammonium cations that act as insulators. This means that carriers can only move in a 2D direction, which is the same direction as the [PbX4]2− layers. In solar cells, the best way for carriers to move is usually perpendicular to the ground.

Even though it was hard, Mohite and his colleagues reported a PCE of 12.5% in a reverse-structured quasi-2D perovskite cell that was made by hot-cast spin-coating a heated base. Using the quasi-2D perovskite (C4H9NH3)2(CH3NH3)3Pb4I13 (m = 4 Ruddlesden–Popper phase), a fairly high PCE was reached by making the [PbX4]2− layers lie more perpendicular to the substrate. This shows how important it is to control the crystallographic orientation when making high-performance PSCs from 2D and quasi-2D perovskites.

Getting the right film thickness and encouraging crystallisation from the solution–air interface at the top (rather than the substrate–solution interface) both raise the degree of vertical rotation.

It is very important to use additive engineering to make spin-coated films with the best shape and single-phase structure, like 3D perovskite materials. Solvent engineering is one way to do this. It involves adding a specific cosolvent to the precursor solution, combining it with the metal (like Pb), and controlling the start and growth of the perovskite phase during the heat treatment after the deposit. Using this method, you can make long films of 3D perovskite materials, which can sometimes form well-crystallized but badly joined films because the nucleation isn’t managed well enough.

Adding thiocyanate salts, like Pb(SCN)2 or CH3NH3(SCN), is another way to control grain growth through additive engineering. These salts can greatly increase grain size in films without the need for cooling after the fact. It is thought that the change in grain size is partly caused by a change in the colloid size in the first precursor solution. When adding CH3NH3(SCN), adding more of it makes the colloids bigger, which makes it easier for bigger grains to form in the perovskite film.

Another useful tool in additive engineering is adding a small amount of a big molecule (like aminovaleric acid) to a solution that already has the 3D perovskite in it. It is thought that adding the small large-molecule part to these systems makes the organic material beautify the film grain edges and surfaces, which makes the perovskite films more stable overall.

To sum up, spin coating is an easy, flexible, and not too expensive way to test different solution-processing tactics for producing halide perovskite electronics in a lot of different situations. With this method, a lot of different hard base materials can be used, like silicon, glass, quartz, and sapphire. Other solution-based deposition methods, like doctor blading, slit- or slot-die coating, air blading, spray coating, and printing, are better for large-scale production and a wider range of flexible surfaces. Vault-based methods can also be useful to get around this problem.

Vacuum Evaporation Method

Using vacuum-based layering methods, you can make layers of organic and inorganic perovskite that can come from either a single source or two sources. In single-source deposition, a perovskite compound that has already been synthesised or a suitable mix of organic ammonium and metal halide salts is put into a boat. The boat is then quickly heated under pressure, and the mixture is evaporated or ablated onto a substrate that is placed above the heater. The heating process needs to happen quickly enough for both metal halide and organic salts to emerge at the same time from the source boat. This way, they can both be present on the substrate and react with it.

In dual-source vapour deposition, the precursors RNH3X and PbX2 are put in different boats and sprayed on the substrate at the same time. The deposition rates of each component are controlled separately. A crystal oscillator-type film thickness monitor can be used to find out how thick the thin film that was formed is. Because the two parts evaporate more slowly with dual-source evaporation, it is easier to control the width of the film. To make a smooth film surface, single-source deposition is used. You can get high-quality perovskite films by cleaning the starting salts and carefully controlling the deposition parameters.

While these vapor-phase production methods are useful, they aren’t very flexible because they can only be used when the organic cation species is thermally stable enough to handle the heating process. The organic ammonium halide salt evaporates in a more complicated way than most purely organic or inorganic species because the RNH2 HX salt can split into RNH2 and HX gases instead of disappearing as a single molecular species. Because of this separation, it is harder to control the rates of evaporation and deposition, and skill is needed to get the makeup of the perovskite close to the theoretical value that was wanted.

The resonant infrared matrix-assisted pulsed laser evaporation approach (RIR-MAPLE) is a vacuum-based method that can deal with these problems for organic salt formation. This method gives you more control over the stoichiometry and lowers the chance of hurting the organic cation through heat. By adding a solvent to the vapor-phase coating method, however, there is a chance that the film will still contain organic liquid and leftovers.

Two-Step Deposition Method

For mixed perovskites, a two-step film casting method was created in 1998. For this method, either a solution or a vacuum is used to cast thin films of metal halide. These films are then dipped into a solution that contains the organic cation. This method deals with uncontrolled organic salt deposition while both organic and inorganic components are being deposited in a vacuum at the same time. The van der Waals gap in the organic cation bilayer makes it easier for the reaction and intercalation of the organic cation into the metal halide film to happen quickly in 2D perovskites. The process takes longer for 3D perovskite and can last from minutes to hours. Based on CH3NH3PbI3, this method has been used to make high-performance hybrid solar cells. This process only works if the solvent for the organic cation is carefully chosen. It needs to be a poor solvent for both the metal halide part and the perovskite that is made. Some changes have been made to the method, such as putting the first metal halide films in a vapour solution or covering them with a thick layer of organic salt. High-performance photovoltaic devices have been made, showing that these two-step methods for film formation and solar cell growth work well.

Self-Intercalation Method

A new intercalation method has been created to add large or complicated organic amines, which makes it possible for a 2D perovskite to form. A 2D perovskite can be made by adding lead halide (PbX2) into the stacked structure of a long-chain diammonium salt. A five-minute soaking in a lead halide solution creates a layered structure, as shown by X-ray diffraction (XRD) tests. This method works with big organic ammonium salts, which means that the hybrid perovskite family can be used for more things. Methylamine can also make pictures that are almost flat when n = 2 or 3.

Layer-by-Layer Self-Assembly Method

One way to make 2D perovskite films is to dip them alternately in liquids that contain cationic (organic cation) and anionic (metal halide) structure components. This lets hybrid perovskite films form, and it’s called the self-assembly method [76]. Electrostatic pull between positive (negative) ions in the solution and negative (positive) charge on the substrate caused by ions placed in the previous dip builds up layers on top of each other.
By doing these dips over and over, it’s easy to make thin layered films. In Figure 2.10, [NH3(CH2)12NH3]PbX4 (X = Br, I) perovskite films are used to show how a layered film can be made by soaking the hydrophilic substrate in organic ammonium salt solutions and lead halide solutions back and forth, with a rinse step in between. The films that were made show the unique exciton peaks for the lead bromide and iodide 2D perovskites in their absorption spectra. The power of the absorption features and the film mass, as measured by a quartz crystal oscillator, both go up linearly as the number of dipping cycles goes up [76]. This suggests that the layers are put together roughly one at a time.
Self-assembly is a simple method that doesn’t need complicated or expensive tools for film formation, which is a benefit. Furthermore, this method allows for the creation of multiple designs with different parts in
OH Ahh,
Wash structure of perovskite Wash PbX 2 Wash PbX 2 OH OH
C12N2X2 NH3 NH3X NH3X NH3 OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH
Figure 2.10: A plan for the self-assembly method used to make 2D perovskites. Matsui et al. [76] is the source.
2.4 In the end The baths that contain other metal halides or organic cations are added to make 75 different layers. Lastly, self-assembly can be a good option when trying to work with organic cations or metal halides that are hard to dissolve. This is because the dipping solutions usually only have very small amounts (a few mg/ml) of these two types of molecules.

Langmuir–Blodgett Method

The Langmuir–Blodgett (LB) method is a way to carefully control how molecules are arranged in biological structures. Putting amphiphilic molecules in a trough at the point where air and water meet and then pressing the molecules together to make a single-molecule film is how it works. During the rolling process, this film can be moved to a base. Era and Oka showed how to use the LB method for 2D perovskites by putting a long-chain alkylamine hydrobromide film on top of a watery phase. The layer-by-layer layering method used in the self-assembly method leads to linearly growing absorption with cycle number. The method does have some problems, though. For example, it can only work with a certain number of amphiphilic molecules, and it needs to be used on a bigger scale to make high-quality thin films.

Conclusion

Because they self-organise, halide perovskite substances can be made using a number of different methods. Each method has pros and cons, and the best way to pick one depends on the type of perovskite, the film or crystal that is needed, and the qualities that need to be measured or used. Synthetic methods have made a lot of different fields of science and technology possible by making new materials with different qualities and uses. For instance, a layer-by-layer method can be used to make gradient or heterolayer films while keeping the contact between the layers in good shape. It is also possible to expect advanced organic-inorganic hybrid perovskite materials and films with unique properties derived from custom-designed organic cation components, which could lead to new uses.