The first perovskite, CaTiO3, was found in 1839. It is in the group of materials with the formula ABX3. Most of the first perovskites were oxides that had ferroelectric, piezoelectric, and pyroelectric qualities, but they weren’t good materials for making semiconductors. Metal halide perovskites (MHPs) have high absorption coefficients, direct and tunable bandgaps, high carrier mobility, and production methods that are close to ambient. These properties help the development of MHP thin-film manufacturing for optical devices.
When MHP thin-film was first used, it was in the fields of transistors and light-emitting diodes (LEDs). The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has grown very quickly since 2009. This is a huge step forward in the field of photovoltaics (PVs). In the past ten years, halide perovskite thin films have been used in many areas, including photodetectors, lasers, and spintronics, because they are easy to work with, don’t cost much to make, and have great optical qualities.
The low forming energy of halide perovskites makes it easier to make thin films on a big scale. However, the “soft” quality of these materials makes them less stable in device uses. This chapter gives an in-depth look at MHP processing, covering everything from basic knowledge to synthetic approaches. It starts by explaining the basics of perovskite structure, compositions, and microstructures. It then moves on to discuss current theories of thin-film growth, new fabrication methods, the effects of post-deposition treatments, and the field of halide perovskite thin-film deposition.
Fundamentals of MHPs Thin Films
Crystal Structures and Compositions
We have used the three-dimensional (3D) cubic ABX3 perovskite structure to find different MHP structural derivates, such as the twisted perovskite phase, lead-free perovskites (like double perovskites), and low-dimensional perovskite. A material’s crystal structure and makeup have a direct effect on its electrical bandgap structure. Even small changes in the lattice can cause big changes in the material’s physical features. Three types of perovskites are <100>, <110>, and <111>-oriented, where n, m, and q are different 2D perovskites groups that are orientated in different ways. A lot of different types of MHP libraries are useful in optoelectronics because the crystal structure and make-up of a material have a direct effect on its electric bandgap structure.
3D MHPs
The semiconductor material 3D metal halide perovskite (MHP) is made up of cations that are one-valent (A, B, and X), cations that are two-valent (Pb2+ or Sn2+), and ions that are halide (I−, Br−, or Cl−). A lot of 3D perovskite has a Goldschmidt tolerance factor (t) that is between 0.8 and 1.0, which means that the cation and anion possibilities of MHP work with this factor.
Perovskite is distorted because the RA, RB, and RX ions are not all the same size. This makes an orthorhombic or tetragonal phase, which is caused by the twisting and distortion of octahedral networks and the spins of molecular cations. It’s not as good at predicting unique perovskite structures because it relies on assumptions about the exact ionic sizes of the cations and anions.
Stable perovskite can also be shaped with the help of the octahedral factor (𝼇), and the best value for 𝼇 is between 0.4 and 0.9. With the Goldschmidt tolerance factor (t) and octahedral factor (𝼇), we can see that the sizes of the A, B, and X ions in perovskites have a big effect on their structure and stability. The optical qualities of the material are greatly affected by the size of the body-centered A cation, since its size can make the whole grid grow or shrink.
It is possible to guess if a perovskite will form based on the formation limits shown in the form of a 2D map of ionic radii A and X. The 3D MHPs are affected by the ion radius and makeup. The best structure is chosen by where the ions meet within the creation limits. Changing the A-, B-, and/or X-site ions in perovskites with more than one component can also change the shape of the phase.
Lead-free MHPs
Pb poisoning is a big problem for both people and the world that needs to be fixed in sustainable technology. A lot of study has been done on lead-free products (MHPs) to solve this problem. There are three ways that lead-free MHPs can be made: by replacing cations with different types of cations, by replacing vacancies with higher-valent metal cations, or by replacing vacancies with isoelectronic trivalent cations (Sb3+, Bi3+). These can create four chemical formulas: standard perovskite ABIIIX3, double perovskite A2BI BIIIX6, vacancy-ordered double perovskite A2BVIX6, and layered/dimer perovskite A3BIIIX9.
In the standard MHP structure ABIIIX3, Sn2+ is often used instead of Pb2+. CSnI3, the first perovskite based on Sn, was used as an artificial hole transporter in dye-sensitized solid-state solar cells. In 2014, it was shown that MASnI3 and MASn(I1−xBrx)3 could be used as organic–inorganic lead-free MHPs. They had energy conversion rates of 6.4% and 5.73%, respectively. However, Sn2+ easily changes into Sn4+ in air, which can cause self-doping effects and structure instability.
The double perovskite A2BI BIIIX6 is defined by the B-site cation being occupied by two atoms. The new A2BI BIIIX6 structure is made up of two split B-cations that connect to form a network. It is expected that other double perovskites, like Cs2InSbCl6, Cs2AgInBr6, Rb2AgInBr6, and Rb2CuInCl6, will be very stable and work well as optical materials.
Half of the tetravalent atoms in tetravalent-metal vacancy-ordered double perovskites A2BVIX6 are replaced by vacancies. Compounds with an A2BVIX6 anionic structure are very stable and don’t react with water. Changing Pb2+ to Bi3+ or Sb3+ makes layered or dimer perovskites, which can have a 0D or 2D structure.
2D MHPs
Low-dimensional perovskites are very flexible in how their structure is tuned, unlike most ABX3 perovskites which are very stiff. They are also made up of corner-shared BX6 octahedra, which is the same as their parent 3D structure. Adding a big organic cation A′ to the A-site makes 2D MHPs by releasing the chemical link between the octahedral layers and using van der Waals force to connect the next octahedrons. The shear direction in the 3D MHP crystal is very similar to the formula for the structure of 2D MHPs. Three cuts through the crystal give us three formulas: A′2An−1BX3n+1, A′2AmBX3m+2, and A′2Aq−1BqX3q+3. This ability to change the shape and size of crystals makes it possible to make a lot of different crystal structures with different physical traits.
The most common type of derivates are those that are found along the <100> direction. They give rise to three 2D MHP phases: Ruddlesden–Popper (RP), Dion–Jacobson (DJ), and alternating cations in the interlayer space (ACI). Spacer molecules A′ control the phase structure of the 2D MHPs. They have an organic tail and an amino head with a single or double charge. People have learnt the most about the RP phase of 2D MHP. The DJ phase, on the other hand, is controlled by divalent spacer cations that connect the adjacent organic sheets with hydrogen bonds. This type, ACI, isn’t used very often because it can only use the guanidinium (GA3+) cation.
Layered perovskite structures can also be changed by mixing big (A′) and little (A) cations, which lets changes happen in different ways. By changing the amount of big to small organic cations, a smooth change from n = 1 to n = ∞ perovskites can be made in terms of their physical properties. This is a useful way to improve the optical properties of thin films.
Microstructures
When you use a solution or vapour preparation to make MHP thin films, they are often polycrystalline and have grains and grain borders (GBs). A grain is the smallest part of a microstructure. It is usually a single crystal surrounded by GBs, which are also called interfaces or surfaces. The microstructure of MHP thin films is based on the GB type, the size and spread of the grains, and the crystalline orientations of the grains. These factors have a big impact on the device’s performance, properties, and stability.
Types of the GBs
GBs between grains 1 and 2 have a shape that can be described by tilt and twist misorientation angles, which can be explained by thermodynamic numbers. GBs cause chaos and a lot of empty space, which can be charged in difficult materials with many cations and anion pairs, like MHPs, because they separate ions and impurities. In solid-state GBs, 2D defects are made up of single-crystal grains that are atomically linked and can have any crystalline direction. They have 5 degrees of freedom. GBs are usually made up of grains that are arranged in any way, but sometimes they are sub-boundaries inside single-crystal grains. The crystal phase on either side of the sub-boundary is connected by a certain symmetry operation. You can also find GBs with clear second phases that are amorphous, solid, molecular, or polymer-based.
When GBs are solid, they generally look like a 3D network of linked GBs, like a soap film in foam. The MHP thin film grain size is about the same as the film width, which suggests that the GB network is not completely 3D. It is possible for MHP thin films to have hierarchical microstructures made up of aggregates, which are groups of smaller single-crystal grains and linked GBs. But aggregate bounds are not GBs, so thermodynamic numbers can’t be used to figure them out.
Grain Size and Distribution
Grain size is a popular way to measure the architecture of MHP thin films. Images of the top surface taken with scanning electron microscopy (SEM) or atomic force microscopy (AFM) are usually used to do this. The size of the grain is determined by where each grain meets the surface. These points are not chosen, but happen naturally. After processing, however, MHP thin film surfaces can get lines on the grain surfaces that aren’t there in perovskite films that were formed during processing. There may also be wrinkles on the grain surface that can be mistaken for grain lines and cause the grain size to be overestimated. The range of grain sizes is also very important, but it isn’t talked about much in MHP texts because the mean grain size is usually used. A narrow distribution, which means abnormal or too much growth, is a sign of normal grain growth.
Crystallographic Orientations
The structural orientations of microstructures are often very random. A pattern, on the other hand, is decided by the desired orientation between grains that are close to each other, between groups of grains, or between all grains. It’s still not clear how devices made from MHP thin films work or what their properties are, but the effect of crystalline alignment is stronger for stacked MHP thin films. Because the crystal structure isn’t uniform, moving electrons is easier along corner-sharing metal halide octahedra planes but harder across these planes. To meet the needs of different applications, like in-plane charge transport for parallel transistors and out-of-plane charge transport for perpendicularly structured solar cells and LEDs, it is important to change the orientations of stacked perovskite films in relation to the substrate.
Thin Film Growth Mechanism
Crystal Nucleation Mechanism
To make good MHP films, the crystallisation process is very important. It can be broken down into two parts: the nucleation process and the growth process. Different growth factors can change these processes, which leads to different shapes. To sum up nucleation and growth theory, basic ideas must be summed up, and these ideas are looked at individually to keep things simple. Also, intermediates that have a big effect on how well high-quality films are made will be shown.
Nucleation Theory
Metal halide perovskite thin films (MHP films) grow and change qualities in very important ways that depend on nucleation. According to classical nucleation theory, when the precursor gets too full, nucleating phase molecules are often formed. These molecules may then condense into small single crystal nuclei. Two common methods are heterogeneous nucleation and homogeneous nucleation. Heterogeneous nucleation happens where the substrate solution meets the precursor solution and/or inside the precursor solution.
When the content (C) of a perovskite solution is higher than its solubility (Cs), nuclei are made. This is called homogeneous nucleation. These nuclei made by changes in temperature are dissolved again if supersaturation is not high enough for nucleation to happen. It is a thermodynamic process that is caused by the change in the total Gibbs free energy (ΔG).
The highest value of ΔG (ΔGHom) is seen at the critical nucleus radius (r) in a homogeneous nucleation process. Those nuclei that are smaller than r* dissolve back into the precursor solution, but those that are bigger than r* stay stable and keep growing. r* = 1/ln(S) when the temperature stays the same. As nuclei grow on foreign surfaces, the nucleation process is uneven, and the energy barrier ΔG*Het is dropped a lot because the contact energy is lowered as well.
Both types of nucleation events are common in the formation of MHP thin films. Heterogeneous nucleation is more common at low supersaturation (S), while uniform nucleation is more common at high S. The rate of nucleation is fastest at a temperature in the middle. At this temperature, the first exponential term goes up as the temperature goes up, while the second exponential term goes down. This means that the general rate of nucleation as a function of temperature is at its highest point.
Even though there is a strong mechanical force pushing for nucleation at low temperatures, diffusion slows it down. Nucleation is slowed down by high temperatures, but diffusion speeds up. It is important to know how nucleation works in order to control the quality of MHP films and figure out how nucleating phase molecules act in the end result.
Influences on Nucleation
It’s not as simple as basic nucleation theory to explain how solid MHPs form from a precursor fluid. Perovskite has a lot of ions, and polar aprotic fluids are very complicated. These make the precursor solution interact with itself when it’s fully saturated, which causes nucleation behaviour. When precursors are fully used up, they make intermediate entities that can be either amorphous or rigid solid stages. The actions of these intermediates rely on the system and the conditions, so more study is needed.
The colloidal structure and presence of a solvated phase play a big role in the formation of perovskite, controlling nucleation, slowing crystallisation, and changing the flaw density. The Gutmann donor number (DN) of a liquid can show how well it can coordinate. DN becomes important when it goes above 18.0 kcal mol−1. Before the perovskite phase forms, there are phases that are dissolved in the fluid. A study looked at the crystal shapes of PbI2-solvent and MAI-PbI2-solvent in common solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The solvated phases have PbI6 octahedral chains that share edges in a flat plane. Solvent molecules separate them through Pb–O interactions. This crystal grows quickly in one direction, making a 3D structure of fibres.
Crystal Growth Mechanism
Basic Growth Theory
There are two steps in the process of crystal growth: molecules move from the solution to the crystal’s surface, and molecules then deposit on the crystal, which causes it to grow even more. There are three main steps in the process of perovskites nucleation and uniform formation. The solution gathers small molecules in the first step of uniform nucleation. In the second stage, the temperature rise and loss of the liquid can make an environment that is too saturated for nucleation to start. During saturation, the concentration (C) rises in a straight line with time until it reaches a maximum called “supersaturation,” which is higher than the minimum concentration needed for nucleation (Css).
In the third step, crystals grow and get ready to be used. As more nuclei are formed, fewer “monomers” are available, so concentration drops below Css. This stops any more nucleation from happening. When the rate of solute consumption is higher than the rate of solvent evaporation, the concentration of the solution drops below the critical radius value. This means that the growth stage has moved on to the next stage, which is nucleation.
Nucleation or growth control could be used to change the growth of MHP films. It is potentially possible for mixed nucleation to happen at the condition of C ≥ Css. It is possible for secondary heterogeneous nucleation to happen on colloidal particles during thin film nucleation. These nuclei can settle on the substrate and be taken into the thin film during future growth. As long as the concentration of the solution is higher than the solubility limit, stable nuclei will grow, which lowers the total free energy.
Island growth (Volmer-Weber model), layer growth (Frank-van der Merwe model), and layer-island growth (Stranski-Krastanov model) are the three main traditional ways that nuclei grow. During island development, monomers connect to nuclei and make them grow both vertically and horizontally. If there are enough nutrients in the precursor film, the islands will finally join together to make a thick, polycrystalline thin film. In epitaxial systems, where the link between the monomer and the substrate is stronger, layer growth happens more often. It’s likely that the island growth method can be used for perovskite thin films since it doesn’t usually need or involve epitaxy.
Grain-coarsening Theory
Grain growth in solids and matrix-phase-mediated Classical grain-coarsening methods like Ostwald ripening are two important ones that are relevant to MHP thin films. When solid-state grains grow, it is very important to think about the structure of each grain. It is a type of coarsening, and Ostwald ripening is specifically coarsening that happens when there is a constant second phase. The thermal push for both processes comes from the fact that small and big grains are not curved the same way.
Grain stability is reached when it has exactly six sides. Grains with fewer than six sides shrink because their GBs are curved, which hurts close grains with more than six sides. The curved curve becomes more noticeable as a result, and more species move to the bigger grain. In the end, the smaller grain goes away, which makes the bigger grain grow.
When the average grain size hits the film thickness, coarseness in thin films happens slowly and usually stops. This means that all GBs cross the top surface and bottom touch with the substrate. It is thought that this coarsening standstill is caused by isotropic GB energy, which comes from the pull forces that the surface and contact put on the moving GBs.
In anisotropic thin films, secondary coarsening happens later on. This is when a few grains that are orientated in a good way quickly turn into large-grained, textured thin films. The low-energy crystalline planes of these films form the surface of the thin film and the contact with the substrate.
One-step Growth
Growth From Solutions
The easiest way to make perovskite films is with the one-step solution process. All of the initial parts are mixed together to make a single solution, which is then put on surfaces. As the liquids disappear, the solution gets full, and perovskite starts to form nuclei on the surfaces. These nuclei grow until they form a film. Other extraction methods, like heat processing, can get rid of the leftover liquid and make the film more crystallised. This part talks about well-known one-step solution methods used in the lab and the main things that affect solution-based synthesis.
Spin-coating
For making metal halide perovskite thin films in labs, spin-coating is the best way to do it. In this method, the base is quickly rotated to spread the solution evenly, which can change the properties of the film. However, films made with this method often don’t cover the whole area because they crystallise quickly because of the strong ionic reactions between PbI2 and ammonium iodide. This also affects how well the device works. Spin-coating rarely meets mass production standards for things like perovskite makeup and phase purity, full substrate covering, pinhole removal, interface optimisation, controlled grain size, and customised GBs because it’s hard to keep an eye on the quality of the film.
To improve the quality of spin-coated MHP thin films, scientists looked into a number of factors that affect the crystallisation process. These included solvents, antisolvents, chemicals, and the temperature and solvent vapour pressure of the working environment. Solvents like DMF and DMSO are often used to create perovskite films. Since DMF dissolves faster than DMSO, the film it makes is much thicker. When the antisolvent is dropped from the top, the DMF at the top can be quickly removed, making the surface compact right away. For now, there may still be some DMF at the bottom of the perovskite film. After the next step of heat processing, holes would appear at the bottom of the perovskite film because the remaining DMF would evaporate. It’s possible for DMSO to make stronger links with perovskite intermediates. The much slower creation of the packed surface makes it possible to remove all of the DMSO from the film without creating any holes.
To fix problems with the crystallisation process, some solvent extraction methods were used to quickly get rid of the solvent from the precursor solution. This made the crystallisation process better. Seok et al. were the first to describe the antisolvent washing method. They made a thick, solid MAI-PbI2-DMSO intermediate-phase film by washing the toluene antisolvent onto the liquid precursor film. This method got around the problem of limited solubility and cut down on the need for precise antisolvent dripping time.
In the crystallisation processes of perovskite films, temperature also plays a big part. Mohite et al. described a “hot-casting” method in which the substrate is heated to 180 °C and then coated with a 1:1 PbI2: MACl solution in DMF. The solution is kept at 70 °C and the substrate is then heated to 100 °C. Through this process, a unique shape is made with leaf-like structures and big grains that cover the ground.
Drop-casting
Drop-casting is a simple way to work with solutions to make perovskites. It involves putting a precursor solution on a base and letting the solvent evaporate on its own or with some mild heating. How thick the film is and what it’s made of depend on the content and volume of the dispersion solution, how well the droplets stick to the substrate, and how fast the solvent evaporates. However, compared to the spin-coating method, this one can’t usually make thin plates that are well-formed.
This cast-coating method has been used successfully on devices with an all-mesoporous structure, which includes the mesoporous electron transport layer (ETL), the porous graphitic carbon electrode, and the layer that divides the ETL from the electrode and is not conductive. Han and his colleagues reported the first drop-casting method for making MHP film-based solar cells that work well. The solar cell made of (5-AVA)x(MA)1−xPbI3-TiO2 works better than MAPbI3-TiO2 because the exciton lifetime is longer and the quantum yield for photoinduced charge separation is higher.
This cast-coating method has recently been shown to work well for making MAPbI3 PSCs under 88% humidity, with a PCE of 18.17 %. When made at different temperatures, the MAPbI3 crystals have different orientations. The MAPbI3 PSCs can be made below 90 °C so that pinholes don’t form.
Growth from Vapor Phase
Dual-/single-source vapour deposition (SSVD) and pulsed laser deposition are two perovskite thin-film deposition methods that use vapor-based methods. You can use either dual-source co-evaporation or single-source evaporation. Both of them only need one step. Single-source evaporation is less flexible when it comes to controlling deposition than dual-source co-evaporation.
Thermal Evaporation
In 2013, the dual-source co-evaporation method for making perovskite films was unveiled. In this process, two precursors are extracted from two different sources and then mixed with the substrate. The rate of evaporation changes with temperature, and the perovskite film’s makeup is based on the ratio of two evaporation rates and precursor materials. The time it takes for the water to evaporate determines how thick the film is. After evaporation, the film is taken out of the room and heated to help two precursor components bond. This makes the final perovskite film.
Dual-source co-evaporation is a powerful method for depositing halide perovskite thin films, which makes it possible to make solar cells, LEDs, lasers, and photodetectors. It can be used from a number of different sources to produce more complicated perovskite compositions, such as bandgap-tunable compositions for tandem devices.
One of the first uses of single-source vapor-phase deposition is to make stacked perovskite patterns on a tantalum sheet. This process is called single-source thermal ablation (SSTA). Rapid heating is important for SSTA to work because precursors can escape and rejoin without breaking down too much. SSVD is a sped-up version of co-evaporation that happens in minutes instead of seconds. It works by adding a powder of already-made perovskite or a mix of its building blocks to a thermal crucible that has a hot coil on top of it.
Pulsed Laser Deposition
Pulsed laser deposition, or PLD, is a way to carefully control how fast organic parts are formed. In this method, a UV laser is used to heat up the precursors and turn them into vapour. However, this makes things more complicated because the energetic particles may fly off the top of the material. This problem can be fixed by mounting surfaces either on-axis or off-axis. MAPbI3 films with pure phases can be made with an MAI:PbI2 stoichiometry of 4:1 because high-energy particles are less likely to hit the film when it is set up off-axis. The shape of these pictures shows that they are made up of pillars.
For the resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) method, the target and laser interact in a more complex way. The liquid takes in most of the heat and disappears to move the precursors to the substrate. This gives you more control over the make-up of the film. This method makes films that look like metal halide perovskite thin films, with grains that look small but are actually very close together.
Two-step Growth
For perovskite solar cells to work well, the MHP thin-film material must be of good quality. The one-step spin-coating method works, but it’s hard to control the rate of crystallisation, which causes the covering to be uneven. Controlling the stoichiometric mix of organic salts and metal components is also hard with the dual-source vapour deposition method. The two-step method makes it easier to make the picture the same way every time. Using methods like absorption, spin-coating, and vapor-assisted methods, an artificial part (usually a metal precursor) is deposited on the base. The substrate is then put in contact with organic salts that are vapour, liquid, or solid. This starts the process that makes the MHPs.
Growth from Solutions
Immersion Method
In 1998, Mitzi and his coworkers were the first to describe a two-step process for making MHP pictures. To do the process, vacuum evaporation or spin-coating is used to put metal iodides on a substrate. The substrate is then submerged in organic ammonium iodide. This method was used by Grätzel and his colleagues in 2003 to make MHP solar cells with a 15% PCE. The base is covered with the metal bromide, and then it is immersed in organic ammonium salts. To keep it from breaking down, the organic ammonium salt is generally mixed with isopropanol. After the MHP layer has been immersed and dried, isopropanol is used to wash the surface. The finished MHP film quality is based on the shape of the metal halide film, the strength of the organic ammonium salt solution, and the reaction time in the second step.
Tailoring metal halide film morphology
When PbI2 is put on a mesoporous material, MAI can get through the PbI2 film and the reaction can happen fully. When placed on a flat surface, however, only the top part can connect with MAI, making it hard for the bottom part to do so. This problem can be solved by changing the shape of the PbI2 so that it is porous. Wu et al. switched out DMF for DMSO, which has a higher boiling point and a slower rate of loss. This made a solid PbI2 film. Liu et al. created a way to age PbI2 films by putting them in a closed Petri dish right after they were formed. This makes the films porous because of the small amount of DMF present. This amorphous or porous PbI2 film can let MAI move through it, which leads to a full reaction and a pure MHP film.
The concentration of the organic ammonium salt solution
Bi et al. looked into how metal bromide changes into perovskite in different amounts of organic ammonium salt solution. There are two parts to the process: the reaction between the solid and liquid at the contact and the growth from breakdown to recrystallisation. The isopropanol-organic ammonium salt mixes with the metal halide layer and makes MHP. This is the result of a solid-liquid reaction. The metal halide moves to the structured PbI2 layer.
When there are small amounts of organic ammonium salt, the solid-liquid reaction starts the change from MAI to MHP. MAI moves to the organised PbI2 layer and directly makes MHP. Equation (7.7) shows the equation for the change. When the amount of organic ammonium salt is higher (>10 mg ml−1), MAPbI3 forms right away on top of PbI2, stopping MAI from spreading further and leaving the MHP conversion unfinished. Under high MAI concentration, unreacted PbI2 tends to produce the lead-iodine complex PbI4-2−. This breaks down the initially formed MAPbI3 and reacts with the unreacted PbI2. PbI4-2− complexes will react with CH3NH3+ ions to make MAPbI3 when this reaction is full.
Reaction time in the second step
In the second step, Wei and his colleagues found a link between the form of MHP and the time spent immersed. MAPbI3 crystals were made when the PbI2 film mixed with the MAI solution. As the soaking time went up, flaws appeared on the film surface, and then MHPs began to dissolve and fall off.
Spin-coating Method
Using the spin-costing method, metal iodides are first put on the substrate, and then an organic ammonium salt solution is spun over them. There are different types of solids and liquids in this process. Solid metal iodides and organic ammonium salt react directly with each other. When compared to the dipping method, this one causes crystallisation to happen faster, which makes the films more regular, small, and smooth. Heat-stable devices with a high PCE of 25.6% have been made with this method. Panzer and his colleagues found five steps that happen one after the other to make a film. One of these steps is the formation of MAPbI3 capping, which stops MAPbI3 from crystallising more as the MAI solution evaporates over a few seconds. The concentration of iodine rises as the solvent disappears, which breaks down the MAPbI3 covering layer. A lot of MAPbI3 is made when the crystals break down and then come back together again. It is stable once all the PbI2 is moved into it. For solar cells, this method works better and more efficiently.
Electro/Chemical Bath Deposition
Yang and his colleagues created an easy, flexible, and scalable electrodeposition method for making a lot of MHP devices. There are three steps in the process: the PbO2 layer is electrodedeposited at room temperature, the PbO2 layer is changed into a PbI2 layer using an HI ethanol solution, and the PbO2 layer is changed into a MAPbI3 layer using an IPA solution of MAI. With this method, a carbon electrode-based solar cell device can be made that has a PCE greater than 10%. Other experts have made this method better by changing the PbO2 layer straight to MAPbI3 by immersing it in an MAI isopropanol solution. This makes the MAPbI3 film less dense than spin-coating.
Growth From Vapor Phase
Vapor-assisted Solution Processing
This method has two steps. The first step changes the metal halide film into metal halide pyrophosphate (MHPs). Spin-coated PbI2 films were exposed to MAI vapour by putting MAI powder around them in a Petri dish with a lid and heating it to 150°C. This vapor-assisted solution process (VASP) is a common solid-vapor reaction that lets MHP grains grow from different nucleation sites while keeping the smooth shape of the original PbI2 film. The PbI2 and MHP stages cohabit in the picture as the response time goes up, and all of the PbI2 is turned into MHP during the whole stage. The MHP film’s cross-section shows how PbI2 and MAI moved around due to heavy diffusion as the film formed.
The VASP method, on the other hand, takes four hours, which limits how often it can be used. In the second step, Li et al. used a low-pressure MAI vapour at the same time to make the halide MHPs they wanted. This cut the reaction time from four hours to two hours compared to the usual VASP method.
Sequential Vapor Deposition
Spin-coating is usually used to make high-efficiency photovoltaic solar cells (PSCs), but this method is only good for lab-scale products. Solvents used in solution processes, on the other hand, can harm functional layers below and harm the environment. The thickness of the film is controlled by a vacuum evaporation method that deposits precursors evenly on surfaces. This makes manufacturing scalable. For layer-by-layer sequential vapour deposition, the lead halide and organic ammonium salt are evaporated separately in different pressure tanks to keep them from mixing. Lin et al. reported the first successive vapour deposition work. They used heat to melt PbCl2 films onto PEDOT:PSS-coated indium tin oxide (ITO) glass, and then they evaporated CH3NH3I. As the CH3NH3I disappears, the film’s colour changes from clear to dark, which shows that an MHP layer is forming. Recenty, Yi and his colleagues reported a sequential vacuum deposition method based on Cl-alloys for high-efficiency PSCs with 24.42% efficiency, which is the best efficiency of MHP solar cells that release heat. The study suggests that chlorine reacts with lead halide to align the MHP face-on with the chemical element. A Cl embedded in the MHP helps the solid-state diffusion of organic ammonium halide salt and phase transition from the ε phase to the α phase, which makes the MHP nanocrystals more crystallin and reduces the number of defects.
Scalable Growth Methods
A cell with a small size (0.04 to 0.2 cm2) and spin coating methods (0.01 cm2) have the best PCEs for PSCs. The film quality and consistency get worse as the device area gets bigger, which causes PCE to drop dramatically. For real business uses, it is important that large-area cells (>1 cm2) have PCEs that are similar to small-area cells. Researchers have been trying to make large-scale MHP manufacturing methods better. Recent progress in making processes compatible with large-area deposition has shown that PCE can be reasonable for industrial products.
Blade Coating
Blade coating, which is also called knife coating or bar coating, is a large-scale manufacturing method that is cheap, can be used on hard or flexible materials, and is safe for the environment. As a general rule, the precursor is dropped on the substrate and in front of the blade. The substrate is then swept by the blade, and the thin film is coated on the substrate. The meniscus that forms between the blade and substrate affects the width of the film. The meniscus is affected by the blade’s speed, the viscosity of the precursor, the blade’s shape, and how easily the substrate can absorb water.
Huang et al. developed blade covering for PSC manufacturing in 2015, and their device worked 15.0% of the time. Several studies have looked into different ways to improve the quality of films. It has been shown that preheating the substrate works because the crystallisation time is too long when the substrate temperature is much lower than the boiling point of the precursor liquid. This makes the film rough and broken up. Huang et al. used a high temperature coating to study how temperature affects the shape of a film and how nucleation and grain growth happen. They discovered that solvates with different compositions and bad PCEs form at low to middle temperatures. However, when the temperature goes above 100 °C, the solvated compositions can’t be seen, which causes the material to directly crystallise. So, they made an MHP film that was continuous and had big crystal pieces.
Another good way to make a uniform large-area MHP thin film is to use gas cooling. After the blade, a fixed-distance N2-gas knife is added to apply a steady flow of N2 and speed up the drying process of the MHP layer at room temperature. Huang et al. demonstrated that using this method and customising the liquid part could speed up the drying process at room temperature. This led to the creation of PSC modules with an average PCE of 16.4% and an effective area of 63.7 cm2.
Yang et al. looked into how the shape of a film is related to the speed of wind. When the airflow was raised from 150 to 225 l min−1, the film had fewer pin holes and smaller dendrite grains. The MHP’s shape was the most compact and free of pinholes when the gas speed was up to 300 l min1. This was because the solvent evaporated faster, which led to higher supersaturation levels and burst nucleation. The PCEs of the devices that went with them were up to 11.7% (1.0 cm2) and 17.71% (0.1 cm2).
Slot-die Coating
Because it can keep adding ink, slot-die covering is used for rolling-to-roll processes. It has two metal blades that cut holes in the bottom of the substrate, and a solution pumping system is linked to a container built into the slot-die head. The solution constantly sends ink to the head lips and base, creating upstream and downstream meniscuses as it does so. The rate at which the solution is pumped also affects how thick the film is.
There are two kinds of slot-die coating: one-step coating and two-step coating. In the first one, the MHP precursor is put directly on the substrate, and the MHP layer is made after it is heated. Usually, a slot-die coating is used to put down a PbI2 layer first, and then an organic ammonium salt is applied or the base is soaked in an organic ammonium salt solution.
A lot of research has been done on making high-performance MHP solar cells using slot-die technology. But more care is taken to control cooling and crystallisation, since that’s what makes the MHP thin film good or bad. Researchers have looked into things like changing the solvent, raising the temperature, cooling the fluid or gas, and vacuum-assisted crystallisation.
Deng et al. used detergents to change how fluids dried, which made it easier for the MHP precursor to stick to the non-wetting charge transport layer below. Zhu and his colleagues made the precursor processing window up to eight minutes longer. This made it possible to make high-quality MHP films that were very regular.
Spray Coating
A common way to make Multi-Height Polycrystalline (MHP) solar cells is by spray painting them. The spray head goes up to 5 m min−1, which makes this method faster than others for making things on a big scale. It has several benefits over traditional antisolvent dropping methods, such as using less antisolvent, spreading it out quickly, speeding up supersaturation and nucleation, and making the mixture more uniform. However, the MHP film quality is bad in all single-step spray-coating tests. The crystallisation is not regular, and the shape is mostly branching. This makes the fill factors, open-circuit voltages, and short-circuit currents lower than with spin-coated PSCs. This could be because the shunt resistance is smaller. Also, this method leaves flaws on the surface, and overspray makes it hard to control the thickness of the film.
Spray-coating can be broken down into four steps: the precursor is cut into a mist of micrometer-sized droplets as the spray head moves; the N2 gas jet guides the droplets to the substrate, and when they reach the surface, they form a smooth, continuous wet film. As the solvent disappears, the wet film dries on the base.
The first study of spray-coated MHP solar cells was released by Lidzey et al. in 2014, and it had a PCE of 11%. So far, the best PCE for spray-coated MHP solar cells is 19.4% for small surfaces, 16.3% for 15.4 mm2, and 12.7% for 1.08 cm2. The devices were made using an antisolvent exposure method and vacuum-assisted solution processing. This shows that spray-coating can be used to quickly make MHP solar cells that are both high-efficiency and low-cost.
Meniscus-assisted Solution Printing
One way to print that blends parts of blade coating and slot-die coating is called meniscus-assisted solution printing (MASP). By limiting the flow of perovskite ink between a lower base and an upper plate through capillary action, a meniscus is made. As the solvent quickly evaporates at the meniscus’s edge, it moves perovskite solutes towards the contact line to make up for the solvent that is lost through evaporation. How the die/print head is shaped and how it moves in relation to the material influence the shape of the meniscus.
As the solvent disappears, a convective flow of solution forms at the meniscus. This flow carries the solute to the line where the substrate, solution, and air meet, where it crystallises. Putting the bottom substrate on a controlled translation stage lets the retreating meniscus be moved slowly and steadily across the whole substrate, which makes it possible to make a single MHP film.
The MASP method was used by Lin et al. to make suitable FA0.85 MA0.15PbI2.55Br0.45 solar cells that have a PCE of almost 20%. If the substrate moves quickly enough, it should be close to the speed at which the meniscus edge recedes due to fluid loss. This keeps the MHP’s shape regular. When the speed of the covering is faster than the speed of the meniscus fading, the edge of the meniscus quickly hits the edge of the upper plate. This lets the perovskite crystals connect the upper plate to the lower base.
Inkjet Printing
Inkjet printing is a way to print without touching the paper. A solution is ejected from a tip, which can be moved to make exact patterns. Continuous inkjet printing (CIP) and drop-on-demand inkjet printing (DOD) are the two most popular ways to make ink drops. In CIP, fluids are constantly pushed through a tube and split because of surface tension. Droplets are constantly moved to land on a target medium because of Rayleigh instability. A fluid is pushed out of the opening and breaks up into small drops that fall on the ground. As the droplets fall through the opening, they each get a small charge. A plate that bends bends the droplets in the right direction as they move between the charged droplets.
Drop-on-demand inkjet printing, or DOD, is the most common type. In DOD systems, a computer software or substrate motion moves the print head to the right place on the desired surface. As the printer tip sends out pressure bursts, a chain of events builds up and liquid is ejected. The pressure pulse is caused by a quick decrease in the volume of the chamber, which lets the ink leak out of the tip.
When Yang et al. made MHP layers with a PCE of 11.6% in 2014, they used inkjet printing. In 2018, Gheno et al. showed a fully inkjet-printed PSC (without wires) that worked 10.7% of the time. Inkjet-printed PSCs have broken the record for efficiency by going over 21%. A mix of high-boiling-point solvent GBL and polar-aprotic liquids DMF and DMSO was used in the manufacturing process. This led to better even cooling and better wetting during the ink application step.
Postdeposition Treatments
Post-deposition treatment improves the shape of the film and the crystallisation process. Two important ways that are being studied are heating and organic gas dosing.
Annealing
Solvent Annealing
Solvent annealing is a very good way to control MHP flaw states and help MHP films grow grains and crystals in the right place. During the melting process, this method exposes the MHP film to a solvent atmosphere made up of DMF, DMSO, or a mix of the two. It is clear that the MAPbI3 MHP films made by liquid annealing have bigger grains and better crystallinity. For solvent annealing to work properly on optimised films for optoelectronic devices, specific solvents must be used because microstructural traits are likely to trade off. Single-component solvents like DMF, DMSO, and GBL are used. So are alcohol-based solvents like methanol, ethanol, and isopropanol, as well as mixes of these. Solvent annealing of PSCs and related optoelectronic devices needs to be done by finding the best solvents for the job. This is because the finished films have different microstructural properties. We also have SEM pictures of MAPbI3 films that were heated and annealed and films that were heated and annealed with a solvent.
Vacuum-assisted Annealing
During MHP layer annealing, vacuum can be used as an extra step to crystallise by changing the chemical change of the precursor and the crystallisation process. This method can also get rid of extra parts that are hard to get rid of with heat processing. It was shown by Song et al. that this method can be used to make a CsPbI3 film from a mixture of PbI2, CsI, and DMAI at a 1:1:1 molar ratio in DMF. If the released DMAI isn’t taken off before crystallisation, it could make holes and cracks in the MHP film. The vacuum speeds up the melting of DMAI, which speeds up the formation process of CsPbI3. This makes the PCE go from 17.26% to 20.06%.
Organic-gas Dosing
Metal halide perovskite thin films (MHPs) often have flaws like holes and empty spaces in them. A better way to make big, high-quality MHP pictures is to use post-processing morphology restoration. Organic gases can pass through MHP thin films because they are soft. In 2015, Zhou et al. said that MA gas can change the phase and shape of MAPbI3, which can be undone when the gas is taken away.
It is possible for perovskite crystals to melt and turn into a liquid when MA gas is added. The perovskite crystal starts to re-crystallize when the MA gas is taken away, which shows that MAPbI3MA has been changed back into MAPbI3. For thin plates, this process only takes a few seconds. If you use the phenomenon on a rough, porous MAPbI3 MHP film, it can turn it into a fully thick, smooth film.
The same changes happened to MHP layers when formamidine (FA) gas was added. Zhou et al. said that FA gas took the place of MA in MAPbI3 and changed it into FAPbI3. The cation-displacement reaction happens when MAPbI3 films are introduced to FA gas at a high temperature (150 C). This changes the MAPbI3 perovskite phase in the thin film to the FAPbI3 perovskite phase while keeping the good shapes and microstructures of the original thin film. This quick reaction doesn’t need a fluid and can’t be undone easily, showing that this method for changing the shape of perovskites works.
Summary
PSCs have had a big effect on studies into solar cells and optoelectronics. To find out what the boundaries of MHP materials are, different ways of making them and their chemical make-up have been studied. This chapter gives an overview of the basics of MHP thin films and the most common ways to make them, from the lab to the marketplace. Photovoltaics and other optoelectronics can hit their theoretical efficiency limits with the help of new mechanical guidelines for creating MHP materials that are more stable and have better carrier qualities. But it takes more work to look into high-quality, large-scale MHP thin film production processes. Some of the problems that need to be solved are accurate control of precursor components, scalable preparation control, and strange device occurrences.
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