Halide perovskite (HP) thin films are often polycrystalline and are used in devices like perovskite solar cells (PSCs) [1–3], light-emitting diodes (LEDs) [4], and detectors [5], which are talked about in Chapters 1, 5, 8, 10, 12–15. So, HP thin films have a substructure made up of grains and the edges (GBs) of the grains where they meet. (The word “microstructure” is used here in a broad sense, even though the grains may be very small.) For devices, these thin films need to have a thick microstructure that covers the whole base and has few pinholes, empty spots, and rough surfaces. This situation [6] makes it possible to (i) increase the active area of the device, (ii) get rid of any shunt lines that leaking current could use, and (iii) place next functional layers in a conformal way. A grain is the smallest unit in a microstructure. It is made up of a single crystal and is surrounded on all sides by gateways, interfaces, or surfaces. That is, everywhere inside a grain has the same crystalline direction or a form of it (like a twin) in relation to every other place inside the grain. As long as the HP has the common AMX3-based three-dimensional (3D) perovskite crystal structure or a stacked crystal structure (see Chapter 2), this description stays the same. The architecture of HP thin films comes from the way they are made using solution- and/or vapor-based methods, where single-crystal nuclei start to form, grow, and then join together [6, 7]. Along with the chemical and phase makeups, the microstructure of HP thin films is mostly determined by 6, 7 the type of GBs, (ii) the size and spread of the grains, and (iii) the crystalline orientations of the grains (texture). It has been found that the microstructures of HP thin films have a big impact on the devices’ features, performance, and stability [6, 7]. What microstructure does to PSCs is much more important than it is to dye-sensitized solar cells (DSSCs) and organic photovoltaics (OPVs), two other new photovoltaic (PV) technologies that are competition. However, the HP literature is full of claims that microstructural/GBs effects don’t exist, studies that aren’t good enough, and guesses.
This is the first edition of Perovskite Photovoltaics and Optoelectronics: From Fundamentals to Advanced Applications. It was edited by Tsutomu Miyasaka.
2022 The WILEY-VCH GmbH. Published by WILEY-VCH GmbH in 2022.
82 There have been claims, and sometimes wrong readings, about the 3 microstructures and grain boundaries of halide perovskite thin films. To make real scientific and industrial progress in HP thin films and the products that come from them, it is very important to study their microstructures and how they change over time using good materials science concepts.
In this chapter, we talk about the features of microstructures in HP thin films, as well as how they formed and changed over time. Next, we looked at and gave examples of how microstructural changes can affect the stability and function of HP thin films and devices in a critical way. Finally, a look ahead is given at where study in HP thin-film microstructures and GBs could go in the future, which would help PSCs and other HP-based devices reach their full potential.
Microstructure Characteristics
The Nature of Grain Boundaries (GBs)
Solid-state GBs are two-dimensional flaws made up of single crystal grains that are bonded together by atoms. The crystal grains have random crystalline orientations. These GBs have five degrees of freedom: two more angle coordinates show the orientation of the GB itself, and three more coordinates show the misorientation between grains next to it. Thermodynamic numbers can be used to explain them. GBs are very different from the body of the grains because they are more disordered and have more free space. In HPs and other complicated materials with many cations and anionics, solid-state GBs can become charged when extra ions and/or impurities are separated. This creates a “space charge” in the grains near the GBs that eventually disappears.
GBs are not fully relaxed like grain surfaces because of the constraints from the grains on either side. This makes them very different from both bulk and external grain surfaces. These GBs can have structures on the inside (called “chromes”), and GBs can be thought of as “phases.” GBs have a big impact on carrier transport, recombination, mass transport, and reactivities in HP thin films, even though they are small 2D channels compared to the large 3D grain bulk and 2D grain surfaces. This might make HP’s thin sheets and gadgets less stable and less effective, but it might also help in some situations.
When there is a specific misorientation between grains next to each other, the GBs are called “special” or “low-angle” GBs. The regular GBs between randomly orientated grains, on the other hand, are also called “general” or “random” GBs. There may be sub-boundaries within the single-crystal grains. The diffraction of the areas on either side of the sub-boundary is linked by a certain symmetry operation. One such boundary is the twin boundary, which can form easily in some HP thin films.
There may be separate second phases for some or all of the GBs in HP thin films. These can be amorphous, solid, molecules, or polymers. You could say that these kinds of GBs are functionalised, whether they were meant to be that way or not. The GBs can be solid state, functionalised, or a mix of the two. They can be thought of as a 3D network of linked GBs, like soap film in foam.
HP thin films can also have hierarchical microstructures made up of aggregates, which are groups of smaller single-crystal grains and the GBs that go with them (also called “domains” in the HP literature). There are borders between the groups that are not GBs and can’t be explained by thermodynamic values.
To sum up, solid-state GBs are two-dimensional flaws with five degrees of freedom that can be described by thermodynamic values. To the contrary, HPs are very easily damaged by electron beams and break down in just a few minutes. Because of this, extra care and methods must be used when characterising HPs with electron imaging.
Grain Size and Distribution
Grain size is an important way to measure the microstructure of HP thin films, and the best way to do it is to look at SEM pictures of the top surface and analyse them. But this might not be true because how the grains grow and stick together affects how lines form during processing. When GBs evaporate or react chemically, they leave these grooves, which tend to form on grain surfaces.
New research on HP thin films using electron-backscatter diffraction (EBSD) in the SEM has shown that most of the grain sizes given in the papers are probably too big. Aside from that, grain surfaces in as-deposited HP thin films may have lines that look like grain border marks, which can lead to a wrong estimate of grain size.
Even though it’s important, the range of grain size isn’t usually talked about in HP material; only the average grain size is given. When grains grow normally, their sizes tend to be spread out fairly evenly, and the biggest grain must be less than twice the size of the smallest grain. This is called the “Hillert” requirement. If this condition is broken, it means that the grain growth is odd or too big.
The grains’ form can also be important. The grains in Figures 3.2–3.4 and 3.6 are said to be equiaxed, which means they grew normally. If the grains in a microstructure look like plates or needles, that means the grain growth is not uniform.
Crystallographic Texture
The crystalline directions of grains in microstructures are usually all over the place. The microstructure, on the other hand, is said to have texture if there is a preferred direction between grains that are next to each other or between a group of grains. To measure the general texture of halide perovskite thin films, X-ray diffraction (XRD) is used. Figure 3.8 shows an example of a MAPbI3 HP thin film that has been grooved. EBSD in a SEM is the best way to map texture at the tiny level, because each pixel’s crystalline direction matches a spot on the stereographic project.
Microstructural Evolution in HP Thin Films
Genesis of Microstructure
Solution deposition is usually used to make HP thin films. This process starts with the beginning of the HP phase and continues with its growth. It is possible for nuclei to be homogeneous or heterogeneous, and they can form when antisolvent drips, evaporates, or degasses, which causes the material to become too saturated. In vapour formation, the base always has different kinds of nucleation. The main reason for nuclei growth is that the system wants to lower its total free energy. The Volmer-Weber process, also known as “island growth,” is the most common way this happens.
The growth centres join together to make a thick, full-coverage microstructure if there are enough “nutrients” in the system. From the point of view of the whole system, both nucleation events and growth can happen at the same time. Nucleation rate usually reaches its highest point as a function of temperature because of other factors that are at play. On the other hand, growth rate always rises exponentially with temperature.
In “nutrient”-limited solution processes, you need to increase the nucleation rate as much as possible to get full-coverage HP thin films. However, this makes HP thin films with small grains because the grain size relies on how dense the nuclei are, which might not be what you want. As we talked about in Section 3.4, the grains need to be treated after they are deposited in order to grow.
Grain Growth
If you give enough heat and time to a thick, polycrystalline halide perovskite thin film, the grains will get bigger, as long as the film doesn’t break down. Solid-state grain growth and matrix-phase-mediated Ostwald hardening are two important traditional ways that grains get coarser. The difference in chemical potential caused by the shapes of smaller and bigger grains is what drives both processes in terms of thermodynamics.
In solid-state grain growth, grains with six sides stay steady. Larger grains with more than six sides will grow (concave GBs, 𝼙 <120∘) at the cost of smaller grains with fewer than six sides that are nearby, which will shrink (convex GBs, 𝼙 >120∘). Rn^t – rn0 = K2t is the formula for the grain-coarsening rate [6, 7, 9]. Here, r0 and rt are the grain radii at time zero and t, respectively, and n = 2. The chemical potential difference causes important species to move from the convex curvature grain to the neighbouring concave curvature grain across the grain. This causes the grain to move in the opposite way.
In thin films, the coarsening happens slowly and usually stops when the average grain size (2r) hits about the film width (d), which is the point where all the GBs meet at the top and bottom of the film. It is thought that this “stagnation” in coarsening is caused by drag forces acting on the moving GBs from the surface and the interface, assuming that the grain-boundary energy is the same everywhere. In particular, the top grain-boundary lines should have a big effect on the drag force.
Secondary coarsening, which is also called abnormal or excessive grain growth, can happen in anisotropic thin films when a few grains that are orientated in a good way grow quickly. This makes the thin films have big grains and a rough surface. The system tends to make the surfaces and contacts as big as possible by letting the grains with better orientations grow at the cost of the grains with worse orientations. This means that grain sizes that are a lot bigger than the film thickness are possible. It does, however, always come with a strong crystalline structure in the thin film, which can be a good thing.
For Ostwald ripening to work normally, there needs to be a matrix phase (usually a liquid or vapour) between the grains. This is where the smaller grains dissolve because they have a higher chemical potential because they are more curved. In some places, the matrix gets too full of the right species, which stick to bigger grains with smaller chemical potential. This makes the grains bigger overall.
The processing windows (temperature, time) for HP thin film methods are naturally small because HPs are not very stable at high temperatures. As a result, the grain size of the perovskite thin films that are formed is usually very small (a few hundred nanometres). In thick perovskite thin films, this means that grain growth or coarsening after depositing is important.
Influence of Microstructures and GBs
on Performance and Stability
Grain Size Effects
The study talks about how important grain size is in making high-performance photodetectors and perovskite solar cells (PSCs). Most of the time, big grains are better for moving carriers, but GBs might get in the way of this. It is important for the grains to cover the whole length of the thin film in PSCs and photodetectors, since horizontal GBs can make it harder for carriers to move and be extracted in the normal vertical device setting. Charge-carrier delivery is less likely to be harmed by vertical GBs.
The size of the grains may change other features, like the dielectric constant and transmission. Because GBs have a lot of traps, they can act as recombination centres. This makes carrier diffusion shorter and shorter, which hurts the performance of PSCs by lowering their power conversion efficiency (PCE) and increasing photocurrent hysteresis. Even though the GBs are fully passivated, the HP layers in LEDs may benefit from a fine-grained microstructure. This is because the small grain size can help contain electron-hole pairs in space and make radiative recombination easier.
Some people aren’t sure if the big grain size in coarse-grained MAPbI3 thin films is needed for PSCs to work well. In the past, computer models have shown that flaws at GBs in HP materials aren’t too bad, but results from experiments haven’t been completely persuasive. It has been shown that HP films are less strong when they have small grains. This is because smaller grains make it easier for PSC delamination failure in the MAPbI3 thin film because they are not as tough. A bigger grain size moves the failure to the much tougher interlayer contact. Because of this, a high GB density can be linked to less stable and reliable thin films as well as worse PSC performance.
A number of papers on HP thin films, PSCs, and other devices talk about grain size and how it affects these things. However, there are many concerns about the studies and the stated grain size, which may be too small. Different processing, dosing, and post-treatment methods used to make grains grow significantly mean that grain size alone can’t be changed in a way that is self-similar. This is true even within the same study.
One problem with GBs is that they can pull in airborne particles that can break down perovskite and speed up the release of breakdown products, which can make the optical qualities of HP thin films worse. It is possible to fix these problems and make HP thin films more stable by using coarse-grained microstructures with low GB density. For example, Ji et al. made PSCs that were more stable by using MAPbI3 HP thin plates with grains about 3 μm in size. Figure 3.13 shows a very interesting example of how coarse-grained α-FAPbI3 HP thin films are more stable in humid conditions. A 7.6-μm grain-sized thin film is much more stable than a 1.2-μm grain-sized α-FAPbI3 thin film that was left in a humid environment (70% relative humidity) at room temperature for 40 hours.
The degradation process, which changes the α-FAPbI3 HP phase to the ε-FAPbI3 “yellow” non-HP phase, only happens through GBs when H2O gets in. This amazing result is caused by a low GB density. Different methods were used to make MAPbI3 thin films, and the breakdown product in this case is PbI2. The effect of grain size on this degradation shows how important GBs are, even though they are only narrow 2D channels compared to the large 3D grain mass and 2D grain surfaces.
Effects of the Nature of GBs
A lot of study has been done on the properties of grainy graphene (GBs) in MAPbI3 thin films. Some experts think that GBs might help separate and collect photogenerated carriers, which would make recombination less likely. Others, though, have seen less photoluminescence (PL) at GBs, which suggests that these may be places where unwanted trap states come from. The PL may be lower at GBs, but the SRH recombination lengths can be the same as or even longer than those at the film surfaces or grain cores. This rise in PL at GBs could be caused by carriers getting stuck and piling up close to them, which makes the radiative recombination rate go up where there are more carriers. Based on these findings, GBs may have a positive effect, but carriers may not be able to cross them at all, which is a negative effect.
Reports often don’t talk about the types of GBs that are used, and the way that different types of GBs affect carrier movement or recombination can be very different. One study that stands out is that by Jiang et al., who used photoconductive atomic force microscopy (PC-AFM) to find that GBs between grains with similar photocurrent-generating capacity often work better than the grain interiors, while their behaviour between different grains is in the middle of what it is like in the adjacent grain interiors. They think that the changes in the photocurrent from different types of grains might be because of how the grains are arranged across their GBs. It’s possible that smaller misorientations, or “special” GBs, are better than bigger ones.
The study talks about adding GBs to perovskite thin films (PSCs) to improve their optical qualities and stop harmful molecules from moving along them. In MAPbI3 thin films, two kinds of GBs have been found: one has higher carrier mobilities (type I), and the other is not damp (type II). Adding functionalities to GBs has the opposite effect. This means that the “best of both worlds” can be achieved in PSCs by carefully adjusting the GB phase and its width.
Zong et al. used Pluronic P-123®, a tri-block polymer that is hydrophilic, hydrophobic, and hydrophilic, to continuously GB functionalise MAPbI3 thin films. The hydrophilic ends of the polymer attach to nearby perovskite grains, and the hydrophobic centre of the polymer (polyethylene oxide) keeps water from getting in. The PSCs that were made, with the amount of GB functionalisation managed, were found to have better PCE and better operating stability. Chen and others used C6H5(CH2)2NH3PbI4 (PEA2PbI4) molecules to continuously functionalise GBs in MAPbI3 thin films. This slowed down the passage of ions (I−) and made the PSCs more stable.
Recently, molecules of 4-(amniomethyl)piperdine (4AMP) have been used to modify GBs of Pb-free FASnI3 thin films in order to make the resulting PSCs much more stable and effective. GBs can also be changed by artificial second stages, which can do things like store useful species. Such as, oxidation of Sn(II) to Sn(IV) is a problem for Sn-based HPs thin films used in eco-friendly low-Pb or Pb-free PSCs. By supposedly giving Sn(II) to the grains, functionalising the GBs in these HP thin films with SnF2 can help with this problem in some ways.
Sub-GBs can be found in single grains. For example, MAPbI3 grains have a lot of twin borders inside them. It’s still not clear, though, what effect twins have on the success of MAPbI3-based PSCs. In the case of α-FAPbI3 thin films with large grains, sub-GBs block photocarriers along with random GBs, which will lower the performance of the PSC. Sub-GBs in HP thin films are not likely to let ions and external species get in quickly like random GBs do.
Crystallographic Texture Effects
Researchers have found that the grain crystalline orientations of 3D high-performance thin films (HPs) may be just as important as the size of the grains. PC-AFM shows that some types of grains are better at collecting photocurrent than others. It’s possible that the “best-performing” grains only make up a small part of the whole film. Different parts of grains work better or worse at their solar (PV) functions, which causes variations in the way carriers are collected within the grains. When it comes to MAPbI3 films, a desired (110) tetragonal shape is often linked with better PV performance. This can happen when Cl is present.
In general, it’s not clear how much performance and film properties are controlled by how the grains are arranged, as opposed to how those arrangements affect other layers of the device. One type of stacked perovskites (like Ruddlesden–Popper and Dion–Jacobson) thin films are more affected by crystallographic direction. This is because the crystal structure is very uneven, which means that carrier motion is uneven as well.
There are doubts about the purity of the phases in layered HPs because most thin films are unknown mixes of 3D and layered HP phases. It is very important to make sure that the layered HP thin films are phase pure and that the direction of the layered perovskite thin films with respect to the substrate is optimised for the needs of the application (for example, LEDs, PSCs, etc.).
Outlook
Halide Perovskite (HP) materials are a new way to connect “soft” and “hard” materials that has never been done before. HPs have a “hard” artificial framework that gives them nanostructures and grain boundaries (GBs), which are studied in materials science. It might be tempting to get rid of GBs in order to get single-crystal HP thin films for devices, but it is better for science and technology to use the depth that HP microstructures and GBs offer and benefit from them.
The fact that HPs are both “soft” and “hard” makes them difficult to work with and gives them new chances. Ceramics and other “hard” materials are well understood, but not much is known about GBs in HPs, especially functionalised GBs. The main reason for this is that high-resolution electron imaging methods (TEM, STEM) and related spectroscopies can hurt HP materials. HPs are very easy to hurt with electron beams because they have low creation energies. This is a big problem.
To learn more about GBs in HP microstructures, we will need new imaging methods, low-dose TEM, high-sensitivity cameras and detectors, cryo-TEM, and low-dose TEM. In situ and operando TEM will be needed to study how GBs move, which will give us new information about the nature and features of GBs and how HP microstructures change over time.
Instead of haphazardly trying things out and seeing what works, GBs in HPs need to be designed and customised in a smart way. Because HPs have a low formation energy, they can be made and processed at temperatures close to room temperature. This means that more species (amorphous, solid, molecules, and polymers) can be used to modify GBs. To make the sensible design of GB functionalisation species work well, we will need computational methods along with new tools like materials informatics.
Finally, new ways of synthesising and processing will be needed to enable the “programmed” customisation of the created HP thin-film GBs to achieve the desired qualities and help HP-based products reach their full potential.
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