Making halide perovskites and other types of material synthesis is an important part of finding and customising new useful materials for energy uses. Most studies focus on characterising the perovskite material after it has been made, usually when it is close to equilibrium or when it is used in optoelectronic devices. In situ readings let you directly describe how the reactant changes into the result chemically in real time during reactions. In situ characterisation during the production of a perovskite thin film can tell us about how crystal phases and optical properties change over time, as well as the formation paths and possible metastable phases that are driven by thermodynamics and kinetics.
Knowing how a material changes over time and being able to control how it forms can help with the deliberate creation of halide perovskites and materials. To record the fast changes in features during solution deposition and consecutive cooling of halide perovskites, you need fast data collection tools like fast detectors with ms or faster integration times and high-flux beam sources like X-rays. In the past ten years, advances in technology have made it possible to characterise soft materials in situ. This includes growth rates, which means how traits change over time, and structure transformations.
To connect how functional qualities like bandgap and crystal phase change over time during development, in situ multimodal characterisation is needed. These insights into how things work help designers choose the right synthesis settings to make syntheses work better and be much more reproducible. In situ characterisation is one step towards automating synthesis, and it is becoming more popular and being used in experiments. There are several ways to study halide perovskites in this chapter, including using synchrotron-based X-ray scattering and fluorescence, optical spectroscopy, and a mix of these methods at the same time (multimodal characterisation).
Fundamentals of X-Ray Scattering
and Fluorescence Techniques
This review talks about the basics of the X-ray scattering and X-ray fluorescence methods that are used to characterise perovskite thin films in situ. It talks about grazing incidence wide-angle X-ray scattering (GIWAXS) and grazing incidence small-angle X-ray scattering (GISAXS), as well as how they can be used to look into how crystals form at the small length scale and how perovskite grains form. A lot of attention is paid to GIWAXS because it tells us a lot about the crystal packing structure, direction, relative degree of crystallinity, material makeup, and crystalline correlation length. GISAXS finds out about the average domain size by looking at the difference in contrast between domains of different stages. X-ray scattering methods from both the lab and the synchrotron have been used to characterise perovskite thin films. However, measures done in the lab can only be steady because the photon flux and energy are low, which takes a long time. Synchrotrons give off strong, focused X-ray streams with a lot of photons. This lets scientists study how fast films form in real time.
Grazing Incidence Wide-Angle X-Ray Scattering (GIWAXS)
Low peak levels may happen in polycrystalline thin films with random crystal orientation when X-rays are scattered in a symmetric way, like in Bragg–Brentano geometry. This is because the X-ray path lengths are short. Grazing impact X-ray diffraction (GIXRD) is used to make the X-ray path length and signal longer. In this method, the arriving X-ray angle is kept constant while the detector is moved. Periodic crystalline planes bend a part of the beam. When Bragg’s rule is met, as shown in Equation (15.1) below, diffraction takes place.
The main goal of GIWAXS is to study Bragg diffraction at large angles using hard, high-energy X-rays in a grazing incidence shape. It is possible to guess axial scattering patterns by building reciprocal space and Ewald’s sphere. The reciprocal space vector (q‗) has a length equal to the inter-planar spacing of the lattice planes, which is given by q = 2π/d. Its direction is perpendicular to the lattice planes. The Ewald sphere is made up of a sphere whose radius is equal to the wavelength of the X-ray beam that comes in at 2π/𝼆. This sphere holds all the possible incoming and diffracted vectors.
The crystal structure of perovskites has a big impact on their electronic and photonic qualities. The periodic nature of a crystal lattice is used to calculate the electronic band structure. When it comes to two-dimensional (2D) perovskite crystals, these qualities can be different along different crystallography lines. To fully understand the connection between the microstructure and optical qualities of perovskite thin films, it is important to find out the texture or crystalline lattice orientation distribution.
GIWAXS is the most common way to find out the direction of the texture. The form and polar distribution of the scattering peaks in a reciprocal space map can show this. Different crystallographic planes will give rise to different peaks in a 3D lattice. For instance, the diffraction pattern of the 2D perovskite (PEA)2PbI4 shows two groups of very well-aligned peaks in the out-of-plane (qz) and in-plane (qxy) directions. These correspond to crystalline planes that are parallel to and normal to the substrate.
Grazing Incidence Small-Angle X-Ray Scattering (GISAXS)
The GISAXS method uses the difference in refractive index that happens when the electron density of different materials or phases is not uniform. This is likely to be present when the film is being formed because different phases change over time. In situ GISAXS can be used to measure the average domain size of new phases as films form, and it can also be used on liquid samples to look into how they clump together in solution.
The reciprocal space, which is found by taking the Fourier transform of the crystalline lines in Euclidean space, shows the X-ray diffraction condition. GIWAXS of the 2D perovskite (PEA)2PbI4 shows that out-of-plane scattering is related to its scattering angle (20), and in-plane scattering is related to its scattering angle (2Φ0). The radial distance (qr) can also be found.
The left pattern shows diffraction patterns that are crystallographic lines that are parallel to the substrate, the middle pattern shows patterns that are orientated with an angle distribution around the horizontal alignment, and the right pattern shows patterns that are orientated randomly in all directions.
X-Ray Fluorescence (XRF)
As a way to find out about chemicals, X-ray diffraction (XRD) uses X-ray energy to send out specific X-rays. Interactions and ionisation of atoms happen during this process, which can remove an electron. The atom’s electronic structure becomes unstable if the radiation energy is higher than the ionisation energy. This lets electrons from higher orbitals fill the inner state. This change sends out a photon whose energy is unique to the atom and equal to the difference in energy between the two orbitals. The X-rays that are made can find elements while the film is being made, which lets you figure out how strong each element is compared to the others.
Selected Examples for In Situ X-Ray Scattering
and Fluorescence
In this section, we provide a few selected examples of deploying X-ray characterization techniques in an in situ mode to probe the fabrication of halide perovskite thin
films
In Situ GIWAXS to Study Crystallization Kinetics and A-Site Doping
Qin et al. used in situ GIWAXS to look into the crystallisation processes of a FAPbI3 perovskite thin film that was made in two steps, with Cs+ and GA+ doping the A-sites one after the other. In this process, a PbI2 layer is made first, and then it is changed into perovskite in a second step by adding organic salts and heating the layer to make the PbI2 and salts mix together. This process was changed by Qin et al., who added CsI and guanidinium iodide (GAI) to the precursors for the first and second deposition steps, respectively. They discovered that adding CsI changed the way the crystals formed in the first step, which is what made the ε-CsPbI3 and PbI2 phases coexist.
In the second step of synthesis, the crystallisation route changed based on whether Cs+ and GA+ were present. In Stage I, the PbI2 and ε-CsPbI3 phases were present. In Stage II, the perovskite α-phase formed, and both the PbI2 and ε-CsPbI3 phases broke down. The change from the ε-CsPbI3 phase to the perovskite phase happened much more slowly in Stage III than in Stage II. It finished when the peak for the ε-CsPbI3 phase disappeared. In Stage IV, more pure perovskite α-phase crystals formed until they were fully formed.
GA+ speeds up the change from ε-CsPbI3 to the perovskite phase, which helps get rid of pinholes in Cs-doped films. Adding GA+ to Cs+ GA-doped films in the second step helps get rid of pinholes by promoting Ostwald hardening and better grain boundary movement.
In Situ GIWAXS to Probe Film Evolution via Antisolvent and Gas Jet Treatments
Abdelsamie et al. used in situ GIWAXS to figure out how the crystals of Cs0.15FA0.85PbI3 formed during antisolvent-assisted synthesis or N2 treatment during spin coating in this work. The study took phase fractions from the in situ data to show how the makeup changed during synthesis. When N2 was used, hexagonal non-perovskite (εH) FAPbI3 crystallites formed first, and they were found to be highly orientated in the out-of-plane direction along qz. This was followed by the formation of crystallites that were more randomly orientated. Adding N2 speeds up the removal of the solvent from the contact between the air and the liquid. After some time, the lower part of the film becomes supersaturated, and crystallisation begins on the top surface.
It was found that vapor-interface-induced crystallisation caused the first formation of highly orientated crystallites. Later, in addition to vapor-interface-induced crystallisation, uniform crystallisation happens in the bulk solution. When CB was dropped, the first and later εH-FAPbI3 crystallisation had a wider direction than the N2-treated films. This showed that isotropic crystallisation was happening more in the bulk of the wet film.
We were able to get a better understanding of how the composition changed during thermal annealing by separating the roles of thermal expansion and chemical change in the change seen in the (100) peak point of the perovskite phase (also known as α-phase). The changes in the amounts of α-phase, αH-phase, and amorphous phases were found by looking at how the εH- and α-phase peaks changed in strength during heat annealing and using Equation (15.4).
It was interesting to see that the pictures didn’t have a uniform spread of Cs and FA when they were first made. Cs was separated into the α-phase and FA was separated into the pure µH-FAPbI3. Afterwards, annealing happens, which causes cation (Cs, FA) diffusion, which creates a more uniform microstructure with an average stoichiometry of about 15% Cs, Cs0.15FA0.85PbI3. The α-phase crystallises at the expense of the εH-FAPbI3 and the amorphous phase at the same time.
As the phase fractions changed over time, it became clear that the crystallisation processes were different for each synthesis setting. In the sample that wasn’t treated with an antisolvent or N2, the perovskite α-phase mostly formed from amorphous phases instead of a solid precursor. When CB or N2 was used during spin coating, crystallisation mostly happened from more solid predecessors. This was linked to getting a better shape and covering all of the perovskite thin films.
In Situ X-Ray Diffraction (XRD), XRF, and GISAXS to Probe the PbCl2-Derived Formation of MAPbI3
Stone et al. used in situ XRF and XRD to look into how the chemistry changed from a liquid starting material to a solid perovskite phase. They discovered that the crystallisation process goes through an intermediate stage where the peaks of the Cl-containing precursor phase (MA2PbI3Cl) stay pretty much the same. This is followed by a transformation stage where MA2PbI3Cl continues to be lost while the perovskite phase grows. In situ XRF readings showed that Cl was losing its strength in a straight line. This was because MACl was being absorbed from a surface area that stayed the same, possibly from the disorganised MACl phase in the film. Perovskite crystallisation doesn’t happen until 50% of the Cl is taken away. To add to wide-angle characterisation, another work used in situ GISAXS to measure small-angle scattering down to q values of 10−3 Å−1. It was seen that as the perovskite film forms, there are more and more structures that are 30 to 400 nm in size. This is because the q value gradually went up when the temperature was raised to 80 °C.
In Situ GIWAXS to Probe the 2D Perovskite Formation on 3D Films
Thin layers of perovskite can be made passivate by adding thin 2D or almost 2D perovskite layers. Usually, the 2D/3D interface is made by solution processing, in which the 2D perovskite layer is placed on top of the fully formed 3D perovskite film. We measured in situ GIWAXS during spin-coating to get a better understanding of how the 2D/3D contact forms.
A VBABr-based 2D perovskite was put on top of MAPbI3 and (MAPbBr3)0.05(FAPbI3)0.95 sheets in this case. It was seen that 3D perovskite and PbI2 crystals scattered light in situ at t = -6 s and t = -7 s, respectively. Before n =1 of the 2D phase (VBABr)2MAn−1PbnI3n+1 forms, an intermediate state with a higher q (≈0.50 Å−1) than any VBA-based 2D structure is seen for MAPbI3. The middle state was caused by 2D PbI2 sheets surrounded by IPA molecules, which was named i-PbI2.
On the other hand, the 2D/3D interface for (MAPbBr3)0.05(FAPbI3)0.95 forms by the 2D structure (VBABr)2(FA0.95MA0.05)n−1Pbn(I0.95Br0.05)3n+1 gradually losing dimensions from n =3 to n =1, instead of n =1 forming right away. The different ways that the 2D and 3D interfaces were made were because of the different levels of stability between the MAPbI3 and (MAPbBr3)0.05(FAPbI3)0.95 films. The less stable MAPbI3 is more likely to change into PbI2, which creates PbI2-intercalated phases with fluid molecules.
GIWAXS was also used in situ while the sample was being heated to study how the 2D/3D interface changed over time and find out what part 2-TMAI or PEAI played in stopping degradation. The in situ GIWAXS results showed that the 2D crystalline structure changed into a mixed 2D/3D phase. This stopped the 3D bulk from breaking down into lead iodide and kept the 3D bulk below whole. These results gave engineers ideas on how to make stable connections for devices.
In Situ Optical Spectroscopy
This part talks about UV-vis-NIR optical spectroscopy and how it affects perovskite materials, causing them to absorb, transmit, reflect, and give off light. It talks about how light can pass through and reflect, how photoluminescence works, and how it can be used to make halide perovskite. The book also talks about cases of how the knowledge can be used and gives details on phase development and formation dynamics. In Chapter 12, you’ll find more information.
Fundamentals of Absorption and Emission of Light in Halide Perovskites
Optical spectroscopy is a key tool for figuring out how light works and what its qualities are. After photoexcitation, the main species in halide perovskites is free charges, and the excitons are Wannier-Mott excitons. The temperature energy that is given might be enough to separate excitons during synthesis, especially when the material is being heated. Free charges made by photons can move around. At room temperature, carrier diffusion lengths have been measured to be between 100 nm and a few micrometres.
There is a sharp optical absorption edge in halide perovskites that show they can absorb a lot of light. Their absorption coefficients are more than 104 cm−1 above the bandgap. Measurements of wavelength-dependent absorbance along A𝼆 = −log10(T𝼆) with the transmitted radiation T𝼆 can be used to describe how light is absorbed.
Taking measurements of in situ transmission during synthesis is a good way to look at visual aspects connected to the band structure, like the absorption edge and how it changes as the material is made. Photoluminescence (PL), also known as radiative recombination of electrons and holes, is when light is released on its own when it is excited by light. For instance, the absorption edge for MAPbI3 moves steadily to higher energies as the temperature rises, which is the opposite of what happens with other semiconductors that are tetrahedrally coordinated. This behaviour can be explained by an electronic state ordering on the opposite band edge.
PL spectra also have peak strength and width, which are typically described by their full-width at half maximum (FWHM). The PL FWHM shows how the PL peaks get wider, which can be due to temperature changes, changes in the bandgap, or the number of separate states. A high PL density is usually a sign of high-quality products.
During the synthesis process, especially during the thermal annealing stage, the PL intensity changes in complicated ways. These changes are caused by trap states, temperature-dependent recombination dynamics and rates, thermal quenching, phonon-assisted nonradiative recombination, and thermally activated carrier trapping. The band-edge absorption and PL spectra show a red shift. This is called the Stokes shift.
Setup Design for In Situ Optical Spectroscopy
For studying how halide perovskite is made, in situ spectroscopy data are necessary. Characterisation is done during casting and heat cooling as part of these measures. Low-power class lasers or light emitting diodes with a narrow emission range are often used. PL optics can be used directly in a normal synthesis setting, like a glovebox, as long as the light source doesn’t pose any safety risks. When the PL signal is excited, it is picked up by gathering lenses, which are usually connected to a spectrometer by fibre. The signal is split up into its different wavelengths and sent to a group of detectors. To see fast changes happening during the halide perovskite synthesis process, you need short acquisition times. This is especially true for solution engineering techniques like antisolvent dripping during spin-coating and early crystallisation stages. The shortest amount of time needed to acquire something is determined by the excitation energy, density, quantum yield, and setting requirements. A photon flow similar to the 1-sun AM 1.5 spectrum can be used to get capture times of less than a second. A spectrometer, collection lenses, and a white light source are needed to measure UV-vis penetration and reflection in real life. When you do tests in a real working setting, you can use what you find in normal halide perovskite production methods, since most thin films are made by spin coating and cooling in a glovebox with a controlled atmosphere.
Selected Examples for In Situ Optical Spectroscopy
In this section, we provide a few selected examples of optical spectroscopy characterization techniques in an in situ mode to probe the fabrication of halide perovskite
thin films.
Fast In Situ Reflectance Measurements to Characterize the Perovskite Formation
We used fast in situ optical reflection readings to look at how different halide perovskite compositions were spin-coated and annealed. These compositions were (CH3NH3PbI3, MAPI), (CH3NH3PbBr3, MAPBr)), (HC(NH2)2PbI3, FAPI)), and the triple cation perovskite (TripleCat). Reflectometry measures light that bounces back and depends on the absorption rate, the refractive index, and the scattering processes. It was set up in a glove box with a tungsten lamp that was connected by fibre.
The light was parallel to the film surface during spin-coating but 15° off-axis during cooling to catch light that was spread out. During spin-coating, integration times were between 0.2 and 0.5 ms and 10 to 40 ms. Figure 15.9 shows the interference patterns and fringes that were found when specular reflection readings were taken during spin-coating. When t is longer than 28 seconds, the surface gets rougher and specular reflection goes away. At this point, only diffuse light scattering (shown as tend) can be measured. For MAPbI3, the study found that the complex formation energy is the lowest. For triple cation precursors, it was the highest.
The interference fringes from Equation (15.5) were used to figure out the layer thickness (d) during synthesis. This is because the solvent has a lot to do with the changes in n(𝼆) dispersion between 500 and 1100 nm for the perovskite precursor solutions. At first, the layer width drops quickly, but after about 10 seconds, it evens out. The TripleCat wet films were thicker in the end than the other precursor solutions. This was because the precursor ions interacted with the liquid molecules more strongly.
In situ absorption readings were taken during a long spin-coating process to find out how much the films crystallised. The MAPI and FAPI films did not show any reflectivity changes that are usually seen in perovskite formation, but FAPbI3 did still show interference patterns. The absorbance of MAPBr and TripleCat films changed over time, which was thought to be the perovskite bandgap growing as the solvent evaporated. Diffuse in situ absorption readings during annealing showed that perovskite formation happens in about 7 seconds at 100 °C for MAPI but takes about 60 seconds at 165 °C. We didn’t see any big changes in the MAPBr and TripleCat films during heating because the perovskite formation happened during spin-coating.
In Situ UV–Vis Absorbance Characterization During the Drying Stage
A group of researchers led by Hu used a laminar air-knife to help coat a meniscus at room temperature in order to make the solution process of halide perovskites bigger. We used in situ UV-vis absorption spectroscopy to find links between supersaturation, nucleation, and growth rate as the liquid turned into a solid. Natural air drying and laminar flow N2 knife in a N2 glovebox were put side by side to see which one worked best.
In situ UV-vis absorption tests showed that natural air drying goes through three stages: the solution, the intermediate, and the solid stages. During stage I (solution), only the absorption of the starting solution was seen. In the middle stage, the absorption edge slowly moved to the red, which was linked to a higher precursor content after the solvent evaporated. This caused perovskite crystallites and/or solvent-containing intermediates to form and grow. In the solid stage, stable absorption was seen, which showed that the precursor had completely changed into solid perovskite.
When the laminar N2 knife was used, the absorption spectra changed colour 10 times faster, and saturation was reached almost instantly. It was possible to figure out how fast the wet film dried by plotting the first derivative of the absorbance at 500 nm for both drying methods. It was discovered that using the laminar N2 knife speeds up the liquid drying rate by two orders of magnitude compared to natural air drying. This makes it possible to make a perovskite film quickly and under control. With the laminar N2 knife, quick nucleation and high-quality films with a small, smooth shape were possible over a large working window.
In Situ Photoluminescence Characterization to Investigate the Role of the Precursor
A study by Song et al. used a multimodal in situ method to look into how lead sources affect the physical and chemical changes in MAPbI3 thin films. They used three different lead salts, namely PbI2, PbAc2, and PbCl2. They used a 532 nm laser diode as a light source and a home-built setup in the fume hood to measure PL. Then, they used a single Gaussian to fit each PL spectrum and looked at how the strength changed over time and where the peak was located during thermal annealing.
During annealing, all three predecessors showed a similar PL-evolution. However, the Pb-salt had a big effect on the size of the changes seen in the PL peak positions, intensities, and dynamics. There is an energy change ΔE, which means that the first PL signal happens at a higher energy than the average bandgap energy of MAPbI3. The Pb-salt has a big effect on the size of the changes seen in the places of the PL peaks.
The halide salts (PbI2 and PbCl2) had an energy shift ΔE that was ten times bigger than that of PbAc2. The PL point moves red to the position of bulk MAPbI3 during annealing. The PbCl2 precursor showed a longer delay than the other two predecessors. The first change in PL colour was thought to be caused by quantum-confined PL radiation from small nanocrystallites. The redshifting was thought to be caused by the nanocrystallites getting bigger.
In situ diffraction tests showed that the perovskite formed right away without an intermediate phase when the predecessors came from PbAc2. The Pb-halides-derived precursors, on the other hand, crystallised through the formation of a precursor-solvent complex. This caused perovskite formation to happen more slowly than with the PbAc2-derived precursors. When heated, all of the predecessors showed double peaks in the PL strength.
It has been used to quickly and without damaging the sample to watch perovskite formation. However, there is a fast charge carrier funnelling that could stop the PL analysis from getting all the information it needs.
Examples of In Situ Multimodal Characterization
During Solution-Based Fabrication
Taking measurements in real time is a great way to see how the useful qualities of perovskite materials change as they are being made. Multimodal characterisation is the use of more than one method to get a better understanding of how and when chemicals are changed into the end product. Correlative studies can also be done with this method. Combining two in situ optical methods and in situ PL and synchrotron-based XRD are two examples of multimodal in situ characterisation in action. They show how MAPbI3−xClx and MAPbI3 are formed, with a focus on the antisolvent falling step.
This study used both in situ reflectometry and PL readings to describe a one-step deposition process that included spin-coating of a (MAI+PbCl2):DMF precursor solution, cooling of the spin-coated film, and then heating. Not much was known about the exact process by which chlorine was lost and added to the MAPbI3−xClx thin film. A Beer–Lambert-law-based model was used on the in situ optical reflectometry data to figure out how the content of the chlorine-containing MAPbI3−xClx phase changed over time in the thin film.
It was seen that the formation of the MAPbI3−xClx from the precursor stages took longer than expected. It took 35 minutes longer than expected because the chlorine had to be added to the perovskite.
The study is mostly about making MAPbI3-xClx-based thin films by measuring PL in real time. The in situ PL readings backed the idea of delayed chlorine absorption, showing that the formation goes through a complicated change process. The delay in adding chlorine to MAPbI3 was because there was too much chlorine in the film that was drying. The chlorine is added to the perovskite phase after enough of the extra chlorine evaporates and the solid precursor phase (MA2PbI3Cl) breaks down.
The absolute PL energy was used to find the quasi-Fermi level (qFL) splitting versus heating time. The qFL splitting is a measure of the solar cell’s highest open-circuit voltage. A peak in the qFL splitting was seen, which shows the best cooling time for this material. This best value is linked to a certain amount of the crystalline precursor phase that contains chlorine and the total amount of chlorine in the thin film.
It was possible to separate the kinetic parameters of the MAPbI3−xClx formation process using a multimodal method at different heating temperatures. According to the multimodal measurements, the films at the start of the annealing process have MACl, MA2PbI3Cl, and MAPbI3−xClx (with very small x). MACl evaporates during annealing with an activation energy of about 84 kJ mol−1, and MA2PbI3Cl starts to break down, adding some of the remaining chlorine to the MAPbI3−xClx phase with an activation energy of about 94 kJ mol−1.
The GIWAXS data showed that the material being studied changes in several steps while it is being spun-coated and then heated.
It is possible to figure out how halide perovskite grows by using in situ GIWAXS findings. When you put these data together with in situ PL, you get more information about how phases change. The first nucleation that happens when antisolvent drips shows a strong blue-shifted PL emission. This is because quantum-confined MAPI nanocrystals start to grow right away. The lack of MAPI diffraction bands in GIWAXS data is because they have a low volume density in the film compared to the solvent-complex phase, which is most likely what the film is made of at this point. But it’s easy to see the PL signal because it’s very sensitive to even small amounts of MAPI crystallites because they have a high quantum yield.
When antisolvent was dripped, broad, uneven, and blue-shifted PL signals were seen, which showed that the nanocrystals were a wide range of sizes. You could also think of these signals as coming from a very bright mix of precursor and solvent molecules, or these parts could be arranged in a low-dimensional luminous cluster. Later, the nanocrystals get bigger and more uniform because the clusters stick together during the rest of the spin-coating process. They are also mixed with solvent complexes that don’t glow.
When the cooling temperature is reached, PL data adds to the information from GIWAXS data. This shows that the change from nanocrystals and solvent complexes to the solvent-free perovskite thin film happens through processes of re-dissolution and nucleation. The solvent is taken away from the top to the bottom of the film, which causes highly emissive, quantum confined MAPI-clusters to grow. The PL signal and its full width at half maximum (FWHM) change to show this growth and homogenisation. The PL signal that was seen is a lot like the first nucleation event that was seen when the antisolvent was dropped.
Characterising something in more than one way helps us understand how a complicated and quick crystallisation process works. This study gives us new information about how phases and structures change during the production of halide perovskite. It may also help us understand why it’s hard to repeat results from one lab to the next, even when making the same high-quality devices in the same lab, because processes that are driven by thermodynamics and kinetics change quickly. Multimodal in situ studies can help choose the right synthesis parameters and make the best films.
Probing Beam–Sample Interaction
This chapter is all about the relationship between the probe beam and the sample. This includes sending energy to the sample, which can change its qualities and/or the speed of the processes being studied. Because halide perovskites are soft and their parts are very volatile, they are more likely to get damaged by beams. For in situ readings to get the time precision needed while keeping the signal-to-noise ratio acceptable, high photon rates are often needed. So, one important thing for in situ characterisation during synthesis is to find the right experimental conditions that let you record changes at the right time scales while minimising photon flux so that the characterisation doesn’t damage anything. If the second option isn’t possible because of the need for very high time resolution, probe-induced changes should be described and understood.
This chapter mostly talks about measurement methods that depend on how the perovskite film changes when it comes in contact with photons from white or monochromatic light sources and X-ray sources. Both of these can mess up the perovskite sample while it is being made. To get a good picture of time, most in situ diffraction studies are done at synchrotron sources. Beam damage from X-rays can show up as less scattering peak strength and peak shifts, and it can also cause changes in the structure caused by the radiation. X-ray photon energies can often be changed at synchrotron X-ray sources, which can also change the amount of energy stored in the object.
It depends on the sample properties, the stimulation power, and the measurement environment how photons from the visible spectrum interact with the sample. These interactions can have a number of different effects on the sample’s properties. Looking at mixed-halide perovskites with visible light can change the way the elements are distributed, causing light-induced halide segregation. This can make the PL signal go up or down based on the energy of the photons used.
When measures are taken on-site while the film is being formed, they can change the dynamics of the process being studied. During in situ characterisation, changing the sample spot and lowering exposure times by cutting the incoming light are two more ways to reduce the damage. This, however, will only give an average of the property being studied over a certain area and won’t allow for the study of the development of potentially different useful properties across a wider area.
Summary and Outlook
It is very important to study the changes in functional traits over time and how they relate to synthetic parameters and solution engineering methods while the different manufacturing steps are still happening. Real-time characterisation can help with choosing synthetic factors, making materials work better, and proving theory theories. Robots and automatic synthesis platforms that are combined with in situ characterisation methods can speed up the process of optimising the synthesis process. But when synthesis is done, high temporal resolution means poor or no horizontal resolution. To fully understand synthesis, we need to characterise samples both in situ while they are being made and in depth afterward using high spatial resolution. One major worry is that high-energy X-rays or lasers could hurt the objects being studied by the wave. Because of this, it is important to pick probing beam factors that don’t get in the way of the measurement variable too much. It is important to keep changes in the measuring variable separate from changes in the beam.
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