This chapter is written for students who use electron microscopes to work with perovskite solar cells. It gives you a basic idea of what electron microscopes are, how they work, and what they can see before talking about recent research results. This chapter also talks about problems that can happen when photoactive perovskites are studied using electron microscopes. This is because these materials are unstable in many situations, and the high vacuum and strong electron radiation in electron microscopes can be harmful to them. The writers make notes about how to prepare samples, move samples, set up image settings, and spot beam damage. They use examples of studies that used different electron microscope methods to show how flexible electron microscopes are and to encourage people to use these microscopes for more fun things.
Fundamentals of Electron Microscopy
Electron microscopes are tools that use the fact that electrons can be both waves and particles and how they interact with things to learn a lot about a subject. This kind of information can include the shape of the surface, the arrangement of the elements, the crystalline direction, the quality of the crystals, and their chemical states. With a precision of a few nanometres, SEM is often used to look at the shape of objects. TEM, on the other hand, is a long, empty tube with a sample in the middle, an electron source at one end, and different detectors at the other end.
With a precision of just a few nanometres, SEM is often used to look at the shape of objects. Through X-ray spectroscopy, it can also give very specific details about the elements in an object. Through cathodoluminescence (CL), it can show details about the material’s electronic state. A TEM is much more complicated than this reduced picture shows. It has many important parts that surround and interact with the tube, such as vacuum pumps, cooling systems that use liquid nitrogen, and high voltage sources.
TEM is transmissive, which is the main difference between how SEM and TEM work. Most of the time, electrons are recorded from the surface of a specimen in a SEM. In a TEM, electrons are recorded after they have passed through the specimen. The contact with the specimen can then be analysed, and a lot of information about the specimen can be gathered, from simple images of its surface to maps of its elements at the atomic level.
TEMs usually work in either a broad-beam mode (TEM) or a narrow, scanning beam mode (scanning TEM or STEM). Each method has its own benefits. In broad-beam mode, the electron beam is directed in a way that makes it spread out over the sample, lighting up a certain area all the time. Electron detectors, which are usually CCD, CMOS, or single-electron detectors, record the spatial strength of the transmitted electrons and make a contrast picture, also called a micrograph, from the whole area that is lit up and within the detector’s field of view.
Broad-beam mode also lets you use selected area electron diffraction (SAED), which is a great way to get crystallographic data about a specific part of a material.
Signal Generation
Electron microscopy (SEM) and transmission electron microscopy (TEM) are two ways to look at changes in the qualities of objects and learn more about them. Secondary electrons (SE) and backscattered electrons (BSE) are the main signs that electron scanners make. SE and BSE are mostly picked up in SEM, while elastically and inelastically forward-scattered electrons are more often picked up in TEM.
There are different kinds of signs used in SEM and TEM, but some are used in both. In SEM, secondary electrons (SE) and BSE are most often seen. In TEM, on the other hand, elastically and inelastically forward-scattered electrons are more common. Some SEMs have secondary electron detectors that can pick up transmitted electrons, and some TEMs do the same.
Each kind of signal is made in a certain way and tells us something different about the thing we are studying. The signal that is most often picked up by a SEM is the SE that the specimen gives off when arriving electrons hit electrons in the specimen in a way that is not elastic. The beam is moved across the specimen’s surface in a raster pattern. The number of SE released in each pixel is noted, and an electron intensity map can be made.
Most of the interactions between the electron beam and the specimen are rigid, but some electrons bounce off the atoms in the specimen and bounce back to the electron source. The BSE strength can be measured, typically by a round detector placed close to the electron emission pole piece, and a picture can be made at the same time as the SE image. Heavy atoms spread more electrons than light atoms, so the BSE signal can be used to make a picture of how the atomic numbers of different parts of an object are distributed.
Electrons coming in and interacting with electrons in the object can give useful information about the chemistry and electronic features of a material. This is in addition to structural processes. Electron dispersive X-ray spectroscopy, or EDS, can give very specific details about the atoms that make up an object by collecting the unique X-rays that it gives off when electrons are irradiated on it. X-rays from each element are different because as powerful incoming electrons push out ground state atomic electrons and higher orbital electrons relax into the ground state, they give off their own unique pattern of X-rays.
An interesting process called cathodoluminescence (CL) can also make things give off visible light. When the electron beam makes SE, it moves electrons in the object from the valence band to the conduction band. These electrons then join with holes in the valence band, which can release a visible photon. The colour or wavelength of emission is directly linked to the material’s bandgap, which includes the presence of trap states. CL can give a lot of information about a material’s electronic features.
SEM
Cathodoluminescence
Through high-energy electron stimulation, CL can show the visual and electrical features of halide perovskite materials on the nanoscale level. It can take very clear pictures, and when used with SEM or TEM, it can show where long-lasting flaws like grain boundaries and dislocations are spread out in semiconductors like perovskites at the nanoscale level.
Comparison of CL and Photoluminescence (PL)
CL is a way to look at how charge carriers recombine in perovskite thin films by using electrons with energies between hundreds of eVs and tens of keVs as an exciting source. A high energy density electron beam moves valence electrons into the conduction band in CL. These electrons then join with holes in the valence band and give off photons with an energy equal to the characteristic bandgap (Eg). The three-dimensional clarity of CL microscopy is based on the electron beam probe’s size and how much the sample scatters the electrons. PL studies, on the other hand, use an optical microscope objective to focus the source laser beam. The size of the laser beam at focus determines the spatial resolution. It is possible to study large differences in charge carrier recombination on the nanoscale because CL has a much higher spatial resolution than PL. The electron beam accelerating voltage controls the generation volume of electron-hole pairs (EHPs) by CL. This lets the entry depth of main electrons be tuned.
Working Principle
Figure 14.3 shows how CL readings are usually set up in studies that use a SEM. The setup has lenses for catching light and sending it to the detector, a spectrometer, and a detector. Most of the photons that are made or leave from the sample surface are collected by a parabolic mirror with a big solid angle. There is a hole in the mirror that lines up with the microscope column. This lets electron beams pass through and hit the sample surface. The light that is collected is sent from the vacuum chamber into a monochromator for spectrum detection or into a photomultiplier tube (PMT) detector for a panchromatic CL signal. The light is directed parallel to the paraboloid axis. Spectrally resolved CL is the name of this method.
CL for Perovskites
Changesin CL spectra can be associated with trap states atsurfaces and grain boundaries, as well as local changes in stoichiometric ratio in halide perovskites
CL for All Inorganic Perovskites
It was found by Li et al. that the body of the CsPbIBr2 film had iodide-rich phases. This was because the phases separated near the grain boundaries, creating iodide-rich “clusters.” The panchromatic CL picture showed that the emission strength was higher at the edges of the grains than in the middle of the grains. The writers did CL spectrum imaging over the same area to find out why there was more CL light from the edges of the grains. At the grain boundaries, the lower-energy emission is strongest, and the PL spectra that are made after light causes phase separation are very similar to the lower-energy spectra in Figure 14.4f. The higher spatial precision of CL, on the other hand, proves that this mostly happens at the film’s edges.
In their study, Davila et al. found secondary phases in fully solid CsPbBr3 that was formed by spin coating and co-evaporation. They also showed where these phases were located. By comparing CL spectrum imaging and EDS composition analysis, they found a spot in the middle of the bottom where there were no CL emissions that could be measured. In the area around the source, there is an emission peak at about 2.35 eV, which is caused by CsPbBr3. The EDS data showed that the material was made of Cs4PbBr6.
Finally, iodide-rich phases can be seen in the CsPbIBr2 film because the phases separate near the grain borders and iodide-rich clusters form. Several methods, such as electron imaging and EDS composition analysis, have been used to study the presence and spatial distribution of secondary phases in the CsPbIBr3 film.
Beam Damage
Electron microscopy (CL) signals need to be generated and injected at high rates, which can damage materials that are sensitive to radiation. When an electron beam shines on organic-inorganic hybrid perovskites, they become delicate and can lose their shape, makeup, and structure. This means that electron imaging can’t be used to look into the shape, microstructure, makeup, crystal structure, and electric features of mixed materials on the micro- or nanoscale level.
Xiao et al. looked into how the emission strength and range changed when the e-beam conditions changed for different perovskite films. They discovered that the peak value of photon energy, which is equal to 1.6 eV, is clear for at least one second of exposure. The spectrum peak moves to higher energies and gets a little wider as the exposure time goes up. This shows that the electron beam causes a new phase to form. A higher acceleration voltage makes these effects much stronger.
Damage Reduction
Reduce Acceleration Voltage
Cai et al. discovered that the knock-on atomic displacement damage in CH3NH3PbI3 is big and depends on the energy present. They discovered that H, C, N, and I move when the impact energy goes up to 2.3, 26.4, 70.4, and 249.4 keV, in that order. Even at 1 MeV, pb doesn’t move. This explains why radiation with an energy level of 5 keV or higher changes the CL spectrum a lot, but radiation with an energy level of 2 keV only changes it a little.
Reduce Exposure Time
There were 200 ms of dwell time and 40 nm × 40 nm pixels used to make the CL spectrum maps. This is 667 times longer than the dwell time used for PMT mapping. Different accelerating voltages and beam currents were used to measure the photon energy (eV) normalised strength. A spectrometer and a spectral camera are used to determine the wavelengths of the photons that are released. This is different from PMT mapping in that the total CL intensity information is still collected. This makes the exposure time longer and the damage from the electron beam worse. Putting PMT and a band pass filter together, on the other hand, might get about the same amount of information with a little less spectrum precision and less exposure time and beam damage. There are boosting powers of 2 kV, 5 kV, 8 kV, and 10 kV, and the beam current ranges from 0.2 nA to 14 nA. The method of spectrum mapping is used to look into the photon energy (eV) and photon energy (eV) in materials for perovskite solar cells.
Reduce Dose Rate
Pulsed beams are useful for optical devices because they can lower beam current to tens to hundreds of pA. Low input conditions can be changed to match operando conditions, which keeps the beam from getting damaged. Between bursts, the low-current pulsed beam lets the material relax electrically and thermally. Orri’s research used pulsed mode (PM) to get CL on organic hybrid perovskite films. They did this by using an electron gun and the third harmonic of a Nd:YAG laser with a frequency of 80.6 MHz and a pulse width of 7 ps.
Electron-Beam-Induced Current
In SEM or STEM, electron-beam-induced current (EBIC) imaging is used to look at semiconductor materials and circuits. It lets you find out about different kinds of recombination activity and charge carrier collection that happen because of differences in makeup, structure flaws, or imperfections. EBIC can also find charge separation points in device cross sections and measure the length of carrier diffusion, which gives us useful information about how the device works.
Working Principle of EBIC
A method called electron beam imaging (EBIC) creates pairs of electrons and holes by exciting electron beams. It is possible for these charge carriers to move to the depletion region with an electric field if the diffusion length is long enough. Electrons and holes move in different directions because of this field, which makes current flow. This current, which is usually around µA, is measured by EBIC as it connects to an outside circuit. Cross-sectional and plan-view are the two ways to measure EBIC. View of the plan EBIC can help us figure out what part chemical makeup or flaws in structure play in creating and collecting carriers.
Applications
Cross-sectional
Cross-sectional EBIC, or emission beam interference, is a way to look into how well solar devices receive charge carriers. When the e-beam hits the depletion region of the p-n junction, the signal gets stronger. When the e-beam scans away, the signal gets weaker. There is a strong link between this difference and the diffusion length of minority carriers in the absorber layer of a solar device. If you fit the EBIC current to a simple exponential decline, you can get the diffusion length from the data.
Cross-sectional EBIC was used by Edri et al. to confirm the p-i-n working process in CH3NH3PbI3−xClx perovskites. They found two prime places with high efficiency: one was near the interfaces between the absorber and the hole-blocking layer, and the other was near the interfaces between the absorber and the electron-blocking layer. The first one is clearer, and p-i-n solar cells usually have this “twin-peaks” shape.
A flat CH3NH3PbI3−xClx device with hole transport material was studied using EBIC. It was found that the EBIC signal gets stronger as the e-beam moves away from the depletion region of the p-n junction and weaker as it moves closer to it. “Twin-peaks” is a shape that p-i-n solar cells often have.
Plan-view
Planar EBIC is a way to take pictures of the nonradiative centres in semiconductor devices. The current taken at each scan point is plotted on a map, which shows possible sources of current reduction. Less collection current and darker pictures can be caused by flaws like grain borders or dislocations. Researchers have discovered that Rb clusters don’t do anything optoelectrically, which stops current flow, causes nonradiative recombination, and makes devices work less well. They used EBIC along with nanoscale X-ray fluorescence (n-XRF) technology to connect weak EBIC signals with Rb clusters.
Electron Beam Induced Damage
Halide perovskite materials and systems that use electron imaging can break down when they are exposed to electron beams. Luo et al. discovered that scanning the same area of perovskite solar cells over and over again caused very little damage to the electronics. They saw two different EBIC responses at higher beam powers: a response that was almost steady at medium power and a straight loss that led to lasting degradation at high power. Even at low energies, the electron beam hurts the sample over time as it is exposed to it more. To keep things from breaking, the beam voltage can be changed slightly based on whether the hole transport layer (HTL) or back contact is bigger or thinner. Luo suggests starting the plan-view EBIC measurement at 7 kV/6.3 pA to keep the samples from getting damaged quickly. The layers should be about 70 nm thick for Au and 180 nm thick for HTL.
Electron Backscatter Diffraction
EBSD is a method used in SEM to find out the orientations of individual grains, the local texture, the relationships between orientations at different points, and the identification and spread of phases in polycrystalline materials. When used with other methods like electron beam induced current (EBIC), atom probe tomography (APT), PL, CL, and scanning probe microscopy (SPM), it is useful for solar cells. It is based on Kikuchi diffraction patterns. When used with other methods like electron beam induced current (EBIC), atom probe tomography (APT), PL, CL, and different scanning probe microscopy (SPM) methods, EBSD is especially helpful for solar cells. It also lets you look at how structural defects, local-orientation distributions, charge carrier separation, chemical defect segregation, and nonradiative recombination are related to each other. This method works best for solar cells when used with other methods like Kelvin probe force (KPFM), scanning spreading resistance (SSRM), or scanning capacitance microscopy (SCM).
Differences Between EBSD, XRD, and TEM
Crystallography methods like TEM and XRD are often used to look at the atomic structure of thin films. For TEM, samples must be a certain thickness, which can make them have different qualities than larger samples. It is usually only done in a few places, which makes it hard to get big amounts of data about grain size spread, grain direction effect, and grain size. XRD, on the other hand, doesn’t damage the crystals and can find out about the qualities of big areas of thin films. However, strain and particle shape make it more difficult to use. Standard XRD doesn’t give you local measures, which are needed to figure out how the grain sizes are spread out and how the optical or electrical qualities are affected by specific grain orientations or borders. Because it needs to be done in a synchrotron, nanoscale resolution XRD isn’t often used to link local crystal properties with photonic characteristics.
Working Principle of EBSD
Electron microscopy (EBSD) is a way to look at the crystal structure of perovskite solar cells. A cleaned sample is put in a scanning electron microscope (SEM) tube and tilted 70 degrees away from the usual path of the electron beam for standard readings. Because of the steep tilt angle, electrons can move along a longer path near the surface, which increases the scattering volume. The usual detector is a camera with a phosphor screen and a digital frame grabber built right in. The phosphor screen or straight detector is put close to the sample, which lets the local crystal direction be found. Lattice plane diffraction makes a diffraction pattern on the screen called a Kikuchi pattern. It is made up of bands that cross each other. These lines are automatically digitised and can be identified by their unique numbers. Crystal direction can be found by looking at the angles between the bands. To index, patterns are collected per scan pixel and compared to generated patterns. The microstructural picture that is made shows the phase, strain, crystal direction, and grain borders.
EBSD for Perovskites
Beam Damages and EBSD Detector
EBSD is a useful method for seeing the grain direction, structure, and phases of crystalline films. However, because it damages halide perovskites, it has been hard to use it for solar purposes. To get enough difference in the Kikuchi diffraction lines made by backscattered electrons, traditional EBSD detectors need higher amounts, which causes a lot of damage from the beam. Right now, EBSD mostly uses two kinds of detectors: the old-fashioned phosphor screen and camera, and the direct electron detector. Charge-integrating sensors like charge-couple devices (CCD), complementary metal-oxidesemiconductor (CMOS), phosphorous screen/camera, or a mix of these are what these detectors are based on.
There is a complicated link between beam energy and the ability to analyse samples that are sensitive to beams. Lower beam energy lowers the risk of damage to the sample, but it slows things down because the phosphor scintillator works less efficiently. An increasing voltage of 8 to 15 keV works best for most samples and improves the spatial precision of EBSD results and EDS measures that go with them.
This issue can be fixed with a more sensitive direct electron EBSD detector, but it has been said that it still damages electron beams. High-quality EBSD patterns are needed to find the right direction. These designs take a lot of electrons and hurt the film even more.
EBSD for Grain Size
To get a good picture of CH3NH3PbBr3 perovskite films with different grain sizes, Adhyaksa et al. used a 30 keV accelerating voltage and a straight detector to do electron backscattered diffraction (EBSD). The EBSD maps for 32±7 µm grains showed jumbled areas close to the grain border (GB), which did not show the crystalline direction. The different features of grain boundaries showed that in two cases, the border that could be seen under SEM wasn’t actually a GB from the EBSD, and there was an unseen GB inside the grain that looked smooth under SEM. The bottom panel showed Kikuchi bands that were made by combining several diffraction patterns for each pixel in a 200–600 pixel scanning area. The Kikuchi bands in scanning areas 1 and 2 were the same, but there was no clear backscattered diffraction in scanning area 4. The researchers showed examples of times when SEM contrast showed a GB that wasn’t there and times when SEM contrast was smooth even though EBSD showed a clear GB.
EBSD for Orientation
A solid-state EBSD detector was used by David et al. to look into how grain direction spread and PL intensity are related in perovskite. They discovered that there was a negative relationship between the grain orientation spread and the PL intensity. The PL intensity was lower when the grain orientation spread was higher. This shows that there is a direct connection between the amount of nonradiative defects in a certain area and the amount of strain within that area. Grains with larger grain orientation spreads have lower PL intensities and higher trap densities, which help with nonradiative recombination.
EBSD for Intragrain Defects
To study the crystalline direction and ferroic features of CH3NH3PbI3 twin stripes, Liu et al. used electron EBSD and advanced piezoresponse force microscopy (PFM). The research showed that the orientation relationship between the two walls of CH3NH3PbI3 is a 90° turn around ▨1 10▩, with the ▨030▩ and ▨111▩ directions running parallel to the surface. This EBSD image of a twin stripe structure gives us important information about the crystal lattice that we need to understand this kind of twin structure. The study also found that the PL map and the GB network from EBSD were not related in a good way in CH3NH3PbI3. This suggests that differences in the crystal structure may lead to more nonradiative recombination loss in that area.
EBSD for Crystal Quality
Erick et al. used EBSD to look into how the quality of the crystal CsPbBr3 affected the quality of the images they took. They found that the Kikuchi line was sharper when the IQRT number was greater, which meant that the crystals were of higher quality. They also used rapid pump-probe imaging and EBSD together to find a link between the movement of charge carriers and the difference in local diffraction patterns, which shows how good the crystals are. Researchers discovered that even on single crystal CsPbBr3 domains, tiny differences in crystal quality have a big effect on how well charge carriers can move.
To get more accurate EBSD data from the CH3NH3PbI3 thin film, the study used standard numbering and a measurement step size of 750 nm. The forescatter electron picture showed domain structures in the film that looked like twins, and the EBSD’s IPF-z orientation map made the domain structures very clear. It was easy to see that the inverted pole figure lined up with the <030> and <111> lines.
TEM
Sample Preparation and Transfer
For a SEM sample, the most important things are that it is somewhat sensitive and can fit inside the vacuum chamber. On the other hand, the TEM’s transmissive nature and shape limit the types of samples that can be studied. Most market TEM sample holders are made to hold very thin metal grids that hold samples. These grids are usually about 3 mm in diameter and a few tens of micrometres thick. A few nanometres of porous material, such as carbon or silicon nitride, cover these grids. However, there are also sample holders with different sample shapes that can be used instead.
Most detectors in a TEM are below the sample in relation to the electron gun. This means that the sample itself needs to be thin enough to let at least some of the electron beam pass through it. Usually, samples with a thickness of a few nanometres to a few hundred nanometres can do this. However, the thickest samples that can be used for each method vary.
To get a sample ready for TEM studies, it is usually necessary to either cut down a large sample to the right thickness or put a thin layer of the material directly on the TEM grid. Focused ion beam (FIB) milling, pulling a thin film off of a material, or just grinding a large crystal are common ways to do the first step. There are pros and cons to each preparation method that should be carefully thought through when choosing the method.
In FIB milling, heavy ions move material off the surface by transferring energy from the ions to the material on the surface. This is done by using the ion beam to dig into the sample. It is possible to make TEM-compatible samples from large specimens by carefully controlling the beam’s spread and position. This lets a very thin layer of the specimen be cut, to a thickness of less than 100 nm. However, the milling process can be very bad for the material in the lamella because gallium ions have a lot of motion and are charged. It can lead to both Ga+-implementation and the introduction of a lot of crystalline flaws.
When you scrape a film off of a base, you can use a film that could be used in a solar cell device. This lets you study a variety of chemicals that are important for the device. But removing a film manually is likely to hurt the film, change its shape, and maybe even add flaws at the crystalline level. This method can also lose relative spatial information, like the relative direction of grains or the make-up of GBs, because the film has to be put back on a TEM grid in a somewhat broken state.
A common way to get TEM samples ready is to crush a large crystal. This works especially well for hybrid perovskite single crystals. A mortar and pestle are used in this process to break up the crystal into very small pieces, about a few hundred nanometres across. It’s added a liquid that doesn’t dissolve to keep the powder inside and make moving the particles to a TEM grid easier. The dispersion can then be put on the grid with a pipette after the grid is locked in place with self-closing TEM tongs and a single drop is put on the grid. Putting an absorbent wipe between the tweezers and letting capillary forces pull the solvent away will get rid of the extra liquid. By putting the grid on a hot plate or in a pressure room, you can help get rid of the last of the solvent. Under an electron beam, the solvent likes to evaporate and blow up.
Putting perovskite thin films right on TEM grids is a good way to look at native film features like grain borders, relative grain angles, extended flaws, and secondary phases. Because the preparation methods are similar, there is a clear link to the traits found in devices. The films can only be studied from the top down, though, and information about charge carrier features in a solar cell device that is crosswise can be hidden or hard to see.
Both solution preparation and steam drying have been used to make TEM samples. The photoactive films in the most efficient single junction solar cells are solution-processed films. However, it has been hard to keep a full film with widths below a few hundred nanometres. Photoactive layers that are dissolved by heat tend to be less efficient, but you can better control the width of the film, which makes this method perfect for making TEM samples. It has been said that films of about 30 nm can be made, which makes them very good at letting electrons pass through and making pictures of native perovskite thin films with atomic precision.
Imaging Conditions
In a TEM (Transmission Electron Microscopy), the electromagnetic lenses are not fixed like the glass lenses in a visible light microscope. Often, alignment is needed at the start of each session. This can involve irradiating some parts of the sample with very strong electrons. Alignments can cause radiation that is very bad for beam-sensitive materials and can change the crystalline nature of a sample. When electrons hit hybrid perovskites, they break down quickly, losing their organic molecules and turning into stable lead halide crystals. This makes them very subject to beam damage. These crystals are much more stable under the beam than perovskites, and they need a higher amount of electrons to damage them.
For the user, it is very important that different parts of the sample be used for viewing and aligning. The damage caused by the electron beam could cause the specimen to lose material, making it smaller and changing the focal height of the sample. This is especially noticeable in samples that are very thin (about 30 nm), and it can make it hard to match them exactly.
A TEM usually works in either a broad-beam mode (TEM) or a targeted, scanning beam mode (STEM or scanning TEM). Each method has its own benefits. In broad-beam mode, the electron beam is set to be spread out over the sample, lighting up a certain area all the time. An electron detector measures the spatial strength of the electrons that are being transferred and makes a contrast picture, also known as a micrograph, from the whole area that is lit up and within the detector’s field of view.
In scanning mode, an electron beam is directed and moved across the sample. Signals are logged using a raster method, which is similar to how a SEM works. The electron beam can be directed to a width smaller than an atom’s, which means it can be used to collect a lot of information that is localised to the atomic level.
Beam Damage
When fast electrons hit a material with an electron microscope, they combine with the atoms in that material to make a picture. This contact can happen either elastically or inelastically, which can damage the object in different ways. Beam damage comes in many forms, such as electron charging in SEM, opening of the grain boundary in a FAPbI3 thin film, damage to the crystal structure after a single run of atomic resolution STEM imaging, and GB splitting seen in STEM.
Usually, lower amounts of electrons with lower accelerating voltages at lower temperatures are used to protect solid materials from beam damage. Possibly the best way to keep the beam from getting damaged is to lower the electron dose. However, this can reduce the number of signals that can be made and affect the precision. To lower the dose in the TEM, you can either lower the emission current, use a smaller condenser opening, or spread the beam. This needs to be balanced, though, by the exposure time of the device. A longer exposure time can record a better signal, but it also makes the total dose higher.
For the most accurate results, you should always take a picture of a “fresh” place that hasn’t been exposed before. To do this, line up the beam on one part of the sample, turn it off, and move it to a new part of the sample that is “in the dark.” As soon as the beam goes on, the picture is recorded.
In electron microscopes that scan, you can change the sharpness and stay time of the scan, which in turn change the total dose and beam current. The scan resolution of the coordinates where the beam is stopped is equal to the micrograph’s resolution. The dwell time is the amount of time that the beam stays in each coordinate.
When the increasing voltage is lowered, the electrons that hit the sample have less kinetic energy. This can lower the energy that is stored in the sample and the beam damage. Most of the time, 5 kV acceleration voltage is a good mix between resolution, signal strength, and beam damage for simple morphological imaging of surface and cross-section objects. Lowering the sample’s temperature can lessen some of the damage that burning does, but it may also cause phase changes in the material being studied.
Examples of Applications of TEM
We present four studies that used scanned TEMs to look at photoactive perovskites. Three of them are about low-dose imaging and diffraction of films that are cast directly onto TEM grids, even though these methods are likely to introduce errors. The fourth study looks at the features of perovskite solar cell devices even though the beams are damaged. It does this by using high-dose methods like FIB preparation and chemical EDX mapping. Using (S)TEM to look into hybrid perovskites in thin films, each study takes a different method. These include atomic resolution imaging, low-dose scanning diffraction, low-dose broad beam diffraction and imaging, and FIB preparation mixed with EDX mapping.
Atomic Resolution Imaging of FAPbI3 and MAPbI3
A scanning transmission electron microscope (ADF) can see individual stacks of atoms in a small sample. This makes it useful for studying all kinds of perovskites. The crystal structure creates unique picture intensity patterns that make it possible to accurately figure out what kind of atoms are in a certain area. Rothmann et al. were able to take the first pictures of a hybrid perovskite thin film with atomic precision using a JEOL ARM-200 analytical STEM with an aberration corrected LAADF setup. The total impinging dose per picture was lowered to about 66 e É−2 by dropping the emission current and using a circular detector. The atomic resolution stayed the same. Due to the low electron dose, the signal-to-noise ratio wasn’t great. To fix this, a linear Butterworth filter was used in Digital Micrograph software to get rid of high-frequency information and improve contrast. This method makes it possible to study mixed perovskites in a reliable and accurate way.
Correlating the Composition of MA1−xFAxPbI3 with Structure, Defects, and Device Properties for a Range of x
Single-Assisted Electron Microscopy (SAED) was used by Li et al. to look at the structure of single grains in MA1–xFAxPbI3 thin films. They found that when the amount of FA+ to MA+ changes, so does the “average” crystal structure of the perovskite material. The structure changes to a “average” cubic shape when the A-site cations are made up of 80% MA+ and 20% FA+.
Crystallographic twinning happens in the picture when the amount of FA+ goes up, and the number of twins grows as the amount of FA+ goes up until there are a lot of stacking flaws. Low-dose SAED was used to see these changes in the crystal structure because it only records electrons that pass through a single crystal grain. By spinning the perovskite thin film directly onto TEM grids and blowing nitrogen gas on them, samples of the different compositions were made from solutions.
The writers discovered that the solar cells that worked the best were the cubic perovskite ones that had the most order and no long crystal flaws. TEM is the only tool that could help the researchers connect changes in performance factors with local crystals and the presence of flaws. SAED made this possible.
Correlating Charge Carrier Traps with Crystallography Using Scanning Electron Diffraction and Photoemission Electron Microscopy
Doherty et al. used scanning electron diffraction and photoemission electron microscopy (PEEM) together in this work to find a link between the crystalline features of a thin film and the number of charge carrier traps in that film. When a sample is hit with UV rays, PEEM records the electrons that are released from the surface. These electrons can be excited from trap states within the bandgap without the valence band electrons being excited. An electron beam with a width of about 4 nm was then used by the authors to scan across the same sample in a scanning electron diffraction (SED) setup. They put special gold bits on the sample, which let them connect the PEEM signals with the SED data.
SED lets you record diffraction patterns with a very fine spatial resolution, about 4 nm for the beam’s width. According to the authors, charge carrier traps are present at certain surfaces and when there is a non-cubic phase. This was done by scanning places with high and low trap densities. One way that electron microscopy’s high precision and ability to work with crystals can be used to explain important things happening in perovskite solar cells is shown here.
A 300 kV JEOL ARM300CF microscope was used in the study. It was set up in nanobeam alignment with a convergence angle of about 1 mrad, an electron probe width of about 4 nm, a probe current of about 2 pA, a scan rest time of about 1 ms, and a camera length of 15.
Observing the Diffusion of Sodium from Glass Substrates into the Perovskite Layer
Kosasih et al. used STEM-EDX and other spectroscopy methods to look for sodium in the charge transport and photoactive layers of a perovskite solar cell device that had been ground with FIB. The sodium spread out from the glass base, which let it move through the device over several hundred nanometres. It mixed with bromine in the perovskite film to make bromine-poor perovskite that is high in iodine. This helps protect flaws and lowers non-radiative recombination. Even though these effects are good for the performance of solar cells, the authors warn that the longer-term effects are not known and can lead to sodium diffusion that doesn’t stop while the cells are working. In this case, it doesn’t matter if the perovskite crystal is perfect because sodium isn’t in the perovskite structure; if it is there, it must have come from the glass base. The FIB preparation may have caused some sodium to be redistributed, but the way the sodium is spread out in space suggests that this is due to diffusion and not beam damage. This research is a great example of how to use high-dose methods to clearly explain important things happening in perovskite solar cell devices.
Conclusions
Electron imaging is a strong method that can tell you important things about photoactive perovskites. This chapter talks about the basics of scanning and transmission electron microscopy. It also talks about how to set up an electron microscope so that it works best with materials for perovskite solar cells, how to avoid damaging the beam, and how powerful and flexible electron microscopes are. We still need to learn a lot more about perovskite solar cells, so electron microscopy is likely to stay an important tool for studying and making them better.
Leave a Reply