The chemical makeup of halide perovskites (HaPs) is talked about in this book. HaPs include methylammonium lead tri-iodide, organic-inorganic halide perovskite, and complicated systems with many organic and/or inorganic cations and halide mixes. If you change just a few of these materials’ chemicals, you can change how they behave in optical devices and how easily they can be shaped. For example, you can change their electrical bandgap and how stable they are in different temperatures and environments. Figuring out and understanding the crystal structure is important for the growth and improvement of perovskite materials. X-ray diffraction (XRD) is still a useful tool for checking the quality and structure of these materials.
More research needs to be done on the atomic and electronic structure of these materials, especially where they meet other materials used in solar cells. This is important for making the materials’ qualities even better. When you look at the surface, the mass, or the point where one part of a solar cell meets another part, like the metal contact or hole or electron carrying materials (HTM, ETM), you can see chemical changes. These differences or unevenness in the material could have a big effect on how well the solar cell works as a whole.
Spectroscopic methods, like direct photoemission spectroscopy (PES), can be used in experiments to learn a lot about the chemical makeup of the surfaces and contacts of materials at the atomic level. PES can tell you about the electronic structure, like how the valence band maximum (VBM) or the highest occupied molecular orbital (HOMO) changes based on the material, and how the energy of the different parts of the device are aligned. When used with theoretical calculations, PES can give atomic-level details about how the structure of electrons may explain how optical devices work.
Photoelectron spectroscopy
Photoelectron spectroscopy (PES) or X-ray Photoelectron Spectroscopy (XPS) was created in the 1950s by Prof. Kai M. Siegbahn and others at Sweden’s Uppsala University. PES gives information about chemicals. It was first called ESCA, which stands for Electron Spectroscopy for Chemical Analysis. Siegbahn won the Physics Nobel Prize in 1981 for his work on high-resolution electron spectroscopy. This part uses sources and the writers’ past work to explain the basic ideas behind PES.
Basic principles
Photoelectron spectroscopy, or PES, is a way to look at how materials, like halide perovskite materials used in solar cells, let electrons escape. The photoelectric effect is used, which means that when a photon with the right amount of energy (hv) is received by matter, an electron is likely to be released. A photo-emitted free electron, also called a photoelectron, has a certain amount of kinetic energy (Ek) that can be tested in the lab.
The idea that energy doesn’t change forms a link between the bond energy, the photon energy, and the moving energy of the electron that is released by the photon. To do a PES experiment, you choose the photon energy and measure the kinetic energy Ek. It is possible to find the energy difference (Ef(N 2 1) 2 Ei(N)), which is known as the photoelectron’s binding energy (E0b). When a photon comes in, it can only leave a material if its binding energy (E0b) is higher than the photon’s energy (hv).
The binding energy E0b is based on the vacuum level Evac, which makes things more complicated for solid-state samples. In the next step, the kinetic energy is adjusted for a work function () that shows the difference in energy between the vacuum level outside the sample and the sample’s Fermi level.
A radiation source, a sample, and an electron analyser are the most important parts of a PES experiment. It looks like the sample is a solid, but it could be in a different phase, like a gas or a liquid. Most of the time, PES studies are done in an ultra-high vacuum because the photoelectrons need a big enough mean free path to get to the electron analyser.
Figure 5.2A shows a broad spectrum of a mixed-composition lead halide perovskite (MA0.15FA0.85Pb(I0.85Br0.15)3). This spectrum can usually be broken down into two different energy regions. There are clear photoelectron peaks in the higher binding energy region that match to the atomic core levels. Core-level spectroscopy is the study of core levels. Valence-level spectroscopy, on the other hand, looks at more complicated electronic structures that are found at lower binding energies (0–20 eV).
Core-level photoelectron spectroscopy
Photoelectron spectroscopy, or PES, is a way to find out what the binding energy values are for core level peaks in materials for solar cells. It is easy to tell which elements are which in a spectrum because each has its own set of core levels and bound energies. The binding energies of lead (Pb 4p, 4d, 4f, 5p, and 5d) are all in the range of 0 to 700 eV. The core level with the most strength, the best energy clarity, and the obvious chemical shift is found for a certain element.
As shown in Fig. 5.4, the main elements found in halide perovskite solar cell absorbers have core level peaks that can be broken down into cations, metals, and halides (A), (B), and (C). It will also be possible to use these bands to help explain the ideas of “chemical shift” and “spin-orbit splitting.”
A big part of the success of PES’s growth has been the drive to get information about chemistry states. In a range close to 10 eV, the peak point can change based on the chemical surroundings of the photo-emitting atom. Sometimes, the idea of electronegativity can help explain which way the chemical shift is going. Iodine takes electrons away from bismuth, which means that the bismuth in BiI3 (formerly Bi31) has fewer electrons than bismuth that is solid. Due to this, the binding energy for the Bi 4f peaks in BiI3 is likely to be higher than that for Bi0.
The “charge potential model,” a simple electrostatic theory, has been used to explain chemical shifts. It looks at the electrical potential around the atom and how the charge of nearby atoms changes it. For newer and more complex theories, rest or “final state” effects are taken into account, and theoretical tools let us understand and predict chemical changes using the (Z 101) method.
The main atomic quantum number (n 5 = 1, 2, 3, etc.) and the angular momentum quantum number (l 5 = 0, 1, 2, 3; s, p, d, and f, respectively) of the electron in the photoemission process determine spin-orbit splitting. As seen in a PES experiment, this breaking can be seen for all levels except s-states (l6¿0). The formula nlj is then used to get a more accurate picture of a measured core level.
As a rough guide, the ratio (2j 1 1) shows how intense the two split components are compared to each other. This ratio is based on the amount of different spin state degeneracy pairs that can give j. The doublet split depends on the element, its redox state, and the core levels that are being looked at. It can be anywhere from a few meV to several 10s of eV.
A study by Grattel et al. used photoelectron spectroscopy to look into the chemical distribution of perovskite materials containing different cations (Rb1, Cs1, MA1, and FA1). The Cl 2p spectrum was measured from a perovskite called MAPbI3 that was made by mixing MAI with PbCl2. A BiI3 film was used to measure the Bi 4f core level, and a photon energy of 4000 eV was used to get almost all of the spectra. A photon with an energy of 1486.6 was used to measure the Sn 3d spectrum from MASnI3. A sample of mixed perovskite (Cs, FA, MA)Pb(I0.85Br0.15)3 was used to measure the K 2p spectrum at 758 eV. The precursor solution had 5% KI added to it.
When you measure the core-level photoelectron peaks and add up their strength and area, you can get quantitative information about the sample’s surface chemical makeup. The amount of an element A that makes up a material with up to i elements is shown in Equation 5.4: CAº%Ú 5 X IA = SA i.Ii = Si. The atomic sensitivity factor (SA) measures how likely it is that a photoemission event will happen and that the photoelectron will be found. It is based on H, the spectrometer transmission function,, a factor that describes how the emission is not uniform,, the angle between the polarisation direction of the incoming photon and the direction of the emitted photoelectron,, the cross-section that measures the chance of photo-ionization of a core level of a certain element, and, the inelastic mean free path.
In general, as photon energy rises, photo-ionization cross-sections fall. To do photoemission events at higher photon energies, you need a source of high photon flux, like a synchrotron. This is the IMFP, which is also called the electron escape depth. It is a probability-weighted distance that shows how far a photoelectron with a certain kinetic energy will move in a solid before it scatters because of interactions between electrons and phonons and/or between electrons.
Most people think of photoelectron spectroscopy as a surface-sensitive method. But what makes it surface-sensitive? Berglund and Spicer’s simplified three-step model of photoemission includes (1) an electron being excited by light, (2) travelling to the surface, and (3) passing through it. If the photoelectrons’ ability to pass through the surface depends only on their energy and not on where they came from, then step 2, which is how they get to the surface, determines how sensitive the method is to the surface. Due to inelastic scattering, energy loss means losing information about where the photoelectron came from. The IMFP determines the part of the material from which useful photoelectrons with information can be gathered.
A method called photoelectron spectroscopy (PES) is used to look into the chemicals that make up things, especially perovskite solar cells. The surface sensitivity of PES is mostly because of the scattering length of photo-emitted electrons. It can only identify things that are 100 to 1022 at%. Because it is so sensitive to the surface, PES can be used with methods that are more sensitive to the mass, like XRD or Raman spectroscopy.
When a photoelectron is bound but almost free, the IMFP depends on how strong the interactions between electrons and phonons are, which depend on the photoelectron’s kinetic energy. For instance, about 1 2e63% of photoelectrons travelling normally towards the sample have not been inelastically scattered up to one λ from the free surface. This means that information about where they came from at the electronic level is preserved, and it adds to the intensity at the core-level energy. The in-elastically spread photoelectrons will add to the spectrum’s background level if they are found.
The IMFP for a photoelectron that is bound but almost free is the strength of the interactions between electrons and phonons, which depend on the electron’s kinetic energy. For this reason, the IMFP for a certain photoelectron kinetic energy might be different for different materials. However, it has been shown that the IMFPs of many materials are similar at a given moving energy. This has led to the creation of a “universal curve” for IMFP as a function of energy.
It is usual for recorded readings of the IMFP to be a factor of ten different for the same molecule and the same amount of energy. As the electron’s moving energy rises, the differences between stated IMFP readings begin to shrink. If users want to do absolute measurement, they should be aware that IMFP differences could lead to errors. It is more correct to use relative measurement, which is the ratio of the amount of element A to element B. This is especially true if the IMFPs for the core levels from elements A and B are similar, which usually happens when XPS is done with higher photon energies because λA and λB may be very close to each other.
It is possible to do detailed studies of the chemical make-up of materials using PES. The surface sensitivity of PES is largely due to the scattering length of the photo-emitted electrons. PES has low detection limits (100–1022% at%), which makes it useful to use with techniques that are better at detecting bulk, like XRD or Raman spectroscopy.
Valence band photoelectron spectroscopy
In experiments, valence band spectra show or get close to a material’s filled valence electronic states, which determine its physical features. It’s harder to figure out what valence band spectra mean than core-level spectra because core-levels are more like atomic levels and include inputs from more than one element and orbital. Calculations of the electronic structure help us understand valence band spectra. Valence band spectra are very important to study in solar cell materials research because they show where the valence band edge is in terms of energy for artificial or mixed organic materials or the HOMO for polymeric and molecular organic materials compared to Fermi and/or vacuum levels. With this new knowledge, energy level graphs for optoelectronic devices can be made, and chemical treatment or change can be used to control how well materials fit energy levels.
From UPS to HAXPES: variation of the photon excitation energy
Photoelectron spectroscopy is a broad field that includes many sub-techniques that are grouped by the amount of photo-excitation energy they use. Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) are widely used in labs, and full systems are available for both. A helium gas discharge lamp is used by UPS to make XUV photon energies of 21.2 eV or 40.8 eV. Neon or other neutral or noble gases can also be used.
Because the IMFP depends on how strong electron-electron and electron-phonon scattering are, the low photon energies used in UPS make it hard to say how deep the probe is. At lower kinetic energies, the “universal” curve splits for different materials. This means that the depth of analysis is very material-dependent and could change by an order of magnitude.
Scientists have used UPS to study useful materials. It has been used to find the valence band density of states, the energy position and shape of the valence band edge, and the materials’ work function. It is possible to understand how opto-electronic devices work by making energy level graphs. It is possible to figure out the interfacial energy-level/band alignment by slowly forming an interface between two different materials and then taking sequential UPS measurements. This is necessary to know whether the interface makes it easier or harder to efficiently extract or inject charge.
XPS uses photons with more energy than UPS, usually Al-K± (1486.7 eV) and Mg-K± (1253.7 eV). XPS uses X-ray sources that can be paired with a (crystal) monochromator to lower or get rid of the satellite photon lines that the X-ray sources produce. This makes the recorded spectra more accurate by giving them more energy precision. XPS is sometimes used to get valence band spectra, like UPS, but it is more often used as a core-level spectroscopy method to look into the features of different materials, such as their chemical makeup.
The growth of synchrotron radiation has led to the creation of spectroscopic methods where the energy of the photons that hit the sample can be changed from a few eV to several keV. These techniques can be used for both standard UPS and XPS measurements. PES beamlines that give soft-X-ray energies (20–1500 eV) are sometimes called SOXPES (Soft X-ray Photoelectron Spectroscopy) beamlines. On the other hand, HAXPES (Hard X-ray Photoelectron Spectroscopy) beamlines are used for photon energies from 2 to 15 keV.
In conclusion, there are a number of things to think about when changing the incoming photon energy used in PES and switching from UPS to HAXPES. When electrons’ moving energy goes up, their IMFP goes up too, which means that more depth can be explored. Depending on the element and orbital state, the cross-section changes at very different rates as the photon energy changes. With careful selection of the input energy, the photon energy can also be changed and tuned to bring out or hide certain orbitals.
PES investigations of halide perovskite materials
A common way to study halide perovskite materials is with photoelectron spectroscopy (PES). But care must be taken both before and during the measurement to get useful details about the electronic structure, chemical makeup, how it interacts with other materials, and how the energy levels line up at the HaP/buffer layer surfaces. These safety measures include checking the binding energy, properly preparing and handling samples, and protecting against radiation damage. PES has been used in studies that have taught us new things about HaP materials, showing how important it is to handle and prepare them carefully.
Precautions
Binding energy calibration
Angle-integrated photoelectron spectroscopy studies find out how strong photoelectrons are based on how fast they are moving. The zero eV Fermi level is used to connect the kinetic energy of a photoelectron that has been identified to the electronic level where the photoelectron came from. The kinetic energy of electrons photo-emitted from the Fermi level can be used to measure the kinetic energy of a photoelectron that has been found.
The Fermi level can only be found in materials that are in balance, and the electrical system is only slightly changed during photoemission. A sigmoid function fit can be used to get a spectrum of the Fermi step, usually from a metal sample.
An outside sample can be used for samples that don’t have a measurable Fermi level. A piece of gold foil that is electrically connected to the sample stand or stage that is being used to measure the real sample is often used as an external sample. As the Au 4f7/2 peak is at a known value of 84.0 eV in relation to the Fermi level, adding 84.0 eV to the kinetic energy of the Au 4f7/2 photoelectrons will give you the photoelectron kinetic energy that is equal to the Fermi level.
Using the sample itself for internal energy assessment works the same way, but readings of reference core levels from the sample are used instead. Thin layers of HaPs are usually put on top of an electrical substrate. If the substrate can’t be found, a reference like a “adventitious” carbon (C-C hydrocarbon bond) is sometimes used.
When samples are insulating, things get even more complicated because after photoelectrons are released, a leftover charge can show up on the sample’s surface. This can cause the peaks to become wider and the number to move up.
Sample preparation and handling
Solution-coating methods are used to make halide perovskite thin film examples in gloveboxes that are filled with a dry, harmless gas. Photoelectron spectroscopy studies are done in places with a very high or very low vacuum. The thin films are in clean environments when they are being made and measured, but there is a chance that they could be exposed to dirty environments when the sample is moved from one environment to another. Water vapour, air, and sun are all environmental factors. Iodide moves through MAPI films when they are exposed to light, and both MAPI and FAPI films break down when they are exposed to water vapour. After one month of being exposed to air, MAPI films break down a lot. When stored in a low vacuum, they break down less quickly than when exposed to air. To get useful results, it’s important to keep the sample from breaking down by accident.
The main goal of the sample shipping and handling process is to lower the chance that the sample will break down because it is exposed to the environment without being protected. Getting rid of as much wetness, air, and light as possible is usually the best thing to do. A double (air) barrier system is used to protect samples that were made away from the spectrometer system before they are sent to it. Before the measurement, the sample is taken out of a place with no air and put into a transfer bag that is hooked up to the measurement system’s load lock.
Radiation damage
In photoelectron spectroscopy, photons are used to hit a sample and make photoelectrons, which are then gathered and analysed. Soft X-rays have been shown to break down MAPI films after long-term contact. For many HaP solar cell materials, changes in the nitrogen and lead core level spectra can be used to tell when a sample is breaking down. MAPbI3 breaks down into lead diiodide (PbI2), which can be seen by the N 1s peak strength going down and the Pb core level peak position moving around. The Pb spectrum can also be used to track the formation of solid Pb0 over time. Damage from radiation can be more minor, and the amount of elements changes depending on how much radiation is absorbed. This behaviour might happen in complex HaP compounds because of ion movement and flaw creation. We need to do more thorough preliminary work in which we measure each core level range for a longer time. Using screens to lower photon flux, setting the beamline parameters, and shifting the treated spot on the sample on a regular basis are all useful ways to limit radiation harm.
Selected results
The way that halide perovskite (HaP) materials are studied has changed over time as they have become more complicated. The most common type of HaP is MAPbI3, which is made up of four elements: carbon, nitrogen, lead, and iodine. It also has one cation and one bromide. At the moment, the best HaP solar cells have at least four cations (MA, FA, Cs, K) and two halides (I, Br). To get more detailed answers about where the cations are located, research has moved from lab-scale PES sets to more advanced experimental stations with photon sources that can be changed, like synchrotron beamline/end-station. In this way, we can get a better idea of how the cations are spread out in HaP solar cells.
Chemical characterization
Photoelectron spectroscopy (PES) has been used to describe chemicals since the early days of developing halide perovskite solar cells. Researchers using PES have added a lot to the conversation about chlorine’s role and where it is located in the perovskite film. It was tried to make thin films of the mixed halide MAPbI2Cl by combining MAI with PbCl2 in a 3:1 ratio. PES made it clear that there was no chlorine on the surface of the products. Dopants and chemicals were the centre of later studies because they were easy to find. Elements like Rb1, Cs1, or K1 in Fig. 5.4A, as well as Ag, Cu, and Na, are good examples. It has also been said that solid lead or bismuth can be found in HaP pictures, and this has been talked about through PES.
The growing process (Vapour Assisted Solution Process, or VASP) that turned PbI2 into MAPbI3 over time could be seen and proven with PES. First, lead di-iodide was spin-coated onto a TiO2/FTO substrate. Then, it was put in a vapour of MAI for a longer and longer time. As the film went from yellow to dark grey, the creation of the dark, light-absorbing perovskite phase could be seen. The iodine-tolead (I/Pb) intensity ratios went from 1.9 (PbI2) to 3.0 (MAPbI3) after 60 minutes of reaction time. These ratios were found by measuring the iodine and lead core levels with soft-XPS. By measuring the carbon 1s and nitrogen 1s core levels one after the other, we saw that the amount of methylammonium increased compared to the iodide and lead in the PbI2.
Because its surface sensitivity can be changed, PES is the only way to clearly show how much a HaP has broken down. When MAPbI3 comes into close touch with water, it breaks down into PbI2. The sample changes colour from dark grey to yellow (PbI2), which makes this kind of bulk and surface breakdown easy to see. Optics, on the other hand, don’t always make it possible to see small changes in the sample’s makeup at the surface.
PES has been used to connect how changes in processing affect the make-up of the surface and nearby areas of HaP films that have many cations and halides. When looking at a mixed perovskite, there are clear changes between the surface and the bulk. The main difference is probably the iodine excess seen near the surface and the relative variation in the first few nanometres of the film surface. As shown in the diagram of Fig. 5.10D, changing the amount of PbI2 that is added changes the surface in a big way.
Electronic structure
A valence band spectrum tells you more than just where the VBM’s energy is; it also tells you about the hybridised atomic levels that make up the valence band quasi-continuum of states. Calculation methods like DFT, which are explained in this book, are very important for getting a clear picture of what a valence band means. After 45 and 83 days of keeping in argon or air, Fig. 5.11 shows how a MAPbI3 sample that was formed on a TiO2/FTO substrate changed over time. After that, the information was used to figure out the structures of an experimental valence band of a mixed perovskite. The conduction band (unoccupied sates) is also important to the electronic structure of a material. This can be studied directly with Inverse Photoemission Specroscopy (IPES) or X-ray absorption (XAS). Resonant Inelastic X-ray Scattering (RIXS) is a newer and more hopeful method that gives information on both the border electronic states and the detailed picture of a material’s electronic structure.
Energy level alignment
Photoelectron spectroscopy (PES) is an important tool in solar cell study for figuring out how different materials, like light filters, electron/hole transport materials, and metal contacts, line up in terms of their energy levels. For devices to work well, the energy must be aligned perfectly at the points where the different parts of a solar cell meet. Before you can draw an energy map, you need to do an exact energy assessment. PES spectra are usually shown after being calibrated to the Fermi level (EF), and linear extrapolation is used to find the location of the valence band peaks (VBM). When looking at the energy difference between materials that are similar, this method works well. However, it can be harder to figure out the difference between the VBM and EF, particularly for materials that have n-doped elements. When this happens, the VBM is better explained by looking at the lower number of states at the real edge of the valence band. This can be done with a logarithmic scale.
UPS has been the most common way to guess where the VBM is because it is an in-house tool that makes it easy to get spectra and measure both the VBM and work function of a material at the same time. This makes it possible to plot band alignment maps directly against the vacuum level (Evac). This picture shows how the VBM position changes with six different samples and how the UPS spectrum and DFT calculation of the number of states of MAPbBr3 are compared.
It is also talked about how the valence band spectra (14–2 eV binding energy range) of mixed perovskite materials change with photon energy. It is also talked about how the valence band spectra (14–2 eV binding energy range) of mixed perovskite materials change with photon energy.
The study talks about how to characterise perovskite solar cell materials, with a focus on how sensitive the surface is to UPS and how different the perovskite materials are on their surfaces. Most of the time, the VBM found by XPS and HAXPES is more reliable and repeatable than the one found by UPS. This picture (Fig. 5.12C) shows how the valence band spectra of mixed perovskite materials change with photon energy. The noted differences in the valence structure are mostly due to changes in the cross-section of different orbitals that happen when the photon source’s energy is changed.
When the photon energy is higher than 500 eV, the valence band is dominated by the iodine, lead, and bromine. When the photon energy is lower, the nitrogen and carbon will be easier to see. It is possible for carbon-based pollution on the samples’ surfaces to get worse when low photon energies are used.
Figures 5.13-5.15 show examples of how HAXPES, XPS, and UPS can be used to match band energies. Look at Fig. 5.13A to see the valence band spectra of various perovskite solar cell materials and compare them to the one of a pure TiO2 base. A close-up look at the valence band edge in Fig. 5.13B shows how the VBM was found. The absolute difference (EVBM 2 EF) is also different based on the method used, such as linear projection or the actual edge.
Finally, Fig. 5.15 shows a band alignment that was found by measuring UPS on a mixed perovskite that was formed on two different ETMs, either TiO2 or SnO2. The writers were able to explain why SnO2 is better than TiO2 because it has an easier way for electrons to move from the perovskite conduction band to the SnO2 CB band than TiO2 does, which has an energy barrier.
Conclusions and outlook
Halide perovskites are a very interesting material for scientists and engineers to study. The photoelectron or photoemission spectroscopy (PES) method is a strong way to find out about the materials’ chemical make-up and electronic structure. PES uses the photoelectric effect and has two types: core-level and valence-level spectroscopy. These are based on the binding energy region that is studied. Core-level spectroscopy measures the surface’s elemental make-up with a low detection limit of 100–1022% at%, and valence-level spectroscopy pulls out electronic levels important for the operation of optoelectronic devices.
The photo-excitation used in each sub-technique makes them different from each other under the name “PES.” These studies have taught us a lot about the chemical makeup of the surface and interface of HaP materials and their electronic structure. They have also helped us understand how the energy levels line up at the interfaces between HaP and buffer layers in HaP-based solar cells. To get useful data from PES measurements, it is important to adjust the binding energy scale, make sure that photo-, oxygen-, and water vapor-induced degradation doesn’t happen while the samples are being prepared and transported, and to carefully watch for damage caused by beams of radiation.
PES tests on halide perovskites showed that chlorine wasn’t added, that PbI2 slowly changed into MAPI, and that MAPI’s surface was breaking down. We found out what parts of the valence electronic structure lead, iodine, and other elements play by comparing density-of-states calculations to valence band measurements. It was found that PES can match the energy levels of several types of halide perovskites with materials that are usually used as electron-selective contacts in solar cells.
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