Due to the fact that they don’t damage the materials being studied, optical absorption and photoluminescence spectroscopy are very important. To find out how well solar cells convert light into electricity and what their photovoltaic properties are, it is important to measure the bandgap and the amount of light that is received. To understand losses in optoelectronic devices, you need to look at light in semiconductors.
Optical absorption spectroscopy
The absorption of a photo-absorber semiconductor is important for its use in optoelectronic devices because it sets the optical bandgap, which is needed to figure out the theoretical limits. Thin film-based perovskite works well because metal-halide perovskite has a straight bandgap. For semiconductors with an indirect bandgap, like silicon, to receive the same amount of light, they need bigger plates.
Steady state UV-Vis-NIR spectroscopy
ultraviolet-visible-near infrared spectroscopy, or UV-Vis-NIR, is a way to measure how much light is absorbed in the ultraviolet, visible, and near infrared range. It works with fluorescence/photoluminescence spectroscopy. In this type of spectroscopy, photoluminescence happens when an excited state changes to a ground state, and photon absorption happens when a ground state changes to an excited state. The Beer-Lambert rule is used to measure the absorption. It says that A is the absorbance, I0 is the intensity of the light that hits the material, and I is the intensity that is transferred. It is possible to get a full absorption spectrum of the perovskite semiconductor by scanning all bands from UV to visible to near infrared. The transmittance (T) is equal to I/I0, which is written as %T. The absorbance (A) is equal to 5 log10ºT times %T. Reflectance (R) is equal to I/I0, which is generally shown as %R. Because perovskite samples in solar cells can be crystalline or polycrystalline, simple UV-Vis readings may be off because of how much they reflect and scatter light.
Photothermal deflection spectroscopy (PDS)
Photothermal Deflection Spectroscopy (PDS) is a very sensitive way to measure absorption. It can find absorption coefficients as low as 1 cm21 or an absorbance level as low as 1025, which is a dynamic sensitivity range of 4-5 orders of magnitude. Jackson et al. were the first to show this effect in 1981. It is mostly used to find flaw states in artificial semiconductors such as a-Si, GaAs, InGaAsP, and others. Transverse PDS techniques and collinear PDS methods are the two major types. Both systems have a single-color excitation source (or heating beam) that can be changed and a continuous wave (CW) probe laser source with a set wavelength that is used to look into how the sample absorbs light at a certain wavelength.
Standard methods for measuring absorption, such as UV-Vis-NIR spectrometers, are prone to mistakes because of optical effects such as light splitting, reflection, and interference, which lowers their sensitivity. The PDS is not affected by light because the measurement is based on the sample heating up because of absorption, which happens when excited species relax without giving off radiation.
Depending on the lamp used, the measurement range changes. For example, an ozone-free Xenon light would make it easier for the PDS setup to measure absorption in the 380–2100 nm wavelength range. Most of the samples used for PDS readings are thin films that are coated on quartz surfaces.
In a 1D setting, the angle (φ) of the probe laser beam displacement is given by
φ 5 s^n^d^dT ˠ dTðz; t^dz (3.3), where n is the sample’s fluid’s refractive index, dT dz is the temperature difference, and s is the length of the interaction path.
A lot of information about the sample thickness, absorption coefficient, heat diffusion coefficients, and measurement frequencies on the displacement signal was given by Rosencwaig and Gersho.
When using the PDS to measure, it’s important to remember that (1) absorbance is directly related to the strength of the PDS signal for an optically clear sample, (2) the PDS signal stops working for thermally thin samples (l, ¼s) at ±l 5 1, and (3) the PDS signal stops working for thermally thick samples (1, l, ±, 1μs) at ±μs 5.
Estimation of the bandgap
Estimating the bandgap is an important part of measuring optical absorption. There are different ways to do this by using absorption data from UV-Vis-NIR or PDS techniques. To get the Urbach energy, a linear fit is made on the PDS absorption bands. The equation E = hc^2λ is used to find the band gap energy. Here, h is the planks constant, c is the speed of light, and λ is the length of a light wave in meters. This method is necessary to describe perovskite solar cell materials.
Tauc plots
As a result of measuring absorbance and transmission with UV, Vis, Nir, PDS, or other methods, such as Diffuse Reflectant Spectroscopy (DRS), the absorption coefficient () is found. What is the absorption coefficient? It’s ± 5 A hv2Eg, where A is the absorbance and hv is the photon energy. To find the bandgap (Eg), you need to plot a graph of (±hv 1¾n against hv) and the hv axis slope. Indirect bandgap semiconductors have n 5 2 and 3, while direct bandgap semiconductors have n 5 1 2. In very good perovskite devices, most of the photoabsorbing films have straight band gaps. The derivative method is a simple way to find the bandgap. It involves taking the derivative of the absorption spectrum from different techniques and finding the point of the peak with the highest strength.
Near band edge trap states
Near bandgap flaws in semiconductors can be caused by many things, including how the sample was processed, the crystallinity, the disorder in the structure, defects, and doping. These flaws have a big effect on how charges move and other electrical features that show how good a semiconductor device is. You can get a rough idea of these close band ends by guessing the Urbach energy. This can be done with PDS data or any other sensitive absorption measurement method. If you put the absorption bands on a natural logarithmic scale and find the negative slope of the linear fit to the Urbach tail (or Urbach front), you can figure out the Urbach energy EU.
The Urbach energy rule has been explained by a number of different theoretical models. However, there is still no single theory that explains where Urbach energy comes from; it is best thought of as an observational measure. There are a few other measurement methods that can be used to accurately measure absorption, but they all depend on measuring photocurrent and can only do so for exciton species that can create a free charge pair and doped samples that have a fixed ion and a free opposite charge. As shown in Fig. 3.4, PDS, on the other hand, figures out how much each of these species contributes.
When tin (Sn) is added to perovskite samples, natural doping and free charge carriers that can’t be removed by electrical measures cause absorption data that isn’t accurate because it doesn’t show the input from doping that is picked up correctly by PDS. For examples that haven’t been doped, like lead-only perovskite CH3NH3PbI3, both electrical and PDS results are similar. So, picking the right sensitive absorption measurement is very important because the results have a big effect on the study of the quality of the semiconductors and their electrical features.
The PDS method is widely used to find out the near band edge trap states of many semiconductors. It is especially useful for making titanium dioxide better because it is a key semiconductor for the electron transport layer in perovskite solar cells and dye-sensitized solar cell architectures. It also helps create an interface with the fewest defects possible. The absorption spectra near the band-edge of TiO2 have changed a lot after two different treatments on the titania mesoporous layer (mp-TiO2): the subgap level and Urbach energy of the titania went down a lot with the TiCl4 treatment [20] and the lithium treatment [23].
Adding different modifiers to the top surface of mp-TiO2 can also passivate the intrabandgap states within TiO2 and make the contact with the perovskite top layer better. We used the PDS method to look at the normalised PDS and photocurrent (EQE) spectra of CH3NH3PbI3, CH3NH3Pb0.4 Sn0.6I3, and CH3NH3 Pb0.2Sn0.8I3 perovskites.
Absorption properties of metal-halide perovskites
Metal-halide perovskites are very good at absorbing light, which is very important for making solar cells work well. This absorption lets a thin photoactive layer be used in the solar cell to collect light from the sun, which lowers the amount of photo-generated charges that mix again. Metallic-halide perovskite can absorb light much better than silicon, and it can absorb light about the same as CIGS, CdTe, and GaAs, which are all thin film solar cell materials.
Strong light absorption is one thing that metal-halide perovskites do. They also show a clear optical absorption line that is close to the material’s stated band gap energy. In order to look at the optical absorption edge and a small Urbach energy of 15 meV for CH3NH3PbI3, photo-thermal deflection spectroscopy (PDS) and Fourier transform photocurrent spectroscopy (FTPS) were used. The low Urbach energy means that the structure is not very disorganised and there are no deep band gap states that can be seen with the naked eye.
The high open-circuit voltage for CH3NH3PbI3-based solar cells may be linked to the sharp absorption edge and low Urbach energy. Materials for solar cells with low Urbach energies also tend to lose less voltage. It has been shown that the real and imaginary parts of the dielectric constant for CH3NH3PbI3 at 300 K change with frequency, as do the absorption coefficients for different solar cell materials.
Light absorption process in metal-halide perovskites
There are two types of bands in CH3NH3PbI3: the top band is made up of halide p-orbitals mixed with lead states, and the bottom band is made up of lead p-orbitals mixed with a smaller amount of halide states. When light is absorbed close to the band gap energy, it mostly moves between these bands. Organic cations don’t play a part in light absorption close to the band gap energy.
But if you change the organic cation, the Pb-I octahedrals will change, which will change the band gap energy. When the organic cation is changed from methylammonium (MA) to formamidinium (FA), the perovskite crystal structure goes from being tetragonal to being almost cubic. The visual features of the perovskite with formamidinium cations change because of this change in structure. The band gap value goes down.
There were changes in the structure because the organic cations were different sizes and there were electrostatic interactions between the cations and the negatively charged Pb-I matrix. There are changes in the Pb-I matrix, such as bond length, bond angle, and octahedral tiltings. These changes affect the electronic structure and band gap value.
Different cations have been added to the perovskite structure, which changes how well it absorbs light. To get more stable perovskite materials, like Caesium, it’s important to look into the role of different cations in the perovskite structure. When the P-I grid changes, light absorption changes too. This is important for the features of the solar cell.
Excitons in metal-halide perovskites
To make a photocurrent, solar cell devices need an excited electron to move away from a positive hole. The perovskite is in between an electron and hole extraction layer in perovskite solar cells, so it’s important to understand the exciton and how it splits into free charges. When the first perovskite solar cells were made, it wasn’t clear how the charges would be separated at the points where they met the electron or hole extraction layers. At room temperature, experiments showed that charge separation can happen directly in the perovskite material, creating a photocurrent in the perovskite layer. At 4 K, the exciton binding energy was found to be about 16 meV, which is about the same as in other III-V semiconductors with the same band gap energy. The exciton binding energy in the perovskite is even lower at room temperature, which means that photo-induced carriers are the only ones that can move the exciton.
Tuning of the light absorption spectrum via chemical modifications in metal-halide perovskite
Metal-halide perovskites are very important in solar cells because they can change the band gap, which is necessary for single, tandem, or multi-junction solar cells to work at their best. Adding iodide to bromide can make the band gap of CH3NH3PbI3 bigger. The pure bromide perovskite CH3NH3PbBr3 has a band gap energy of about 2.3 eV, which is a lot higher than the iodide-based perovskite’s band gap energy of about 1.6 eV. This change in band gap energy is caused by changes in the valence band structure, with the halogen taking over the electronic structure.
Changes in the bromide or cations can be made to tune how much light lead-based perovskites absorb. Lead cations, on the other hand, can be switched out for other metal ions in the structure, like tin, silver, or bismuth halides. Tin-based perovskites have a light absorption edge that is moved to the red. This lets perovskites absorb light over a wider range of wavelengths, including those close to infrared. The band gap in bismuth halides is bigger than that in lead-based perovskites. The band gap in Cs3Bi2I9 is about 2 eV.
The material is interesting for solar uses even though it has an indirect band gap because it absorbs a lot of light. Putting bismuth and silver halides together has interesting optical and solar qualities. These halides can be used to make double perovskite materials. Sun cells can also use mixed bismuth and silver halides, which have interesting visual qualities. Copper- and germanium-based perovskites also absorb light in interesting ways that could be used in photovoltaics.
In general, metal-halide perovskites seem to absorb light very well. It is likely that many more new metal-halide perovskite materials with different light absorption bands will be found that are useful for solar cells.
Photoluminescence spectroscopy
The high performance of perovskite optical devices can be seen in the emission properties of perovskite filters. Depending on their bandgaps, perovskite semiconductors can absorb a wide range of light. This creates charge carriers that move towards the lowest point in the conduction band and the highest point in the valence band. They then give off a strong signal through radiative recombination. Perovskites made of iodide have a wide spectral response; they take in photons with different energies and mostly create free charge carriers. Around the top of the valence band and the bottom of the conduction band, these charge carriers come back together. This makes an emission that is bright and narrow. If you change the source of stimulation, you can study either photoluminescence, electroluminescence, or cathodoluminescence. Understanding the photophysical processes going on inside working perovskite solar cells is possible by studying photoluminescence features. This is because photovoltaic uses light from the sun’s range.
Processes involved in photoluminescence
In a solar cell, the charge carriers that are made in the conduction and valence bands of perovskite create a unique charge density profile within the film. This profile is based on the absorption coefficient at each wavelength. Part of the carriers are either collected at charge selective contacts or stay fixed at the film to build up a certain Fermi level. This depends on how the device is working. There are two types of recombination processes: radiative recombination and non-radiative recombination. Charge carriers will join back together after a certain amount of time, which is called carrier life time, if they are not divided and collected at contacts. When there is radiative recombination, one pair of electrons and holes gives off a photon instead, which can be seen in the Photoluminescence (PL) spectrum.
Light rays with more energy than the band gap are used in PL spectroscopy to excite the film and record the emission spectrum. It is important to be extra careful with samples that are delicate to air and humidity. For example, the spectrum should be measured in a neutral environment or safety layers should be used. The strength of the PL peak at the band ends shows that charge carriers are radiatively recombinating. This can be used to figure out how charges are recombinating in the film and how they are collecting at the contacts.
The PL spectrum shows the band edge, whether there are excitonic peaks or trap states, and the strength of the interactions between electrons and phonons. The temperature-dependent PL spectrum can be used to figure out the exciton binding energy. This is important for characterising and improving solar cell devices. The traps seen in metal-halide perovskite films can also be caused by light. This has an affect that can be undone and can happen in the dark or with oxygen present. PL spectroscopy that changes with temperature can be used to look at the energy and number of trap states.
Diffusion length and carrier lifetime
Time-dependent photoluminescence (PL) patterns are caused by the exchange of free electrons and holes when there isn’t a charge selective layer present. But when there is a charge selective layer present, the movement and collection of charge carriers towards contacts mess up the mixing of electrons and holes, which changes the expected PL decay times. A good decline time is the time it takes for the PL peak to drop to 1/e of its original value.
The perovskite PL decay time shows the free electron-hole recombination time, also known as the electron life time (τrec) in the device. When charge extraction layers are present, the PL decline time (τ) is affected by both charge movement (to extraction layers, τtrans) and charge recombination (τrec).
There should be a big difference between the collection time and the synthesis time for a photo-absorber to work right. In very efficient devices, the time it takes for charges to move is much faster than the time they combine again, so it makes sense to connect the PL decay time to the time it takes for charges to move. From the carrier travel time and film thickness, the Einstein relation can be used to find the charge diffusion length.
Photon recycling in metal-halide perovskites
Radiative recombination can be reabsorbed in perovskite films, which makes it possible for charge carriers to be made. According to Pazos-Quton et al., the Beer-Lambert’s law of photon absorption should hold true for perovskite films, but the actually recorded PL spectra don’t match up with this. Higher yields are possible because of this effect, but it doesn’t have a big impact on a 300 nm perovskite film. It has been shown that the photon recycle effect lowers the external photoluminescence quantum yield (PLQE) from the internal yield in metal halide perovskite. This is proven by the fact that PLQE is much higher when perovskite thin films are deposited on textured substrates. This is because perovskite films formed on textured substrates efficiently let light out. Using a textured active layer to boost the PLQE of the halide perovskite layer in solar cells and light emitting diodes (LEDs) is a good way to improve the efficiency of conversion.
Exciton binding energy and excitonic peaks
Exciton binding energy is an important factor in solar performance because it shows how well charges are separated in the excited state. When the exciton binding energy is low, like 25 meV for lead halide perovskites, only certain charges can be collected at the contacts. Most of the time, Elliot’s theory is used to fit the near band edge absorption spectrum and find the exciton binding energy. The temperature-dependent reduction of photoluminescence can also be used to find the exciton binding energy. As you get closer to the band edge, you can see sharp peaks that are excitonic. There are no such peaks in the absorption spectrum for perovskite phases like CH3NH3PbI3 that are good at photovoltaics and charge separation. In the UV-Vis band, however, MaPbBr3 or alloyed perovskites with a lot of Br can show an exciton absorption peak. In the photoluminescence spectrum, these excitons also leave marks at wavelengths that are close to each other.
Tunability and stability of PL in alloyed perovskites
The perovskite absorption material, which has the general formula ABX3, can be changed to change the wavelength of photoluminescence in a controlled way. Light goes through about 30 internal echoes in a curved solar cell, and it can move a lot farther in the active layer. We measured and figured out the external photoluminescence quantum efficiency (PLQE) of CH3NH3PbI32xClx films that were cast on a rough and flat ground while being excited by CW light. This was done for a range of photon escape probabilities. It was found that the rough base has a 50% chance of escaping, which gives it an external PLQE of 57%.
Changing A-cation, B-cation, and/or halide(X) or using a mixture are ways to characterise materials for perovskite solar cells. For example, APbI3 can give off light in the 780–850 nm range when MA1 is switched out for FA1. The bandgap changes more when you use a mixture of halides than when you use a mixture of A-cations. For instance, the radiation wavelength in MAPbI3-xBrx can be fine-tuned from 550 nm to 780 nm by changing the iodide-bromide ratio. However, replacing MA1 with FA1 from MAPbBr3 has little effect on the bandgap because the λmax of photoluminescence moves from 560 nm for FAPbBr3 to 550 nm for MAPbBr3. It is only with perovskites that you can change the bandgap by changing the amounts of precursors in chemical solutions. This is different from other thin film solar cell technologies, like silicon and CIGS, which need more expensive methods and give you less control over the bandgap.
The most up-to-date perovskite makeup, which is made up of various halides and single-valent cations within the perovskite structure, has the best recorded stability and efficiency. However, when light hits it, a photo-induced phase segregation has been seen that breaks down a mixed perovskite made up of I and Br halogens into iodine reach and bromine reach perovskite regions. A Photoluminescence (PL) mapping method can help us understand this kind of phase segregation. This method takes the PL spectrum from various parts of the film, which lets us see the fingerprints of areas that are rich in iodine or bromine compared to the mixed phase. If you keep the film dark for long enough, this kind of phase segregation can be undone.
PL mapping data in pure MAPbI3 shows a similar pattern. The iodine anions can move towards the grain boundaries when exposed to light and return when left in the dark. This is known as photo-induced ionic movement. A lot of people are interested in mixing iodide and bromide because it makes it possible to precisely control the emission wavelength. This is mostly for light emission uses. Different uses of these mixed-halide mixtures have not been able to separate the iodide-rich and bromide-rich perovskite regions under constant lighting. This photo-induced partition limits the amount of bromide that can be found in the iodide-based perovskite crystal structure.
Abdi-Jalebi et al. recently came up with a new method called potassium passivation. It works well to stop photo-induced phase segregation and bandgap instability in alloyed perovskite materials and also keeps the high luminesce yield stable. In the potassium passivation process, the photo-induced segregation of mixed iodide-bromide mixtures is totally stopped by adding extra halide to the perovskite crystal structure and painting the hybrid grains with K1.
To get an optical device to its maximum efficiency, it is important to get the light quantum yield as high as possible and stop any non-radiative losses in the active layer when it comes into contact with the wires of the device. With the potassium passivation method, Abdi-Jalebi et al. were able to get a much longer PL lifetime and a high PLQE in the device stack without changing how the perovskite films carry charges.
Impact of perovskite crystalline quality, fluence and charge extraction layer on PL
In perovskite solar cells, the movement of charge carriers is controlled by non-radiative processes like Auger recombination and trap-assisted recombination (SRH). You can control SRH recombination by making crystals better and lowering the material’s trap-state density. There are two types of flaws or traps: deep traps and surface traps. Deep traps come from halide gaps or interstitial ions, while surface traps come from unsaturated bonds at grain borders or impurity phases that touch the absorber layer.
The speed at which charge carriers recombine is directly affected by the types and amounts of flaws, along with the recombination factors that go with them. The PL fluence changes the number of charge carriers, which has a direct effect on how fast the particles combine again. Auger recombination only makes a difference when there are strong effects and the charge carrier density is higher than 1017 cm23.
When the flux is low, the gathering of charges at different flaw spots is very important for how well solar cells and light producing devices work generally. There are specific contacts, such as electron and hole carrying layers, that can change how charge carriers recombine inside the perovskite absorber layer. By making the energy gap bigger between the valence band of the perovskite layer and the HOMO of HTM, holes can be taken out of the perovskite layer more quickly. Similarly, by making the properties of the interface between the absorber layer and ETL better, electrons can be taken out of the perovskite layer more efficiently. Time-resolved photoluminescence spectroscopy can be used to look into how things move and spread in the perovskite film.
Temperature dependent PL in metal halide perovskite
The photophysical processes in organic-inorganic lead halide perovskites change a lot with temperature because the chemical bonds between the atoms or ions in the material affect these processes. This changes the lattice constant, the motions of the lattice, and the interactions between electrons and phonons. These changes have an effect on the bandgap and the movement of charge carriers.
In contrast to most semiconductors, the band gap of perovskite materials grows as the temperature rises. The Varshni model says that as the temperature rises, the bandgap should reduce because the lattice will become less compact. However, this strange behaviour can’t be explained by this model. The strange bandgap shift, on the other hand, can be explained by the fact that the valence and conduction band orbitals don’t connect with each other.
Dual emission is another interesting property of perovskite emitters that can be seen at low temperatures. This is especially clear in MA-cation based lead halide perovskites. Classical molecular dynamics (CMD) shows that when the temperature is low, the organic cation can have two different arrangements: (1) exactly antiparallel (ordered) and (2) random (disordered). The first one is called the ordered orthorhombic domain and gives off light around 750 nm at 15 K. The second one, on the other hand, creates the disorderly orthorhombic domain and releases light around 788 nm.
The band gap difference between orderly and disordered systems is B85 meV. This is because MA1 has a stronger dipole moment than FA1. Because of this difference in energy, it is hard to separate the emission traits that are linked to ordered or disordered regions. It’s interesting that the energy difference doesn’t seem to depend on the type of halide anion because switching all the iodides in the MAPbI3 system for bromides doesn’t change the band gap difference between ordered and disordered systems; it stays the same at B85 meV.
In conclusion, the photophysical processes in organic-inorganic lead halide perovskites change a lot with temperature. These changes are caused by things like the lattice constant, the movements of the lattice, and the interactions between electrons and phonons. The unique bandgap shift and dual emission in perovskite solar cells can be explained by the fact that the valence and conduction band orbitals don’t stick to each other.
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