More research needs to be done on the optoelectronic features of mixed halide perovskite materials before they can be used in solar cells or other applications. Single crystals (SC) with fewer flaws are the best ones to study further because they don’t break as easily when exposed to air. This part talks about the main features and uses of SC made from perovskite materials, focussing on three main crystal forms and various ways of making them. We talk in detail about the optical qualities of widely used SC and how they can be used in different technologies, such as photodetectors, scintillators, solar cells, light emitting diodes, and memristors.
Crystal Structure
There are ABX3, A, and B cations of different sizes that make up the main structure of perovskite oxide. A used to be bigger than B, and B was held together by an X-site anion to make a BX6 octahedron complex. In order to set up a three-dimensional (3D) system, the octahedron needs to share its sides and put A-cations in the framework’s holes. It is important that the charges of both cations and anion stay neutral. The A-cation in hybrid perovskite materials is an organic amine with one charge, B is a two-charged metal (Pb2+ or Sn2+), and X is a halide element (I−, Br−, or Cl−). Mixed monovalent and trivalent metals could be used instead of the divalent metal to make double perovskites A2BB′X6 structures. The chemistry of perovskite single crystals is controlled by the sizes of the ions that make them up. To make a stable and useful perovskite material, you should think about the Goldschmidt tolerance factor (t) and the octahedral factor (𝼇).
Lead-Based Perovskite Single Crystals
Halide perovskites based on lead crystallise in the ABX3 structure, which is the same structure as the first perovskite oxide CaTiO3. There are 3D corner-sharing cuboctahedral (12-fold coordinated) and octahedral (sixfold coordinated) forms made when the A and B cations coordinate with the 12 and 6 X anions. In the MAPbX3 structure, the methylammonium position is out of order in the tetragonal phase from 160 K to room temperature, but it is in order in the orthorhombic phase below 160 K. So far, the most research has been done on lead-based perovskite single crystals. These are the same perovskite structures that have led to record-high efficiency in thin film solar cells.
It is well known that temperature changes the molecular changes of perovskite materials, which in turn changes their electrical qualities. To learn more about polycrystalline perovskite, it is important to figure out how these two processes relate to each other in single crystals. Ding and his colleagues published a detailed study of the different crystal facets of a single MAPbI3 crystal. They confirmed that the different facets have different atom numbers and showed how this affects the movement of ions.
The crystal of MAPbBr3 is in the cubic Pm3m space group when it is seen at room temperature. As the temperature drops, the crystal’s shape changes several times, going from cube-shaped to tetragonal, then from tetragonal to orthorhombic I, and finally from orthorhombic I to orthorhombic II. When the temperature drops to −116 °C, MAPbCl3 changes from a cubic phase to a tetragonal phase. At about −95 °C, it changes from a tetragonal phase to an orthorhombic phase.
A lot of people know that FA derivatives are unstable at room temperature in polycrystalline devices. This is also true for their monocrystalline partners. FAPbI3 perovskite changes from a cubic phase to a non-perovskite phase, which makes it hard to use in real life. By adding MA to the precursor solution, Kuang and his colleagues made the black phase (cubic) more stable. As you can see in Figure 2.3d, the mixed MA0.45FA0.55PbI3 perovskite single crystal has been stable for 14 days.
Lead-Free Perovskite Single Crystals
Single crystals (SCs) that don’t contain lead can have different crystal structures than the usual ABX3 structure. When Pb2+ ions are replaced, the nanoscale structure changes and the light qualities change because the ions are of different sizes and chemical valences. Changes in the crystal structure are caused by group-14 elements (Sn and Ge), near elements (Sb or Bi), and double elements (Bi mixed with Ag). By making these changes, the lattice systems go from orthorhombic to trigonal, creating quaternary structures A2B+B3+X6. You can also make a vacancy-ordered double perovskite by taking out some of the B atoms from the centre of the octahedron. You can make two face-sharing [M2X9] 3− octahedra by using transition or post-transition elements to get separated groups.
All-Inorganic Perovskite Single Crystals
All-inorganic perovskite, which doesn’t have any organic parts, is very stable at high temperatures and in harsh environments because it doesn’t have any organic cations. There are three main types of these perovskites: ABX3 type, special structures, and doped perovskites. The main character that doesn’t have lead is the CsPbBr3 version, which was grown using both the Bridgman method and the solution-grown method. High purity CsPbBr3 was made after careful optimisation.
At low temperatures, halide perovskite single crystals like CsPbBr3 go through two phase changes that don’t damage them. Around 130 °C, the first change happens, going from a cubic to a tetragonal system. At 88 °C, there is a second-order change to the orthorhombic phase, which stays stable at room temperature.
Another structure that is very stable in air and has great electrical qualities is TlPbI3. Its low freezing point lowers flaws caused by heat during growth, which is usually the Bridgman method. Several changes have been made to this material so that it can be used as a gamma-ray monitor to its fullest.
CsSnI3 SCs behave like p-type metals and have the best hole mobility of all p-type semiconductors. Chung et al. added doped CsSnI3 to solar cells and made all-solid-state dye-sensitized solar cells that had a great PCE of up to 10.2%.
Synthesis Methods
There are three types of crystal synthesis methods based on the phases that change during the growth process: solid-solid, liquid-solid, and gas-solid. Most technology crystals, like perovskite single crystals, are made through liquid-solid processes. This part is a summary of the most popular ways to do synthesis.
Antisolvent Vapor-Assisted Crystallization (AVC) Method
The antisolvent vapor-assisted crystallisation (AVC) method makes it possible to make high-quality perovskite single crystals at room temperature while using less energy. For many years, this method has been used to make micro- and nanoparticles. The most important thing about this method is how well the perovskite precursor solution dissolves in different fluids. With the solute and the solvent (precursor solution) and the antisolvent (AS), which mixes exactly with the solvent but not with the solute, you have a solvent-solute system. As the AS moves through the perovskite precursor solution, it makes the solute less soluble, which causes the material to crystallise as a precipitate.
The AVC method can be used to see, for example, the MAPbBr3 crystal. The precursor solution is mixed with a solvent called dimethylformamide (DMF) in a 1:1 ratio. This is then put in a bottle that is sealed under the atmosphere of an antisolvent called dichloromethane (DCM), which diffuses into the perovskite solution and crystallises it. The AS moves into the solution at a rate that is related to the speed of crystal growth.
Solution Temperature Lowering (STL) Method
Some acid halide liquids, like HI, HBr, and HCl, have different melting points that make it possible to dissolve perovskite precursors. This is called solution temperature lowering (STL). First, the precursors are dissolved at high temperatures. Then, the solution is slowly cooled down to make it supersaturated, which helps crystals grow. There are two types of the STL method: BSSG (seed-assisted) and TSSG (top-assisted). For BSSG-type growth, the seed must be put into the solution as many times as needed to get the crystals to the right size. For TSSG, little seeds must be glued to a base and put in the top half of the crystal precursor. A hot oil bath heats the bottom half of the solution that contains the seed, while air cools the top half. This creates a temperature difference that causes crystallisation. In the lower part, a convection stream that brings in ions is made. By keeping the temperature very low, you can get big single crystals. That being said, this method doesn’t work for crystals that don’t dissolve well at high temperatures and grow slowly. If the temperature drops too quickly, by-products and uneven crystals will form.
Bridgman Method
Making big single perovskite crystals is possible with the Bridgman process, which is a solid-solid way. For this method, the crystal grows inside a sealed quartz vial in a kiln with a vacuum or neutral atmosphere that lets a temperature difference happen. The material with many crystals is heated above its freezing point and then slowly cooled down from one end, where the seed is placed. When the crystallisation front moves through the liquid material, the finished crystal has the same shape as the container it was in. In the past, this method was used for semiconductor crystals like GaAs, ZnSe, CdS, and CdTe. But it doesn’t work for organic substances because they are chemically unstable when they melt.
One problem with this method is that the crystal grows along the ampoule walls, which can cause flaws because of the stress on the crystal. Some of these flaws are small grain borders, which lower the purity and limit its uses in technology. To keep the bars from cracking, it’s important to keep an eye on the crystalline factors, like the temperature difference, the speed at which the temperature drops, and the rate at which it cools. Zhang et al. used a home-made vertical two-zone oven to describe a simpler version of the Bridgman method.
Slow Evaporation Method
This method uses a controlled rate of fluid evaporation from a solution preparation that is close to being fully saturated. It is a liquid-to-liquid method. This state of being fully saturated is what drives things, and it can be reached by changing the temperature or letting the liquid evaporate. The rate of steady-state nucleation on the crystal surface is:
j0 = 𝜔∗ΓN0 exp (−ΔG∗kBT). This equation shows that there is a supersaturation level below which the nucleation rate is zero and a level above which it rises very quickly. To get just the one high-quality crystal that is needed, the temperature needs to be managed to keep the growth factors in check.
This process for crystallisation is one of the easiest ways to make biological crystals in both 2D and 3D shapes. Zhang and his colleagues published a picture of a 2D perovskite single crystal made using this method in 2019. They said that it led to a better growth process that produced well-defined large-sized (2D (PEA)2PbBr4 (C6H5CH2CH2NH3+, PEA+) single crystals.
Synthesised crystals worked so well that they were perfect for rapid optical computing and optical messaging. They were as good as the best materials in their field, like ZnO, TiO2, and GaN.
Inverse Temperature Crystallization (ITC) Method
This method, which is also called “retrograde solubility,” shows how the concentration of a liquid changes above the eutectic temperature. High enthalpy, a changeable distribution coefficient, and the highest chemical potential can all be caused by this event. In 1996, A. L. McKelvey looked at this process from a thermodynamic point of view and came to the conclusion that backward solubility could happen if ΔHmix is big enough to make ΔGmix not mix with itself at temperatures higher than the eutectic temperature. This effect can happen in any two-part system where the enthalpy is higher than the free energy of mixing above a certain temperature. So far, only a few examples of materials with this effect have been recorded. Perovskite is one of them. Bakr and his colleagues showed this by seeing that MAPbX3 perovskites seemed to become less soluble in common solvents (DMF, GBL, etc.) as the temperature rose. The reversed solubility doesn’t happen with all solvents. For example, MAPbBr3 dissolves in DMF but not DMSO. This is because the two solvents can’t coordinate with the lead halide precursor in the same way.
To use the method, all you need is a hotplate with an exact temperature monitor and a bottle with the precursor solution in it. This method speeds up the production process by a huge amount and lets high-quality perovskite single crystals grow. If the temperature is higher than what is needed, the process will go faster, which will make more small crystals. It is very important to keep the ramp temperature and concentration in balance. It is important to keep the crystal separate from the solution once it has grown because if the temperature drops while the crystal is in it, it will dissolve again.
Using a seed is a variation of this method that lets you get bigger crystals with more shape and size control. This ITC method works for a number of different perovskite types, including FAPbI3, MAPbCl3, MAPbIxBr1–x, and MAxFA1–xPbI3.
Methods for 2D and 1D Perovskite Single Crystals
A2Bn−1MnX3n+1 is the structure formula for quasi-2D perovskites, which are also called Ruddlesden–Popper (RP) perovskites. A is an alkyl or aromatic group with a long chain, B is a small organic cation, M is a divalent metal, and X is a halogen. The index n shows how many 3D system layers are placed between the two layers of A+ cations. This number can be changed by changing the stoichiometry of the precursors during synthesis. The big bandgap makes the layers of A+ cations “barriers” for carriers in the low-dimensional system. While in 3D, B+ cations act as “wells,” and the number n determines how thick these wells are. The ITC and AVC methods can’t be used to make 2D perovskite single crystals, but lowering the temperature seems to be a good way to make a concentrated solution less soluble.
Optoelectronic Properties of Halide Perovskite
Single Crystals
Due to their optical qualities in their monocrystalline phase, perovskite single crystals have shown promise in a number of technologies, such as photodetectors, X-ray or γ-ray detectors, high-performance solar cells, and light-emitting devices. Some of these qualities are a long carrier lifetime, a large carrier diffusion length, a wide carrier absorption range, and few carrier flaws. Understanding these traits is important for making tools that work well.
UV–Vis Absorption, Photoluminescence (PL), and Transient
Decays: TRPL and TPV
Single crystals have different optical qualities than polycrystalline ones because they don’t have grain limits. This is because there is less charge exchange and a very ordered pattern in the perovskite single crystal structure. This makes it easier for charge carriers to move and doesn’t trap them. This trait describes the structure and make-up of devices that hold a single crystal.
Single perovskite crystals have a redshifted absorption range compared to polycrystalline films. This is because they absorb light better below the bandgap and have a clear band edge cutoff with no excitonic signal. It is also possible for single crystals to have this signal of low flaw states. Below-bandgap absorption is seen in MAPbI3 single crystals. It starts in a weakly indirect bandgap that is about 60 meV below the direct bandgap transition. The below-bandgap transition’s absorption coefficient is many orders of magnitude smaller than that of the above-bandgap transition. This means that below-bandgap absorption is almost invisible in polycrystalline perovskite films, but it’s clear in single crystals because they are thicker and have longer carrier diffusion lengths.
The photoluminescence (PL) of a single perovskite crystal is weaker in vacuum than in normal conditions. This is because oxygen and water passivate the crystal’s surface. To study phase changes and trap states, XRD and PL measures that depend on temperature have been carried out. In MAPbBr3, the orthogonal phase (Pna21) changes to the tetragonal phase (I4/mcm/or P4/mmm) between 140 and 230 K, and then to the cubic phase (Pm3m) above 230 K.
PL measurements and time-resolved photoluminescence (TRPL) measurements show that bound excitons (BEs) last a few microseconds, while free excitons (FEs) only last a few nanoseconds.
Electronic Properties
This part talks about the electrical features of single perovskite crystals, focussing on their one-crystalline devices using methods like impedance spectroscopy (IS) and space charge limited current (SCLC).
Space-Charge-Limited Current (SCLC)
A steady-state method called Static Charge Liquid Crystallography (SCLC) is used to study how electronics move charges. It can be used to find out the carrier mobility and trap density of a single perovskite crystal by keeping track of the dark current as the voltage is changed. The SCLC model data can be read wrongly, though, if the wrong model is chosen without taking into account the type of the gadget.
The Mott–Gurney (MG) law is what the most common SCLC method is based on. It’s used to figure out the thickness, charge carrier motion, and permittivity of the perovskite semiconductor. To use this model correctly, the device needs to meet certain requirements: (i) the semiconductor material being studied needs to be undoped and free of traps; (ii) the device needs to be configured as a sandwich, with the semiconductor between two ohmic contacts; and (iii) diffusion needs to not have a big effect on the current.
There are three main areas on the current-voltage curve (J–V): (i) the Ohmic region (J ∂ V), which is where the conductivity can be found; (ii) the trap-filling region, where charge carriers fill the trap states and the current rises sharply at the trap-filled limit voltage (VTFL); and (iii) Child’s region at higher voltages, where the current changes in a way that is quadratic with voltage (J ∝ V2).
Here’s how to figure out the number of free carriers from the ohmic part of the jV curve:
nC = 𝟏e𝟇, where 𝜏 is the conductivity taken from the first area. You can also use the following relationship to find the number of deep trap states:
To find ntrap, we need to know 𝼀0, L, e, 𝼀, which stands for the crystal thickness, e for the electron charge, and VTFL, which is the voltage that moves from the ohmic region to the trap filling region.
In the end, though, the results may not be what they seem to be if the model wasn’t properly set to the device. The shape of the device is another thing that needs to be thought about. The MG type can handle a stacked arrangement, but what if the wires have a different shape? You should know that there are three different shapes that can be used to touch the semiconductor being studied.
The carrier mobility is based on Geurst’s SCLC model (Eq. (2.4)) for the gap-type shape, also called the lateral configuration. This is what the Geurst theory says the threshold voltage for a gap-type structure is:
VTFL = 0L 0, where is the carrier mobility, 0 is the surface charge density per unit area, 0 is the vacuum permittivity, and is the perovskite’s relative dielectric constant.
The end results can be changed by both the shape of the wires and the growing method used to make the single crystals. This can also change the electric features of the final device.
Impedance Spectroscopy (IS)
The mixed ionic and electronic nature of perovskite material makes the physical and chemical processes that control it more complicated, which makes them hard to understand. The main difference between polycrystalline and monocrystalline perovskite devices is that monocrystalline devices don’t have any grain boundaries. This makes it easier to understand the different processes that happen in this material, such as how ions move, how it conducts electricity and electricity, how long carriers last, and how it interacts with other materials. It is possible to expect a Debye-type dielectric relaxation in monocrystalline perovskite devices. This relaxation has a single time that is related to the unique grain structure of the crystals and the lack of grain borders.
Ionic migration is one of the main problems in the quest for fully stable perovskite systems. Measuring the ionic diffusion is a necessary step to reduce the negative impacts of this process while it’s running. For monocrystalline perovskite devices to work, the electronic input needs to be slowed down, and the resistance response needs to be studied in great detail. There is a Warburg element and a transmission line in the corresponding circuit that show how the ions move through the impedance spectrum.
It is possible for a Warburg-like coefficient to show up in the HF domain of the monocrystalline device because of charge buildup at the surfaces. The interface between the perovskite and the electrode is very shiny. In the LF domain, on the other hand, it is completely responsive. But in similar devices made with a polycrystalline bulk, the charge carriers can move over the perovskite-metal contact surface with an extra arc at the LF region. This makes the contact partly absorbent.
An inductive behaviour that shows up as a negative capacitance is another common thing that can be seen in the impedance spectra of a single crystal perovskite device. A loop in the LF domain makes this process easy to spot. Several studies have shown that this behaviour is due to vacancy-assisted ionic transport.
The visual features of halide perovskite single crystals show that the contact is reflective, as shown in the inset of the impedance plot. This shows the difference between ionic transport (up to 5 M©) and charge buildup (from 5 to 50 M©). Carriers are injected into perovskite layers through surfaces. This is done through an induction process that takes between 1 and 100 seconds in dark, neutral air.
The time constant in the HF domain is another important difference between the impedance responses of perovskite devices that are monocrystalline and those that are polycrystalline. For MAPbI3, these numbers are between 102 and 103 Hz for monocrystalline systems and between 105 and 106 Hz for polycrystalline systems. This difference is because the single perovskite crystals don’t have any grain borders or uniform contacts, which changes the recombination resistance and geometrical capacitance.
The carrier lifetime tells us how many charge carriers are extracted before they recombine, and IS can tell us what that lifetime is at different light levels. The carrier lifetime of a MAPbI3 SC that is made up of Au/MAPbI3 SC/Au is about 95±8 ms, which is more than 10 times the carrier lifetime found in polycrystalline thin-film devices after surface passivation processes.
Applications
Photodetectors
Perovskite single crystals are very useful for photodetection because they can turn light into an electrical output with different qualities. The efficiency of the photodetector is judged by its reaction time, gain, detectivity, linear dynamic range, noise equivalent power (NEP), and gain. The ratio of photocurrent to incoming light strength is called responsiveness, and it is related to the quantum efficiency of the outside world. The detectivity is the smallest amount of light that the photodetector can pick up. The photoconductive gain is the average number of circuit electrons that are made per photocarrier pair. There is a range of light intensities that the photodetector’s current reaction is linear with respect to. This range is called the linear dynamic range (LDR). The noise equivalent power (NEP) is the noise that the photodetector makes when there is no light shining on it. It is linked to how sensitive the photodetector is. The NEP is a measure of how sensitive the gadget is. It is the opposite of the detectivity.
X-Ray Detection
Because they have a high atomic weight, large carrier movement, and long lifetime, perovskite single crystals might be able to spot high-energy radiations like X-rays. It took Wei et al. to make a single crystal of MAPbBr3, which had a record-high mobility-lifetime product of 1.2 × 10–2 cm2 V–1. By slowing down the surface charge recombination speed with UV-O3 treatment, the carrier lifetime and extraction efficiency were both improved. With an 80 μC Gyair−1 cm–2 sensitivity, the devices could pick up X-ray dose rates as low as 0.5 μ Gyairs–1. This is four times better than α-Se detectors.
𝛄-Ray Detection and Scintillators
Gamma (γ) rays, which have energies between 50 keV and 10 MeV, are made when radioactive atoms break apart. These very powerful gamma rays can go right through matter, making an electric current and an electron-positron pair that can be used to figure out the original γ-ray’s energy and direction. To do this, detectors need to have a high charge-carrier mobility-lifetime (¼τ) product. Perovskite materials have this property. To find γ-rays, there are two main types: spectrometers, which use scintillators or solid-state materials to turn the γ-ray into an optical or electronic signal, and γ-ray imaging, which uses electron-positron production or Compton scattering.
Kovalenko and his colleagues did an example of γ-photodetection using a single perovskite crystal. They talked about the features of detectors for big SCs made from MAPbI3, MAPbBr3, MAPbCl3, FAPbI3, FAPbBr3, and MAPbBr3 that had been treated with I. MAPbI3 was found to be the most sensitive. Ionised photon radiation is changed into UV-visible photons by scintillators. These are used in many areas, such as astronomy, diagnosis, nuclear medicine, and homeland security. According to Ouyang and his colleagues, perovskite single crystals made of MAPbBr0.05Cl2.95 and a silicon photomultiplier (SiPM) worked well for low cost and good time resolution. They got X (γ)-ray excited luminescence (XEL) spectra in the visible range by using a source of 137Cs, a radioactive isotope that was stable at room temperature.
Solar Cells
Polycrystalline perovskites-based solar cells have great photovoltaic performance, but because they are polycrystalline, they are also sensitive to changes in their surroundings. The grain borders in these cells let oxygen and water pass through them, which makes the device work less well. Ionic movement is what causes hysteretic behaviour and instability. High trap numbers also play a part in this. Because there are no grain limits in single perovskite crystals, carriers can move through them more easily. This makes the bulk less dense with defects.
It is important to get rid of grain borders and lower trap densities in solar cells in order to make them more stable and improve their photovoltaic performance. Because they don’t have as many chemical and structural flaws, monocrystalline materials usually have better solar qualities. Single-crystalline perovskites, on the other hand, don’t follow this rule at all. They have a high surface trap density on their single crystals. One of the main problems with solar cells made of perovskite single crystals is that their surface recombination speed is 6 times faster than that of polycrystalline cells.
To get this technology past the proof-of-concept stage, the main goals are to improve the crystallisation method, find good surface passivation methods, and make sure the devices are built to last. There are two main ways for monocrystalline solar cells to grow: from the top down and from the bottom up. Top-down methods focus on making things from a single crystal that is already made and connected to certain contacts, while bottom-up methods work by applying pressure to make things stick better.
Finally, creating thin film single crystals (TFSC) solar cells is hard because it’s hard to control the width and we need better ways to passivate the surface.
Müller-Buschbaum and his colleagues published an improved top-down method for the charge separation process in perovskite solar cells in 2018. They looked at two different structures: one with a p–i–n architecture using ITO\PEDOT:PSS\perovskite\PCBM(spray)\silver paste or\Al, and the other with a n–i–p architecture using FTO\PCBM\perovskite\spiro-OMeTAD\Au and a polymer border. It was necessary to find a material that could both hold the crystal in place and help the charge extraction process. The n-i-p single crystal perovskite solar cell with a PDMS polymer rim solved this problem.
When making polycrystalline devices with the bottom-up method, each layer is added from the bottom base to the top. For single crystals, this method is changed to an in-situ growing method, in which a monocrystalline film forms on the base while the device is being made or while it is already built. Because of this, there are some materials that can’t be used for specific contacts.
In 2017, Jinsong Huang and his colleagues wrote about effective ways to deal with the difficulty of the in-situ growing process. They showed a limited growth method that can handle a mix between single perovskite crystals that are 20 μm thick and crystals that cover a lot of space. The work presented shows how the well-known ITC method can be used to make space confinement easier for ions to diffuse. The way it works is because of how the substrate surface and the liquid in the precursor solution interact with each other.
In conclusion, both the top-down approach and the bottom-up way have shown promise in the creation of perovskite solar cells. These ways might help make solar devices work better and lower the amount of feedback they have.
The gap between substrates that are used to limit ions is very important for ion diffusion in the solvent because it controls how fast precursor ions move through the solvent. When solvent molecules move forward in a plane direction near the substrate, surface tension also plays a part. When a less hydrophilic material like PTAA raises the surface tension, the contact between the solvent and the substrate weakens. This lets the solvent spread out and keeps the ions from being pulled during the process.
The finished device, which was made of ITO, PTAA, MAPBI3, PCBM, C60, BCP, and copper (Cu), had a PCE of 16.1% and a jV curve shape that changed depending on the width of the crystals. Because there are more traps on the sides of single perovskite crystals than in the bulk, they are the hardest to solve. It was treated with MAI to stop those flaws from acting up and keep the number of surface traps under control, which improved the end performance.
The best mix so far has been the bottom-up way with the ITC technique used in a half-made device, which allows a PCE of about 23%. According to Turedi et al., MAPbI3 perovskites were used to make single crystals that were 20 μm thick and grew straight on a PTAA film. They used the solution-limited inverse-temperature crystal growth method, which led to up to 21.09% power transfer efficiency. To improve their results even more in 2021, they used a mixed-cation mixture and got the highest PCE ever, up to 22.8%.
To make two types of thin-film single-crystalline perovskite solar cells, Bakr and his colleagues came up with a cavitation-triggered asymmetrical crystallisation (CTAC) method for other perovskite formulas. This method uses an ultrasound pulse to hit a perovskite solution, sending a burst of energy through the solution. This causes bubbles to break near the substrate, speeding up the nucleation process. This method doesn’t work with all perovskite formulas, though, because the solubilities of PbI2 and MAI are very different, and the tetragonal system’s uneven growth makes it impossible to get a good result.
Light Emitting Diodes
Perovskites are great for making light-emitting diodes (LEDs) because they have a lot of pure colours, very little Auger recombination, and great photoluminescence. Perovskites are unstable, though, which makes it hard to get long working times. This sets them apart from biological rivals (OLEDs) and slows down the process of industrialisation.
To solve this problem, Pb-free perovskites need to be created without lowering their PLQY. Solution growth with top seed (MAPbI3, MASnI3, and FASnI3) and bottom seed (MASnCl3), antisolvent vapour crystallisation (MAPbI3 and MAPbBr3), inverse temperature crystallisation (MAPbX3, FAPbX3, and FA1–xMAxPbX3), and chemical vapour deposition (CsPbBr3) are the main crystallisation growth methods used in LEDs.
Recently, amazing things have been found in perovskite single crystals. For example, crystals have grown in-situ between two electrodes, a CsCu2I3 perovskite has been used to make a white light, and the ITC method was used to grow a single crystal that is centimetres in size and has two-dimensional layers of (CH3NH3)2MnCl4. When excited at 417 nm, a QE of 2.4% made it possible for a good red emission.
The main goal of the next work shown in Figure 2.29d is still to solve the stability problem. To create a liquid-to-liquid self-encapsulation inkjet printing method, perovskite paints need to be mixed with a PDMS precursor so that MAPbBr3 single crystals can grow in situ. This liquid has an effect that confines space, which slows down the crystallisation rate. It also protects against damage from the surroundings.
To get different radiation bands, Cheng et al. played around with the layer widths of 2D RP phase single crystals. In order to make the LED, Cs–Pb–Br-based perovskites were used. These have wavelengths of 410–420 nm, 10000, 400–500–600 nm, 700–800 nm, 20 000, 30 000, 40 000, and 50 000.
Chen et al. also played around with different layer widths of 2D RP phase single crystals to get a range of radiation bands. The LED was made with perovskites based on Cs–Pb–Br, which had different n layers and a good blue-emission response.
In conclusion, perovskite single crystals have many benefits in photovoltaics, such as high colour clarity, low Auger recombination, and great photoluminescence. However, perovskites are unstable and need to be substituted for lead, which are still big problems in their growth.
Memristors
With their mixed ionic and electronic conductivity, perovskites cause instability during operation, which is shown as hysteretic reactions. However, this mix is useful for using them as memory devices because it combines their ability to conduct electricity with their sensitivity to light radiation. This opens up new possibilities in the field of perovskite, such as optical-erase memory devices, neuromorphic computing, and light-accelerated learning.
An electrical input can be used to control memristors (memory and resistor) in both directions. They’re called inactive devices, and they normally have two outputs. A memristor’s most important feature is that it has a feedback loop when measuring current-voltage (jV) when it is driven by a voltage or current variation. Memristors can be broken down into three groups: ideal, general, and extended. Each group has its own hysteretic behaviour.
The use of perovskites as memristors is still very new, but a number of related papers have shown how useful these materials could be in this area. Most perovskite-based memristors are set up as metal-insulator-metal (MIM), and their reactions can be explained in two ways: by ionic movement or charge trapping and detrapping. Many of the time, these systems rely on the links that are built into them.
Rand and his colleagues released one of the first papers in this area in 2017. It used a single 2D-exfoliated MAPbBr3 crystal as a memory device for neuromorphic computing. Neuromorphic computing needs very little power to process a lot of data at once, which means that memory devices need to be able to move ions around a lot and have very little operation current. 3D perovskites and polycrystalline films have a big internal electronic current and a parasitic leakage current because of the grain borders. This causes a leakage current that is three orders of magnitude higher than what is seen in the best resistive memory devices today.
In order to get to pA, the ionic input must be enough and the electric transfer should be kept to a minimum. Defects are often added to memory material to improve ionic transport while reducing electronic transport. But these flaws can cause leaking currents, which is the opposite of what you want.
Randal and his colleagues looked into the possibilities of 2D exfoliated wide bandgap perovskite single crystals. Because there are no grain borders, leaking currents are not generated. The low carrier density and wide bandgap allow for high ionic migration. The stacked 2D single crystal’s flaking creates a thin film that favours bromine migration. After the ions move, they gather at the anode contact, creating a tunnel that conducts electricity in the presence of an electric field. Finally, an off current of as little as 0.1 pA was reached, which led to an operation current of 10 pA and a 10:1 on/off ratio. This stopped the flow of charge in the direction that wasn’t in the plane of the 2D object.
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