In the last 20 years, scientists have made new materials that can be used to make new electrical, optical, and photonic devices. These include solid-state lasers, sensors, transistors, light-emitting diodes (LEDs), and more. Low-dimensional (LD) electronics are used in some of these devices. These are usually made with fake means. However, natural three-dimensional materials (3D) that are synthesised as two-dimensional (2D), one-dimensional (1D), or zero-dimensional (0D) shapes have been added to these methods in recent years.
One interesting thing about low-dimensional (LD) semiconductors is that their energy band gap (Eg) grows as the material’s dimensions get smaller. As the size of the materials gets smaller, the electrons and holes are forced to move closer together geometrically, which makes the native excitonic oscillator stronger. Common man-made 3D direct energy band gap semiconductors are GaN, GaAlAs, and GaAs. They can be shaped into LD shapes using techniques like molecular beam epitaxy or lithography.
Recently, a group of materials that were well known but had been lost has been brought back to life. These materials are called natural hybrid semiconductors or hybrid organic–inorganic semiconductors (HOIS). Because they are made up of both biological and artificial parts at the same time, these natural things are called that because of how they are made. So far, the most helpful and reliable HOIS are those that use metal–halides complexes with organic molecules, metal–oxides, and metal–chalcogenides. These complexes can have a single or mixed valency.
We chose some interesting single-valence metal cation HOIS to show their structure, which is linked to their qualities that are important for all kinds of electronics. Crystal structures of CH3NH3PbI3, (4F–C6H4CH2CH2NH3)2PbI4, ((NH2)C = I(NH2))3PbI5, and (CH3NH3)4PbI6 are some examples.2H2O are shown as 3D, 2D, 1D, and 0D HOIS, going from left to right.
Most of the time these days, the word “perovskite” refers to the 3D phase. However, all “perovskites” are actually hybrid semiconductors. As the network goes from 3D to 0D, the straight band gap widens and the exciton binding energy rises to levels higher than what would be expected from standard solid-state theory for a very thin 2D material. This is because the dielectric enhancement happens.
At least ten times as much excitonic binding energy can be found in HOIS (perovskite) materials as in 3D materials that are similar, generally around 260 meV. Excitons can stay stable at room temperature because of these high values. This lets new, low-cost excitonic devices be made with HOIS.
If you change I to Br or Cl, the band gap moves towards higher energies. Mixing different amounts of halides with Pb lets you make materials with precise, adjustable band gaps in the 300–800 nm range. Most of the time, the band gap energy is a few hundred meV higher than the excitonic level. You can make different kinds of HOIS with band gaps that can be changed from 800 nm to almost 300 nm by changing the metal cation, the type of halide, and the organic part.
HOIS materials have unique optical and excitonic qualities. This is because they have better quantum confinement and their own LD excitons are dielectrically contained. The amine that covers the inorganic part of HOIS has a dielectric constant that is three times smaller than the inorganic part. This lets the excitons interact through the organic layer, which raises the excitonic binding energy by creating image charges in the organic layer.
If you want to make an electronic device based on HOIS, you should make sure that electrons and holes can pass through the organic barrier. If they can’t, the device won’t work because the semiconducting artificial part is blocked. If the barrier gets a dielectric constant number that is much higher than the quantum well’s, the excitonic energy goes down a lot.
That’s not all. HOIS and their artificial part can take on the shapes of 3D to 0D LD networks as well as networks with dimensions in between, like the so-called quasi-0D, quasi-1D, and quasi-2D HOIS.
This is a type of semiconductor material called halide perovskites. It has been used in many things, like lasers, light-emitting diodes (LEDs), transistors, pressure and chemical sensors, and electromagnetic radiation sensors (photodetectors, radiation detectors). These materials are useful for many other uses besides lithium-ion batteries. They have special qualities that make them useful for photovoltaics, lasers, electromagnetic radiation sensors, transistors, pressure and chemical sensors, and more.
As n goes up, the layer gets thicker. When n = ∞, the 3D semiconductor perovskite is made. When n = 1, the system is the lead halide materials’ smallest 2D HOIS that is possible. The n = 1 form is not completely 2D, though, because each layer is only so thick. When n numbers are in the middle, they make the quasi-2D HOIS. This has strong excitonic visual effects because excitons have a high binding energy in HOIS.
It looks like quasi-2D semiconductors are more interesting than 0D, 1D, 2D, and 3D HOIS, but they are hard to make in pure solid form. Because HOIS can self-assemble, the semiconductor might form in a mix of different n values instead of a single n value system.
Mixtures of quasi-2D and 3D show energy transfer events, with a group of quasi-2D and/or 3D nanoparticles close together taking energy at all energy characteristic peaks that are important for the different n values. Some people think that this effect could help make solar materials work better by directing all the energy from light to a single n-phase emission. It could also help make LEDs work better.
There are different kinds of perovskites, like artificial perovskites, which have big cations like Cs instead of small organic molecules. These are used for different things besides halide perovskites. Some of these HOIS may be totally artificial, but their traits are a lot like those of organic-inorganic mixtures.
Low-Dimensional Hybrid Organic–Inorganic
Semiconductors in Light–Emitting Diodes—LEDs
Halide HOIS (perovskites) have been studied a lot in photovoltaics for more than 30 years. Their main benefit is that their photoluminescence (PL) feature can be controlled, especially the optical band gap. Because of this, halide perovskite looks like it could be useful in LEDs. Only in the last eight years has the promise of HOIS in PVs and LEDs been recognised. More and more research groups are using strong testing methods to study HOIS qualities in a wide range of external factors and LED devices.
In the 1990s, one of the first perovskite LED (PeLED) designs was released. It showed how the energy levels of metal contacts used in the device, like indium tin oxide (ITO) and Mg–Ag alloy, lined up with the Fermi levels. The first report also talked about the 1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl (OXD7) layer that blocks holes and lets electrons move through it.
At first, scientists weren’t interested in PeLEDs because they didn’t work well and weren’t stable; they could only work at very low temperatures and high drive voltages. However, Era et al. were able to make green electroluminescent (EL) devices using phenylethylammonium lead iodide (C6H5C2H4NH3)2PbI4 and 2D-layered perovskite quantum-well structures.
Halide perovskites can be used as active materials in photonic sources that are powered by electricity, and HOIS can work as phosphors in photonic sources that are powered by light. They work well as hole transport layers in LEDs made of plastic materials. There are several major ideas that are thought about in PeLEDs, such as external and internal quantum efficiencies, charge transport, carrier lifespan and radiation recombination, and photon recycling. The front and back electrodes are used to add electrical charge.
There are many groups that study materials that make devices to test these things. Figure 6 shows a modern PeLED schematic. A PeLED is usually made with a back electrode, an emitter, an electron-transporting layer (ETL), and an electron-transporting layer (HTL).
Perovskite-based LEDs (PeLEDs) are often used in electronics because they are cheap, can be broken down naturally, and are not too complicated. It is possible to make these LEDs without using ETL/HTL layers. This can be done with plastic films that have perovskites built into them. But excited electrons can also give off radiation by relaxing to empty states in ETL/HTL layers nearby, which makes the emission spectrum more complicated.
The first oleylamine-based LEDs that worked at room temperature didn’t have a high quantum efficiency and were made from samples that had been dried out for months. The first recorded 2D PeLED made from 4FPA was only briefly talked about because it wasn’t as resistant to humidity. These days, PeLEDs can be protected or covered with layers of protective devices, like MoO3, which stop them from reacting with oxidising conditions.
Vassilakopoulou et al. were the first to look again at how to make a simple 2D hybrid organic–inorganic green colour PeLED based on 4FPA that worked for a few weeks at room temperature and normal conditions. For making a film that could work as a single-layer LED, the doctor blade method worked better than spin coating.
Modern PeLED shapes, on the other hand, can be made by putting a layer of (4-fluorophenethylamine-H)2PbI4 on an ITO base. There is no need to spin coat the film or add any ETL/HTL layers between the layers. But the start voltage was high, working at 5–10 V. This was probably because the film was thick and needle-like phases showed up.
Several studies have shown the recombination kinetic relationship to excitons and defect states. Other studies have also shown that the active material can be used to stabilise LEDs. For instance, 35 W/Sr/cm2 EL can be made at 744 nm with a 1 V turn-on voltage, and pure 2D HOIS can work nonstop for 14 hours, while 3D analogues can only work for 8 hours.
People have been interested in making fluorine-free perovskite LEDs for a few years now. Organic molecules with fluorine atoms cause only a small gap between the molecules. This changes how the material’s phonons connect to its electrons, which changes the EL strength and linewidth. Researchers have revealed that green LEDs made from bromine-based 3D organic–inorganic halide perovskites can give off 1500 cd m−2 of light. High-performance amphiphilic HOIS have also been made for use in LEDs.
This is because methylammonium lead iodide CH3NH3PbI3-based perovskite doesn’t work well in LED uses because it has low exciton binding energies (9–60 meV). Li and his colleagues were able to make a 2D perovskite green LED that works at room temperature using phenylmethanamine lead iodide (C6H5CH2)2PbI4. Because of how the halogen atom affects the end qualities of the gadget, writers have mixed different halogen atoms.
To make color-pure violet LEDs, Liang et al. used quantum-confined 2D perovskite based on 2-phenethylammonium lead bromide (PEA2PbBr4). It was discovered that this substance can form tightly packed quantum wells with a repeating dielectric spacer layer. Changing the halide anion can change the emission colour of a PeLED, which is important because robust PeLEDs have a hard time reaching the EL, especially in the UV range.
Tan et al. showed that halide perovskites’ electroluminescence can be changed in a way that is similar in the visible and near-infrared ranges. They made a PeLED with a 15-nm layer of CH3NH3PbI3−xClx perovskite as the emitter. By switching Br and I in the structure, the emission band can be moved, which can produce red and green colours. Hu et al. wrote about tunable PeLEDs that have fine emission and different colour regions. They did this by using butylammonium iodide (C4H9NH3I) as a spacer to make a 2D perovskite structure.
Mixed quasi-2D/3D HOIS are more stable than single 3D or 2D HOIS, and they can be used as the active layer and a single layer in PeLEDs. It has been seen that energy can be transferred in other semiconductors that are related, like Si-based systems and inorganic perovskites like CsPbBr3. HOIS that can move energy can be set so that the excitonic emission can happen anywhere in the visible spectrum as long as it is based on Pb halides.
Putting lead halide semiconductor units into inactive materials and HOIS has made it possible to make strong compounds that have qualities that are linked to the HOIS. Because the host material protects these compounds, they can keep these qualities over time and in different circumstances. One type of material with holes, like the MCM-41 family, can be used to cover these compounds.
Another interesting area for study is the quantum confinement that is caused on perovskites by limited growth within holes. It is possible to encapsulate 3D perovskite nanoparticles within the holes of mesoporous materials without using chemicals to stop the electrons from moving. This has been seen when the quantum confinement effects of 3D lead halide perovskites in mesoporous materials have been looked at.
The Koutselas group showed a single-layer PeLED made of these mixtures of MCM-41/perovskite. The EL signal comes from energy moving from nanoparticles with a bigger band gap to those with a lower band gap, which are the 3D perovskites. The protective silica structure in these photoluminescent mixtures makes them very resistant to breakdown by chemicals, rust, or dampness. This means that there are possible ways to make HOIS-based LEDs more stable against breakdown.
PeLEDs use trapped perovskites in other materials and polymer films. They are slowly becoming popular because making perovskites is easy and they work in a new way at room temperature. By changing the contact between the HTL and the active HOIS with an amphiphilic polymer, it has been able to get rid of non-radiative charge exchange and uneven charge input. This has led to devices that are 14.4% efficient. It is thought that polymers with quasi-2D perovskites stuck inside them can achieve up to 20% external quantum efficiency, which is the same as 100% internal quantum efficiency.
Another work that was released involved making a PeLED device from a hybrid film that had a 1:1 weight ratio of 1D (Meth-H)CdBr3 and 2D (PhE–H)2PbBr4 HOIS. The two types of film were trapped in a polystyrene matrix. The finished changed enclosed perovskite was a mix of 3D, 2D-like, and 0D perovskites. This made the LED device give off a bright green light.
Low-Dimensional Hybrid Organic–Inorganic
Semiconductors in Lasers
Lasers are a big topic of study these days because they are used in so many consumer goods. Materials for high-energy ion (HOIS) lasers have been made because their excitons and polaritonic states are steady. Solution-processed 3D and LD HOIS make it possible to make cheap materials that are often the same as solid HOIS. At room temperature, these films keep excitonic states that have a high binding energy and oscillator strength, which are needed for commercially new devices.
Nearly 20 years after Kondo et al. released the first low-temperature perovskite-based laser, the first low-threshold perovskite lasing device was made public in 2014. The ability to change the HOIS emission, low cost, ease of synthesis, low flaw density, and higher photoluminescence quantum yield all make them useful as lasing materials. Their effectiveness may make it possible to make devices that use less energy.
Low-temperature growth has made it possible to make low-threshold laser devices that work even when they are in a steady state at room temperature, even though they are naturally unstable. Active perovskite materials can be built on HOIS that is 3D, 2D, quasi-2D, or a mix of quasi-2D and 3D. The lasing process is a complicated thing that can be caused by light or electricity, and it has also been possible to make lasing happen all the time.
It has been said that Whispering gallery mode, Fabry–Perot cavities, or vertical cavity surface-emitting laser (VCSEL) can be used to lase 3D objects. Triple cation mixed-halide perovskites can show lasing when excited for a very short time, which lets them give off light in the visible or near-infrared range. It is also possible to make distributed feedback lasers, which have losses of about 100 cm−1 in 3D materials. At a level of 13 lJ/cm2 at 785 nm, Amplified Spontaneous Emission (ASE) can happen in quasi-2D perovskites.
Low-Dimensional Hybrid Organic–Inorganic
Semiconductors in Sensors
After looking at photo-, X-ray, gamma-ray, pressure, and chemical monitors, this study focusses on how electromagnetic energy changes or causes things to happen in the Honeycomb Interferometer. If you need to find chemicals, scintillators, or pressure, these devices can do it.
The first three groups all depend on the same basic idea: electromagnetic radiation changes the HOIS, which in turn changes its electrical resistance to represent the amount of radiation and/or energy being detected. It can be an electron/hole pair that is excited or something that is made in their conduction and valence bands. Due to collisions between excitons, a high number of excitons will also create free carriers. This means that even plasma made up of bound quasiparticles (excitons) can create free carriers, which can be seen as a change in resistance.
Chemical detectors work by finding molecules that change the structure of the perovskite or cause an electrochemical redox reaction on its surface. This lets them pick up either the HOIS redox or the sensor redox current. When the HOIS unit cell axes get smaller, the visual properties change, which makes the pressure sensors work.
Scientists who study HOIS started looking at photoconductivity spectra from either growing crystals or melted HOIS 20 years ago. This is where the idea of using HOIS as photodetectors came from. We measured the photoconductivity spectra of some 2D and some 1D HOIS, and the optical absorption and photoconductivity peaks were very close to each other. Researchers have looked at the HOIS in both 3D and 2D and found that the energy band gap narrows for the 2D when there is hydraulic pressure, but it widens for some of the 3D analogues. A redshift of 90 meV/GPa is a good number for the 2D perovskites that lets them be used as sensors.
Recently, it was shown that 3D HOIS-based X-ray detectors are four times more sensitive than a-Se detectors. Gamma-ray HOIS-based detectors for energies between 50 keV and 10 MeV are sensitive, cheap, and reliable because the HOIS carriers are highly mobile and have long lifetimes.
In the past few years, there have been new papers about how pressure affects perovskites. For example, CH3NH3PbX3 (X = Br, I) shows piezochromism by first turning lighter in colour and then black. This happens at the same time that pressure lowers the material’s resistivity, which makes it harder to calibrate as a pressure sensor.
Because of how it works electrically, a new kind of perovskite monitor has also come about. The Topoglidis and Koutselas group were the first to use 3D and almost-2D perovskites that were attached to mesoporous TiO2 electrodes and used as sensors. Scientists have shown that HOIS can be dissolved in dichloromethane and tetrabutylammonium hexafluorophosphate that has been weakened. This can then be used to find the pollution CBr4 in the same solution, with a limit of measurement of 20 ppb/mol.
Low-Dimensional Hybrid Organic–Inorganic
Semiconductors in Transistors
In the past, amorphous silicon and polysilicon were used to make field-effect transistors (FETs). However, recent progress in organic-inorganic semiconductors has made people more interested in them as possible alternative channel materials. In FETs, current flows because most of the carriers enter through the source (S) and leave through the drain (D). The gate electrode changes how conductive the gate area is. Mitzi and his colleagues created an organic–inorganic perovskite thin-film field-effect transistor in 1999. It was made with spin-coated 2D organic–inorganic hybrid perovskite (C6H5C2H4NH3)2SnI4.
Later, work was done on tin(II) iodide-based organic–inorganic hybrid perovskites. The cation site was changed to a set of phenethylammonium (PEA) ions to see how changing the cation to an organic one affected the performance of the FET device. Moving ions is the main problem with halide perovskite FETs, though. The work of Chin et al. on light-emitting FETs using the 3D lead iodide perovskite (CH3NH3PbI3) in 2015 showed the connection between screening effects and ionic transport by lowering the FETs’ working temperature. Li et al. looked into halide perovskite phototransistors and found that the transistors didn’t work very well when it came to field-effect motion.
To fix these major issues, Senanayak et al. created a perovskite FET by switching the PbI2 precursor mixture to a Pb(Ac)2 precursor. This made the device more stable and improved the mobility of electron field effects. This made the current go up at both the source and the drain and lowered the loop behaviour. Duan and his colleagues studied FET performance using a MAPbI3 microplate that had been heated and then covered with a boron nitride (BN) layer to make it more stable.
To make an even better FET device, the main problems that need to be solved are charge transport, transport that changes with temperature, and steadiness. People think that the results in the field of halide perovskite FET devices will show more progress in this area, though.
Low-Dimensional Hybrid Organic–Inorganic
Semiconductors in Lithium–Ion Batteries
Hybrid perovskites have both electronic and optical qualities, and they can be used as lithium-ion battery anode materials to store charge. There is still no clear answer to the question of how lithium ions combine with the mixed perovskite structure during the charging process. When a voltage is given to a lithium-ion battery, lithium ions move into the anode and intercalate with the anode material. This shows that the battery is charging. The lithium ions move back to the cathode through the liquid when electrons flow through an outside circuit. This shows that the battery is dead. In 2015, scientists revealed that they could store lithium using CH3NH3PbBr3 and CH3NH3PbI3 as anodes and carbon black as the anode material. They did this using a chemical method and showed that the batteries could charge and discharge in both directions. It took 331.8 mA h g−1 to discharge CH3NH3PbBr3, but only 43.6 mA h g−1 to discharge CH3NH3PbI3. Vicente and Garcia-Belmonte’s recent study showed that electrodes made of perovskite are very stable when they are subjected to electrochemical cycling without causing major changes to the crystal structure. This shows that Li+ can be stored in the perovskite host through topotactic intercalation, without changing or rearrangement of the structure. Materials made from halide perovskite show promise for use in high-power batteries.
Conclusions
A lot of attention has been paid to organic-inorganic hybrid perovskites, especially those made from metal halides, because of the unique optoelectronic qualities they have. These materials have worked very well in many situations, like solar cells, LEDs, lasers, sensors, transistors, and lithium-ion batteries. They do have problems, though, because they are poisonous, stable, and soft. Lead halides look like good options, but they must have no lead in them so they don’t harm the earth. Some of these problems have been partly solved by research groups that have come up with options for high-performance, low-cost optoelectronic devices. The field of metal halide perovskite optoelectronics should have a bright future. However, more research is needed in energy transfer-capable perovskites, quasi-2D materials, and flaw types. These materials work better in places other than labs, like the workplace, where flaws can make the end complex qualities of perovskites stronger.
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