Halide perovskites, especially those made of lead, have a lot of promise for use in optoelectronics, especially as filters for solar cells. But because they are poisonous and don’t handle light or wetness well, people are looking for other materials that have similar traits but don’t have as many of the problems they do. A lot of metals have been looked at as possible alternatives to lead, but only a few have shown promise. Power conversion rates (PCE) of up to 10.37% and 0.57% for tin and germanium perovskites, like [MA]SnI3 and [MA]GeI3, are very high. They are not very stable, though, because they easily change to their +4 oxidation state.
In the past few years, people have been looking for options more and more. Halide double perovskites (HDPs) look like a good option. HDPs are made up of two metals that work together to replace the single B-site metal twice. This change makes it possible to use a lot of different metals that can’t be used in perovskite structures. It also makes it possible to make more halide perovskites. The first mention of an HDP was made in the late 1800s, so they’ve been around for a long time. HDPs are becoming more popular again because people are looking for new materials to use in optoelectronics. They can also be used in a wide range of other ways. This chapter will explain what HDPs are and look at their structure and qualities, as well as their present and possible future uses. It will end with a look at the future for this interesting group of materials.
Definition and Structure
Halide double perovskites (HDPs) are made by replacing a metal B with a single-valent cation B(I) and another trivalent B′(III) in the crystalline sites of a single perovskite, ABX3. They have the general chemical formula A2BB′X6. In HDPs, metal cations B and B′ move back and forth within the lattice sites, creating an arrangement that looks like rock salt and makes FCC sublattices that are twisted and moved half a unit cell in all three directions.
It is possible to make a lot of different materials with a wide range of electronic qualities by adding different metals to the halide perovskite structure and combining metals in different ways. Using the same kind of atom B but changing its oxidation state is another way to substitute. For example, in the double perovskite Cs2Au1+Au3+Cl6, the oxidation state of atom B is changed.
A few types of perovskites, like Cs2Au1+Au3+X6 (where X can be Cl−, Br−, or I−), can crystallise either in a tetragonal or a rhombohedral shape. Changing the make-up of B and B′ creates subnetworks that depend on the charge difference (ΔQ) between B and B′. This, in turn, lets the B sites in the 3D network be ordered in random, rock-salt, or stacked ways. When ΔQ is less than 2, double perovskites have a rock-salt order. In stacked double perovskites, B and B′ make alternate planes of octahedrons.
We talk about double B-site perovskites when we talk about double perovskites because the make-up of double perovskites changes when the A cation is switched out for two different types of cations, namely AA′BB′X6. There are other heterovalent replacements that can be made to the B-site that allow the charge balance of normal ABX3 perovskites to be kept. For example, the structures A3B2 3+◺X6 [3] or A2B4+◺X6 have a void at the B-site [18].
If there is a difference in size between the cations and anions, the cubic shape will become distorted. This can make it so that the octahedrons aren’t connected to each other, so a measure needs to be defined to show which chemicals are most likely to form perovskites phases. Goldschmidt’s tolerance factor is a great way to get a rough idea of how stable a structure will be in simple perovskites. It has been shown, though, that the Goldschmidt factor is not enough to predict how structurally stable HDPs will be. A new probability factor, = rX^rB − nA^rA − rA∖rB^ln (rA^rB)^⎞, has been suggested as a way to predict the formation of both single and double perovskites. This factor has been tested and found to be 92% accurate.
Properties
Lead-based halide perovskites, like MAPbI3, can be used in solar and optoelectronic systems because they have optoelectronic qualities. A lot of people are looking for materials that can mimic these qualities while being less harmful and more stable. Electronic changes happen between the valence band’s (VBM) highest point and the conduction band’s (CBM) lowest point. This is what gives MAPbI3 its features. The VBM is mostly made up of Pb2+ 5s and I− 5p orbitals, while the CBM is mostly made up of Pb2+ 6p orbitals.
When it comes to HDPs, adding both trivalent and monovalent B sites increases the number of less harmful elements that can be used as replacements. This changes the bandgap and makes the structure more stable. HDPs could be used to make materials with properties that are similar to MAPbI3.
The parts of HDPs that can be used to switch out sites A, B+, B3+, and X are shown in Figure 5.4. Depending on what they are made of (mostly B-site metals), HDPs can have either direct or indirect bandgaps with parity-forbidden transitions. As an example, Cs2AgBiBr6 and Cs2AgBiCl6 are the two HDPs that have been studied the most. They both have indirect bandgaps of 1.95 and 2.77 eV. These numbers aren’t good for solar cells with a single absorbing material, but they might work for solar cells that work with each other.
The stated values of the bandgap have changed from time to time, both in experiments and in theory calculations for HDPs. To give you an example, the dispersion for Cs2AgBiCl6 is up to 0.57 eV in the formulas and up to 0.8 eV in the experiments. When spin–orbit interaction is taken into account, the estimated values are closer to those that were found in experiments.
To get allowed transitions, different methods are used to change the band structure in HDPs. So far, chemical doping, choosing the right elements for the B+ and B3+ sites, and making the connections between cations less ordered have all worked well.
Chemical Doping
Adding monovalent or trivalent metals chemically can change phases in alloys that aren’t allowed by parity. In isovalent alloys, stable solid solutions form. In alliovalent alloys, dopants create cation gaps that allow carriers from interstitial flaws to recombine without radiation because the lattice difference is low. For instance, adding small amounts of Tl can change the bandgap of Cs2AgBiBr6 to make Cs2(Ag1−aBi1−b)TlxBr6. This makes the bandgap smaller, from 1.96 eV for Cs2AgBiBr6 to 1.5 eV for Cs2(AgBi1−x)TlxBr6. This change makes the bandgap in CsAgBiCl6 indirect when Tl+ is added. This lets the Tl 6s orbitals change the direction of the interaction, which makes the bandgap direct and lowers the band energy.
In PL, Cs2AgTlCl6 doesn’t give off any measurable emissions because its carriers heat up in the banned area and stay there. Adding HDPs can make emission more efficient, as seen with perovskites Sb3+:Cs2NaInCl6. Adding Sb3+ breaks the parity-forbidden transition and raises the photoluminescence quantum yield (PLQY) to 75.69%. This is a lot more than the best blue PLQY that was found for Cs2SnCl6.
The band structures of Cs2AgBiCl6 and Cs2AgTlCl6 show that the bandgaps are mostly metal-to-metal charge transfers and that the unusually low bandgaps in Cs2AgTlCl6 and Cs2AgTlBr6 come from.
Random Ordering
Changing how the cations are arranged in a semiconductor metal changes its band structure in a big way. The combination Cs2AgBiBr6 is used as an example. When the change in random B sites is simulated, the bandgap goes down from 1.93 eV to 0.44 eV. But it’s hard to do this experiment in the real world because the temperature has to rise to 800 K, which is where phase change starts. At 150 °C, recent tests have been done.
Stability
HDPs try to be more stable than their lead peers, but it’s important to look at both experimental and theory results to find the most thermodynamically beneficial HDPs. We use the energy above the convex hull Ehull to find out how much energy HDPs need to break down in a linear mixture of stable stages. A combination with an Ehull value of zero is in the convex hull and equilibrium stages. Values that are higher than zero mean that the stability is falling.
Even though compounds with a small positive number of Ehull are potentially unstable, they can be made and stay stable for a while. New studies show that the elements used in the A, X, and B+ sites have the most significant impact on the thermal stability of HDPs. The trivalent metal B3+ has almost no effect on stability.
As the mass of A goes up (from Li to Cs) and the mass of X goes down (from I to F), stability slowly rises. When B+ goes up (from Li to Cs), stability goes down. When B+ goes up (from Cu to Ag), stability goes up.
For up to 15 days, Cs2AgBiBr6 stayed stable in light and humidity. For up to 30 days, it stayed stable in darkness. Cs2AgInCl6 is another case to think about. It can stay solid for up to three months as a powder and for up to five months as a single crystal.
Applications in Solar Cells and LEDs
HDPs that can handle a lot of defects and are stable in different environments are being studied for possible uses in solar cells and light-emitting diodes (LEDs). The HDPs that have been studied the most are Cs2AgBiBr6 and Cs2AgInCl6. These have been successfully added to these devices, and many more are likely to follow soon.
Photovoltaic Solar Cells
A big use for Halide Double Perovskites (HDPs) is in perovskite-based solar cells (PSCs). A perovskite absorbs light, and photogenerated electrons are carefully introduced into the conduction band of an electron transporter layer (ETL). They are then removed through a transparent conducting oxide (TCO), blocking holes because of the way the bands are aligned. But making Cs2AgBiBr6-based PSCs is hard because they have a big and indirect bandgap and it’s hard to make thin films that are all the same.
In 2017, Bein et al. reported spin-coated Cs2AgBiBr6 films as an absorber layer for PSCs. These films were very stable and had a power conversion efficiency (PCE) of about 2.5%. We used various techniques for deposition and interface engineering. A low-pressure supported method was used to make a planar heterojunction, which had a PCE of about 1.44%. The antisolvent drip method was used to make an inverted planar heterojunction PSC, which had a PCE of 2.23%.
The bandgap of Cs2AgSbxBi1−xBr6 went from 2.22 eV to 1.97 eV when Bi was replaced with Sb. This led to a PCE of 0.19% and 0.25%, respectively. Tang et al. looked into adding alkali metals, which made the Cs2AgBiBr6 thin films better by lowering flaws and making them more resistant to high temperatures and air conditions without sealing, which raised PCEs by up to 2.57%.
To make HDP-based solar cells work well, we will need to add more buffering, moving, or intermediate layers. Shao et al. added an intermediate layer of the dye N719 to increase the range of light absorption, lower the number of flaws in the thin film, and help remove holes, which led to a PCE of 2.84%.
Light-Emitting Diodes (LEDs)
There are three ways to use HDP (High-Deficiency Polymer) in LEDs: as the only material that emits light, with materials that have different colours, or with a single LED device that has been covered with a phosphor. It makes white light, but the colour rendering index (CRI) is low because it lacks red and has a high correlated colour temperature (CCT). A mix of more phosphors can be added to get around this problem. HDP-based LEDs give off both narrow-band emission from free exciton (FE) and broad-band white-light emission from self-trapped exciton (STE). This is good for high external quantum efficiency (EQE) and colour rendering index (CRI). At the moment, chloride is the halide component in most HDPs. This gives them high quantum efficiency and longevity.
White-LEDs
A study by Yella et al. in 2019 showed that 30% Bi3+ added to Cs2AgInCl6 nanocrystals changed their bandgap to a straight transition and made it smaller than Cs2AgInCl6. In a polymethylmethacrylate polymer framework, the nanocrystals gave off great white light, with a CRI of about 91. This means that Halide Double Perovskites (HDPs) might be able to be used as an emission material in white light-emitting diodes (WLEDs), which is a good sign.
Phosphorus
HPDs can be used as phosphors in white LEDs to make them work more efficiently and give off white light. Bi3+–Er3+ co-doped Cs2AgInCl6 can change the host’s optical absorption and emission spectra, creating a new absorption channel at 372 nm that can be excited by common UV LEDs. Ho3+ doped Cs2(Ag,Na)InCl6 HDPs create energy transfer pathways from STEs to Ho3+, which helps them give off warm white light. The crystals were ground up in a ball mill to make a fine powder, which was then put on to regular UV LED chips that weren’t sealed. A good phosphor for LEDs would keep its high quantum efficiency and wide STE emission while moving the excitation spectrum to longer wavelengths. To get softer white light, phosphophors like Cs2Ag0.6Na0.4InCl6:Bi3+ can be used to move the emission band to the red side. Bi3+ is good for use in LEDs because it can be used as a sensitiser to bring the energy spectrum into the visible range.
Two or More Phosphorus
Different near-UV (NUV) or UV-emitting phosphors can be used to make WLEDs. In order to make a WLED, Liu et al. mixed HDP Cs2AgInCl6:Cr3+ with green and red phosphors in a UV LED. The HDP Cs2NaInCl6:Sb3+ that Woodward et al. used was a blue phosphor mixed with green and red phosphors in a UV LED. This made a device with CIE colour coordinates of (0.890, 0.4009). Adding dopants to HDPs is a good way to change and improve their brightness. Their wide and indirect bandgap, on the other hand, is not good for device application. To make LEDs more efficient at giving off light, HDPs need to be used as the only source of output.
Other Applications
Because they are so flexible, HDPs could be used in a wide range of optoelectronics uses, such as solar cells and LEDs. Photoconductor, photodiode, and phototransistors are all common types of devices. Photodetectors for UV and X-ray rays, memristors, photocatalysts, humidity sensors, and thermochromic sensors are some of the other uses for this material. We will talk more about less well-known uses of HDPs in the future.
Photodetectors
A photodetector is a machine that works like an eye and turns light information into electrical waves. Picture diodes, picture conductors, and picture transistors are the three main kinds. Photodiodes have a joint block that lets low dark current flow. This lets them respond quickly and sense a lot of things. Their EQE and responsiveness are both low. Photoconductors are made up of a semiconductor and two metal contacts. They need a voltage to help separate the charges, but they have a high dark current that makes them less sensitive for detection. Phototransistors have three electrodes at the ends and a thin insulating film that changes how charges move based on the polarisation of the gate.
Parameters like noise-equivalent power, specific detectivity, gain, linear dynamic range, and reaction speed are used to judge how well and how efficiently photodetectors work. Halide perovskites can now recognise X/γ photons in addition to UV, vis, and NIR light. This makes them popular for use in personal electronics, computer vision, machine vision, and biosensing. Due to their ability to absorb light well, hold charge for long periods of time, and move charge around freely, HDPs show promise as an option to lead-based perovskites as photodetectors on the high-energy end of the electromagnetic spectrum.
UV Detectors
Single crystals of Cs2AgInCl6 have been used to identify UV light because they have a low trap density, can only absorb light waves below 400 nm, and have an extremely long carrier lifetime. Tang et al. put high-quality Cs2AgInCl6 single crystals into a photoconductive planar structure with gold contacts. This device had the best ratio of detection to reaction time when compared to metal oxide-based ones.
Oxygen pollution could make the devices less stable, but this can be fixed by putting them away in a vacuum. Shi and others made Cs2AgBiBr6 photodetectors as thin films with the Au/Cs2AgBiBr6/Au structure. These photodetectors were very stable during an ageing test in room-temperature air and could be stored well for two weeks.
Gao and others put together a photodiode device on Cs2AgBiBr6 thin films with bigger grains and fewer trap densities. They were able to make a lead-free perovskite photodetector that was very sensitive and responded quickly to light waves below 450 nm. They improved the layers that move electrons and holes, which led to high dark currents in devices with TiO2 and SnO2 ETL.
Xiao et al. made a flat heterojunction photodetector out of ITO/SnO2/Cs2AgBiBr6/Au that can effectively pick up UV (320–400 nm) and deep blue (435 nm) light. After six months of being kept at room temperature without being sealed, the device didn’t seem to have broken down. HDPs are a good choice for photodetectors because they are stable and don’t harm the environment.
X-Ray Detectors
More and more attention is being paid to X-ray detection in safety, medical diagnosis, and quality control in industry. X-ray holes, electrons, photons, and storage capacitors are some of the tools that can be used to directly or indirectly detect this energy. Scanners that change X-rays into visible light are used for indirect detection. A photodetector then soaks up the light. High-density photopolymers (HDPs) are good for X-ray detection because they have a high absorption coefficient, can handle defects, have a large motion lifetime product, and have a high resistance. But X-ray photodetectors have problems, like noise that makes it hard to identify things and take pictures.
Direct X-Ray Detector
Tang et al. released the first report of an HDP-based X-ray detector that used a single crystal of Cs2AgBiBr6 in 2017. The high average atomic number, high resistance, and low ionisation energy of Cs2AgBiBr6 make it a good material for X-ray detection. The sensitivity and stability of devices made from Cs2AgBiBr6 were very good, showing that HDPs could be used in X-ray detectors. Bismuth oxybromide (BiOBr) was added as a heteroepitaxial passivator to stop ion movement. This resulted in signal drift that was lower than in any other study. Yu et al. reported films of a Cs2AgBiBr6 hybrid mixed with polyvinyl alcohol. This made the photodetector more uniform, flexible, and sensitive.
Indirect X-Ray Detector
Materials in indirect X-ray detectors take in X-ray photons and turn them into visible light. X-ray scintillators with high performance dielectrics (HPDs) have shown a lot of promise. A study by Tang et al. discovered that adding lanthanide ions to Cs2NaLnCl6 makes it produce more light after X-rays are absorbed. The f-f transitions of lanthanide cations make these double perovskites better at PL. X-rays can make 46,600 photons per MeV of light from Cs2NaTbCl6 powder crystals. This is a lot more than lead-based perovskites. Yang et al. made a group of Cs2Ag0.6Na0.4In1−yBiy Cl6 single crystals with different amounts of Bi3+. These crystals had a high scintillator light output and quickly lost their light. Under low-dose X-ray irradiation, high-quality pictures of both still and moving items were made.
Memristors
Memristors are passive devices that have exact levels of resistance that can be changed by applying an electric current. Leon Chua came up with the idea for them in 1971, and they have a lot of promise for use in memory computing, neuromorphic computing, and storing information. Most memristors have a high ratio of ON to OFF, low working voltages, and high resistance. You can find them in sandwich, panel, or cross-array shapes. Cs2AgBiBr6 is the first perovskite-based memristor that has been reported. It is very steady and has great memory ability. The Au/Cs2AgBiBr6/ITO memristor that was made worked well as a resistance switch even in difficult conditions. Because hard charge moves quickly, HDP memristors have a small ON/OFF ratio. Cs2AgBiBr6 is stable because the Ag-Br bonds are strong and the crystals are well-formed. But there aren’t many studies on double perovskite lead-free memristors, and it’s still not clear how the structure affects memory properties.
Photocatalysis
The double perovskite Cs2AgBiBr6 has a lot of promise as a photocatalyst because it can absorb a lot of light, keep its charge for a long time, and work without any problems. In different photoelectrochemical processes, like the hydrogen evolution reaction (HER) and the CO2 reduction reaction (CRR), it has been used to break down organic pollution. The first time HDP was used, it was for HER in a fully mixed water solution of HBr and H3PO2. A mixture with 2.5% reduced graphene oxide (RGO) was made. This made HER about 50 times stronger, hitting 489 εmol g−1 and staying stable.
Photocatalytic water splitting could also work with Cs2BiAgCl6. However, Cs2InAgCl6 and Cs2SbAgCl6 would need to have their energy levels changed to make them work for water splitting. We used Cs2AgBiBr6 nanocrystals (NCs) made by the hot injection method for CRR. They were able to use up 105 μmol g−1 of electrons while being illuminated by 1.5 G of air mass for six hours. The NCs were very stable with water and low-polarity liquids, but the organic layer on the surface made it harder for photogenerated charge to get to the reactants, which limited the yield of the catalyst and poisoned them.
During photocatalytic processes, Cs2AgBiBr6 keeps its high molecular stability in ethanol. This makes it easier for dye or O2 molecules to become active and speeds up the reaction rate between dye molecules and active species.
Sensors
In 2019, Zhan et al. used Cs2BiAgBr6 thin films to make a humidity monitor. Higher relative humidity caused water molecules in the film to physisorbed, which led to the transfer of protons and a rise in electrical conductivity. The sensor responded and recovered very quickly in the RH range of 5 to 75% and stayed stable for up to 110 hours. Gao et al. found that Cs2BiAgBr6 thin films and single crystals have fully reversible thermochromism, which causes changes in the length of the Ag-Br and Bi-Br bonds. The sensors were very stable in both temperature and surroundings after being heated and cooled several times. This opened the door for a new class of smart windows, temperature sensors, and visual thermometers that use lead-free halide double perovskites.
Future Applications
This part talks about how High-Density Polymers (HDPs) are being developed for uses other than solar cells and LEDs. We are still in the early stages of these uses, but HDPs have shown a lot of potential and offer many ways to learn more. As they find new possibilities, HDPs may look into new areas such as piezo- and ferroelectricity, batteries, spintronics, and lasing.
Related Materials: Layered Double Perovskites
and Vacancy Ordered Double Perovskites
Low-dimensional double perovskites are being looked for because bromide double perovskites and stacked perovskites have been made better. HDPs can make the benefits of lower-dimensional materials even better by making them smaller while also increasing their chemical variety and qualities. This part will talk about two types of double perovskites: those that are made by reducing the number of dimensions and those that have ordered gaps.
Dimensional Reduction
Low-dimensional double perovskites (HDPs) are a type of semiconductor material that has been studied in the same way that halide perovskites have been. These HDPs are made by breaking down 3D perovskites. The most well-known and studied are the 2D forms. These shapes are made up of flat sheets of alternate octahedra that share corners. All the known 2D HDPs are in the (001) family, which has n = 1 and 2. This makes the shapes more stable and flexible. Ruddlesden–Popper and Dion–Jacobson type patterns can be found in these kinds of 2D HDPs.
Karunadasa et al. were the first to look into what 2D confinement does to HDPs (BA)4AgBiBr8 and (BA)2CsAgBiBr7. They discovered that when these HDPs go from 3D to 2D, they show a change from an indirect to a direct band. This change is caused by dimensional confinement, but structure errors make the direct and indirect gaps even smaller. They also absorb a lot of energy at similar levels, which is thought to be due to an Ag-to-Bi charge shift. This is what gives these materials their similar yellow colour.
The researchers led by Mitzi used AE2T (5,5′-diylbis(aminoethyl)-[2,2′-bithiophene], an organic molecule with two functions. Aromatic edge-to-face interactions helped to stabilise (AE2T)2AgBiI8, which led to type I/II border quantum alignment. This mix of options creates almost an infinite number of 2D HDPs, giving materials an endless chemical space for many uses, many of which haven’t even been thought of yet.
Vacancy Ordered Perovskites
Ordered vacancies Some people are interested in perovskite because it looks a lot like double perovskites. It is made when M(IV) takes up half of the sites and leaves the other half empty. Cs2SnI6, Cs2PdBr6, Cs2TeI6, and Cs2TiBr6 are all known members of this family. This is a different subgroup made up of three-valent metals. The stoichiometry of A3B(III)◺X9 is given by the occupancy of two M(III) by one ◽. A member of this family that has an extra metal added to it makes a stacked double perovskite. This kind of perovskite has metals in both 2+ and 3+ oxidation states, as well as empty spaces.
A2B(IV)X6: B(IV) Substitution + Vacancies
K2PtCl6-type perovskites are an extension of ABX3 perovskites. They have the general formula A2B(IV)◽X6 and are called 0D perovskites because the [BX6] octahedra are not connected to each other. They are mostly made up of halide p orbitals that are not connected, while the CBMs are mostly made up of both metals. Because of the separate octahedra [BX6], the CBMs are spread out by the charge distribution of the B cations. This makes electrons much more mobile than holes. There is a story that these materials can work as filters in solar cells and have a maximum PCE of 3.3%. They are naturally n-type semiconductors.
A3B(III)X9: B(III) Substitution + Vacancies
Taking out a third of the cations from the B-site gives you the general formula for heterovalent substitutions with trivalent B(III) cations, like Bi3+ and Sb3+. It comes in two different forms: a 0D dimer form with isolated face-shared dioctahedral, and a stacked 2D perovskite structure that can be favoured by picking the right A, B, and X. The 2D objects A3B2X9 are made up of empty spaces along the (111) plane and have more electron dimensions than dimers. The stacked polymorph has a straight bandgap of 2.04 eV, which makes it good for moving charges. They do, however, show PCE up to 3.34 %. If the stacked form is what you want, you can use smaller A-site cations like Rb+, which leads to PCEs of 0.66%.
A2B(II)B2(III)X12: B(II), B(III) Substitution + Vacancies ◽
Quadruple perovskites, whose formula is A2B(II)B2(III)◺X12, are made up of perovskites that are arranged in a longer chain, with a formula of A3B(II)B2(III)X12. They have an artificial slab that is three octahedra thick and is made by adding another layer of B(II)X6 octahedra between two B(III)X6 slabs. The first halide perovskite that was found was Cs4CuSb2Cl12. It had a small bandgap of 1.0 eV and was very good for optical uses because it was non-toxic, had a lot of copper and antimony, had a straight bandgap, and was very stable structurally. Metals like Cu(II), Mn(II), Cd(II), Sb(III), and Bi(III) have been found in other members of this family. Some new ideas say that bigger B(III) ions, like lanthanides, might make it possible for bromide or iodide to join in. These materials could be looked into further for uses in LEDs, phosphors, and spintronics, among other things.
Conclusions
Halide double perovskites were first looked at as possible solutions for solar cell absorbers. They have since shown a lot of promise in many other areas besides photovoltaics. Their comeback has led to big steps forward in fields like photodetection, chemosensory, catalysis, and spintronics. In the future, people will also be making new double perovskites and products that are based on perovskites.
The future looks good for HDPs in many ways. In the short run, the growth of these materials will help us understand the link between structure and qualities better, which will make focused design easier. In the next few years, progress in both applications and materials will push double perovskites to new limits. They will either improve the performance of current devices or open up new uses that haven’t been thought of yet. Long term, HDPs and similar materials will follow the lead of halide perovskites and be used in products that work well.
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