Perovskite Research

All-Inorganic Perovskite Photovoltaics

The power conversion efficiency of organic-inorganic hybrid perovskites has improved quickly. The unique features of hybrid perovskites are most likely due to the mix of organic cations and the inorganic sublattice. These biological parts, on the other hand, don’t seem to be as strong when it comes to the mixed perovskites’ long-term security. The breakdown of hybrid perovskites in damp conditions and the removal of the organic part from the material at high temperatures make it very hard for these compounds to stay stable over time. Because of this, a lot of work and attention has gone into making all-inorganic perovskites that don’t have any organic cations.

The performance of inorganic perovskite solar cells (PSCs) has improved quickly. In about three years, the power conversion efficiency (PCE) reached 19%. All-inorganic perovskites with a bandgap close to 1.8 eV are also interesting because they could be used as the top cells in perovskite/Si or perovskite/CIGS tandem solar cells. All-inorganic perovskites are still a safe option to mixed perovskites because of this.

These are called all-artificial halide perovskites. They have the formula ABX3, where A is an inorganic cation like Cs and Rb instead of an organic cation, B is Sn or Pb, and X is I, Br, or Cl. In the 1890s, those who were interested in halide perovskites first looked into all-inorganic halide perovskites. In 1893, Wells et al. made lead halide chemicals like CsPbX3 (X=Cl, Br, I) [8] or RbPbX3 [9] by mixing lead halide with Cs or Rb in a solution. Later, Møller, a Danish researcher, found that CsPbCl3 and CsPbBr3 have a structure called perovskite [10, 11]. It’s possible that the easy solution method used to make these Cs lead halide ionic crystals led researchers to use other cations instead of Cs. We later learnt that an organic cation called methylammonium (CH3NH3+) can replace Cs+ to make CH3NH3MX3 (M = Pb [12], Sn [9], X = I, Br), and he published the first diffraction study on these mixed organic–inorganic lead halide perovskites.

A lot of perovskite chemicals that are good for PV uses can be made by mixing Cs, Pb, Sn, I, and Br in different ways. A lot of research has been done on CsPbI3, CsPbBr3, CsPbI2Br, CsPbIBr2, CsSnI3, CsPb0.5Sn0.5I3, Cs2SnI6, and other materials that have been found to be promising as PV absorbing materials. CsPbX3 perovskites (X = I, Br, or a mix of I and Br) are useful because they don’t break down chemically at temperatures up to 400 °C. However, it’s hard to keep the black photoactive phase stable at room temperature and this photoactive phase for a long time in a normal atmosphere.

A lot of research has been done on how the black phase forms at room temperature and in normal settings. Some studies also show that other ways to improve cell function include not heating, cooling, flaw passivation, controlled crystallisation, and so on. The geometrical tolerance factor shows that replacing smaller Cs+ ions (r = 167 pm) with bigger cations (MA+, r = 217 pm, and FA+, r = 253 pm) or bigger I ions (r = 220 pm) with smaller anions like Br (r = 196 pm) and Cl (r = 181 pm) can raise the tolerance factor in a way that makes the black phase of CsPbX3 more stable at room temperature.

To sum up, all-inorganic perovskites have shown promise in the creation of solar cells, especially when it comes to organic–inorganic mixed perovskites.

All-Inorganic Lead Halide Perovskites

Cesium Lead Iodide (CsPbI3): Black-Phase Stabilization

Getting the high-temperature dark photoactive phase of CsPbI3 to stay stable at room temperature is the tricky part. There are a number of ways to crystallise and keep this phase stable at room temperature. In Sections 9.2.1.1–9.2.1.4, we talk about the main methods used to keep this phase stable and how well the devices work with them. Table 9.1 shows some examples of these methods. Table 9.1 shows the photovoltaic performance of CsPbI3-PSCs that use the black photoactive phase of CsPbI3 and are stabilised in different ways. The structure of the device includes many different methods, such as adding ingredients, isovalent or aliovalent metal ions, making nanocrystals, controlling crystallisation, and treating the film’s surface after it has been made. The results show that different methods have been used to keep this phase stable and make the gadget work better.

Additive Approach

It was first seen that hydroiodic acid (HI) could keep the CsPbI3 α-phase stable at lower temperatures. The HI-added film only crystallised in the black phase at 100 °C. The formation of the α-CsPbI3 was proven by the UV–vis absorption spectrum and XRD patterns. Several studies suggested that an intermediate HPbI3 could be made from the HI-mixed precursor solution of CsPbI3, which was said to play a part in keeping the black phase stable. But Kanatzidis and his colleagues showed that these results were wrong by showing that HPbI3 is not a step in the process. They learnt that the dimethylammonium iodide (DMAI) released into the precursor solution when the solvent DMF breaks down in the presence of HI helps the crystallisation of the CsPbI3 black phase.

Smaller crystals can usually keep a more even phase, and lattice strain can often lower the temperature at which the phases change. These were most likely the ways that the black phase stayed stable when it was hotter than 310 °C or 100 °C with HI. The dimethylammonium iodine (DMAI) method has been shown to be better at keeping the perovskite phase stable at room temperature. This method has also been used to make cells that work with a Photochemical Energy Efficiency (PEE) of over 19%.

Recently, Sun et al. showed a changed technique called the “additive-involved leaching method” that makes it possible to make artificial CsPbI3 perovskite films at a temperature of 100 °C. What they found is that adding a certain amount of DMAI and CsI to the precursor makes high-quality CsPbI3 perovskite films when isopropanol is used for leaching. This PSC built on CsPbI3 had a PCE of 16%, and the unencased device kept about 93% of its original PCE after being stored for 30 days in air with a 10% relative humidity.

Chang et al. reported a high-efficiency printed CsPbI3 solar cell that used Zn(C6F5)2 as a multipurpose addition. This formed an energy gradient within the perovskite layer and made the matching of the energy bands better at the interface of the perovskite and SnO2. The factors worked together to make CsPbI3 solar cells with a 19% PCE.

Quantum Dot-Induced Black-Phase Stabilization

Making colloidal nanocrystals and quantum dots (QDs) is a good way to keep the CsPbI3 black phase stable for a long time. There are a lot of different ways to make QDs of CsPbI3 that are in the black phase, and these have been used in optoelectronics and PV cells. A solar cell with an array of CsPbI3 QDs was created by repeatedly covering colloidal CsPbI3 QDs made by adding Cs oleate to a PbI2 precursor. This cell has a PCE of 13.43% and was approved by NREL. However, poor carrier separation because of the organic ligands used in the production process is still a problem with these QD-based CsPbI3 devices. More work has been done on changing the ways that CsPbI3 QDs are made to improve their performance. Another way to improve cell efficiency is to design the interface so that carriers can be collected more efficiently. Chen et al. recently reported that CsPbI3 QD devices had a short-circuit photocurrent density (Jsc) of 17.7 mA/cm2 and a PCE of 14.3%.

Stabilization by Surface Treatment

In CsPbI3 films, the black phase doesn’t always have to change to the yellow phase. Big grains can keep the black phase. A small change in the annealing conditions, like letting the precursor films cool to room temperature before heating them to 350 °C, can have a big effect on the stability of the black phase. This makes solar cells with a best performance coefficient (PCE) of 15.7%, and their performance stays the same for 500 hours of constant light exposure. In a different case, adding phenyltrimethylammonium bromide (PTABr) to the surface of the CsPbI3 film made the black phase more stable, which led to cells with a PCE of more than 17%.

B-Site Doping

The α-CsPbI3 stays stable at room temperature thanks to B-site doping, which replaces some of the Pb2+ ions with metal ions such as Bi3+, Sb3+, Ca2+, and Eu3+. By adding 4 mol% bismuth, this doping changes the bandgap of CsPbI3, lowering it from about 1.73 eV to 1.56 eV. The small size of the Bi3+ ion causes a shift in the lattice, which creates microstrain and leads to the formation of the black CsPbI3 phase at room temperature.

Adding the Ca2+ ion to CsPbI3 widens the bandgap and makes the grains bigger. On the surface of the perovskite, Ca2+ cations create high bandgap oxides like CaO and CaCO3. These oxides help to protect the surface and stop recombination. The solar cells with structure glass/FTO/compact TiO2/mesoporous TiO2/CsPb1−xCaxI3/P3HT/Au show an average PCE of 11.0% when the precursor solution has the right amount of Ca2+ (5%). The winning cell had a PCE of 13.5% with an antireflection layer on the glass side.

Adding different metals to the B-site of CsPbI3 can make the black phase more stable by either making the structure more microstrained or cutting down the size of the crystals. Adding Euopium (Eu, both Eu2+ and Eu3+) to CsPbI3 was found to stabilise the black phase at room temperature. This is thought to happen by making the grains smaller and adding microstrain to the crystal structure. The XRD peak moving to a lower 2‼ (lattice expansion) and the XRD peaks breaking show that the crystal is becoming less symmetrical and/or less ordered. This is likely because Eu3+ is being incorporated into the lattice.

In the work that our colleague is currently doing, most of the Eu and a small amount of Cs are pushed to the edges of the grains. This leaves the centre of the grain without any Cs, which keeps the grains to a size of about nanometres. This creates the black γ-CsPbI3 phase, which stops the black phase from turning into the yellow phase. This makes the γ-CsPbI3 phase last longer.

The black phase of CsPbI3 stays stable at low temperatures because crystals can’t grow as big, so they form smaller pieces or grains. Adding the right metal ions or chemicals stops crystals from growing bigger than a few nanometres, which creates the cubic phase of CsPbI3. Some ions or compounds in the CsPbI3 precursor solution change the nucleation and crystallisation process. This causes the thermodynamically less stable black phase to form on top of the thermodynamically more stable ε-phase. The black phase must form and crystallise more quickly than the yellow phase because of the way the ions interact with each other or the chemical environment of the different species in the precursor.

Some findings, like the fact that PEA+ has a strong interaction with CsPbI3, which has a direct effect on black-phase crystallisation, point to the phenomenon. The amount of CsI to PbI2 (non-stoichiometry) has a big effect on the phase that forms. For example, at 80 °C, a precursor with 4 mol of CsI and 1 mol of PbI2 crystallises in the CsPbI3 black phase instead of the Cs4PbI6 phase. Studies have shown that adding a little extra CsI or CsBr makes the CsPbI3/CsPbI3−xBrx black phase more stable. This shows how important solution chemistry is for crystallising the black phase.

It is also seen that the type of substrate (glass, glass/FTO/compact TiO2, glass/FTO/compact TiO2/mesoporous TiO2) affects the formation of the crystal phase and how stable it is. The stretched interface makes the thermal stability of black CsPbI3 thin films much better. It is thought that the phase stability can be improved even more by engineering the thermal strain at the interfaces, which can be done by changing the substrates or the annealing conditions.

It’s also important for the safety of the black CsPbI3 that it has a certain shape. In some cases, low-temperature black phases like ³-CsPbI3 and ²-CsPbI3 have been seen to be more stable for longer. There are controlled ways to make these low-temperature black phases, which can help make all-inorganic CsPbI3 PSCs work better and last longer.

Cesium Lead Bromide (CsPbBr3)

The all-inorganic perovskite CsPbBr3 has a bandgap of about 2.3 eV and is not good for photovoltaic (PV) uses because it is more stable at high temperatures and in different phases. However, its use in PV has been looked into, and the PCE of CsPbBr3 solar cells is higher than 10%. Changes to the interface, chemical engineering, grain boundary passivation, and other methods have been used to make CsPbBr3 solar cells more efficient.

At room temperature, CsPbBr3 perovskite has an orthorhombic shape. Its orange colour, which is caused by an absorption peak at 550 nm, makes it useful as a light collector in PV cells. Between 361 and 403 K, the material has a tetragonal P4/mbm phase. Above 403 K, it has a cubic Pm-3m structure. At 840 K, the stuff melts.

Liang et al. used CsPbBr3 in devices with long-lasting and cheap carbon electrodes to make the devices last longer. The carbon-based device worked with a best PCE of 6.7%, and the cells were found to be more stable in both cold and hot conditions than MAPbI3 solar cells. Duan et al. found that PCE went up by 10.14 percent in carbon-based hole transport material (HTM)-free CsPbBr3 all-inorganic PSCs. The CsPbBr3 had been doped with different lanthanides. In their study, Tong et al. found that the carbon-based CsPbBr3 PSCs made by vapour deposition worked 10.17 percent of the time.

It was reported that controlling the change of two different phases, CsPb2I5 and CsPb2Br6, to pure cubic CsPbBr3, led to a PCE of 9.86% in a carbon-based device without HTM and a PCE of 10.91% in a n–i–p structure device with spiro-OMeTAD as HTM.

The main things that have improved performance are choosing the right ETL and HTL, engineering the interface, and controlling the crystallisation of CsPbBr3 (or phase purity). Using the Shockley–Queisser (S–Q) limit to model, the theoretical PCE of CsPbBr3 can be around 17%. This is about 7% less than the best yields that have been seen so far. The highest open-circuit voltage (VOC) that can be reached with CsPbBr3 is about 1.6 V, which is 0.7 V less than the theoretical limit (2.3 eV bandgap).

Cesium Lead Mixed-Halide Perovskites (CsPbI3−xBrx)

We used the fact that CsPbBr3 is stable at high temperatures and CsPbI3 has a smaller bandgap (black phase) to make all-inorganic lead halide perovskites (PSCs) with carbon electrodes work better as solar materials. Researchers have looked into different mixes of bromide and iodide. Adding brudines to CsPbI3 lowered the temperature at which the black phase forms from 350 °C to about 250 °C and made the phase more stable. For instance, CsPbI2Br crystallises in the black phase at lower temperatures (100 to 150 ∘C), which makes it more stable against phase changes.

When you change the amount of Br in mixed-halide caesium lead perovskites (CsPbI3−xBrx), the bandgap changes. A more well-known all-inorganic perovskite is CsPbI2Br, which is used in many of the most recent works. CsPbI2Br is a good material for perovskite/silicon tandem solar cells when its bandgap is about 1.9 eV. Adding chemicals, metal ions, controlling crystallisation, surface and flaw passivation, and interface engineering are some of the main ways that CsPbI2Br solar cells are made to work better.

In addition to studying how stable CsPbBr3 is at high temperatures, researchers have also looked into the photovoltaic performance of all-inorganic PSCs with carbon electrodes. They have done this by using different mixes of bromide and iodide. The study also looks at the properties of all-inorganic PSCs based on an HTM-free carbon electrode and CsPbBr3 doped with different lanthanides.

Finally, the fact that CsPbBr3 is stable at high temperatures and CsPbI3 has a lower bandgap makes them good choices for making PSCs with carbon electrodes.

A lot of what makes solar cells (PSCs) work depends on films that are regular and dense, with big grains and few grain borders. Controlled crystallisation, precursor solvent engineering, antisolvent-assisted crystallisation, and co-evaporation coating methods can make films and cells work much better. When it comes to PV performance, some PSCs made from CsPbI3−xBrx all-inorganic perovskites have shown promise.

Creating new solvents or improving the ones we already have can also help make high-quality CsPbI2Br films. Wei Huang’s group, for instance, used methylammonium acetate (MAAc) ionic liquid instead of DMSO and DMF. This helped them make a good perovskite film with low trap density. This liquid makes it possible to make the perovskite film in the air, even if it is wet.

As the perovskite grows, MAAc plays a part by creating the Cs1−yMAyPbAcyI2−yBr phase, which disappears during the heating process to leave behind CsPbI2Br. The PSCs that were made with this fluid and intermediate engineering had a best PCE of 15.83% and a Voc of 1.32 V.

It is important for cells to work well that the film is regular and has a thick shape without any pinholes. To improve efficiency even more, phase clarity and low flaw density are very important. To passivate or minimise flaws, different types of faults and their effects on performance are used, among other things.

To sum up, for PSCs to work better, they need films that are regular and small, with big grains and few grain borders.

The study looks into how chemicals, B-site doping, interface changes, and surface treatment can be used in perovskite systems to hide flaws and make cells work better. It has been seen that almost all of the ingredients used in the preparation solution improve the shape of the film and work to seal up defects at the same time. For instance, adding lead acetate (Pb(Ac)2) to the CsPbI2Br precursor makes the black phase more stable over a long period of time and protects against flaws by PbO forming at the edges of the grains.

The voltage kept going up, all the way up to 1.36 V, as the amount of lead(II) propionate added to the CsPbI2Br precursor went up. At the best dose of 1 mol%, the cells using a dopant-free polymer HTM showed a PCE of 14.58%, which was the best. In Pb(Ac)2, PbO (formed at T >300 ∘C) at the grain boundaries helps to hide the defects. But in our case, the propionate stays in the film (annealed at 180 ∘C) and is thought to help coordinate the undercoordinated Pb2+ and lower the defects. The addition also made the black phase more stable for longer.

Because CsPI2Br films with fewer defects are more stable, it’s possible that the flaws, whose types and locations are not yet known, are also to blame for the phase stability and shift. It is also looked into how well PSCs made of all-inorganic mixed halide lead perovskites (CsPbI3−xBrx) work as photovoltaics.

In the end, the study shows how important it is to use chemicals, B-site doping, interface changes, and surface treatment in perovskite systems to make cells work better and hide flaws.

The research is mostly about how to treat the surface of all-inorganic perovskite photovoltaics (PSCs) with tin oxide solution (SnCl2). Putting polythiophene on top of CsPbI2Br has been seen to seal up surface flaws and make hole injection better at the perovskite/HTM interface, leading to little energy loss and a high voltage of 1.32 V. It has also been found that an amino-functionalized polymer (PN4N) can stop interfacial recombination in CsPbI2Br cells when the PCE is above 16%.

The study discovered that even small changes in the tin oxide ETL’s qualities can have a big effect on the Voc output, especially for PSCs that are made up of only inorganic materials. It is possible to greatly improve Voc and PCE by leaving the SnCl2 precursor solution used to treat the surface of SnO2 ETL at room temperature for several days without doing anything else. When an old enough SnCl2 solution was used, the Voc went up by over 1.40 V and the efficiency went up by 15.5%. The winner device had a Voc of 1.43 V, which is the highest Voc ever recorded for CsPbI2Br. This meant that less energy was lost—less than 0.50 eV.

The device made with the old SnCl2 solution was much more stable over time than the one made with the new solution. This method is better because it doesn’t add any dopants, additives, or extra passivating layers to the devices. Wang et al. recently saw that using a SnO2–SnOx (from SnCl2-aged) bilayer ETL made the Voc better, going from 1.2 V to 1.4 V. They also saw that treating the CsPbI2Br surface with methyl acetate, an antisolvent, made the connection between CsPbI2Br and spiro-OMeTAD, the HTM, better. Because of this two-layer change, the cell efficiency went up from 6.96% (for SnO2 ETL, no antisolvent) to 15.86% (for SnO2-TiO2 ETL, methyl acetate antisolvent).

It has been seen that oxygen can also stop flaws in CsPbI2Br films from showing up. Jinsong Hu and his colleagues looked at the perovskite films made in a glove box and dry air. They discovered that the O2 molecule can be broken up into O atoms in the dry-air processing state, which has a strong contact with the perovskite during the growth of the perovskite crystal.

There are only O2 molecules that can combine with the surface of perovskite in the glove box process. The PCE of the device made from O-passivated CsPbI2Br films was 15.17%, which was much higher than the 13.32% of the device processed in a glove box. This increase in device stability shows that all-inorganic perovskites can be processed in air, and the addition of atomic oxygen to the film formation process can have a big effect on getting rid of defects. But it’s still not clear what oxygen does to the CsPbI2Br perovskite film, just like it’s not clear what oxygen does to hybrid perovskites because it depends on the conditions or the structure of the device.

To make the CsPbI2Br phase more stable and less prone to flaws, metal ions such as Sr, Eu, Nb, Mn, Zn, Zn–Mn, Ge, and others have been added to the B-site. Higher Voc and PCE seen in all cases show that recombination is less likely to happen. It is also thought that changing cations at the a-site could help get rid of flaws and make the crystal structure more stable. Smaller A cations, such as K+ and Rb+, can fill up the octahedral gaps, which helps Goldschmidt’s tolerance factor towards ideality and keeps the perovskite phase stable.

By adding Rb+ cations, you can make a high-quality perovskite film with fewer flaws. It works with a high PCE of 17.16%, which is the best number for CsPbI2Br-based solar cells to date. The winner device has 1% Rb in CsPbI2Br. The improved tolerance factor caused by Rb+ doping helps the device keep working well even when it is heated all the time and there is 40% humidity.

There isn’t a good understanding of the types of flaws that exist in all-inorganic perovskites right now, so the word “defect passivation” is used in studies that show better performance as Voc and FF go up. To get a better picture, more research needs to be done to figure out where the flaws come from at the atomic and molecular levels and how they get passivated.

It looks like CsPbI2Br could be useful as an all-inorganic perovskite, but there are some problems that need to be fixed. One of these problems is that the black phase changes quickly to the yellow phase when water is present. This phase change caused by moisture is very different from the breakdown of organic–inorganic hybrid perovskites caused by moisture. Getting rid of small flaws in the CsPbI2Br film may help make the material more stable in its phase. Advanced characterisation using synchrotrons and TEM can help us understand how all-inorganic perovskites break down, and complete containment can successfully stop this phase change caused by moisture.

All-Inorganic Tin Halide Perovskites

Concerns about the safety of Pb-based PSCs have led to the search for Pb-free, safe perovskites or chemicals that are similar to perovskites. Sn-based perovskites are appealing because they have the same ionic radius as Pb and a s2 valence electronic configuration. New developments in Sn-based halide perovskites have made devices work better, but their full potential has not been explored yet because Sn2+ is unreliable. A lot of people are interested in all-inorganic Sn-halide perovskites like CsSnX3 or vacancy-ordered double perovskites Cs2SnX6 because the organic part makes them unstable.

CsSnX3 (X = I, Br, Cl)

CsSnI3 is a good option to Pb-based perovskites that doesn’t contain any lead and works well in single-junction solar cells. However, the PCE achieved with CsSnI3 PSCs is still a long way from what it could be because it has a lot of unwanted qualities, such as a lot of background carriers, a lot of defects, and the divalent Sncation being easily oxidised. The poor performance of CsSnI3 PSCs is because they have a lot of defects. These defects can be found in the mass as Sn gaps or on the top of the CsSnI3 film, where they are made when divalent Sn2+ oxidises to tetravalent Sn4+.

Even though these issues make it hard to use this material to its full potential, there has been progress in improving device performance and stability through the use of additives in CsSnX3 perovskite films, compositional engineering, and the development and optimisation of film deposition techniques. SnF2 has been used successfully as an addition to stop Sn-based perovskites from acting like metals and lower the number of defects. A very high carrier density of 1019 cm−3 was recorded for pure CsSnI3, with holes as the majority charge carriers (p-type conductivity was caused by Sn-vacancies). This density was found to drop as the mole% of SnF2 in CsSnI3 increased. Some other tin halides, like SnI2, SnCl2, and SnBr2, are also thought to raise the chemical potential of Sn and lower the number of Sn vacancies.

Strong inhibitors, such as hydrazine, hypophosphorous acid, triphenyl phosphite, and triphenyl phosphite added during the perovskite film processing, improve the performance of the device by stopping the oxidation of Sn2+. By adding piperazine to the precursor solution of CsSnI3, Kanatzidis and his colleagues were able to lower the conductivity of CsSnI3, which made the cell work better. However, none of these ways have been able to completely stop the oxidation of Sn2+.

The structure of the CsSnX3 PSCs has a big effect on how well the cells work, just like it does with other perovskite devices. It is necessary for the perovskite’s energy levels to match those of the ETL and HTL in order for the performance to be better. For instance, Cahen and his colleagues discovered that adding 20 mol% SnF2 to CsSnBr3 decreased its work function and ionisation potential. This made the voltage loss lower in a device using spiro-OMeTAD as the HTM by bringing the VB level of CsSnBr3 closer to the HOMO of spiro-OMeTAD. This means that the cell works best when it has a PCE of 2.1%.

Over the past few years, a number of new techniques have been used to improve the performance of Sn-based all-inorganic PSCs. For instance, Padture and his colleagues got great results in terms of cell stability and efficiency by mixing CsSnI3 with Ge to make a solid solution that is made up of CsSn0.5Ge0.5I3. The PSCs that used CsSn0.5Ge0.5I3 had the best PCE of 7.11%, and the devices were very stable—their performance dropped by less than 10% after 500 hours of nonstop use (but in a N2 atmosphere). There is a GeO2 layer between the perovskite and HTL that acts as a passivation layer. This makes the PCE and reliability better.

Chen and his team made quantum rods of CsSnI3, CsSnBr3, and CsSnCl3 using the solvothermal method and QDs of CsSnX3. These rods worked very well. The best PCE of 12.96% was found in devices made from CsSnI3 quantum rods. Devices made from CsSnBr3 and CsSnCl3 did not do as well.

Cs2SnX6 (X = I, Br)

Cs2SnX6 is a vacancy-ordered double perovskite that has one filled B-site and one empty B-site that are spread out in a 3D pattern. It is an alternative absorber with a high potential. Its Eg is 1.48 eV, and its absorption coefficient from 1.7 eV is over 105 cm−1. In contrast to CsSnI3, it is not unstable because of the oxidation of Sn2+. The black orthorhombic phase of CsSnI3 can be changed into Cs2SnI6, which is stable in both air and water.

Cs2SnI6 was first used as an HTM in solid-state dye-sensitized solar cells. However, first-principles density functional theory studies showed that iodine gaps and Sn interstitials are the main flaws in Cs2SnI6, which is what makes it n-type. Cs2SnI6 might be useful as an absorber in solar cell devices because it stays stable in damp air and has dispersive border states.

In their first test, Qiu et al. used Cs2SnI6 made using a modified solution process as a light-absorbing layer for a lead-free PSC that used ZnO nanorod arrays as the ETL and P3HT as the HTL. In the best case, they got a PCE of 0.85%. Later, Qui et al. improved the thickness of the Cs2SnI6 film made from unstable B-γ-CsSnI3 that was made using a two-step solution method. They were able to get a PCE of about 1% with a Voc of 0.51 V and a Jsc of 5.41 mA/cm2 in a device made of glass, FTO, TiO2, Cs2SnI6, P3HT, and silver.

However, progress in device PCE may not have been possible because of the material’s structure, electrical qualities, or the fact that there aren’t many ways to make films out of it. You can make device PCE better in the future by making this material’s carrier-diffusion lengths longer. For example, you could make phase-pure high-quality Cs2SnI6 films with big grains and fewer flaws.

All-Inorganic Silver-Bismuth Halides

Bismuth (Bi) is a good lead-free material for solar cells, but it hasn’t worked very well so far because it has a wide bandgap, low dimensionality, and forms crystals quickly. Even though all-inorganic A3Bi2I9 perovskites like Cs3Bi2I9 and Ag3Bi2I9 have been synthesised and used successfully, the device’s photocatalytic efficiency (PCE) has stayed low. In the past few years, people have been interested in some Bi-based halide perovskite versions, like CsBi3I10, AgBiI4, AgBi2I7, Ag2BiI5, and Ag3BiI6. Some quaternary A2MI MIIIX6 double perovskites that don’t contain Pb2+ and swap two of them with one metal ion MI and one metal ion MIII also have promise. The Cs2AgBiBr6 perovskite has become famous because it gives off long-lasting radiation and is very stable in heat and dampness. New progress has also been talked about in Cs2M1(I)M2(III)X6 double perovskite and AgxBiyIx+3y-based solar cells.

Cs2M1(I)M2(III)X6 Double Perovskite

Because they are good at both optics and electronics, inorganic lead-free halide double perovskites, which are also called “elpasolites,” have been used in photovoltaics. To use in photovoltaics, some molecules from the Cs-based halide double perovskites family, like Cs2M1(I)M2(III)X6, have shown promise. If you change the elements at the M1(I) site and the M2(III) site in these double perovskites, you can change their bandgap and optical features. One example is Cs2AgInBr6, which has a bandgap of 1.5 eV and is great for photovoltaics because it absorbs light more efficiently than silicon.

In 2017, Greul et al. were the first to use a spin-coating method to try to make Cs2AgBiBr6 films. They discovered that preheating was important for making thin films with good coverage and that high-temperature annealing (250 ∘C) was needed to make the phase pure Cs2AgBiBr6. Greul et al. were able to make Cs2AgBiBr6-based solar cells with a PCE of 2.43% and a Voc greater than 1 V by tweaking the conditions of synthesis.

Thin-film quality improvement, bandgap engineering, and interface changes are two main ways to make Cs2AgBiBr6-based solar cells work better. To make good thin films, people have tried both solution-based paths and vapour deposition processes. The function of cells made by solution deposition and vapour deposition is different because of the quality of the film and the clarity of the phase.

Igbari et al. discovered that Cs2AgBiBr6 films made using a solution-based method had a more accurate makeup and better cell performance than films made using vapour deposition. The solution-processed film had more accurate soichiometry and better crystallinity, lower bandgap, longer charge carrier lifetime, and higher mobility than the vacuum-processed films. At their best, the solution-processed film had a PCE of 2.51% and the vacuum-processed film had a PCE of 1.41%.

To lower the indirect bandgap of about 2 eV, methods have been used to either lower Eg or switch from the indirect to the direct bandgap to improve performance. Two main ways to change the structure of Cs2AgBr6 are chemically and physically. Chemical changes, like adding or removing elements from crystal sites, change the band structure. Physical changes, like heating or pressing the material, cause an order-disorder transition in the Cs2AgBr6 that lowers Eg.

In conclusion, artificial lead-free halide double perovskites like Cs2M1(I)M2(III)X6 have shown promise in solar uses because they have great electrical qualities and can be made in a number of different ways.

It is possible to use Cs2AgBiBr6 double perovskites in solar cells because they are very stable structurally, have a long charge carrier lifetime, are very flexible, and don’t expand or contract much when heated. Chemical doping is a popular way to change the bandgaps of perovskites. In this method, the trivalent Bi3+ can be partially replaced by In3+, Sb3+, or Tl3+. When Bi3+ is partly replaced by In3+ to make Cs2AgBi1−xInxBr6, the Eg goes up as the amount of In3+ goes up. On the other hand, when Bi3+ is replaced by Sb3+, the Eg goes down. It has the smallest indirect Eg of 1.86 eV when the most Sb3+ is doped.

It is also possible to change the Eg of Cs2AgBiX6 double perovskites by changing the halogens at the X site. In general, Eg goes up in the following order: I, Br, Cl. According to theory, adding I− to Cs2AgBiBr6 makes the Eg value go down. However, Cs2AgBiI6 is thermodynamically unstable as single crystals or thin films, so it couldn’t be used in solar cells. Even so, Cs2AgBiI6 is stable when it is made into nanostructures (nanocrystals) through anion-exchange processes.

Researchers have looked into doping at the Cs and Ag sites. Zhang et al. found that adding Rb to Cs2AgBiBr6 made the device work much better. A rise in the number of flaw states and better absorption at long wavelengths in (Cs0⋅9Rb0.1)2AgBiBr6 caused the PCE to rise by about 15%. The best device had Jsc, PCE, and FF of 1.93 mA/cm2, 1.52%, and 0.788, respectively.

Carrier recombination loss at the edges and on the surface of the absorber can make the cells work less well overall. To fix this, surface flaw passivation and making sure the energy levels are lined up correctly can often help. In Cs2AgBiBr6, interface engineering and surface flaw passivation make their PCEs better. In the case of Yang et al., they added a N719 dye layer between the Cs2AgBiBr6 film and the hole transport material. This made the average PCE 2.77 percent higher than the reference cell’s PCE 2.29 percent.

Wang et al. used a photoactive Zn–chlorophyll derivative as an HTL in Cs2AgBiBr6 cells, along with other HTLs. The zinc chlorophyll derivative (Zn–Chl) not only moves holes but also makes the perovskite more sensitive. The Zn–Chl-sensitized Cs2AgBiBr6 device had a PCE of up to 2.79%, which was 22-27% higher than the PCE of devices using common nonphotoactive HTLs like PTAA, Spiro-OMeTAD, and poly(3-hexylthiophene).

AgaBibXa+3b Rudorffites

The AgaBibXa+3b rudorffites are a group of solar halides that have [AX6] and [BX6] octahedra that share edges. A = Ag, Cu; B = Bi, Sb; and X = I, Br. Walter Redorff found these ABX halides in 1954, and they have been studied as possible ionic carriers. A lot of people are interested in Ag3BiI6, Ag2BiI5, AgBiI4, and AgBi2I7 as possible materials for lead-free all-inorganic photovoltaics these days.

Kim et al. described in 2016 the first solution-based production of air-stable AgBi2I7 thin films, which were then used to collect energy from the sun. In the best case, the AgBi2I7 film that was made had a PCE of 1.22%. The researchers, Turkevych et al., looked into the structure and optoelectronic properties of several very stable and promising Ag–Bi–I photovoltaic rudorffites. They discovered that AgBiI4 and AgBi2I7 exist as single phases, while Ag3BiI6 and Ag2BiI5 contain AgI as an impurity, with Ag2BiI5 being closer to a single phase.

Because AgI doesn’t dissolve well in liquids like DMF and DMSO, it can be hard to work with Ag–Bi–X halides. Zhu et al. reported in 2017 that they used a precursor solution made from 17 weight percent n-butylamine to make high-quality Ag2BiI5 films. These films worked well as an active layer in a cell with TiO2 as the ETL and P3HT as the HTL, and they got a PCE of 2.1% in the best case. However, it’s likely that the makeup of the precursor has a big effect on how well the liquid works.

Jung et al. discovered that adding BiI3 impurities to the Ag2BiI5 phase made the device work much better, up to 2.31% in the best case. The powders that were made were mixed with DMF, DMSO, and HI to make thin films that can be used in solar cells. One solar cell that used pure Ag2BiI5 had a PCE of 1.74%, but the one that used Ag2BiI5 (which is made from Ag:Bi = 1.22:1) with BiI3 impurity worked with a PCE of 2.31%. Instead, Turkevych et al. discovered that it works better when there is a little extra AgI in Ag3BiI6.

It is very important to fine-tune the makeup of Ag–Bi–I rudorffites in order to make them work better in all-inorganic perovskite photovoltaics. There are some techniques that have been shown to work well with Ag–Bi–X rudorffites. These include the dynamic hot casting method, which involves spinning the hot precursor on hot surfaces, and regular spin-coating of a diluted precursor solution.

A study that is still going on has found that regular spin-coating of a low-concentration precursor solution can also work with a PCE > 2.5%, especially when the Voc is close to 0.9 V. Because of the ETL, HTL, and special heating method, which are not the same as what was done before.

A lot of work has been done with doping and chemicals in Pb-based perovskites, but there aren’t many studies on doping of Ag–Bi–I halides yet. It was found by Iyada et al. that adding HI and mixing the III–V elements has big impacts on the movement of charge carriers and the performance of the device. They got a better PCE of 1.82% in the Sb-doped Ag–(Bi/Sb)–I mixture compared to the initial 0.78% of Ag–Bi–I without the addition.

Park et al. looked into adding Cu to the Ag site of Ag2BiI5. They found that adding up to 10 mol% Cu to Ag2BiI5 didn’t change the crystal structure or bandgap of the silver bismuth halide much, but it did make it absorb a lot more light in the 400–700 nm range. It was possible to get a PCE of 2.53% by adding 2.5% Cu-doped Ag2BiI5 to a n–i–p type solar cell.

You can also change some of the anions in Ag–Bi–I halides to change their optoelectronic features. Pai et al. found that replacing even a small amount of I− with S2− causes the bandgaps to get smaller and the valence band edges to move up in a group of Ag–Bi–I materials. This affects the performance of the solar devices that use these materials. The Ag3BiI6−2xSx family has the best and most reliable results. When x was changed from 0% to 4%, the PCE of the solar cells went up from 4.33±0.05 to 5.44±0.07%. Also, the most effective Ag3BiI5.92S0.04-based devices were very stable over time; they kept more than 90% of their original performance after 45 days of keeping at room temperature.

Summary and Outlook

Photovoltaic solar cells (PSCs) have come a long way, and it’s now clear that the organic part of mixed perovskites is what makes them less stable over time. All-inorganic perovskites, like cerium lead/tin halides, are more stable at high temperatures and in harsh environments. They also don’t change chemically like organic–inorganic mixed perovskites do. In just about five years of research, CsPbI3 PSCs have made great strides in PCE, going over 19%.

Stabilising the photoactive black phases, which usually form at high temperatures, at room temperature is not easy, and keeping the phase in damp places is even harder. Because of this, a lot of work has gone into making the black phase at a lower temperature. Adding things to the precursor, adding metals to the B-site, making QDs or nanocrystals, treating the film’s surface after it’s been made, and other methods are the main ways that the black phase of CsPbI3 has been able to lower its crystallisation temperature.

To make progress in stability, it’s important to understand how the black phase stabilises itself in a wet environment. It is important to look into how phase clarity, chemical uniformity, microdefects, and crystal direction affect the change from the black phase to the yellow phase. Also, studying the solution’s properties, figuring out the size and make-up of the clusters, and knowing how ions interact with each other and with additives can help engineers create molecules and additives that make the photoactive phase more stable over time.

Surface cleaning and interface engineering can improve performance, which means that the design of the gadget needs to be improved even more. Using better ETL and HTL that match their bands better with the all-inorganic perovskite can make PV work better. Sn-based all-inorganic perovskites, such as CsSnI3 and Cs2SnI6, look like two great materials for absorbing solar energy. However, their performance is very low because of their inherent bad qualities, such as the fact that Sn2+ oxidises easily in CsSnI3 and has bigger background carriers in Cs2SnI6.

All-inorganic lead-free double perovskites, such as Cs2AgBiBr6 and silver-bismuth rudorffites, show promise as steady and non-toxic PSCs. However, the PCE of these cells is still low (<3%, except in a few rare cases). The clarity of the phase of the Ag–Bi–I halides must be very important for improving the performance of rudorffites-based PSCs.

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