PSCs are made up of materials with the ABX3 structure: A = caesium (Cs), methylammonium (MA), or formamidinium (FA); B = tin (Sn) or lead (Pb); and X = chlorine (Cl), bromine (Br), or iodine (I). They’ve gotten a lot better in the last few years. For example, PCEs went from 3.8% in 2009 to 24.2% in 2019. Recently, changing the makeup of materials has been used to engineer their qualities, especially the bandgap of perovskite materials for use in solar cells.
Different methods, such as cheap solution processing, can be used to work on these perovskites. If you switch the cations, metals, or halides listed, you can change the band gap from about 1.2 to 3.0 eV. By mixing the different halides, you can get band gaps in the middle. The peak for MASn0.8Pb0.2I3 is at 1080 nm (1.15 eV), which is way more red than the others.
When you mix Sn and Pb metals, you can get band gaps between 1.59 eV (MAPbI3) and 1.29 eV (MASnI3). It’s not true that the mixed metals follow a straight Vegard trend. The band gap for an intermediate MASn0.8Pb0.2I3 is the most red-shifted at 1.15 eV.
Germanium is the second-most similar element to lead in the chemical table. Its oxidation state is 21, and it is less poisonous, less electronegative, and smaller than lead. To tune the bandgap of germanium-based perovskites, different monovalent cations, halides, and mixes of halides and cations are used, which leads to bandgaps between 1.3 and 3.7 eV. This is shown in references [13] and [16] for germanium perovskite compounds that are not cube-shaped.
Another choice is Bi31, which has been the trivalent metal cation that has been studied the most for lead exchange in halide perovskite solar cells so far. Bi is next to antimony in the periodic table. It has an oxidation state of 3 1 and a smaller cationic radius. As with bismuth, the antimony cation tends to make perovskites with layers and the formula A2Sb3X9. Trivalent double perovskites, such as Cs2Bi3I9, MA2Bi3I9, and Cs3Sb2I9, have big band gaps of 2.0 eV, 2.1 eV, and 1.96 eV, in that order.
Perovskites are useful for many things besides photovoltaics because they have a wide band gap range. These include lasing, light-emitting devices, sensors, photodetectors, X-ray detection, and particle detection. This also opens up a lot of different solar application choices. Lead-based perovskites that are easy to get and understand, like MAPbI3, can already reach band gaps of 1.55 to 1.62 eV, which are almost perfect for single-junction photovoltaic (PV) uses.
However, this brings about new problems: using the same amount of mixed halides may cause phase segregation with clear Br and I domains, which is not good for making materials that will last for a long time. Then, perovskites with more than one charge can be used to fight this effect.
Complex ion dynamics change the way the film is characterised and have a unique impact on devices like hysteresis. A very important part of perovskite study is careful chemical engineering. As you add more parts to perovskites that are already very complicated, the quality of the film changes, as does the role of the grain boundaries. To make sure that the perovskite and the charge extraction layer can make good contact with each other, high efficiency planar PSCs need charge extraction layers that are fine-tuned even more.
Organic/inorganic ion mixing
Due to their single cation/anion compounds, perovskite solar cell materials have a lot of problems. One of these is MAPbI3, which has a PCE of 21.2% in a steady state. Octyl ammonium (OA), on the other hand, can make single MAPbI3 grains in full armour without layering them, which leads to a stable PCE of 20.1%. MA changes quickly when it comes in contact with water, heat, or light, which makes it a long-term risk factor for stability.
FA-based perovskites are more stable and have a band gap that is farther to the red, but they also have problems, like not being phase-stable at room temperature and having a “yellow phase” that doesn’t react with light. McMeekin et al. ended the “yellow phase gap” by using double-cation CsFA mixes, which stopped the separation of halides.
It is not stable for FAPbI3 and CsPbI3 to be in the cubic (or pseudo-cubic) α-phase at room temperature. The atomistic structures of the ε phases of FAPbI3 and CsPbI3 are very different. At room temperature, FAPbBr3 has a cubic or pseudo-cubic structure and a band gap of 2.3 eV. FA is better than MA because it is more stable at high temperatures and has a band gap that is redshifted in a good way.
The exact mix of Pb-based mixed MA/FA and mixed Br/I perovskites has a big effect on how well the end device works. For example, FA2/3MA1/3Pb(Br1/3I2/3) has the best PCE at 21%.3. Small changes in the stoichiometry of the precursors can have a big impact on the stability, performance, and features of perovskite solar devices.
To sum up, mixing cations and halides is a key design concept for perovskite products because it takes advantage of the good things about each part while avoiding the bad things.
Perovskite “black-phase” stability: role of cations
The double-cation MAFA mixtures show that a small amount of MA makes FA perovskite crystallise more easily into its photoactive black phase. This makes the mixture more stable at high temperatures and in terms of its structure than either pure MA or FA compounds. This is also true for CsFA combinations, which showed even better control of halide segregation, making it possible for tandem uses to use middle band gaps. Using this method, CsMAFA triple cation perovskites were made to make the crystals better.
Halide segregation was slowed down in the double-cation perovskites. But right now, the best results come from complicated perovskite mixes that have a lot of different cations (from Rb, Cs, MA, FA), metals (Pb, Sn), or halides (Br, I). For instance, a PCE of 21.6% (stabilised) was achieved by mixing Rb, Cs, MA, and FA cations with mixed Br and I halides. This is related to a bigger idea: using multi-cation engineering to stabilise the phase and stop the splitting of halides in perovskites.
A better perovskite combination should not have Br because of the “blue penalty” or MA because it makes the compound less stable. Iodine is now the best halide because it can handle different cations and make triple-cation perovskites over and over again. In real life, perovskites with a tolerance factor of 0.8 to 1.0 have a photoactive black phase (solid circles) and a non-photoactive phase (open circles). Rb is very close to this limit, which means it could be added to the perovskite grid using a multi-cation method.
Because Rb, Cs, and FA are more stable at high temperatures, they make RbCsFAPbI3 perovskites very useful. A better RbCsFAPbI3 perovskite (without MA and Br) was found to have a bandgap of 1.53 eV, which is close to the single-junction optimum. It also had high short-circuit currents of 25.06 mA cm22 and a high PCE of 20.44% (stabilised at 20.35%) for planar PSCs. The perovskites don’t need to be heated above 100°C, which means they can be used in perovskite/silicon tandem solar cells or bendable solar cells, which are two of the most promising ways to get these technologies to market. Using only inorganic chemicals is a molecular design approach that can be used to stabilise the phases of FA perovskites and intermediate bandgaps that are relatively stable at high temperatures. This is very appealing for tandem uses.
The Goldschmidt tolerance factor can be used to figure out if a cation is good for a high-performance “black phase” 3D-perovskite. The perfect cubic perovskite structure can be made with a tolerance factor of 0.9. Structures that are orthorhombic, rhombohedral, or tetragonal are made with a tolerance factor between 0.7 and 0.9. t gets bigger than one when A is big, which makes stacked perovskite structures.
In real life, “black phase” 3D perovskites usually form when 0.8, t, and 1.0 are met. Perovskites on the edge of the tolerance factor requirement, like CsPbI3 and FAPbI3 (t B 1), have a twisted structure that makes an extra yellow phase present at room temperature. So that FA-perovskites had a smaller effective cationic radius, the smaller MA cation was added. This made MAFA perovskites. This made the black phase perovskites more stable at room temperature, which led to an amazing success story with all the world records that have been set so far using MAFA mixes.
But, even though MAFA works very well, its XRD data still shows harmful yellow phase impurities. It was thought that adding small amounts of Cs to MAFA mixtures would lower the amount of yellow phase impurities because the t values would be better matched. It is important to note that the CsMAFA black phase forms at room temperature, so there is no need for extra heating steps. For MAFA, there are different solid precursor states. For CsMAFA, on the other hand, there is a clear perovskite peak at room temperature, which shows that the perovskite crystallisation begins with the photoactive black phase.
The random starting conditions of MAFA match how sensitive it is to weather, the surroundings, and liquid vapours. These “hidden variables” are probably one of the main reasons why many groups had trouble getting the same results even though they seemed to follow the same steps and rules. Many follow-up studies using this and similar mixtures show that CsMAFA perovskites greatly improved the performance standard and made it easier to repeat.
For the field to move forward, it is very important to provide scientifically useful data. The stable PCEs of 21.1% and better repeatability were achieved with the triple-cation perovskites. But no other cations have been found to make a single-cation black phase PSC. In this way, the triple cation perovskite was the most complicated mix that could be made.
With a new multi-cation method, adding Cs got rid of the yellow phase impurities in the MAFA spectrum. Twenty different things have a PCE of 20% or more.
Putting rubidium cations into perovskite solar cells has shown promise in making them work better as photovoltaics. The RbCsMAFA perovskite quadruple-cation perovskite showed great conversion efficiency at 21.6% with an open-circuit voltage of 1.24 V and a band gap of 1.62 eV, which is one of the lowest band gaps ever reported for a solar cell material. When Rb is added, the yellow phase is weakened even more. Research using X-ray photoelectron spectroscopy (XPS) shows that the presence of Cs helps Rb fit into the perovskite structure. New studies on perovskites that contain Rb back up the results and show that multi-cation perovskites can stop halide segregation.
Right now, Rb, Cs, MA, and FA are used to show how the multi-cation method works. It is possible to get an extra cation by finding an extra cation. At this point, no more cations have been shown to work with high-efficiency PSCs that are more than 20% efficient. You can choose cations that are even smaller than Rb or ones that are bigger than FA. It could be an alkali metal smaller than Rb, like Na or K, or a cation bigger than FA, like IA, EA, or GA.
You can choose cations that are even smaller than Rb or ones that are bigger than FA. These ABX3 perovskites can combine K with a number of different ions, such as ammonium (Cs1), hydroxylammonium (HA), hydrazinium (DA), methylammonium CH3NH3 1 (MA), formamide NH3COH1 (FM), formamidinium CH(NH2)2 1 (FA), ethylammonium CH3CH2NH3 1 (EA), dimethylamine NH2(CH3)2 1 (DEA), and guanidine amine C(NH2)3 1 (GA) for the A site; Ge, Sn, and Pb group-IV metalloids (MIV) for the B site; and Cl, Br, and I halogens (XVII) for the X site. There are nine groups of molecules with a set A cation, and each column of the matrix shows one of those groups.
For a multication method that keeps the 3D structure, molecules that are a bit bigger than FA, like IA, EA, and GA, would be good choices. For PSCs so far, only EA and GA have been used. But only very small amounts were okay because they didn’t change the qualities of the optical film. IA is an interesting case because it seems to have a cationic radius similar to FA. However, because it is flat and rigid, it might not be possible to incorporate it into a 3D perovskite lattice. It shows that the charged radii for molecules need to be changed more before they can be compared to each other like elements are.
Ion library
A big collection of A, B, and X ions can be used to make PV perovskites. Mixed formulas with more than one ion in each place have shown to be better at photovoltaics. Most stories right now are about Pb and Sn for the metal position and Br and I for the chemical position. FA is the biggest organic cation that has a black phase and a band gap that is farther to the right than MA or Cs. Because FA is more stable at high temperatures than MA, it is the main cation in high-performance effects. The perovskites that have been studied are just a small part of a big collection of possible combinations. Pure and mixed mixtures should lead to even better material qualities.
Perovskite compositions in devices
The largest PCE that has been approved for solar cell materials is 24.2%, but this is not stable. One of the most stable PCEs is 21.6%, which was made with a perovskite that has many cations (Rb, Cs, MA, and/or FA) and halides (Br and I). A simpler method would be better so that it is easier to prepare and characterise. Because Cs is one of the biggest, most stable, and nonradioactive cations, it makes CsPbI3 one of the few artificial perovskites with a good band gap of B1.7 eV. But the volatility of single-cation perovskites isn’t just limited to Cs. All single-cation perovskites that are used today are unstable in terms of phase, temperature, or humidity, and they can’t be made over and over again. Mixing different cations was a key idea for making high-quality pictures that can be made over and over again and are less affected by secret processing factors. Recently, RbFA and RbMAFA were looked into again, which shows that multi-cation engineering has a lot of promise. The idea of a more complicated, multicomponent perovskite has been successfully used to work with different band gaps, like CsFA and MAFA mixed with Sn/Pb and/or Br/I. You can take the mixing method even further by using cations that are smaller than Cs or bigger than FA.
Band gap engineering strategy
One of the best things about metal halide perovskites is that they can be tuned to different band gaps, which makes them very efficient and easy to make. This makes it possible to make colours that are see-through, materials that are less harmful, and even perovskite as a charge-selective layer. Perovskites with a wide band gap can be stacked with other solar cells. Band gap setting also lets you make new materials with interesting qualities. For example, you can use Cs to raise the band gap and get the same band gap with less Br.
Lead replacement
Metal halide perovskites are interesting materials because the ionic makeup of the ABX3 structure can be changed to make them fit specific needs. Because of this, it is interesting to look for other divalent metal species that can change the perovskite’s material properties while keeping its great optical properties and not making it less stable or more poisonous. A lot of alkaline earth and transition metals can reach a steady divalent oxidation state and work well with solution processing. This means they can be used to make new mixed-metal perovskite mixtures.
To look into what possibilities are open with different mixed-metal perovskite formulas, Pb has been switched out for a different divalent metal to make methylammonium mixed-metal triiodide films, which are written as MA(Pb:B0)I3. Snaith et al. found that MAPbI3 can handle Co, Cu, Fe, Mg, Mn, Ni, Sn, Sr, and Zn the best, and that even small amounts of replacement can often make the device work better. This better performance comes from Co’s ability to change the perovskite’s Fermi level and valence band edge (VBE) in a way that isn’t tied to the band gap. This makes the material match its energies better with PEDOT: PSS.
Toxicities, especially with the Pb-compounds, are a big problem for business use. Changing Pb to Sn (or Ge) is one way to avoid this problem, but the chemical oxidation instability of Sn (and Ge) plates makes them less useful for optoelectronics. Also, molecules that are less dangerous and more like perovskites have been suggested. These include Cs2AgBiBr6 or A3Bi2I9 with A 5 Cs or MA.
A lot of stable compounds can be made that have bandgaps between 2.0 and 3.2 eV. These compounds can contain a lanthanide metal (B0: Eu, Dy, Tm, and Yb) instead of lead in MA(Pb:B0)I3 compounds. So, the substances based on lanthanides can be used as either charge selective layers or sun filters at the same time.
But these “perovskite-like” materials still don’t work as well as ABX3 perovskites when it comes to photovoltaics. There aren’t many studies on how poisonous PSCs are, and the lead salts that are in them dissolve easily in water, which means they could pollute groundwater and hurt people’s health. For this reason, PSCs that use full encapsulation and recycling plans need to follow the right steps based on a thorough risk assessment. These steps should include encapsulation strategies and strict rules. In the areas of chemistry and politics, more study is needed.
Anion exchange
Thiocyanic (SCN) is a pseudohalide that has the same ionic radius as I2 and acts chemically like a halide. It makes perovskite solar cells (PSCs) based on the CH3NH3PbI3-x(SCN)x active layer resistant to water. Researchers have looked into how well FTO/TiO2/CH3NH3PbI3-x(SCN)x/Spiro-OMeTAD/Au structures work as solar materials and how stable they are in wet air. The NaSCN-films and KSCN-films that were made this way have big grains and a uniform shape, which makes them better at photovoltaics and makes them more stable. The NaSCN-PSCs and KSCN-PSCs are more stable over time. After 45 days without packaging, the PCE of the KSCN-PSCs and NaSCN-PSCs still had 97% and 93% of their original values, respectively.
Preparing 5-ammonium valeric acid iodide (5-AVAI) helps make good mixed-cation perovskite crystals in stable mesoscopic perovskite solar cells (MPSCs). When MABF4 was added to the precursor solution, high-performance MPSCs up to 15.5% were made, with a high Voc of 0.97 V. The mixed cation/mixed halide perovskites stop carriers from recombining and improve the capture of charge carriers at the surfaces between the charge collection layer and the perovskite. A new mixed cation/mixed halide perovskite has been made using a simple one-step solution processing method.
It has been reported that using molecular ion PF62 in an ion exchange reaction and an anion exchange reaction to make MAPbI32xBrx thin films that are compact, large-grain, and free of pinholes can improve PCE and device stability. Putting a thin layer of FA0.88Cs0.12PbI32x(PF6)x between the perovskite and spiro-OMeTAD HTM makes the carriers last longer and the flaw density go down. This results in higher FF and Voc, less current voltage hysteresis, and better stability.
Conclusions
Using more than one cation has led to stable and effective solar cell devices. This method could be further studied by using even elements or bigger molecular cations. Multi-cation perovskites show promise for strong and consistent solar cells because they are less likely to become unstable due to changes in phase, temperature, and humidity than single-cation perovskites. Polymer-coated PSCs can handle tougher stress tests than what is required by industry standards. However, it is still not possible to make a fully stable product on a business scale. Upscaling reliability and doing more stable tests, like temperature, humidity, and closing cycles, will be part of the multi-cation method.
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