The need for more power around the world and the damage it does to the environment have made people look for new ways to use cheap and clean energy technologies like solar cells. Halide perovskite materials have sparked new interest in solar (PV) study. In just 10 years, their power conversion efficiency (PCE) has grown from 3.8% to 25.5%. These materials are good for PV uses because they can absorb light well, have long diffusion lengths, long carrier lives, and can handle a lot of defects.
Various methods can be used to work with halide perovskite materials, ranging from low-cost solution processing to roll-to-roll methods like blade coating and slot-die coating for making big devices. There are three different types of PSC structures: mesoporous structures, planar standard (n–i–p) structures, and planar reversed (p–i–n) structures. Creating PSCs starts with making a perovskite precursor solution and a substrate. Next, a SnO2 compact layer is deposited on FTO for regular (n–i–p) planar architectures. For regular (n–i–p) architectures, a TiO2 compact plus a mesoporous layer is deposited on FTO. Finally, polybis(4-phenyl)(2,4,6-trimethylphenyl)amine is deposited on ITO for inverted (p–i–n) architectures.
PSC efficiencies are getting close to the Shockley–Queisser thermodynamic limit (31%), but we still don’t fully understand how they work, their structure and photophysical features, or how they break down. It is also very important to know how different metal oxides and perovskites break down and how those breakdowns happen at the metal oxide–perovskite interfaces.
It’s great that PCE has come a long way, but PSCs still have problems because they are unstable when exposed to wetness, air, light, and heat. When choosing materials to get the highest efficiency, people often forget about the important need to get both high steadiness and high efficiency at the same time.
Polyelemental, Multicomponent Engineering
The structure of perovskite materials used in photovoltaics is ABX3. A is a single-valent cation, B is a two-valent metal, and X is a halide anion. Based on its chemical formula ABX3 and the ionic diameters of each ion, the Goldschmidt tolerance factor guesses how stable this structure will be. The size of A is very important for making a stable close-packed structure. The PbI6 octahedra stay in place because their bandgap has moved to the red.
The combinatorial method explained in Reference [7] says that the five cations, two metals, and three halides can be put together in 651 different ways. When A, B, and X are put together, they create different electro-optical qualities and amounts of structure stability.
Individual parts that can change the perovskite structure are hard to choose because they don’t match the right size (tolerance factor), are stable against oxidation, or don’t work with the right fluid systems for the precursor. So, the multicomponent, polyelemental method can be expanded to include new parts or a different set of factors for the experiments. Bandgaps that aren’t good for perovskite PVs, for instance, are great for other uses like light-emitting diodes (LEDs), lasers, and detectors.
Single-Cation Perovskites
If the tolerance factor for perovskite materials is between 0.9 and 1, they can make a cubic structure. On the other hand, for an ideal 3D perovskite structure, the t-value needs to be between 0.8 and 1, which results in a structure that is almost perfect. But structures that are hexagonal or not cubic are made when t-values are higher or lower. Some examples of perovskites in this range are CsPbI3, MAPbI3, and FAPbI3. At room temperature, they crystallise into orthorhombic, tetragonal, and hexagonal shapes.
The atomic structures of the 𝓿 phases of FAPbI3 and CsPbI3 are very different. The -phase is made up of one-dimensional pillars made of face-sharing PbI6 octahedra, with FA-only domains separating them. The -CsPbI3 crystal is also made up of 1D PbI3 pillars surrounded by the cation Cs+. However, it is made up of stacked and moved PbI6 octahedra that share edges.
You can change the non-perovskite phases of FAPbI3 and CsPbI3 that form at room temperature into perovskite phases by heating them up to more than 150 and 335 °C, respectively. Single-cation perovskites are not very stable at high temperatures, which is why scientists are looking into more complicated perovskite mixtures with many cations and anions. In this chapter, we talk about and rate different cation exchange, chemical engineering, and passivation methods that lead to high-performance and stable PSCs.
To get PCEs higher than 20%, it has been talked about how important it is to use more complicated perovskite compositions, like triple-cation perovskites (CsMAFA) and double-cation perovskites (MAFA or CsFA), as well as Rb-modified perovskite and stable perovskites that don’t contain methylammonium.
Double-Cation Perovskites: Stabilizing the Black Phase
Since the beginning of solar power (PV) devices, single-cation perovskites have been used a lot. However, they have had problems with being stable and efficient. When organic single-cation perovskites like MAPbX3 and FAPbX3 are exposed to water, they become unstable. This can cause MAPbI3 to break down into PbI2, CH3NH3, and HI. When it comes to single-junction solar cells, FA is usually chosen over MA because it is more stable at high temperatures and has a redshifted bandgap. On the other hand, FA’s big size changes the perovskite lattice, showing diversity. At room temperature, pure FAPbI3 is not structurally stable. It can crystallise into either a photoinactive, non-perovskite hexagonal ε-phase (yellow phase) or a photoactive perovskite α-phase (black phase), which changes depending on the solvents or humidity present.
People have used two-cation mixes, like CsFA or MAFA perovskites, to make films look better and improve their optical qualities. These mixtures have achieved record PCEs of 25.5%. The main focus of these alloyed polyelemental compositions has been on double-cation mixes, like CsFA or MAFA perovskites, with a set halogen ratio of Br and I. This creates a stable black FA phase at room temperature.
Triple-Cation Perovskites: Stable and Reproducible Devices
Researchers have looked into double-cation perovskites to see how they can change their bandgap to absorb more light and stay stable at high temperatures. MAPbI3 can also work better and be more stable when FA is added to it. It is hard to control the mixed phase of double-cation perovskites, though. A new method was thought of to make the change from the yellow to the black phase easier. It involved adding Cs+ to the mixture to create the common triple-cation Cs0.05MA0.17FA0.83Pb(I0.83Br0.17)3 perovskite formula. This makes the makeup of perovskite films more steady and easy to repeat, and it allows PCE to go above 21%. Later, it can go above 22% by lowering the concentration of bromide.
Improving the security of the structure helps the gadget work better. By raising the tolerance factor, you can reduce local lattice warping and strain. This makes the long-range organising of perovskite crystals better. To make the crystals better, the CsMAFA triple-cation perovskites were made. Adding small amounts of artificial Cs in a “triple-cation” structure creates highly uniform grains of purer perovskite. This makes the films more resistant to small changes that happen during the manufacturing process.
Quadruple-Cation Perovskite: Improvement of Long-Term Device Stability
The parts that make up perovskite are getting more complicated, with triple-cation perovskite having the most complicated parts. However, no other cations have been found to make a single-cation black-phase PSCs. This means that the triple-cation perovskite is the most complicated structure. Even though it doesn’t make a black-phase single-cation perovskite, adding Rb can be done using a multi-cation method. This doubles the number of perovskite compounds that are possible, opening up RbFA, RbMAFA, RbCsFA, and RbCsMAFA perovskites as a new way to make high-quality materials.
With a stable PCE of 21.6%, the quadruple-cation RbCsMAFA perovskite showed the best efficiency. It had an open-circuit voltage of 1250 mV, a bandgap of 1630 mV, and a potential loss of 390 mV. This 390 mV is one of the lowest results, which shows that the RbCsMAFA perovskite is a material that doesn’t recombine very often. Even when the temperature is normal, a solar cell made of this material could work as an LED.
According to Tavakoli et al., the potential loss was lower, at 365 mV. This meant that the PCE was 21.9% and the average Voc was 1.185 V. Polymer-coated multi-cation PSCs can work at high temperatures to stay stable for 500 hours with full load and lighting, which is longer than what is required by industry. New research shows that polymeric hole-transporting materials (HTMs) can stop metal electrodes from moving in ways that are bad at high temperatures.
Methylammonium-Free Perovskite: Staying in the Black Phase with Fewer Components
The work is mostly about making high-efficiency solar cells out of perovskite materials that are made up of many different elements and parts. MAPbIxBr(1−x) compounds can have a bandgap range of 1.58 eV (MAPbI3) to 2.28 eV (MAPbBr3), which is 700 meV. This is done by switching Br with I. But the cation doesn’t have much of an effect on the metal-halide cage. This means that the bandgap change from MAPbI3 (1.58 eV) to FAPbI3 (1.48 eV) is only 100 meV. This moves the bandgap (700 meV) too far to the blue.
Several studies have shown that MAPbI3 is unstable and that the film breaks down because of MA losing gas. The fact that the MA molecule is easily broken down into CH3I and NH3 has been linked to the fact that it can happen at temperatures as low as 80 °C. The X-ray diffraction pattern shows that all MA-based perovskites break down strongly, as shown by the growth of the PbI2 peak at 12° after three hours of cooling at 130°C. This does not happen with the non-MA perovskites. MA’s long-term risk is too high for businesses to handle. For stable reasons, a better perovskite compound should stay away from MA. So, Rb, Cs, and FA are the best and most thermally stable cations.
Most of the time, hybrid PSCs use the more stable FA cation instead of MA, but its bigger size can cause a yellow phase that isn’t working. Most of the time, mixed cations and anions with FA, MA, Cs, I, and Br ions are used to keep the black α-phase of FAPbI3 stable in PSCs. But adding things like MA, Cs, and Br makes the bandgap bigger and the temperature stability worse. When MA and Br are mixed in FAPbI3, problems like low temperature stability from the MA, phase separation from the mixed halides, and less photon absorption happen. This leads to low current density because the bandgap widens, which is not what you want.
A better perovskite combination stays away from Br because of its bandgap and MA because it is unstable. Instead, it chooses iodine as the bromide and Rb, Cs, and FA as the cations because they are more stable at high temperatures. A better RbCsFAPbI3 perovskite (without MA and Br) was studied. It has a bandgap of 1.53 eV, which is close to the single-junction ideal.
The study also looks at how well double-polymer-modified PSCs (pm-PSCs) work with a mix of 5 parts phenyl-C61-butyric acid methyl ester (PCBM) to 1 part polymethylmethacrylate (PMMA) at the contact of ETL and perovskite and 1 part PMMA between the perovskite and HTM. We made a record polymer-modified device with a perovskite composition that worked well at low temperatures (100 °C). It had a high short-circuit photocurrent of 25.06 mA/cm2 and a high PCE of 20.44% (stabilised at 20.35%) for planar PSCs.
Conclusions
Organic-inorganic perovskite is a potential material for solar cells because it can be mixed with crystalline silicon solar cells to make tandem cells that work very well. These multicomponent perovskites make materials that are steady, efficient, and easy to make again and again. These materials are needed for mass production of solar (PSC), lasers, LEDs, photodetectors, and particle detectors. Compositional engineering is a key part of progressing PSCs, and the tolerance factor gives us information about how stable the structure is. By slowly replacing MA cations with FA cations, the security of MA-based mixtures was raised. The stability of inorganic Cs and Rb was better at high temperatures. PSCs with a stabilised efficiency of more than 20% can be made using iodide as the anion instead of bromide and an optical mix of Cs, Rb, and FA cations (without MA). It is possible to make phase-stabilized FA perovskites and intermediate bandgap structures that are thermally relatively stable by adding only artificial materials. These structures are very appealing for tandem uses.
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