The perovskite solar cells (PSCs) are a type of thin-film semiconductor solar cell that works like useful solar cells that use GaAs, CdTe, and amorphous Si in the photovoltaic (PV) industry. These cells are better than the most common crystalline Si solar cells because they can be made into devices that are thin, light, and bendable. Lightweight and mechanical freedom are important features that people want in next-generation PV devices so that they can be connected to curved surfaces of buildings inside and outside.
GaAs has a power conversion efficiency (PCE) of 29%, which is higher than the best efficiency of solid Si cells, which is 26%. But GaAs semiconductor films are very expensive because they are made in a vacuum and at high temperatures. This means they can only be used in small devices or the solar cells of space vehicles. Metal halide perovskite semiconductors were used to make these thin-film solar cells. They were created between 2006 and 2009 as a follow-up to studies on dye-sensitized solar cells (DSSCs). PSCs have become the only type of high-efficiency (>25%) solar cell that can compete with Si cells.
The best perovskites for photovoltaics are in a group called metal halide perovskites, which have their own special chemical and physical characteristics. Halide perovskites are different from oxide perovskites because they are highly ionic crystals that conduct ions. Lead halide perovskites and the materials they crystallise from dissolve in polar solvents. This means that the solution layer can be used as a printing method to make thin films of perovskites crystals that absorb light.
Scientists have made different kinds of lead halide perovskites by switching out the iodine and MA for other organic groups and halides (Br and Cl), respectively. This ability to change visual qualities based on halogens is like other metal halide-type ionic semiconductors. Ag halide is a common example that has been studied a lot in the field of photography materials.
In chemical crystallisation, the balance of cations and anions in the halide perovskite ionic crystal keeps free electrons and holes stable, which lets free carriers move over long distances.
Halide perovskite semiconductors have a special benefit in that they can be made in a variety of ways, which makes it possible to change the bandgap all the time. The crystal structure of MAPbI3 is isotropic in three dimensions (3D), which is good for photovoltaic (PV) materials that need to keep photogenerated carriers from spreading unevenly. One-dimensional (1D) and two-dimensional (2D) structures of perovskites are made by changing the size of the organic cation. The luminous qualities come from photoexcited exciton and quantum confinement effects, which were studied before they were used in PV uses.
Lead iodide (PbI2) affects the conductivity and photovoltaic (PV) qualities of perovskites. Organic cations change the crystalline phase, and the size of the lattice affects the perovskite’s visual properties. Photosensitive halides like PbI2 have been used instead of expensive silver halides in the field of photography. In the past 10 years, perovskite photovoltaics has come a long way. In 2009, the first paper showing a perovskite-based photovoltaic (PV) cell was published.
In 2012, the MAPbI3-based PV cell had an efficiency of more than 10% when the thickness of the perovskite absorber on the solidification of the cell structure was raised. After the efficiency boost, changes were made to the perovskite makeup and the quality of the charge transfer materials. This led to a fast rise in the power conversion efficiency (PCE) of PSCs, which now stands at 25.5%. Over the past 10 years, the PCE of PSCs has grown by more than six times. This is because many research groups in chemistry and physics have been studying this new area of photovoltaics.
Among the benefits of halide perovskites are their naturally high efficiency at switching between photonic and electronic energies and quanta, their wavelength (bandgap) adaptability based on changing the chemical makeup of the perovskite through chemical synthesis, the fact that they have a thin (<1 μm) film of perovskite absorber as the active layer, and the low cost of the perovskite materials and printing processes used to make them. There isn’t another solar gadget that meets all of these requirements and allows for good performance.
The fast progress in perovskite photovoltaics (PSCs) is due to study that comes from different fields. To crystallise perovskite materials using a solution, you need to know a lot about chemistry. On the other hand, physics ideas are used to describe the visual and electrical properties of solid-state crystals (semiconductors). Working together between chemistry and physics has found many uses for halide perovskites besides photovoltaics.
Halide perovskites have unique optical and physical qualities that have led to the search for new photonic uses. One of these is a high-performance light source. When spin-coated thin films of MAPbBr3 are exposed to room temperature air, they give off strong green light. Scientists are very interested in the high photoluminescence (PL) of perovskite films to figure out how defect-induced carrier recombination works.
In optoelectronics, halide perovskites are used for many other things, such as photodetectors, image-sensing devices, X-ray detectors, memory devices, and more. But halide perovskites are naturally stable because their ions move around, which is typical of metal halide materials as soft ionic semiconductors. It is thought that ionic migration is a major reason why the current-voltage features of the device behave in a hysteretic way.
To fix the problem of steadiness, scientists are trying to make perovskite devices better so they can be used in real life. This book talks about the basic chemical and physical features of halide perovskite materials, as well as how they work in solar conversion when paired with partner charge transport materials. It also talks about current efforts to make perovskite devices more stable and efficient, as well as problems that come up and ways to solve them.
History of Halide Perovskite Photovoltaics
Discovery of the Perovskite Crystal Form
The German mineralogist and crystallographer Gustav Rose found perovskite materials in 1839. They are a black mineral made up of calcium titanate (CaTiO3). Mineral hunter August Alexander Kämmerer found the mineral in a piece of chlorite-rich skarn in Russia’s Ural Mountains. In 1925, researchers in Norway looked into the crystal structure of perovskite (CaTiO3). This was done after Victor M. Goldschmidt got the first business patent for the CaTiO3 pigment in 1922.
Because of research into these oxide perovskites, they are now used in electronics for things like fuel cells, memory storage devices, capacitors, superconductors, piezoelectric devices, and proton conductors. It was found that BaTiO3 has a high dielectric constant, which has led to the development of ferroelectrics used in capacitors, superconductors, piezoelectric devices, proton conductors, fuel cells, and memory storage devices.
Other metal oxide perovskites, like BiFeO3, LiNbO3, PbTiO3, and SrTiO3, can also be used as ferroelectric ceramics. Some of them, like BiFeO3, can even be used as photovoltaics. However, these oxide perovskites have big bandgaps and aren’t very good at collecting free charges. Because of this, they aren’t good enough to be used as solar emitters because they aren’t semiconducting.
Discovery of Metal Halide Perovskites
It has the chemical formula ABX3, where A and B are alkali and metal cations, and X is an anion. Perovskite is a type of crystal structure. It used to be called artificial metal oxides. Halide perovskites are different from oxide perovskites because they have halide anions instead of oxide anions. This makes the ionic crystallinity greater. In halide perovskites, charge neutralisation is achieved by coordinating a lot of cations and anions. This causes some structure strain because the distance between the ions and the coordination number changes.
In the 1890s, these kinds of halide perovskites were first found. In 1893, Wells did a thorough study on how to make lead halide crystals from liquids that contained caesium (Cs)-based CsPbX3. Møller, a Danish researcher, discovered in 1957 that CsPbCl3 and CsPbBr3 have a perovskite structure, with a twisted structure that changes to a pure cubic phase at high temperatures. It was Weber who discovered that an organic cation (CH3NH3+) can replace Cs+ to make CH3NH3MX3. This was the start of the first diffraction study on organic–inorganic hybrid lead halide perovskites.
David Mitzi made a lot of different halide perovskites with both small and big organic cations near the end of the last century. In his research, Mitzi looked at the physical features of low-dimensional perovskites, like 2D perovskites that have a big organic group attached to them. In the late 1990s, Professor Kohei Sanui used a Japanese national study program (JST–CREST) to look into the visual qualities of both 2D and 3D crystals. Researchers have found ways to use low-dimensional perovskite crystals in nonlinear optics and electroluminescence by using bright, single-color optical absorption and luminescence. However, it wasn’t known that these materials could also be used to collect solar energy.
Beginning of Halide Perovskite Photovoltaics
The first and second generations of solar cells, which include silicon wafer-based and thin-film semiconductor-based solar cells, have improved in terms of efficiency and stability. However, the high process cost of wafer fabrication has prevented them from becoming the most popular alternative to fossil fuel-based energy sources such as thermal power generation. As a result, there has been a great deal of research and development of third-generation photovoltaics, including organic PV cells (OPVs), DSSCs, and quantum dot (QD) solar cells.
During 2005-2006, researchers explored the use of organic–inorganic lead halide perovskite as an absorber replacing the organic dye in DSSCs. This research began in 2005 after Miyasaka established a venture company Peccell Technologies on the campus of Toin University of Yokohama (TUY), specializing in applications of DSSCs. Akihiro Kojima, then a graduate student at Tokyo Polytechnic University (TPU), began visiting Miyasaka’s laboratory to study DSSC through a collaboration between TPU and TUY. The goal of the study was to examine the possibility of using lead halide perovskite as a sensitizer on mesoporous TiO2 electrodes.
After initiating experiments using methylammonium lead halide perovskites as a DSSC sensitizer, preliminary results were obtained that demonstrated the visible light sensitization of TiO2 with deposited nanocrystals of the perovskite. In October 2006, the team presented their initial results at the Electrochemical Society’s Annual Meeting in Mexico. Kojima joined the group as a doctoral course student at the Graduate School of University of Tokyo (UT), where Miyasaka served as a guest professor (2005–2010).
The perovskite photovoltaic cell first employed CH3NH3PbX3 (X = I, Br) as a sensitizer on a TiO2 mesoporous electrode used in conjunction with a lithium halide-containing electrolyte solution. After optimizing the cell structure, they fabricated cells that yielded a PCE barely reaching 3.8%, which was published in the first peer-reviewed paper on perovskite-based photovoltaic cells in 2009. They later realized that such poor performance was apparently due to the significantly low loading amount of perovskite.
In 2011, researchers started a collaborative work with Henry Snaith (H. S.) of Oxford University for creating solid-state perovskite cells. This opportunity was arranged by Dr. Takurou Murakami (presently in AIST, Japan), who had joined the EPFL (Swiss Research Background and Recent Progress of Perovskite Photovoltaics).
The research by H. S. Murakami and his team at the Federal Institute of Technology in Lausanne focused on developing a photoelectrochemical cell using nanocrystalline CH3NH3PbBr3 (MAPbBr3) deposited on the TiO2 surface. They used MAPbBr3/TiO2 and MAPbI3/TiO2 photoanodes with a liquid electrolyte, LiBr and Br2, dissolved in acetonitrile for the former and 0.15 M LiI and 0.075 M I2 dissolved in methoxyacetonitrile for the latter photoanode. The researchers also studied the fabrication of perovskite-based PV cells as an extension of their DSSC study.
In 2011, Michael Lee, a PhD student of the H. S. group in Oxford, spent several months learning the method of preparing the perovskite. They aimed to solidify the perovskite-sensitized cell with a spin-coated layer of an organic HTM, spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-dimethoxyphenylamine)-9,9′-spirobifluorene). However, they faced difficulties in stabilizing perovskite crystals against dissolution into the solvent used in the spiro-OMeTAD solution. They later found a way to suppress the dissolution of perovskite by mixing chloride with the composition of perovskite, MAPbI3−xClx, which eventually produced cells with a PCE of 10.9%.
The long carrier diffusion length of the lead halide perovskite, confirmed by opto-physical analysis and carrier mobility characterizations, has brought radical changes in our understanding of the functioning of photovoltaic solar cells (PSCs), distinguishing this technology from other organic and hybrid material-based solar cells. PSCs have grown larger and shone brighter in the shortest time measured in the history of PV technology.
Semiconductor Properties of Organo-Lead Halide
Perovskites
A lot of research has been done on the solid-state physics of halide perovskite semiconductors, mainly on their semiconducting qualities, how carriers move through them, and ideas of how defects affect charge recombination. These are ionic crystals that have a strong negative charge because of the bromide anion. The unique behaviours of perovskite-based PV devices, such as slow reaction, hysteresis in current-voltage performance, and material instability, are caused by ions moving around in these materials.
Because halide perovskites are semiconducting, changing the halide ions makes it easy to tune the bandgap and optical absorption. MA lead halide (CH3NH3Pb(BrxI1−x)3−yCly) and formamidinium (FA) lead halide (HC(NH2)2PbBr1−yIy, y = 0–1) are added to the perovskite structure. The FAPbBr1−yIy shows a redshift that goes from 550 nm for pure bromide (FAPbBr3) to -830 nm for pure iodide (FAPbI3). In the PL spectrum, there is also a steady change of the emission peak.
These optical features show that mixed-halide perovskites can make good solid solutions when they are formed as ionic crystals. Halide perovskites are very good at absorbing sunshine because they have strong bandgap absorption at the edge wavelength and broad flat absorption covering visible light wavelengths. MA lead iodide (MAPbI3) is a common perovskite absorber in PV cells. It is also a rare intrinsic semiconductor that lets photogenerated electrons and holes move around very easily.
The better photophysical features of MAPbI3 have been confirmed by density functional theory (DFT) and first-principles theory-based studies. This is because free carriers can move along long diffusion lengths. Its high bandgap energy and wide, flat absorption range for visible light wavelengths make up its electronic structure.
Finally, halide perovskite semiconductors show great promise for use in solar power generation, especially in 3D materials that allow for isotropic carrier transfer, which is good for power production. Their ability to carry electricity, move carriers, and ideas of charge exchange caused by defects make them useful in many areas, such as solar energy storage and photovoltaics.
For photovoltaic uses, perovskite solar cells (PSCs) look good because they have a high optical absorption coefficient, a long carrier diffusion length, and well-balanced charge transfer. Perovskite’s ability to work with flaws is very important in photovoltaic (PV) uses because it makes high voltage. In PSCs, the high photovoltage output is caused by the high photovoltage output, not the intensity of the photocurrent.
The open-circuit voltage (VOC) of all types of solar cells loses a lot of heat from their bandgap energy. This loss is usually more than a third of the bandgap energy (Eg). For instance, VOC drops a lot in Si p/n junction cells (Eg = about 1.1 eV, VOC = about 0.7 V), especially when they are working with little light. On the other hand, PSCs can make a VOC of more than 1.2 V (1.26 V with MAPbI3) compared to their Eg of 1.55–1.6 eV.
It is known that a single crystal of GaAs has the highest PCE value of 27.8%, which is higher than the highest PCE value of crystal silicon (26.1%). This means that it has the lowest deficit. When the GaAs cell has an Eg of 1.42 eV, it can make VOC values of up to 1.12 V. There is a small shortage of 0.3 eV, which is close to the theoretical limit set by Shockley and Queisser (9). Perovskites semiconductors and their solar cells are also better at lowering the VOC imbalance to between 0.35 and 0.45 V.
Of course, there is still a big drop in VOC. The flaw (trap) density in halide perovskite crystals is thought to be between 1010 cm−3 (single crystals) and 1016 cm−3 (polycrystals). This is one of the things that is still limiting the VOC and efficiency of PSCs. This shows that the clarity of solution-processed polycrystalline films can play a big role in lowering the amount of defects and traps and making the efficiency even better. To get even better speed, it’s also important to know how PSCs work in standard designs and how to use optimised electron and hole transport layers.
To sum up, perovskite solar cells (PSCs) work very well and efficiently because they have a high light absorption coefficient, a long carrier diffusion length, and well-balanced charge transfer. But there is still a big drop in VOC because of the flaw density in halide perovskite crystals, which is thought to be 1010 cm−3 for single crystals and 1016 cm−3 for polycrystals. To make things even better, we need to know how PSCs work in standard designs and how to use optimised electron and hole transport layers.
Working Principle of Perovskite Photovoltaics
It is possible for perovskite photovoltaic (PVC) devices to work without a mesoporous support layer. One example is a flat heterojunction with a compact TiO2 layer that gathers electrons. It is now possible to tell the difference between the PSC and the sensitisation process. It is also clear that the PSCs work more like solid-state p–n junction solar cells, where perovskite acts as a natural semiconductor between two selective (p and n) contacts.
One type of material that moves electrons in the n–i–p structure is TiO2. Another type of material that moves holes, like spiro-OMeTAD, acts as a p-type contact. A p–i–n-type structure, also called a “inverted structure,” has perovskite in the middle of a p-type material (like conductive polymers like poly(3,4-ethylenedioxythiophene [PEDOT-PSS]) and an n-type layer (like a fullerene derivative like [6,6]-Phenyl-C61 butyric acid methyl ester [PCBM]) at the top. This creates a junction of FTO(ITO)/PEDOT-PSS/perovskite/PCBM/metal.
It is important to note that the high PCE of PSCs is a major cause of the high open-circuit voltage (VOC). Some perovskite cells have a VOC of 1.18–1.26 V, which is close to the SQ theoretical limit of VOC (∼1.32 V). This is because the bandgap of the perovskite semiconductor conductors is 1.6 eV for MAPbI3. In PV cells, VOC is usually set by the difference in energy levels between the Fermi levels of the p-type and n-type electrical layers that make up the selective contacts. In Figure 1.10, you can see a normal junction structure. The HTM is spiro-OMeTAD, and the ETM is TiO2.
It is the open-circuit voltage (VOC) that plays a big role in the high PCE of PSCs. Some perovskite cells have a VOC of 1.18–1.26 V, which is close to the SQ theoretical limit of VOC (∼1.32 V). This is because the bandgap of the perovskite semiconductor conductors is 1.6 eV for MAPbI3. In PV cells, VOC is usually set by the difference in energy levels between the Fermi levels of the p-type and n-type electrical layers that make up the selective contacts. In Figure 1.10, you can see a normal junction structure. The HTM is spiro-OMeTAD, and the ETM is TiO2.
Even though SnO2 has a low CB level, both ETMs can produce a relatively high VOC (>1.0 V). This may mean that mesoporous metal oxides (TiO2 and SnO2) mostly support perovskite crystals, and the material that acts as an ETM is a thin, dense oxide layer (usually less than 10 nm thick) that sits on top of the FTO and below the mesoporous layer. But this kind of thin, packed layer isn’t thought to have the properties of a semiconductor, even though it can stop electron flow at the contact by stopping hole injection. To this end, the energy gap that is linked to VOC is thought to be the difference between the perovskite absorber’s CB level and the HTM’s HOMO level.
In VOC generation, the bandgap of perovskites only loses a small amount of heat. This is because halide perovskites have better semiconductor properties, such as an ultralong carrier diffusion length, a long carrier lifetime, moderate carrier mobility, and high defect tolerance. All of these properties show that they effectively stop carrier recombination. Theoretical modelling of MAPbI3’s electronic structures shows that the formation of defects that affect carrier recombination is limited to places that are energetically shallow, like near the CB and VB levels. There are no deep defects in the bandgap that strongly trap carriers.
Compositional Engineering for the Halide Perovskite
Absorbers
Halide perovskite semiconductors are useful because they can have their crystal structure changed using solution-based production. Changing the A-site cation and B-site anion to different organic or metal cations and halogen anions has not only changed perovskite’s visual features and PV efficiency, but it has also made it more stable. In terms of effectiveness and stability, perovskites with a mix of cations at the A-site and halides at the B-site were better than those with only cations or halides.
The most well-known organic charge The problem with MA is that it is not thermally stable because this small molecule evaporates at temperatures above 120 °C. Currently, the most stable perovskite PV cells are made with compositions that don’t contain MA and/or all artificial compositions. Researchers have looked into different combinations of cations and anions, including MA, FA, Cs, Rb, and I, Br, and Cl. They did this by looking at the ionic size and the geometrical tolerance factor (), which is a common way to guess the structure of perovskite crystals.
The mixed perovskite that has become the most common in recent years is (MA/FA/Cs)Pb(I/Br)3, which is also known as triple-cation perovskite. People are also interested in the quadruple-cation perovskite, which has Rb (i.e., (MA/FA/Cs/Rb) Pb(I/Br)3) as the fourth cation and is very stable and good at making cells work. It is thought that Rb is located in the edges between grains in perovskites that have Rb added to them, not in any lattice sites.
The DFT calculations show that the cations in the A-site don’t directly affect the band structure, but they do play a big part in keeping the structure of the crystal stable by balancing the charges within the PbI6 octahedra. This is mostly because of the electrostatic (van der Waals) interactions they have with the inorganic cage. One thing that can change the optical qualities of the perovskite is the size of the A-site cation. It can either help the crystal lattice shrink or expand. It is thought that small metal cations like Cs and Rb will make the lattice smaller, which will raise the bandgap energy (Eg). On the other hand, large organic cations like FA+ will make the lattice bigger, which will lower the Eg.
The short-circuit photocurrent density (Jsc) is higher in FAPbI3 cells, which results in a PCE number of over 20%. MA and FA change how stable perovskites are on their own. When the temperature is below 120 °C, MAPbI3’s crystal structure stays mostly the same. At room temperature, FAPbI3 easily crystallises into a phase that is not photoactive (ε-FAPbI3). At high temperatures, this phase changes into the photoactive black phase (α-FAPbI3). This phase instability often changes the PV properties by making the fill factor (FF) of current density–voltage (J–V) graphs smaller.
MAPbI3 is much more stable at high temperatures when FA is partially replaced with MA. It has become possible to use artificial cation Cs instead, as Niu et al. [87] reported. It was found that CsxMA1−xPbI3 with about 9 mol% of Cs had better PV performance and thermal stability than pure MAPbI3 (PCE of 15.8%), but stability dropped surprisingly when the concentration of Cs went up. Adding Cs can also help keep FAPbI3 stable.
Strategies to Stabilize Halide Perovskite Solar Cells
Bridging the Gap Between Efficiency and Stability
To get a better record of Photocurrent Efficiency (PCE) in terms of FF and output voltage, halide photovoltaic solar cells (PSCs) have been made. Most of the time, though, these PCE record numbers don’t work with the gadget being stable enough. This is different from commercialised high-performance devices that use solid solar semiconductors like Si, GaAs, and CdTe, which can keep up high stability while still being very efficient.
The main reason why halide perovskite materials become unstable at high temperatures and under constant light is because of the ionic mobility that is built into ionic semiconductors. It is also thought that ionic diffusion can lead to strange hysteretic behaviour of the photocurrent in J–V performance. This can get worse if there are structural flaws or gaps where the perovskite and charge transport layers meet.
To solve this problem, molecular engineering should be used to slow down the movement of ions inside the perovskite crystals and change the chemical structure of heterojunction surfaces so that mobile ions can move past the interfaces and interact with the layers next to them. By making the shape and quality of solution-processed perovskite films better, both efficiency and reliability have been increased at the same time.
High-efficiency triple-cation PSCs have been made in normal settings. The cells kept their high PCE even after being stored in normal air for more than five months. A mixed perovskite with four cations, including Rb as the fourth cation, was used. It achieved stable efficiencies of up to 21.6% and kept 95% of its original performance after 500 hours of operation at the maximum power point (MPP) at 85 °C.
Enhancing Intrinsic Stability of Halide Perovskites
We need to know how stable the structures of perovskites are in order to figure out how long photovoltaic (PV) solar cells (PSCs) will last. For more than 20 years, these cells must be able to produce steady electricity in real sunlight, at high temperatures caused by heating, and with oxygen and wetness in the air. Environmental pressures like heat, light, humidity, and air can change how stable perovskites are both inside and outside the material.
This number, called the Goldschmidt tolerance factor (𝼏), changes depending on the size of the ions in ABX3. It shows how structurally stable perovskites are on their own. In the range of 0.8 to 1, , perfect cubic perovskite structures or twisted perovskite structures with tilted octahedra are most likely to form. Values of less than 0.8 and greater than 1 make it less likely that the perovskite structure will form.
By changing the value of close to the middle of the perovskite zone, compositional engineering of perovskite by mixing different cations and anions makes the structure more stable. When Cs or MA are added to FAPbI3, the 𝼏 number drops from 1 to keep the cubic phase of FAPbI3 stable. A simple mixing method can be used for mixed perovskites.
To sum up, the structure stability of perovskites is very important for how long they last in outdoor setups like Si PV panels. Changing the number of close to the middle of the perovskite zone can make perovskite solar cells more stable.
The research is mostly about how perovskite, a type of solar material, is made. Researchers have found that the creation of perovskite is affected by several things, such as the additives’ make-up (Na, K, and Rb), the matrix system (FA/MA mix), and the ions’ phase clarity. These numbers can be used to figure out the effective tolerance factor (𝼏eff) of a mixed perovskite. A triple-cation mixed perovskite with 5% Cs (Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3) is one example. It is thought to be around 0.97.
This could be because mixed perovskites (FA-MA-Cs) have a crystal structure that is different from MAPbI3’s (tetragonal), which could also be why they are more stable. Pure MAPbI3 is less stable than mixed perovskites with cubic structures. The thermodynamic stability of various perovskite mixtures gives us more information about the structure and fundamental stability of these materials.
Ion movement in the halide perovskite crystal also breaks down materials, leading to strange J–V hysteresis, phase splitting, and a loss of PSCs’ long-term stability. Ions move through the cation and anion flaws (MA+ and I− holes) in perovskites. Your ion’s activation energy of migration can tell you how easy it is for it to move through a perovskite film.
Experiments have shown that iodide is the ion that moves around the most easily in perovskite crystals. In perovskite crystals, MA+ cations also move around. In films with many crystals, ions like to move along the edges of the crystals rather than inside the crystals. So, ion movement can be slowed down by either cutting down on the number of grain borders or making the grains bigger. However, the fact that ion migration can happen within single crystals of MAPbI3 and MAPbBr3 shows that stopping ion migration needs a major change in the perovskite’s structure.
External and Environmental Stability
Perovskite-based devices are affected by things like air, wetness, heat, light, and external bias. These things can change the surrounding carrier transport materials and weaken the structure of the device. People are worried about how stable perovskite will be in the long run because it is easily damaged by water (H2O). Hydrogen bonds make it easy for water molecules to attach to perovskite and form hydrated compounds. These compounds change the qualities of perovskite in specific areas. It is possible to undo the damage this hydrated substance does, but adding more water can break down perovskite into PbI2 and other parts that can’t be fixed.
To keep PSCs from breaking down because of water, they need to be properly enclosed in films that act as gas barriers. Many attempts have been made to make the materials that make up the cell structure more resistant to water. These have included adding hydrophobic layers like polymer/carbon composites instead of the commonly used spiro-OMeTAD, putting non-hygroscopic layers between the perovskite and HTMs, or passingivating (changing) the surface of the perovskite with small molecules. Tetra-ammonium zinc phthalocyanine was used to stabilise the grain limit of MAPbI3, and the device had a long life of 1000 hours, keeping 90% of its photoelectric effect (PEC) while being stored at 85 °C and 45% humidity.
Adding a 2D perovskite material with an organic group that doesn’t like water to general 3D perovskites has become a popular way to change the surface and grain boundaries of perovskite films recently. This protects against wetness getting in and allows the traps to work normally. Large organic cations in 2D perovskites don’t like water as much as MA and FA do, which helps make them more resistant to water. But adding a little 2D perovskites to 3D perovskites can make them work better and be more stable over time.
Table 1.3 shows a number of 2D and 3D mixed perovskite materials along with their cell yields and stabilities. In this case, adding 0.8 mol% of ethylenediammonium iodide (EDAI) to a 3D MAPbI3 structure raises the PCE by lowering recombination and allowing the device to work steadily at 50 °C, 50% relative humidity, and light. To make halide perovskite solar cells work better, FEA-based perovskite was also made available.
A lot of study has been done on perovskite photovoltaics (PSCs), and recent success includes making planar PSCs out of pure MAPbI3 and MA1–2xEDAxPbI3. These have been tried in full sun at 50 °C and 50% relative humidity. Alkyl diammonium, butanediamine iodide (BEAI2) creates a 2D perovskite interlayer that protects the grain boundaries of perovskites and keeps high-performance cells stable at 85 °C in room air and with constant light. Since it was used in dopant-free carbon-based PSCs, 5-aminovaleric acid iodide (AVAI) has become a useful way to stabilise MAPbI3. Adding three mole percent of AVAI to MAPbI3 perovskite makes a carbon-based PSC device stable. A big cell (10 × 10 cm2) of the device stayed stable for more than 10,000 hours, which is more than a year, at 55 °C with one sun shining on it.
You can also make PSCs more resistant to water by adding hydrophobic materials to the perovskite surface. For example, putting a very thin layer (less than 10 nm) of polyvinylpyridine (PVP) on the perovskite surface (MAPbI3) and putting it in touch with spiro-OMeTAD makes MAPbI3 more resistant to water for up to 100 seconds. Devices that have been changed by PVP also have higher open-circuit power and performance. The Lewis base side chain of PVP might help coordinate the undercoordinated Pb2+ on the perovskite’s surface, which lowers the surface trap density as seen by PL enhancement. Another study by Tres and colleagues showed a similar outcome. They used PVP as a thin intermediate to stop non-radiative recombination and were able to raise VOC to 1.20 V. It was also discovered that adding triblock copolymers to the grain borders of MAPbI3 films, which are both hydrophilic and hydrophobic, helps increase the PCE (>19%) and stability of MAPbI3 cells throughout the process.
On the other hand, the perovskite film is very sensitive to environment (O2, N2, or vacuum), which could be a bigger problem than humidity when it comes to the need for gas barrier films to protect the device. Researchers have found that perovskite films have weak PL when they are in a vacuum or N2, but strong luminescence when they are aged in O2. O2 is adsorbed on the surface of MAPbI3 (Cl) in a way that can be undone. This stops the deep surface trap states from working, which makes the glow stronger.
Perovskites’ ability to handle oxygen depends a lot on the number of charge carriers they have. Cells that are aged in open-circuit conditions break down faster than cells that are aged in short-circuit conditions. Adding an effective layer for collecting electrons (TiO2) to the structure makes it more stable by lowering the production of superoxide ions.
The history and most recent success in perovskite photovoltaics are based on how light changes perovskite and how that changes its long-term stability. Photo-intability is a big problem because it changes chemistry processes like ion movement, flaw formation, and phase splitting that make perovskites less stable. Some of the reasons why devices don’t work as well when they are constantly lit up are photocatalytic breakdown by TiO2, photoinduced ion movement, photoinduced trap-state generation, photoinduced phase segregation, and photoinduced cation or halide redistribution.
It is possible to stop UV light from breaking down the most common electron transport layer (ETL), TiO2, by switching it out for an alternative ETL that is less photoactive, like Al2O3 or SnO2. Even devices that don’t use ETL and use bathocuproine (BCP) as a layer between FTO and perovskite have been seen to be more stable under light than devices that use TiO2. Putting an organic film or another UV filter on the front of the base has also been shown to make the device more resistant to UV exposure.
But, photoinduced phase segregation or ion redistribution in mixed-halide perovskites (I and Br) is a big problem that needs to be fixed. A photoinduced phase split that can happen both ways creates iodide-rich and bromide-rich regions in mixed-halide perovskites. This creates low-bandgap trap states, which lowers the performance of the solar cells when they are lit. It is thought that flaws like halogen gaps are what cause these phases to separate.
Recent research has shown that MA-containing mixed perovskites can be stabilised for practical use. This was shown by analysing the outgas and breakdown processes for a device covered in gas barrier films. When used for sealing, a polyisobutylene- or polyolefin-based polymer–glass mix gives great longevity that goes beyond the IEC61215:2016 damp heat and humidity freeze tests.
Materials that move carriers and come in touch with the perovskite layer also change the stability of the device in a big way. Because the perovskite/HTM joints are the “heart” of the whole device, HTMs can be very important for protecting it over time. Spiro-OMeTAD is a great HTM because it has the right amount of HOMO and can be used as a fairly thick film that can be made by covering it. But spiro-OMeTAD needs ionic dopants to be mixed in. This helps the HTM oxidise in normal air to keep the hole conductivity normal. Device decay can also be caused by ionic dopants, such as the passage of ions from perovskites. In order to make PSCs more thermally stable, one way is to stop ions from moving between the layers.
People have worked hard to make different kinds of organic HTMs that can be used in PSCs instead of spiro-OMeTAD. Most of these HTMs are either small molecules or hole-conducting (p-type) polymers, like P3HT, PTAA, PEDOT:PSS, and PTAA. But their dopants still have the potential to make the device less stable.
Creating dopant-free hole transport materials (HTMs) is a way to stop the damage that biological HTMs cause. Inorganic HTMs, like oxide or compound semiconductors like NiOx, CuOx, Cu2O, CuI, Cu(thiourea)I, CuSCN, CuGaO2, CuCrO2, and Cuphthalocyanine, work just as well as spiro-OMeTAD and last longer. Dopant-free organic HTMs are quickly becoming one of the most important ways to keep perovskite photovoltaics (PSCs) stable. Organic HTMs are not as stable at high temperatures as artificial HTMs, but they may be more resistant to oxidation and attack by the halide anions of perovskites in the chemical and electrical realms.
There are a lot of dopant-free organic HTMs in small molecules or polymeric materials that can get PCEs above 18%. One example is an OMeTAD version with a truxene core that has a high PCE of 18.6% without any dopants. A small molecule HTM that was linked and had a benzothieno-pyrrole core made it possible for an effective PSC device (PCE>18%) to last for 33 days. Cao et al. used a spiro-based dopant-free HTM that has redox-active triphenyl amine units that organise themselves in a way that is similar to the substrate. The PCE for a device with this organised HTM layer was 20.6%.
New HTMs have donor–π–acceptor-type (D–π–A) molecules. The D–π–A molecule is thought to remove the need for dopants or chemicals in the HTM because it allows charge to move within the molecule. An HTM molecule with a quinolizino acridine donor and terthiophenes and malononitriles as acceptors works in a device with a PCE of 18.9% and better long-term stability after 1300 hours of illumination. It was shown as a D–A–D-type HTM, which had a PCE of 19.27% and a high VOC of 1.11 V. The molecule had a triphenylamine donor and a thienopyrazine unit as the acceptor. Phthalocyanine (Pc) compounds are p-type semiconductors and organic colours that don’t change much when heated. They also work well as dopant-free HTMs that are very resistant to wetness and have a high temperature stability.
Polymer HTMs are also sought after because they don’t absorb water, which means they don’t get damaged by moisture. Dopant-free polymer HTMs are different from PTAA-type polymers because they are mostly made up of thiophene backbone chains, which is similar to the structure of P3HT. For electronic devices to work well, HTM needs to have a hole movement of more than 10−4 cm2/(V s). It is possible for the polythiophene family to have this amount of mobility without any dopants, which is higher than PTAA’s mobility (10−5 cm2/(V s)).
A new copolymer called poly(DTSTPD-r-BThTPD) doesn’t have any dopants added to it. It has a high hole mobility of 1.5 × 10−3 cm2/(V s) and stays stable at high temperatures up to 330 °C. When the TiO2 ETL was switched out for an amorphous SnO2 ETL, this dopant-free CsPbI2Br solar cell showed a PCE of 15.5% and a very high VOC value of 1.43 V.
To sum up, one way to make perovskite devices more stable by stopping ion diffusion across the junction surfaces is to mix these high-efficiency, dopant-free HTMs with 2D-based perovskites that have been stabilised. This trend should make it possible to make high-performance, reliable PSCs that can be used in real devices.
Progress of All inorganic and Lead-Free Perovskites
Interfacial engineering with 2D materials can make organic-inorganic hybrid perovskites better, but they can only be thermally stable for a short time if they contain flammable organic molecules like MA. Creating photovoltaic (PV) filters with organic-free formulas can help make sure that devices will work well in hot weather. A lot of research has been done on Cs-based lead perovskites that are stable at high temperatures. But, CsPbI3’s chemical balance changes when the temperature does. For example, the photovoltaically active black phase of α-CsPbI3 (cubic) can stay stable at temperatures above 310 °C, while the inactive yellow phase ε-CsPbI3 (orthorhombic) tends to take over the crystal at room temperature.
From 2.9% in the first experiment by Snaith and coworkers [182] in 2015 to 18.4% in the most recent study by Wang et al. [183], the PCE of CsPbI3 PSCs has gone up. Adding Eu2+/3+ to CsPbI3 can make the black α-CsPbI3 phase more stable at room temperature by making the polycrystalline film better [185]. A different study was able to create a stable black phase by making α-CsPbI3 better and reaching a PCE of 15.7% [186].
Another area of study for Cs-based artificial perovskites is to improve the PV functions of Br-mixed CsPbI3, which has bigger bandgaps depending on how much Br and I it contains. When Br is added to CsPbI3, the temperature at which the black phase forms drops from 350 °C to about 250 °C. This makes the material more stable during phase transitions. It looks like bromoiodide perovskites CsPbI3Br3−x with smaller bandgaps could be useful in the top cell of tandem solar cells or indoor PV cells.
When used indoors, photovoltaic (PV) systems that power internet of things (IoT) devices and items work very well by collecting LED light (wavelengths, <700 nm) through a semiconductor filter that is only sensitive to visible light (Eg >1.8 eV). The ratio of I to Br can be used to exactly control the bandgap of CsPbI3−xBrx.
The group got a PCE of 15.5% by raising the device’s VOC to 1.43 V with CsPbI2Br. This is the highest VOC that has been achieved with visible light-harvesting PSCs so far.
To make metal halide compounds that don’t contain lead, which is one of the ten most poisonous elements for human health and bad for the environment, researchers need to make progress on all-inorganic perovskites. Organic tin halide perovskites, like MASnI3, MASnI3−xBrx, and FASnI3−xBrx, have shown the best performance in lead-free perovskite photovoltaics. By changing the makeup and adding organic cations, Sn-based perovskites have reached a PCE of 10%. But, Sn-based halide perovskites can work well as PV semiconductors as long as the reduced divalent form of Sn(II) stays stable in an oxygen-free environment.
Researchers have also looked into Sn-based and all-inorganic perovskites for use in PV systems. Compounds based on Cs, like CsSnI3, CsSnBr3, Cs2SnI4Br2, and CsGe0.5Sn0.5I3, work better in devices than organic hybrid Sn perovskites. However, their device performance is not as good as that of organic mixed Sn perovskites, and Sn(II) has a stability problem.
A lot of research has been done on lead-free, all-inorganic perovskites, with a focus on group 15 metals like Sb and Bi. There are similar electronic structures and ionic radii between these materials and Pb, which lets them effectively join the perovskite lattice. Researchers have looked into the crystal structure and visual qualities of several types of ternary bismuth halide perovskite materials, both in theory and in practice.
PV devices that use Bi perovskites haven’t been as efficient as those that use lead- or Sn-based perovskite films because a lot of energy is lost when carriers combine, which lowers JSC and VOC. To get around this problem, Zhang et al. created a non-equilibrium vacuum deposition method for making high-quality MA3Bi2I9 films. These films formed a large-grain, uniform layer without any pinholes, with a charge-diffusion length and a number of trap states that were similar to lead perovskite films. It was created by Jain et al. who used methylammonium iodide (MAI) vapour to help the solution-processed BiI3 film work better. This improved the PCE of the MA3Bi2I9 device by up to 3.17 percent with VOC = 1.01 volts.
All-inorganic versions of Bi halide perovskites are more important because they are very stable. AgBi2I7, a silver bismuth halide, is one possible option for lead-free PSCs that are made of only inorganic materials. Sargent et al. made an AgBi2I7 (non-perovskite cubic phase) thin film from BiI3 and AgI. The PV cell they made with this film had a 1.2% PCE and was more stable over time when exposed to light and air. Due to the mixing of different phases, it is hard to get high-purity single-crystal forms.
The work is mostly about making perovskite solar cells (PSCs) with artificial perovskites that don’t contain lead. The researchers made the solvent engineering process better so that they could get phase-pure AgBi2I7, which gave them a PCE of 2.1%. However, there is a big PCE difference between Bi-based and Pb-based PSCs that use all-inorganic perovskite compositions. This is because ionic dopants (LiTFSI) break down Ag/Bi perovskites chemically. As a dopant-free HTM, this problem is fixed.
If you use P3HT as a dopant-free HTM, the device J–V performance of the Bi perovskites shows that the efficiency of photocurrent production is very low, giving you an EQE value between 20% and 40%. The main reason for this drop in VOC is a rise in trap density, which is high in areas where grains meet and there are lots of flaws. To improve the device’s function, it is important to make the Bi perovskite film better by making it have a regular, pinhole-free structure made up of big grains.
Photovoltaic reactions can also be seen in antimony (Sb)-based halide perovskites, though their best efficiencies aren’t even close to those of bi-based perovskites. The straight bandgap of Cs3Sb2I9 is 2.05 eV, and it has a high absorption rate. Rb3Sb2I9 stays stable in its 2D stacked form when Cs is switched out for Rb, giving it a PCE of 0.66%. With organic Sb perovskites like MASb2I9, you can get PCEs that are much higher.
Some halide perovskites based on Ti(IV), like Cs2TiBr6, have been suggested as possible collectors for photovoltaics. To make pure Cs2TiBr6, on the other hand, very careful chemical and physical steps are needed. Au-based perovskites, like Cs2Au2I6, have also been suggested as possible absorbers for photovoltaics. However, attempts to make devices have not been successful because the Au-containing material has relatively high conductivities that can make it hard to rectify photocurrent generation.
In the end, the study shows how important it is to make lead-free artificial perovskites better for PV cells, especially when it comes to their uniformity, tightness, and phase clarity.
Enhancing Efficiency of Low-Cost Tandem Solar Cells
Photovoltaics (PSCs) that can be tuned to different wavelengths can make it easier to design tandem cells. These are made up of more than two cells that are sensitive to different wavelengths. They work together to collect more sunlight and get a Photo-Electrical Efficiency (PEE) that is higher than the best single cells, like crystalline silicon (Si) cells. The first perovskite-based tandem cells were made in 2015, when the PCE of PSCs hit about 20%. Triple-junction tandem cells like InGaP/InGaAs/Ge have been used in space ship projects so far because they have very high PCEs.
In the business world, making tandem solar cells costs more than making a single cell that makes up the structure. This is because of the materials needed and the process for stacking unit cells. Cost-effectiveness is a very important factor in bringing solar cells to the public and customers in general. Because of this, it is a good idea to increase the PCE by using tandem cells, which don’t cost much more. For instance, it’s okay to pay 20% more if it means improving the PCE by 50%. But if the PCE goes up by 50%, a cost increase of more than 50% might not be okay, based on how much demand there is for that product.
For making tandem cells, it would be best to use a mix of inexpensive perovskite and solid Si cells or two perovskite cells together. Technically, the basic rule for making a tandem cell is to make sure that the photocurrent density of each unit cell that makes up the tandem is the same as the photocurrent density of the other unit cells. In dual-junction tandem cells, the top cell is changed so that it absorbs light with a shorter wavelength. This creates a photocurrent density that is exactly the same as the bottom cell’s photocurrent density, which is caused by light with a longer wavelength passing through the top cell.
Review pieces sum up the latest progress made in perovskite-based tandem solar cells. This type of solar cell shares the photon flux with a perovskite top cell (Eg = about 1.6 eV) that can produce 1.2 V and a solid Si bottom cell (Eg = about 1.1 eV). The PCE of this type of cell can reach almost 30%, depending on how much VOC and FF are increased.
Duong et al. reported a Photo-Electrochemical Efficiency (PCE) of 26.4% using a 4-terminal tandem cell made up of Rb-containing quadruple-cation perovskite and silicon cells stacked on top of each other. More PCE was found in the Si cell, and the VOC gain was 1.81 V (1.12 V at the top and 0.69 V at the bottom). The highest PCE that Oxford PV reported was 27.3% with a perovskite and crystalline Si tandem cell. This was higher than the highest PCE that a single-junction Si cell could reach, which was 26.1%.
Because they are cheap, all perovskite-type tandem cells have been studied in great detail. Using Cs0.4FA0.6PbI1.95Br1.05 (Eg, 1.78 eV) and Cs0.05MA0.45FA0.5Pb0.5Sn0.5I3 (Eg, 1.21 eV) as the top and bottom cells, Huang and his colleagues made an all-perovskite tandem cell. The ETM was SnO2−x, and the HTM was fullerene (C60). With a VOC of about 2.0 V and a PCE of 24.4%, the tandem cells were very stable and kept their efficiency almost the same after 1000 hours of constant sunlight.
The team from Nanjing University got a verified PCE of 24.2% with a 1 cm2 cell size by using a mixed perovskite of Pb and Sn for the bottom cells. According to Oxford PV, the all-perovskite tandem cell is the best way for top and bottom cells to join because it has the lowest production cost and can be made into thin-film devices.
Power tools used indoors need to have bodies that are as light and flexible as possible. By changing the bandgaps of the perovskite top and bottom cells, it is possible to make a perovskite-based thin-film tandem cell that is sensitive to light from LED lights and can work with a high output voltage. The Internet of Things (IoT) is likely to use a thin, flexible perovskite device with a plastic film base as a high-voltage power source soon.
Space Applications of the Perovskite Solar Cells
Solar batteries with a high efficiency have been used on space missions since 1958. Si solar cells drove the first American satellite. For satellite solar cell modules, high-efficiency solar cells with tandem structures based on solid Si and GaAs are usually used. A group of researchers from the Japan Aerospace Exploration Agency (JAXA) looked into perovskite solar cells (PSCs) in 2015 because they are lightweight, flexible, and could be used in the future.
The main purpose of the test is to see how stable PSCs are in the hard conditions of space. The materials in most industrial solar cells break down in space, mostly because of intense particle energy like electron and proton beams. So, all of the electronics on satellites should be very stable when they are constantly exposed to high-energy space radiation. The sun shines on solar cells all the time, which can heat the device up to 100 °C.
In a lab experiment using high-energy electron and proton beams, experts worked with JAXA to look into how well PSCs can handle being irradiated in space. It was discovered that organo-lead halide perovskite semiconductors can handle flaws, which lets PSCs stay alive when exposed to high-energy light on perovskite films. Several studies have shown that perovskite materials (MAPbI3) are pretty stable when exposed to proton radiation with a particle energy tuned at 68 MeV.
To see how stable perovskite devices are at high and low temperatures, 100 °C and -80 °C, which are like a satellite orbiting with and without sunlight, were used to test them. Instead of the perovskite absorber, this test showed how fragile organic HTMs are. It’s pretty good that both MAPbI3 and mixed-cation PSCs using P3HT HTM can handle high fluence and big amounts of protons (50 keV) and electrons (1 MeV), which can damage solid Si and GaAs-based solar cells.
Other study groups have proven that lead halide perovskites and their solar cells can handle radiation. This means that PSCs could be used in the future as long-lasting power sources in space station projects. Lang et al. created a perovskite/copper indium gallium selenide (CIGS) tandem solar cell that is all one piece. It has a PCE of 18.0% at air mass (AM) 1.5 (15.1% at AM0) and 2.1 W/g of specific power.
Conclusion and Perspectives
The photophysical features of metal halide perovskites have sparked many study topics in organic-inorganic hybrid ionic semiconductors and given engineers ideas for many device uses. Fundamental and practical study both use methods from different fields. For example, making materials is mostly a chemical process that uses solutions, while describing them needs physical methods. In industrial settings, the solution-based printing method used on thin perovskite films allows for high-throughput and low-cost coating processes, which makes it possible to make flexible modules roll-to-roll.
Perovskite materials can handle a lot of flaws, which is a key part of making optoelectronic devices work well, especially in solar power generation. This is especially helpful when it’s used as an indoor power source for IoT apps, since IoT devices and their backup batteries need a higher voltage input when there isn’t much light to power the electronics.
In perovskite photovoltaics, one goal is to get the great photophysical features of GaAs semiconductors. These have an open-circuit voltage and PCE close to the SQ limits, with a bandgap of 1.42 eV at the absorption edge around 900 nm. To keep this bandgap, lead must be totally or partly swapped out for other group 14 or 15 metals. Perovskites based on Sn(II) are the most likely options, but they are not very stable against oxidation, which makes them hard to use in real life.
It has been hard for engineers to change the perovskite makeup and improve the film shape in order to get reasonably high gadget performance, but theory models suggest that solar functions will have a lot of promise. Based on chemical engineering, more research will be done to find new formulas and ways to make solid films.
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