A lot of work has been made in research on halide perovskite-based solar cells, and they are now close to having a power conversion efficiency (PCE) of over 23%. Large-scale commercial uses of these cells, on the other hand, meet the problem of long-term stability, since their starting efficiency drops quickly in working settings like light, biases, and exposure to the air. Because they have been stable for over 20 years, current technologies with similar levels of efficiency can be used to make a lot of energy. This chapter is about how stable perovskite solar cells (PSCs) are in normal working conditions on Earth. It focusses on tests for stability and talking about the role of contact layers and wires in a device. Initial results can get worse because of things inside the devices, like the natural stability of the perovskite material, other layers inside the devices, or outside causes, like water and air breaking down. The chapter tries to separate the impact of each stressful factor on each device part. However, degradation is usually caused by a mix of outside factors and device parts, which makes it hard to fully understand the stability of PSCs.
Stability testing
Conventional testing
Solar power (PV) technology goes through a lot of tests to make sure it works well and is reliable. To find out how well a PV panel works in the solar energy business, they follow a thorough process that includes standard testing conditions (STC) and power conversion efficiency extraction from a current density voltage scan. International Electrotechnical Commission (IEC) guidelines include wet heat tests, thermal cycles tests, and rapid stress tests to make sure that devices work well in the real world. The module’s T80 is found, which is the amount of time it takes for a panel to lose 80% of its original PCE. For well-known PV systems, the T80 can last up to 2025 years. The perovskite group has used information from both inorganic and organic PV (OPV) tests to figure out how stable it is. The T80 is when the device’s efficiency drops to 80% of what it was at first, and the TS80 is the stabilised efficiency, which doesn’t take into account the burn-in time or the fast initial loss of efficiency that is common in OPV. When compared to existing artificial PV systems, PSCs break down much more quickly. There aren’t any standard tests for PSCs, but modern testing methods have been suggested to describe how stable they are.
Perovskite testing
Initial efficiency testing
To find out how efficient Perovskite Solar Cells (PSCs) are at first, they are usually put through standard tests using JV scans. Because of the well-known feedback effects in JV curves, it is important to record both forward and backward scan power conversion rates (PCE). Forward and backward scans give different yields, and all photovoltaic factors rely on scan rates, pre-bias, and the temperature of the device. It has been said that most JV scans are done with an AM1.5 synthetic sun spectrum that follows STC. However, light bias can change hysteresis. The strength and range of the light that goes into the device affect how it works. According to Pazoki et al., devices made from mixed cation perovskite films have a faster rate of ionic movement. This is because high-energy blue photons cause more ionic movement and bigger hysteresis in JV scans than red photons. So, it’s important to give specific testing conditions for measuring JV for PSCs and fully characterising the device’s JV with a steady power output that is taken out while following the MPP for a few minutes.
Laboratory long-term stability testing
It is common to measure JV scans until the efficiency drops to 80% of the original value to find out how stable a perovskite solar cell (PSC) is. However, this way isn’t good enough because it doesn’t give a true picture of how the device will work in long-term use. There isn’t yet a set maximum power point (MPP) tracking method for PSCs, but people are working hard to find the best one.
As this discussion goes on, the standard tweak and observe (P&O) method is used because it is both accurate and easy to use. The method looks at the highest power point voltage (VMPP) from earlier or first JV scans and estimates the power output. Then it changes the applied voltage and figures out the difference in power. If the power flow goes up, the applied voltage will keep going up or down, and the other way around. Eventually, the applied voltage levels off around VMPP, achieving a nearly steady state that can be thought of as the device’s stable efficiency.
On a longer time scale, the total power of PSCs generally drops by a lot. The MPP curves can be fit by a double exponential decline, where the device’s efficiency drops quickly at first and then slowly but steadily over time. Compared to other PV systems, this degradation is fundamentally different, though the rates of decline rely on the makeup of the perovskite.
As Domanski et al. showed, PSCs partly regained the efficiency loss after resting in the dark for the night after working for a few hours in direct sunlight. Because of an internal electric field that was present during the working situation, ions and ion flaws moved around. It has been seen in other works that PSCs heal in the dark. It has been said, though, that PSCs can get better over time. This might be because the contact’s contacts with the perovskite layer get better, especially when NiO is used.
Outdoor testing
For PV technology to move out of the niche market, it needs to be tested outside in real working settings. This is important for figuring out how much energy it will produce and how long it will take to pay for itself. It has been possible to make perovskite solar cells (PSCs), but studies in the field have been slow to start. In 2016, Bella et al. did a large test in Italy using fluorinated photopolymer coats to keep PSCs clean and protected from UV rays, water, and dirt. After 5 weeks of tests outside, the gadgets still worked at over 90% of their original level of efficiency. But well-known PV systems say that they lose less than 1% of their starting PCE in one year when they are outside. The industrialisation of perovskite PV technology needs to be improved. When figuring out how much power halide perovskite-based devices and modules put out, you have to take into account both reversible and nonreversible losses. It’s important to have a standard way to test the safety of halide perovskite solar cells outside, including MPP tracking, dark storage lifetime, damp heat test, and MPP tracking. PSCs are very responsive to stress from outside sources, and monitoring needs to be standardised.
Perovskite stability
Atmospheric water and oxygen stability
Due to their high performance and adaptability, perovskite solar cells (PSCs) look like a good option to regular solar energy systems. The oxygen and wetness in the air, on the other hand, can really hurt the function of PSCs. This is especially scary for other PV systems, since water and air can break down some parts of PV screens. Even though people are working hard to find the right encapsulation for PSCs, the fact that they are becoming more intrinsically stable can make the strict standards of encapsulation less important.
It is known that moisture can break down the perovskite layer, even if it is encased. DFT studies show that the main part of the perovskite solar cell family, CH3NH3PbI3 (MAPbI3), is unstable when water is present because the MA cations are very reactive and dissolve in water. When put in water, MAPbI3 breaks down into HI, CH3NH2, H2O, and PbI2. As little as 10% relative humidity (RH) makes the perovskite film quickly absorb water. It is likely that MAPbI3 turns into light yellow hydrate crystals when it comes in contact with water. Over time, the hydrogen bonds between the organic cation and the octahedral structure of PbI426 get weaker. This lets water form strong hydrogen bonds with the organic cation, which creates the hydrate phase.
Another idea for how moisture can break down halide perovskite in a way that can’t be fixed is through the trap state. Photogenerated charges can get stuck in defects, especially at the edges of grains. This can create local electric fields that deprotonate organic cations and break down into amine products. A small amount of water added to the perovskite precursor or being exposed to wetness during the crystallisation process, on the other hand, is helpful. During heating, water molecules can help the MAPbI3 film form a more uniform nucleus and a better crystallinity, as well as protect loose bonds.
While there is light, PSCs break down very quickly because MA loses its proton. The rate of breakdown speeds up when both wetness and air are present. Adding water raises the quantity of protons and changes the balance towards broken down products, which speeds up the process of deprotonating superoxide species.
It has been shown that it is possible and sometimes even helpful to process PSCs in the atmospheric state (normal RH is 5070%), even though air and wetness are bad for them.
Thermal stability
Halide perovskites, which are described by the formula ABX3, have different solar active phases that depend on the elements A, B, and X. The mixed organic-inorganic perovskite MAPbI3 is in the α-phase at room temperature, but it is not very stable chemically when exposed to outside forces. At room temperature, FAPbI3 can be in the photoactive black phase, but the material is more energetically beneficial when it is in the unwelcome, photo-inactive β-phase or yellow phase. If you heat pure artificial perovskite CsPbI3 above 300°C, it only has a black phase. The A site in a 3D perovskite structure can be used in a lot of different ways. An intermixing cation approach has been used to make the material more stable while still getting high efficiency.
The PV performance of perovskite solar cell materials changes a lot based on the working temperature. The working temperature range is from -40°C to 85°C. The cell’s PCE drops quickly at 65°C, and after less than 50 hours of light exposure, the devices are only 20% as efficient as they were at first. At 20°C, the cell still has 90% of its original value after 500 hours of sunlight.
When water and air are present, methylammonium lead iodide can’t handle heat as well. Perovskite is more stable at high temperatures when the organic cation is changed from methylammonium to formamidinium. Because FA is less acidic than MA, it is more stable. This could mean that FA is less likely to give up electrons to form HI. Also, inorganic perovskite, like caesium perovskite, is more stable at high temperatures.
Light stability
MAPbI3 breaks down in light when oxygen is present, but it can also happen when oxygen is not present. Photons excite a “Pb1…Io” complex, creating I2 and 2Pb1 that respond with light. The unstable lead species can mix with oxygen and water to make lead iodide and methylamine. This causes the perovskite 3D structure to break down.
Changes in the structure of MAPbI3 can explain why it breaks down when exposed to light. When there is light, the hydrogen bond between MA1 and hydrogen can weaken. This lets MA1 spin more easily. When there is an electric field, the dipole cation can react or line up by interacting with other dipoles in a way that depends on temperature. Ionic diffusion speeds up when light hits a material, along with heat. This makes the lattice bigger and starts the diffusion of CH3NH31, which breaks down the material into PbI2. Ion movement raises the number of defects in the material, which speeds up the breakdown even more.
If light is shining on Pb-I-Pb for a long time, the bond angle can change. This can cause the structure to break down forever into lead iodide. To stop this process, adding formamidinium, a bigger cation, can make the device more stable because it has a higher activation energy for moving ions.
Light causes reversible phase segregation in the mixed cation/anion halide perovskite, which is linked to halide movement in the material. The material can recover some of its shape after being left in the dark for a while.
Electric field stability
When a certain voltage is given to solar cells, all the ions in the perovskite film can move in reaction to electric fields from the outside. The least amount of energy is needed for iodide ions and their gaps to move, while the most energy is needed for lead components. It’s easy for cations to move out of the hole in the octahedral cage of PbI42-6. This causes the lattice to stretch and twist, which breaks down the material over time. Leijtens et al. suggested that the material breaks down in a way that involves applied bias and humidity. When there is water, the methylammonium cation moves to the negative electrode, which makes lead iodide form at the positive electrode. At the positive electrode, CH3NH3-1 built up, leaving behind an unstable PbI426 octahedral that broke down into PbI2 over time. The movement of ions from perovskite to electrodes is another thing that makes PSCs less stable. For example, I2 reacts with Ag to make AgI, which leads to metal electrode corrosion. Charge flaws can build up at the points where perovskite and other layers meet, which makes the material break down faster because of forces from the outside. To fully understand how ion movements affect the long-term health of perovskite solar cells, we need to do a lot of research.
Mechanical stability
It has been possible to make perovskite solar cells on bendable surfaces, and these cells work very well at bent angles. But because they don’t have a metal oxide support in their flat structure, they are weak and easily broken. Grain sizes have a lot to do with how hard a material is. Changing the temperature from day to night can cause more mechanical stress, which can damage the perovskite cell permanently. Not enough is known about the mechanical stability of PSCs, which is important for the technology’s long-term security and usefulness in business. To make sure that PSCs will last and be useful in business, it is important to know how stable they are mechanically.
Device and interface stability
Charge selective contacts
Since the beginning, perovskite solar cells (PSCs) have been improved. Mesoporous TiO2 was the first electron selective layer (ESL) for PSCs. However, TiO2 in PSCs can be damaged by UV light, leaving TiO2 with empty, inactive deep electron traps that lower the performance of PSCs over time, especially photocurrent after a few hours of being in full sunlight. To make devices more stable, different interlayers and tweaked TiO2 or the Al2O3 scaffold have been looked into.
As an ESL for more stable PSCs, metal oxides have been used, especially SnO2-based PSCs. The researchers Anaraki et al. showed that improving devices with a SnO2 layer and a triple-cation CsMAFA perovskite absorber led to a stable efficiency of about 21%. The band ends of SnO2 are lined well with those of perovskite, which makes it better at collecting electrons and more stable under light than the TiO2 analogue.
Researchers who were ahead of their time have used phenyl-C61-butyric acid methyl ester (PCBM) as an ESL for reversed p-i-n structure/architecture. Some buffer materials, like ZnO, have been used to make PCBM more stable. Metal oxides have also been used in contact layers to stop water and oxygen from getting into the perovskite and to stop dangerous organic components from leaving the material. When it comes to temperature stability, metal oxides are better than their biological cousins.
Organic HSLs like PTAA, Spiro-OMETAD, and their derivatives are used by high-efficiency PSCs right now. The solution-processed Spiro-OMETAD layer, on the other hand, is porous, which lets water and air get into the perovskite layer. The hygroscopic quality of the Li-TFSI dopant found in Spiro-OMETAD and PTAA also speeds up the breakdown of PSCs when they are wet.
A great deal of work has gone into finding other dopant-free HSLs, like tantalum doped tungsten oxide (TaWOx), that are not π-conjugate polymers or perovskite. The device kept more than 90% of its original power after being exposed to sunlight for one hour without being sealed in N2. To improve its conductivity without adding dopants, another method is to change pure Spiro-OMETAD with its dicationic salt, Spiro-OMETAD(TFSI)2.
PEDOT: PSS is another HSL that is often used, especially in a reversed p-i-n form. However, because it is hygroscopic, PEDOT: PSS can take in water from the air, which makes solar cells less stable. Metal oxide, especially NiO, is used to make the whole stack of devices more stable, especially when it comes to heat protection. For instance, Cu-doped-NiO MAPbI3 devices could make PSCs more stable in air, but they still lost more than 10% of their original power after 250 hours in air, even though they weren’t exposed to the sun the whole time.
To make devices more stable, researchers have looked for PSCs without contact layers in addition to using artificial contact layers. For instance, Mei et al. used a mixed cation perovskite made of methylammonium (MA) iodide and 5-ammoniumvaleric acid (5-AVA) in a device that didn’t have a hole selective layer. This made the device more stable than Spiro-OMETAD, which had a 12.8% efficiency after more than 1000 hours of sun exposure.
Metal contact
Top metal contacts are very important for solar cells (PSCs) to collect charges well. For high efficiency, metal electrodes need to be well aligned with the contact layers and the perovskite absorber layer in terms of energy. Aluminium is widely used because it is cheap and easy to find. However, water and air can oxidise it because they can move through the holes in the aluminium layer. Silver can be used as a back reflector in thin-film solar cells because it is both a good conductor and a good reflector. But it has been said that Ag breaks down when used in PSCs.
After being left out in the open air for three weeks, silver electrodes change into yellow plates (AgI) in a device that also has TiO2/MAPbI3/Spiro-OMETAD/Ag. Devices made of copper are more stable in air because the copper layer keeps its colour for up to one month when stored in normal air. Gold is mostly used for n-i-p structures in the most advanced PSCs. When the temperature is high, gold can move through the thin Spiro-OMETAD and into the perovskite film. This makes the solar performance worse in a way that can’t be fixed.
One of the main reasons why metal electrodes break down is that mobile iodine anions can move around. One way to deal with this issue is to lower the number of mobile ions using a buffer layer or a changed contact layer. Back et al. used metal oxide that had been changed by an amine to neutralise the mobile ionic component of perovskite. This made devices last up to 9000 hours on a shelf.
People have tried to fully replace the metal electrode with cheaper carbon-based electrodes, but they are still not as efficient as PSCs that use metal contacts.
Conclusion and outlooks
This chapter gives a review of how stable perovskite solar cells (PSCs) are over time, which is an important factor for making a lot of energy. Because the material is both complicated and solid, it needs to be tested in a certain way for PSCs. To find out how efficient a PSC is, you need to do current density-voltage scans and highest power point tracking. For studying long-term stability, it is important to keep track of MPP for hundreds or thousands of hours. To figure out how different things break down, stability tests must be done under a variety of stress situations. This part also gives a general overview of how stable perovskite material and other layers are in device stacks when they are stressed from the outside by things like water, air, and an electric field. It is thought that the best way to make PSCs steady is to look into new perovskite formulas, the right interlayers and encapsulating, and strong charge contact layers. Stable devices in one sun light for 1000 hours MPP tracking has been reported in N2 atmospheres or devices that are enclosed. Stability tests should be more accurate now that they use MPP tracking instead of shelf lifetime testing. This shows that the field is making progress, especially in areas like photostability and heat resistance. Soon, perovskite-based devices that are stable enough to use for 20 years may be made available for PV applications.
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