Perovskites are compounds that have cubelike crystal lattices and are highly efficient light harvesters. Their potential has been known for years, but they present a conundrum: They’re good at converting sunlight into energy, but sunlight and moisture degrade them.
Perovskite solar cells have demonstrated competitive efficiencies with potential for higher performance, but their stability is quite limited compared with that of leading PV technologies: They don’t stand up well to moisture, oxygen, extended periods of light, or high heat. To increase stability, researchers are studying degradation in both the perovskite materials and the contact layers. Improved cell durability is paramount for the development of commercial perovskite solar products.
Despite significant progress in understanding the stability and degradation of perovskite solar cells, current operational lifetimes are not commercially viable. Mobile markets may tolerate a shorter operational life, but stability during storage (prior to use) is still a key performance criterion for this sector. For mainstream solar power generation, technologies that cannot operate for more than two decades are unlikely to be viable regardless of other benefits.
Early perovskite devices degraded rapidly. A few years ago, typical perovskite devices would degrade within minutes or hours to non-functional states. Now multiple groups have demonstrated lifetimes of several months of operation. For commercial, grid-level electricity production, The Solar Energy Technologies Office (SETO) is targeting an operational lifetime of at least 20 years, and preferably more than 30 years.
The perovskite PV R&D community is heavily focused on operational lifetime and is considering multiple approaches to understand and improve intrinsic and extrinsic stability and degradation. Efforts include improved surface passivation of absorber layers; alternative materials and formulations for absorber layers, charge transport layers, and electrodes; and advanced encapsulation materials and approaches that mitigate degradation sources during fabrication and operation.
One issue with assessing degradation in perovskites relates to developing consistent testing and validation methodologies. Research groups frequently report performance results based on varied test conditions, including variability in encapsulation approaches, atmospheric composition, illumination, electrical bias, and other parameters. While such varied test conditions can provide insights and valuable data, the lack of standardization makes it challenging to directly compare results and difficult to predict field performance from test results. This affects the entire perovskite research and development (R&D) community, independent of any specific research area, material set, or stability improvement approach.
Improving the stability of perovskite solar cells is one of the most pressing issues faced by the field right now. Like the causes of degradation, the approaches to increase stability fall into two broad categories. By making intrinsic improvements – for example by changing the perovskite stoichiometry – you can reduce innate vulnerabilities of the perovskite itself. Extrinsic improvements (such as encapsulation) can reduce exposure to degradation factors. However, there is still much work to be done in both these areas to make a reliable and stable perovskite solar cell.
One of the main causes of perovskite instability is the hydroscopic nature of the organic cations, especially methylammonium. Additionally, methylammonium lead iodide (MAPbI3) can show high temperature vulnerability, which makes it unsuitable for use in commercial solar cells.
Saliba et al. demonstrated a mixed-halide, quadruple-cation perovskite solar cell that achieved efficiencies of 19% on 0.5 cm2 area, and held 95% of its original performance at 85°C for 500 hours under illumination. Using mixed-halide and mixed-cation perovskites, solar cells that have good efficiencies and (relatively) good stabilities can be achieved.
Another stoichiometric change that can improve perovskite solar cell stability is replacing iodine with other halides (such as chlorine or bromine). Perovskite crystals are most stable in the pseudo-cubic state (e.g. halfway between cubic and tetragonal), and MAPbI3 creates perovskites in the tetragonal state. Bromine (Br) and Chlorine (Cl) are different sizes to iodine, and therefore create perovskite crystals with a different lattice structure. By varying the ratios of these halides, band structure and stability can be changed.
Many additives have been trialled to increase the stability of perovskite solar cells. Some additives, such as butylphosphonic acid 4-ammonium chloride (4-ABPACl), can form cross-links between adjacent perovskite grains, which reduces moisture vulnerabilities at grain boundaries in the perovskite layer.9 Other additives can provide scaffold structures or nucleation sites to aid in producing uniform films or reducing external penetration.
2D perovskites can be made by using larger A-cations (e.g. PEA+) acting as a spacer cation. In pure 2D perovskites, only spacer cations are used, leading to single sheets of separated perovskite crystals – in this case the number of layers (n) of perovskite material is . In a 3D perovskite structure, n-> ∞.
As is often the case with perovskite solar cells, there is a trade-off between efficiency and stability. 2D perovskites are more stable. However, they have a larger band gap compared to their 3D counterparts, so they have poorer optical properties. By mixing different stoichiometric quantities of MAI and spacer cations (like PEAI), the n can be tuned. This can be utilised to create 2D-3D hybrid perovskites with enhanced optical properties and stability.
An n=3 layered perovskite achieved an PCE of 4.7% in 2016 and showed no signs of decomposition after 46 days without encapsulation. Recently, encapsulated 2D/3D perovskites have sustained PCEs of 11.2% over 10,000 hours in controlled conditions.
A key element of improving perovskite solar cell stability is the full encapsulation of devices. This will – at least partially – protect them from external degradation catalysts, such as ambient moisture and UV light. A common method of encapsulation is to encompass the cell in a UV-curable epoxy resin, followed by a glass cover slip. Some studies have used a hydroscopic substance to absorb moisture before it can reach the perovskite layer. These all significantly improve the solar cell stability.
However, when considering the scalability of perovskites, it is important that devices can be compatible with roll-to-roll processing. There has been promising work looking at polymer encapsulation methods. When encapsulated using polyethylene terephthalate (fully sealed around the device), 10,000-hour lifetimes have been achieved. This polymer layer is clearly effective in preventing moisture and oxygen penetration.
Additionally, luminescent photopolymers can be used in polymer encapsulations to reduce UV degradation. These photopolymers downshift the UV light – which may be absorbed by the perovskite to increase efficiency, and also protects the perovskite. It has been shown that by encasing a perovskite solar cell with a fluoropolymer coating, cells can maintain 3-month lifetimes in outdoor conditions at high efficiencies
Rice lab finds 2D perovskite compound for stable and efficient solar cells
Rice University engineers have achieved a new benchmark in the design of atomically thin solar cells made of semiconducting perovskites, boosting their efficiency while retaining their ability to stand up to the environment.
The lab of Aditya Mohite of Rice’s George R. Brown School of Engineering discovered that sunlight itself contracts the space between atomic layers in 2D perovskites enough to improve the material’s photovoltaic efficiency by up to 18%, an astounding leap in a field where progress is often measured in fractions of a percent.
“A solar cell technology is expected to work for 20 to 25 years,” said Mohite, an associate professor of chemical and biomolecular engineering and of materials science and nanoengineering. “We’ve been working for many years and continue to work with bulk perovskites that are very efficient but not as stable. In contrast, 2D perovskites have tremendous stability but are not efficient enough to put on a roof.
“The big issue has been to make them efficient without compromising the stability,” he said. The Rice engineers and their collaborators at Purdue and Northwestern universities, U.S. Department of Energy national laboratories Los Alamos, Argonne and Brookhaven and the Institute of Electronics and Digital Technologies (INSA) in Rennes, France, discovered that in certain 2D perovskites, sunlight effectively shrinks the space between the atoms, improving their ability to carry a current.
“We find that as you light the material, you kind of squeeze it like a sponge and bring the layers together to enhance the charge transport in that direction,” Mohite said. The researchers found placing a layer of organic cations between the iodide on top and lead on the bottom enhanced interactions between the layers.
“This work has significant implications for studying excited states and quasiparticles in which a positive charge lies on one layer and the negative charge lies on the other and they can talk to each other,” Mohite said. “These are called excitons, which may have unique properties.
“This effect has given us the opportunity to understand and tailor these fundamental light-matter interactions without creating complex heterostructures like stacked 2D transition metal dichalcogenides,” he said.
Experiments were confirmed by computer models by colleagues in France. “This study offered a unique opportunity to combine state of the art ab initio simulation techniques, material investigations using large scale national synchrotron facilities and in-situ characterizations of solar cells under operation,” said Jacky Even, a professor of physics at INSA. “The paper depicts for the first time how a percolation phenomenon suddenly releases the charge current flow in a perovskite material.”
“In 10 years, the efficiencies of perovskites have skyrocketed from about 3% to over 25%,” Mohite said. “Other semiconductors have taken about 60 years to get there. That’s why we’re so excited.”
Both results showed that after 10 minutes under a solar simulator at one-sun intensity, the 2D perovskites contracted by 0.4% along their length and about 1% top to bottom. They demonstrated the effect can be seen in 1 minute under five-sun intensity.
“It doesn’t sound like a lot, but this 1% contraction in the lattice spacing induces a large enhancement of electron flow,” said Rice graduate student and co-lead author Wenbin Li. “Our research shows a threefold increase in the electron conduction of the material.”
At the same time, the nature of the lattice made the material less prone to degrading, even when heated to 80 degrees(176 degrees ). The researchers also found the lattice quickly relaxed back to its normal configuration once the light was turned off.
“One of the major attractions of 2D perovskites was they usually have organic atoms that act as barriers to humidity, are thermally stable and solve ion migration problems,” said graduate student and co-lead author Siraj Sidhik. “3D perovskites are prone to heat and light instability, so researchers started putting 2D layers on top of bulk perovskites to see if they could get the best of both. “We thought, let’s just move to 2D only and make it efficient,” he said.
“We’re on a path to get greater than 20% efficiency by engineering the cations and interfaces,” Sidhik said. “It would change everything in the field of perovskites, because then people would begin to use 2D perovskites for 2D perovskite/silicon and 2D/3D perovskite tandems, which could enable efficiencies approaching 30%. That would make it compelling for commercialization.”
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