As the world races towards a sustainable energy future, solar energy has emerged as a promising solution to combat climate change and reduce our dependence on fossil fuels. Over the years, solar cell technology has made significant strides in improving efficiency and affordability. Among the latest breakthroughs, Perovskite Solar Cells (PSCs) have captured the attention of researchers and industries alike. These innovative photovoltaic devices hold the potential to revolutionize solar power generation by combining exceptional efficiency with improved durability, thus bridging the gap between these two crucial aspects.
Understanding Perovskite Solar Cells
Perovskite solar cells are a class of photovoltaic devices that utilize materials with a distinctive crystal structure known as perovskite. The most commonly used perovskite material is a hybrid organic-inorganic compound, often based on lead or tin halides. One of the main reasons behind the growing popularity of PSCs is their exceptional light-absorption properties. They can be engineered to efficiently convert a wide spectrum of light, including visible and near-infrared, into electricity.
Perovskite solar cells have emerged as a promising technology for generating cheap and efficient solar energy. Perovskites are compounds that have cubelike crystal lattices and are highly efficient light harvesters. With their high conversion efficiency, perovskite solar cells can power everything from small electronic devices to entire buildings and electrical grids. Moreover, the manufacturing methods used to produce these cells have improved significantly, making them even more accessible and cost-effective.
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Bridging the Gap: Efficiency
Efficiency is a crucial parameter in determining the viability of solar cell technology. The efficiency of a solar cell refers to the percentage of sunlight it can convert into usable electricity. Perovskite solar cells have shown impressive progress in this regard. Since their discovery in 2009, PSCs have achieved impressive power conversion efficiencies (PCE), climbing from initial single-digit values to over 25% in a relatively short period.
The remarkable efficiency of PSCs can be attributed to several factors. First and foremost, perovskite materials offer direct and efficient charge carrier transport, resulting in minimal energy loss during the conversion process. Additionally, their tunable bandgap allows researchers to optimize the material to match specific solar spectrum regions, enhancing overall energy conversion efficiency. Furthermore, advancements in the device architecture and interface engineering have also contributed significantly to improving efficiency.
Bridging the Gap: Durability
While efficiency has been a focal point in solar cell research, durability has been an equally critical aspect. Solar cells operate in harsh environmental conditions, including exposure to moisture, heat, and UV radiation. Traditional perovskite materials were known to be susceptible to degradation under such conditions, which hindered their practical application for long-term use, particularly in outdoor environments.
However, substantial progress has been made in enhancing the stability and durability of PSCs. Researchers have explored various strategies to address this issue, such as encapsulation techniques, improved material composition, and engineering better device interfaces. By carefully selecting suitable protective layers, researchers have managed to shield the sensitive perovskite layer from external factors, significantly prolonging the cell’s operational lifetime.
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. Improved cell durability is paramount for the development of commercial perovskite solar products.
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.
Recent developments in perovskite solar cell technology have focused on improving their durability, specifically by addressing their sensitivity to moisture and heat. 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.
New manufacturing methods have been developed that incorporate protective layers and encapsulation techniques to shield the cells from the elements. Moreover, researchers have also discovered new perovskite materials that are more stable and resistant to degradation. These materials have been shown to maintain their high conversion efficiency for longer periods, even when exposed to harsh environmental conditions.
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.”
Alternate Substrate Drives Drastic Efficiency Increase in Perovskites
University of Rochester researchers have found that the use of a metal substrate can boost the photoelectric conversion efficiency of perovskites. The Rochester researchers, led by professor Chunlei Guo, devised a method that, rather than glass, uses a substrate of either a layer of metal or alternating layers of metal and dielectric materials.
In a solar cell, photons from sunlight need to interact with and excite electrons, causing the electrons to leave their atomic cores and generating an electrical current, Guo said. Ideally, the solar cell would use materials that are weak to pull the excited electrons back to the atomic cores and stop the electrical current.
The researchers showed that such a recombination could be substantially prevented by combining a perovskite material with either a layer of metal or a metamaterial substrate consisting of alternating layers of silver, a noble metal, and aluminum oxide, a dielectric.
“A piece of metal can do just as much work as complex chemical engineering in a wet lab,” Guo said.
The result was a significant reduction of electron recombination. In effect, the metal layer served as a mirror that created reversed images of electron-hole pairs, weakening the ability of the electrons to recombine with the holes.
According to the researchers, suppressing the recombination processes in lead-halide perovskites without the use of chemical treatments is itself a challenge.
Using a simple detector, the researchers observed the resulting 250% increase in efficiency of light conversion.
Several challenges remain before perovskites become practical for their applications. Their tendency to degrade quickly is among the hurdles. Researchers are working to find new, more stable perovskite materials.
Researchers have made a technological breakthrough and constructed a perovskite solar cell with the dual benefits of being both highly efficient and highly stable.
The work was done in collaboration with scientists from the University of Toledo, the University of Colorado-Boulder, and the University of California-San Diego.
A unique architectural structure enabled the researchers to record a certified stabilized efficiency of 24% under 1-sun illumination, making it the highest reported of its kind. The highly efficient cell also retained 87% of its original efficiency after 2,400 hours of operation at 55 degrees Celsius.
The paper, “Surface Reaction for Efficient and Stable Inverted Perovskite Solar Cells,” appears in the journal Nature. The authors from NREL are Qi Jiang, Jinhui Tong, Ross Kerner, Sean Dunfield, Chuanxiao Xiao, Rebecca Scheidt, Darius Kuciauskas, Matthew Hautzinger, Robert Tirawat, Matthew Beard, Joseph Berry, Bryon Larson, and Kai Zhu.
The researchers used an inverted architecture, rather than the “normal” architecture that has to date yielded the highest efficiencies. The difference between the two types is defined by how the layers are deposited on the glass substrate. The inverted perovskite architecture is known for its high stability and integration into tandem solar cells. The NREL-led team also added a new molecule, 3-(Aminomethyl) pyridine (3-APy), to the surface of the perovskite. The molecule reacted to the formamidinium within the perovskite to create an electric field on the surface of the perovskite layer.
“That suddenly gave us a huge boost of not only efficiency but also stability,” Zhu said.
The scientists reported the 3-APy reactive surface engineering can improve the efficiency of an inverted cell from less than 23% to greater than 25%. They also noted the reactive surface engineering stands out as an effective approach to significantly enhance the performance of inverted cells “to new state-of-the-art levels of efficiency and operational reliability.”
In conclusion, the development of perovskite solar cells with improved durability is a promising advancement in renewable energy technology. The use of protective layers and encapsulation techniques, along with the discovery of more stable perovskite materials, has addressed one of the main challenges in their practical application. This brings us closer to a future where perovskite solar cells can be used as a reliable and sustainable source of energy for a wide range of applications.
Applications and Future Prospects
The unique combination of high efficiency and improved durability opens up a world of possibilities for perovskite solar cells. With their potential to be manufactured using low-cost, scalable processes, PSCs hold the promise of becoming a cost-effective alternative to conventional silicon-based solar cells.
PSCs can find applications not only in traditional solar panels but also in various niche areas. For instance, they can be integrated into flexible and lightweight solar modules for portable electronics and wearable devices. Building-integrated photovoltaics (BIPV) and solar windows are also exciting avenues where PSCs could revolutionize energy harvesting from infrastructures.
Perovskite solar cells represent a remarkable advancement in the field of photovoltaic technology, showcasing a unique balance between efficiency and durability. Their rapid progress in efficiency, combined with efforts to enhance stability and operational lifetime, has made PSCs a frontrunner in the quest for sustainable energy solutions. As research continues, and commercialization efforts gain momentum, perovskite solar cells have the potential to transform the global energy landscape, making clean and renewable solar power more accessible to all
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