The steep drop in the cost of solar power since the early 2000’s is set to continue as the solar industry matures, including the introduction of perovskite material. Hurdles remain to be crossed before perovskite technology hits the mainstream market with full force, though. In particular, the mystery of perovskite solar cell stability is still a mystery, but a multinational team based at the Georgia Institute of Technology is among those hot on the trail of the answer.
The Rise Of The Perovskite Solar Cell
The crystalline mineral perovskite is a 19th-century discovery, leading to the creation of lab-grown variations with a similar structure for use in the electronics industry in the 20th century.
The application of perovskites to photovoltaics is a 21st century phenomenon. A team of researchers in Japan won the credit for publishing the first perovskite solar cell study in 2009 under the title, “Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells.”
Ever since then, perovskite solar cell technology has attracted researchers around the world, lured by the promise of creating high-efficiency solar cells that are more widely available, at less cost, than their silicon counterparts.
If you’re wondering why perovskite solar cells are not yet widely available 14 years after that first paper was published, that’s a good question. Silicon is a more expensive energy-capturing material, but its durability has made it the go-to photovoltaic material of choice. Perovskites are relatively inexpensive, but they are unstable. Their crystalline structure shifts when exposed to water and oxygen.
That’s a big problem, considering that solar panels are continually exposed to water and oxygen in the form of air.
Since science loves a challenge, a number of solutions have already emerged. Among the strategies to surface on the CleanTechnica radar is a perovskite solar cell sandwiched into a silicon cell (here’s another example). More stable perovskite formulations have also emerged, including an adhesive solution developed by the Canadian startup XlynX Materials.
Perovksite Solar Cell Mystery Solved…
Still, knowing precisely why perovskites destabilize when exposed to water and oxygen would provide researchers with a more precise understanding of how to build even more stability into a solar cell, and the Georgia Tech team has come up with a plan.
In a newly published paper titled, “Synergistic Role of Water and Oxygen Leads to Degradation in Formamidinium-Based Halide Perovskites,” the team presents a detailed description of the chemical interactions that cause perovskites to shift states, and they also laid out the solution.
The key was investigating the interplay of water and oxygen, rather than separately investigating the impact of water and oxygen.
“We find that the degradation rate is considerably slower when the perovskite is exposed to H2O/N2 when compared to H2O/air,” the team wrote. “Our results show that a critical synergy between H2O and O2 in air is needed to accelerate the undesired phase transformations in perovskites.”
“Before this paper, people thought if you expose them to just water, these materials degrade. If you expose them to just oxygen, these materials degrade,” explains Assistant Professor Juan-Pablo Correa-Baena, the senior author of the paper. “If you prevent one or the other from interacting with the perovskites, you mostly prevent the degradation.”
“The researchers found the complex interplay of both water and oxygen with the perovskites leads to instability; taking away one of those preserved the perovskites’ energy-capturing crystal structure,” explains the Georgia Tech College of Engineering.
…Now Comes The Hard Part
As for preventing either water or oxygen from encountering a perovskite solar cell, that’s a tough nut to crack considering that both are found in ambient air.
The sandwich strategy is one solution, though balancing the cost-cutting opportunity of perovskite with the durability of silicon is a tricky business.
The Georgia Tech team tried another approach. They applied a thin film of the common industrial chemical PEIA (phenethylammonium iodide) to their perovskite solar cell.
PEIA repels water, and that did the trick — partly. The researchers found that heat from the solar cell excites the PEIA molecules, causing them to lose their ability to repel water.
The next step is to create a heat-resistant version of PEIA. For that, Correa-Baena — who is head of the Materials for Solar Energy Harvesting and Conversion research progam at the Georgia Tech Institute for Materials and Strategic Energy Institute — is recruiting a top materials scientist onto the team.
Pushing The Cost Of Solar Cells Down, Down, Down
Meanwhile, other research teams have also been investingating the use of PEIA to stabilize perovskite solar cells.
In 2020, for example, the Royal Chemistry Society published a study titled, “Evaluating the role of phenethylamine iodide as a novel anti-solvent for enhancing performance of inverted planar perovskite solar cells.”
In 2021, a paper titled “Phenyl Ethylammonium Iodide introduction into inverted triple cation perovskite solar cells for improved VOC and stability” appeared in the journal Organic Electronics.
Last year a research team based in Taiwan also published an investigation of PEIA under the title, “Efficient Perovskite Solar Cells via Phenethylamine Iodide Cation-Modified Hole Transport Layer/Perovskite Interface.”
Perovskite solar cell researchers are also working on ways to cut costs. Over at the US Department of Energy, the National Renewable Energy Laboratory notes that a thin layer of gold or silver is known to improve perovskite solar cell efficiency, but it also raises the price.
Working with researchers from Northern Illinois University, the NREL team came up with an alternative described as a ” nickel-doped graphite layer coupled with a bismuth-indium alloy layer.”
“The two layers can be easily integrated into the perovskite device through painting them on, offering a low-cost fabrication method,” NREL notes.
If you caught that thing about painting, that’s another part of the low-cost perovskite solar cell solution. In contrast to the precision fabrication processes required for silicon solar cells, perovskites are formulated in a solution that can be painted, sprayed, deposited or printed onto a surface.
That kind of flexibility provides for high-volume, low-cost production, helping to make solar power more accessible.
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Image (cropped): “Georgia Tech engineers showed in a new study that both water and oxygen are required for perovskites to degrade. The team stopped the transformation with a thin layer of another molecule that repelled water” (courtesy of Juan-Pablo Correa-Baena).
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