Published on October 25th, 2016 | By: Stephanie Liverani0
It takes two: New tandem perovskite solar cell could outrank silicon’s efficiency and stabilityPublished on October 25th, 2016 | By: Stephanie Liverani
[Image above] Close up of a perovskite solar cell. Credit: Stanford Precourt Institute for Energy; YouTube
The sun provides one of most powerful, readily available natural sources of energy. But that power is variable—when the sun sets each night, we lose that power if it’s not efficiently stored.
Enter: perovskite solar cells. They’ve been making news for years for their potential for high efficiency and low manufacturing cost. The challenge, however, lies in perovskites’ well-documented instability.
At ACerS Materials Challenges in Alternative and Renewable Energy (MCARE) conference in Clearwater, Fla., in April, Md. K. Nazeeruddin (École Polytechnique Fédérale de Lausanne, Switzerland) presented his research about perovskite solar cells and the challenges associated with developing them for efficient solar energy storage.
“Stability is the biggest problem with perovskites and we have to come up with solutions,” Nazeeruddin said.
Current perovskites have proven photophysical properties and power conversion efficiencies in excess of 20%, but they don’t stand up well to light, heat, and humidity. So researchers have accepted the challenge to develop something that trumps the status quo.
Last week, researchers from Stanford University (Stanford, Calif.) and Oxford University (Oxford, England) joined forces to create a new perovskite design they say “could outperform existing commercial technologies,” according to a Stanford News article.
And this new design uses inexpensive, readily available materials that could give silicon solar cells a run for their money.
The team used tin and other abundant elements to create a new photovoltaic crystalline material that’s thinner, more flexible, and easier to scale up than silicon, according to the article.
“Perovskite semiconductors have shown great promise for making high-efficiency solar cells at low cost,” Michael McGehee, a professor of materials science and engineering at Stanford and co-author of the study, tells Stanford News. “We have designed a robust, all-perovskite device that converts sunlight into electricity with an efficiency of 20.3%, a rate comparable to silicon solar cells on the market today.”
Credit: Stanford Precourt Institute for Energy; YouTube
The novel design leverages two perovskite solar cells stacked on top of each other. The researchers printed the cells on a glass substrate, but the technology could easily swap glass for plastic, McGhee adds in the Stanford News article.
“The all-perovskite tandem cells we have demonstrated clearly outline a roadmap for thin-film solar cells to deliver over 30% efficiency,” Henry Snaith, a professor of physics at Oxford and co-author of the study, explains. “This is just the beginning.”
Standard perovskite solar cells generate electric current by harvesting high-energy photons from the visible part of the solar spectrum that cause electrons to effectively jump across an energy gap, Stanford News explains.
“The cell with the larger energy gap would absorb higher-energy photons and generate an additional voltage,” co-lead author Giles Eperon and an Oxford postdoctoral scholar currently at the University of Washington, explains. “The cell with the smaller energy gap can harvest photons that aren’t collected by the first cell and still produce a voltage.”
With this concept in mind, the team used a unique combination of tin, lead, cesium, iodine, and organic materials to develop an efficient cell with a small energy gap first.
“We developed a novel perovskite that absorbs lower-energy infrared light and delivers a 14.8% conversion efficiency,” Eperon adds. “We then combined it with a perovskite cell composed of similar materials but with a larger energy gap.”
When the team combined the two perovskite cells, it created a novel tandem device capable of producing a combined efficiency of 20.3%.
“There are thousands of possible compounds for perovskites,” explains co-lead author Tomas Leijtens, a postdoctoral scholar at Stanford. “But this one works very well, quite a bit better than anything before it.”
But what about the stability concern?
The researchers exposed the experimental tandem perovskite solar cells to temperatures of 212°F (100°C) for four days.
“Crucially, we found that our cells exhibit excellent thermal and atmospheric stability, unprecedented for tin-based perovskites,” the authors explain in the article.
“The efficiency of our tandem device is already far in excess of the best tandem solar cells made with other low-cost semiconductors, such as organic small molecules and microcrystalline silicon,” McGehee tells Stanford News. “Those who see the potential realize that these results are amazing.”
Optimization is on deck next for the team—they’re currently working to fine-tune the tandem perovskite solar cell technology so it can absorb more light and generate an even higher current.
“The versatility of perovskites, the low cost of materials and manufacturing, now coupled with the potential to achieve very high efficiencies, will be transformative to the photovoltaic industry once manufacturability and acceptable stability are also proven,” McGehee says.
The study, published in Science, is “Perovskite-perovskite tandem photovoltaics with optimized bandgaps” (DOI: 10.1126/science.aaf9717).
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