Researchers at Princeton Engineering have developed the first perovskite solar cell with a commercially viable lifespan, marking a major milestone for a new class of renewable energy technology. The team predicts that their device can operate above industry standards for about 30 years, far more than 20 years used as a threshold for the sustainability of solar cells.
Xiaoming Zhao, a postdoctoral researcher in the Department of Chemical and Biological Engineering, examines perovskites in Loo Laboratory on June 7, 2022. Watching highly stable perovskite solar cells under magnification during the accelerated aging process helps researchers predict extended lifespans of advanced designs. Photo by Bumper DeJesus from Princeton University.
The device is not only very durable, but also meets the usual standards of efficiency. It is the first of its kind to compete with the performance of silicone-based cells, which have dominated the market since their introduction in 1954.
Perovskites are semiconductors with a special crystal structure that makes them very suitable for solar cell technology. They can be produced at room temperature, using much less energy than silicon, which makes them cheaper and more sustainable to produce. And while silicon is solid and opaque, perovskites can be made flexible and transparent, extending solar energy far beyond the cult panels that inhabit slopes and roofs across America.
But unlike silicon, perovskites are notoriously fragile. The early Perovskite solar cells (PSCs), formed between 2009 and 2012, lasted only a few minutes. The projected lifespan of the new device is a fivefold increase over the previous record set by a lower efficiency PSC in 2017. (This device operated under constant lighting at room temperature for a year. The new device would operate for five years under similar laboratory conditions.)
Princeton’s team, led by Lynn Loo, Theodora D. ’78 and William H. Walton III ’74 professor of engineering, unveiled their new device and their new testing method in a paper published June 16 in Science.
Loo said the record-breaking design highlighted the enduring potential of the PSC, especially as a way to push solar cell technology beyond the boundaries of silicon. But, in addition to the title result, she also pointed out the new technique of accelerated aging of her team as a deeper significance of the work.
“We may have a record today,” she said, “but someone else will come along with a better record tomorrow. What is really exciting is that we now have a way to test these devices and know how they will behave in the long run. ”
Due to the well-known weakness of perovskites, long-term testing has not been a major concern so far. But as devices get better and last longer, testing one design against another will become key to introducing durable consumer-friendly technologies.
“This paper is likely to be a prototype for anyone looking to analyze performance at the intersection of efficiency and stability,” said Joseph Berry, a senior fellow at the National Renewable Energy Laboratory who specializes in solar cell physics and was not involved in the study. “By making a prototype to study stability and showing what can be extrapolated [through accelerated testing], is doing a job that everyone wants to see before we start field testing on a large scale. It allows you to design in a way that is really impressive. ”
Although efficiency has accelerated at an extraordinary rate over the past decade, Berry said, the stability of these devices has been improving more slowly. In order to become widespread and introduced in the industry, testing will have to become more sophisticated. This is where Loo’s accelerated aging process comes into play.
“These types of tests will be more and more important,” Loo said. “You can make the most efficient solar cells, but it won’t matter if they’re not stable.”
How they got here
In early 2020, Loo’s team worked on a variety of device architectures that would maintain relatively high efficiencies — converting enough sunlight into electricity to be valuable — and survive the heat, light, and moisture that bombard the solar cell over its lifetime.
A series of perovskite solar cell designs sit under strong light at high temperatures during the accelerated aging process and testing developed by Princeton Engineering researchers. The new approach to testing marks a major step towards the commercialization of advanced solar cells. Photo by Bumper DeJesus from Princeton University
Xiaoming Zhao, a postdoctoral researcher at Loo’s lab at the Andlinger Center for Energy and the Environment, has worked on a number of designs with colleagues. Efforts have piled up different materials to optimize light absorption while protecting the most sensitive areas from exposure. They have developed an ultra-thin protective layer between two key components: an absorbent layer of perovskite and a charge-carrying layer made of copper salt and other substances. The goal was to prevent the perovskite semiconductor from burning out in a few weeks or months, which was the norm at the time.
It is difficult to understand how thin this protective layer is. Scientists use the term 2D to describe it, meaning two dimensions, as in something that has no thickness at all. In reality, the thickness is only a few atoms – more than a million times smaller than the smallest thing the human eye can see. Although the idea of a 2D coating layer is not new, it is still considered a promising emerging technique. Scientists at NREL have shown that 2D layers can greatly improve long-distance performance, but no one has developed a device that would push Perovskites even close to the commercial threshold of 20 years.
Zhao and his colleagues have gone through a multitude of permutations of these designs, changing tiny details in geometry, changing the number of layers, and trying out dozens of material combinations. Each design went into a lighting box, where they could irradiate sensitive devices with relentlessly strong light and measure their decline in performance over time.
In the fall of that year, when the first wave of the pandemic subsided and researchers returned to their laboratories to devote themselves to their experiments in carefully coordinated shifts, Zhao noticed something strange in the data. It seemed that one set of devices was still working close to its top efficiency.
“There was basically zero decline after almost half a year,” he said.
It was then that he realized that he needed a way to test his device for stress faster than his real-time experiment allowed.
“The lifespan we want is about 30 years, but you can’t take 30 years to test your device,” Zhao said. “So we need some way to predict this lifespan within a reasonable time frame. That’s why this accelerated aging is so important. “
The new test method speeds up the aging process by illuminating the device as it explodes with heat. This process accelerates what would naturally happen during years of regular exposure. The researchers selected four aging temperatures and measured the results in these four different data streams, from the baseline temperature of a typical summer day to extreme Fahrenheit, higher than the boiling point of water.
They then extrapolated from the combined data and predicted the performance of the device at room temperature during tens of thousands of hours of continuous lighting. The results showed a device that would operate over 80 percent of its peak efficiency under continuous lighting for at least five years at an average temperature of 95 degrees Fahrenheit. Using standard conversion metrics, Loo said it was the laboratory equivalent of 30 years of outdoor work in an area such as Princeton, NJ.
Berry of NREL agreed. “It’s very credible,” he said. “Some people will still want to see it happen. But this is a much more credible science than many other attempts at prediction. ”
Michael Jordan Solar Cells
Researchers have built their device based on perovskite from layers that perform various tasks, including an innovative ultra-thin layer that protects the most sensitive elements. They then illuminated the device under strong light and ignited it with extreme heat to understand how it would behave during tens of thousands of hours of exposure. The result was a record-setting device and a revolutionary method of aging and testing. Animation Bumper DeJesus, via Princeton University.
Perovski solar cells were a pioneer in 2006, and the first published devices followed in 2009. Some of the earliest devices lasted only a few seconds. Other minutes. In the 2010s, the lifespan of the device increased to days and weeks and eventually months. Then, in 2017, a group from Switzerland published a revolutionary paper on the PSC that lasted a full year of continuous lighting.
Led by postdoctoral researcher Xiaoming Zhao (center), engineers in Professor Lynn Loo’s lab tested dozens of material permutations and design combinations to try to improve the life of their devices. Rudolph Holley, III (left), a graduate student, and Quinn Burlingame (right), a postdoctoral researcher, contributed. Photo by Bumper DeJesus from Princeton University.
Meanwhile, the efficiency of these devices has risen sharply over the same period. While the first PSC showed a power conversion efficiency of less than 4 percent, researchers increased that metric nearly tenfold in the same number of years. It was the fastest improvement scientists have seen in any class of renewable energy technology to date.
Then why push for perovskites? Berry said the combination of recent advances makes them uniquely desirable: new high efficiency, outstanding “tuning” that allows scientists to make very specific applications, the ability to produce them locally with low energy inputs, and now a credible forecast of extended life. with a sophisticated aging process to test a wide range of designs.
Loo said PSC will not replace silicone devices so much that new technology will complement old ones, making solar panels even cheaper, more efficient and durable than they are now, and expanding solar energy into indescribably new areas of modern life. For example, her group recently demonstrated a completely transparent perovskite film (which has different chemistry) that can turn windows into energy-producing devices without changing their appearance. Other groups have found ways to print photovoltaic inks using perovskite, enabling form factors that scientists are only now dreaming of.
But the main advantage in the long run, according to Berry and Lowe: perovskites can be produced at room temperature, while silicon is forged at about 3000 degrees Fahrenheit. That energy has to come from somewhere, and that currently means burning a lot of fossil fuels.
Berry added this: Because scientists can easily and widely adjust the properties of perovskites, they allow different platforms to work together seamlessly. This could be crucial for wedding silicon with new platforms such as thin-film and organic photovoltaic devices, which have also made great strides in recent years.
“It’s like Michael Jordan on the basketball court,” he said. “Great in itself, but it makes all the other players better.”
The paper “Accelerated aging of completely inorganic, perovskite solar cells stabilized at the interface” was published with the support of the National Science Foundation; U.S. Department of Energy, through Brookhaven National Laboratory; Swedish Government Strategic Research Area in Materials Science on Functional Materials; and the Princeton Imaging and Analysis Center. In addition to Loo and Zhao, the contributors are Tianjun Liu and Feng Gao, both from Linköping University; and Tianran Liu, Quinn C. Burlingame, Rudolph Holley III, Guangming Cheng and Nan Yao, all from Princeton University
Scott Lyon. Courtesy of Princeton University School of Engineering and Applied Sciences.
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