Princeton Engineering researchers have developed the first perovskite solar cell with a commercially viable lifespan, a major milestone for the emerging class of renewable energy technology. The team estimates that their equipment can operate above industry standards for approximately 30 years, which is much more than the 20 years used as a viability threshold for solar cells.
The device is not only highly durable, but also meets common efficiency standards. It is the first of its kind to compete with the performance of silicon-based cells, which have dominated the market since its launch in 1954.
Perovskites are semiconductors with a special crystal structure, which makes them well suited for solar cell technology. They can be produced at room temperature, consuming much less energy than silicon, making them cheaper and more sustainable to produce. And while silicon is rigid and opaque, perovskites can be flexible and transparent, extending solar energy far beyond the iconic panels that inhabit slopes and roofs across America.
But unlike silicon, perovskites are notoriously fragile. Early perovskite solar cells (PSC), created between 2009 and 2012, lasted only minutes. The life expectancy of the new facility is a fivefold increase from the previous record set by the lower efficiency of the PSC in 2017. (This facility operated under continuous lighting at room temperature for one year. The new facility would operate for five years under similar laboratory conditions.)
The Princeton team, led by Lynn Loo, Theodora D. ’78, and Engineering Professor William H. Walton III ’74, unveiled their new equipment and testing method in an article published on 16. Science.
Loo said the record design highlighted the enduring potential of PSC, especially as a way to push solar cell technology beyond silicon. But she also pointed to the title of the new technique of accelerating the aging of her team as a deeper meaning of the work.
“We could have the record today,” she said, “but tomorrow someone else will come with a better record. It’s really exciting that we now have a way to test these devices and know how they will work in the long run. ”
Due to the known fragility of perovskites, long-term testing has not been a major problem until now. But as devices improve and last longer, testing one design against another becomes essential in implementing robust, consumer-friendly technologies.
“This document is likely to be a prototype for anyone who wants to analyze performance at the intersection of efficiency and stability,” said Joseph Berry, head of the National Renewable Energy Laboratory, which specializes in solar cell physics and was not involved in the study. “By creating a prototype to study stability and demonstrating what can be extrapolated [through accelerated testing], does the work everyone wants to see before we start testing on a large scale. It allows you to project in a way that is truly impressive. ”
While efficiency has accelerated at a remarkable rate over the past decade, Berry said, the stability of these devices has improved more slowly. Testing will need to be more sophisticated in order to spread and implement industry. This is where Loo’s aging process begins.
“These kinds of tests will become more and more important,” Loo said. “You can make the most efficient solar cells, but it doesn’t matter if they are not stable.”
How they got here
In early 2020, Loo’s team worked on a variety of plant architectures that would maintain relatively high efficiency – convert enough sunlight into electricity to be valuable – and survive the onslaught of heat, light and humidity that bombard a solar cell over its lifetime.
Xiaoming Zhao, a postdoctoral researcher at Loo’s Laboratory at the Andlinger Center for Energy and the Environment, has worked on a number of proposals with colleagues. Efforts have layered various materials to optimize light absorption while protecting the most fragile areas from exposure. They developed an ultra-thin cover layer between two key components: a perovskite absorbent layer and a charge-bearing layer made of copper salt and other substances. The goal was to prevent the perovskite semiconductor from burning out within a few weeks or months, which was the norm at the time.
It is difficult to understand how thin this cover layer is. Scientists describe it as 2D, which means two dimensions, as something that has no thickness at all. In fact, it is only a few atoms thick – more than a million times smaller than the smallest thing the human eye sees. Although the idea of a 2D cover layer is not new, it is still considered a promising, emerging technique. NREL researchers have shown that 2D layers can significantly improve long-distance performance, but no one has developed a device that would move Perovskites almost to the commercial limit of 20 years.
Zhao and his colleagues went through a number of permutations of these designs, shifting small details in the geometry, changing the number of layers, and testing dozens of material combinations. Each design went to a lighting box where it could irradiate sensitive devices with relentless bright light and measure their power drop over time.
In the fall of that year, as the first wave of the pandemic subsided and researchers returned to their labs to conduct their experiments in carefully coordinated shifts, Zhao noticed something strange in the data. One set of devices still seemed to work close to its maximum efficiency.
“After almost half a year, there was basically zero decline,” he said.
That’s when he realized he needed a way to stress test his device faster than his real-time experiment allowed.
“The lifespan we want is about 30 years, but testing your equipment can’t take you 30 years,” Zhao said. “So we need some way to predict this longevity within a reasonable time frame.” That is why this accelerated aging is very important. “
The new test method speeds up the aging process by illuminating and heating the device. This process accelerates what would naturally happen during the years of regular exposure. The researchers selected four aging temperatures and measured the results across these four different data streams, from the base temperature of a typical summer day to the extreme Fahrenheit, higher than the boiling point of water.
They then extrapolated from the combined data and predicted the device’s performance at room temperature for tens of thousands of hours of continuous lighting. The results showed that the device would operate above 80 percent of its peak efficiency in continuous lighting for at least five years at an average temperature of 95 degrees Fahrenheit. Using standard conversion metrics, Loo said it is the laboratory equivalent of 30 years of outdoor operation in an area such as Princeton, NJ.
Berry of NREL agreed. “It’s very believable,” he said. “Some people will want to see how it’s played.” But this is a much more credible science than many other attempts at prediction. “
Michael Jordan from solar cells
Perovskite solar cells were promoted in 2006 and the first published devices followed in 2009. Some of the first devices lasted only a few seconds. Other minutes. In 2010, the life of the equipment increased to days and weeks and finally to months. In 2017, a group from Switzerland published a groundbreaking documentary about the PSC, which lasted one full year of continuous lighting.
Meanwhile, the efficiency of these devices has skyrocketed over the same period. While the first PSC showed an energy conversion efficiency of less than 4 percent, researchers have increased this metric almost tenfold in so many years. It was the fastest improvement scientists have ever seen in any class of renewable energy technology.
So why the pressure on the Perovskites? Berry said a combination of recent advances made them uniquely desirable: the newly high efficiency, exceptional “tunability” that allows scientists to perform highly specific applications, the ability to produce them locally with low energy inputs, and the now reliable prediction of extended life with a sophisticated aging process for testing wide range of designs.
Loo said it is not the case that the PSC will replace silicon plants so much that the new technology will complement the old ones, making solar panels even cheaper, more efficient and more durable than they are now, and expanding solar energy into unspeakable new areas of modern life. For example, her group recently demonstrated a completely transparent perovskite film (with a different chemistry) that can turn windows into energy-producing devices without changing their appearance. Other groups have found ways to print photovoltaic inks using perovskites, enabling the formfactory that scientists are still dreaming of.
But the main advantage in the long run, according to both Berry and Loo: Perovskites can be produced at room temperature, while silicon is forged at around 3000 degrees Fahrenheit. That energy must come from somewhere, and that means burning a lot of fossil fuels at the moment.
Berry added: Because scientists can easily and widely tune the properties of perovskite, they allow disparate platforms to work together seamlessly. This could be key for emerging wedding silicon platforms, such as thin-film and organic photovoltaics, which have also made great strides in recent years.
“It’s like Michael Jordan on a basketball court,” he said. “Great in itself, but it also makes all the other players better.”
The article “Accelerated Aging of Completely Inorganic, Interface-Stabilized Perovskite Solar Cells” was published with the support of the National Science Foundation; US Department of Energy through Brookhaven National Laboratory; Swedish Government Area of Strategic Materials Research in Functional Materials; and the Princeton Imaging and Analysis Center. In addition to Loo and Zhao, contributing authors include 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
From Scott Lyon. Article courtesy of Princeton University School of Engineering and Applied Science.
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