An emerging class of solar energy technology, made with perovskite semiconductors, has passed the long-sought milestone of a 30-year lifetime. The Princeton Engineering researchers who designed the new device also revealed a new method for testing long-term performance, a key hurdle on the road to commercialization. Credit: Photos by Bumper DeJesus
30-year perovskite solar cells and the new technique for testing them for the long haul.
Princeton Engineering scientists have developed the first perovskite solar cell with a commercially viable lifetime, marking a major milestone for an emerging class of renewable energy technology. The research team projects their device can perform above industry standards for around 30 years, far more than the 20 years used as a threshold for viability for solar cells.
The device is not only highly durable, but it also meets common efficiency standards. In fact, it is the first of its kind to rival the performance of silicon-based cells, which have dominated the market since their introduction in 1954.
Perovskites are semiconductors with a special crystal structure that makes them well suited for solar cell technology. They can be manufactured at room temperature, using much less energy than silicon, making them cheaper and more sustainable to produce. And whereas silicon is stiff and opaque, perovskites can be made flexible and transparent, extending solar power well beyond the iconic rectangular panels that populate hillsides and rooftops across America.
An array of perovskite solar cell designs sit under bright light at high temperatures during an ac
celerated aging and testing process developed by Princeton Engineering researchers. The new testing approach marks a major step toward the commercialization of advanced solar cells. Credit: Photo by Bumper DeJesus
But unlike silicon, perovskites are notoriously fragile. Early perovskite solar cells (PSC), created between 2009 and 2012, lasted only minutes. The projected lifetime of the new device represents a five-fold increase over the previous record, set by a lower efficiency PSC in 2017. (That device operated under continuous illumination at room temperature for one year. The new device would operate for five years under similar lab conditions.)
The Princeton team, led by Lynn Loo, the Theodora D. ’78 and William H. Walton III ’74 Professor in Engineering, revealed their new device and their new method for testing such devices in a paper published on June 16, 2022, in the journal Science.
Loo said the record-setting design has highlighted the durable potential of PSCs, especially as a way to push solar cell technology beyond the limits of silicon. But she also pointed past the headline result to her team’s new accelerated aging technique as the work’s deeper significance.
Looking at a highly stable perovskite solar cell under magnification during an accelerated aging process that helps researchers forecast the extended lifetimes of advanced designs. Credit: Photo by Bumper DeJesus
This paper is likely going 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 the physics of solar cells and who was not involved in this study. “By producing a prototype to study stability, and showing what can be extrapolated [through accelerated testing], it’s doing the work everyone wants to see before we start field testing at scale. It allows you to project in a way that’s really impressive.”
While efficiency has accelerated at a remarkable pace over the past decade, Berry said, the stability of these devices has improved more slowly. For them to become widespread and rolled out by industry, testing will need to become more sophisticated. That’s where Loo’s accelerated aging process comes in.
“These kinds of tests are going to be increasingly important,” Loo said. “You can make the most efficient solar cells, but it won’t matter if they aren’t stable.”
How they got here
Early in 2020, Loo’s team was working on various device architectures that would maintain relatively strong efficiency — converting enough sunlight to electric power to make them valuable — and survive the onslaught of heat, light, and humidity that bombard a solar cell during its lifetime.
Xiaoming Zhao, a postdoctoral researcher in Loo’s lab, had been working on a number of designs with colleagues. The efforts layered different materials in order to optimize light absorption while protecting the most fragile areas from exposure. They developed an ultra-thin capping layer between two crucial components: the absorbing perovskite layer and a charge-carrying layer made from cupric salt and other substances. The goal was to keep the perovskite semiconductor from burning out in a matter of weeks or months, the norm at that time.
It’s hard to comprehend how thin this capping layer is. Scientists use the term 2D to describe it, meaning two dimensions, as in something that has no thickness at all. In reality, it’s merely a few atoms thick — more than a million times smaller than the smallest thing a human eye can see. While the idea of a 2D capping layer isn’t new, it is still considered a promising, emerging technique. Scientists at NREL have shown that 2D layers can greatly improve long-haul performance, but no one had developed a device that pushed perovskites anywhere close to the commercial threshold of a 20-year lifetime.
Zhao and his colleagues went through scores of permutations of these designs, shifting minute details in the geometry, varying the number of layers, and trying out dozens of material combinations. Each design went into the light box, where they could irradiate the sensitive devices in relentless bright light and measure their drop in performance over time.
In the fall of that year, as the first wave of the pandemic subsided and researchers to returned to their labs to tend to their experiments in carefully coordinated shifts, Zhao noticed something odd in the data. One set of the devices still seemed to be operating near its peak efficiency.
“There was basically zero drop after nearly half a year,” he said.
That’s when he realized he needed a way to stress test his device faster than his real-time experiment allowed.
“The lifetime 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 lifetime within a reasonable timeframe. That’s why this accelerated aging is very important.”
