One drawback of most solar cells made with efficiency-boosting perovskite is the presence of hazardous lead in the materials that turn sunlight into electricity. Next-generation solar cells made with the most promising, lead-free replacement — non-toxic tin perovskite — have had their own major weakness: they don't hold up against the elements.
Now, researchers at the University of Wisconsin–Madison, the National Laboratory of the Rockies and other collaborating institutions have designed a new tin-based perovskite with built-in protection against air and moisture. It's a coating that gives the solar cells protection like a raincoat.
The advance, published recently in the journal Nature Materials , addresses one of the biggest challenges facing tin-based perovskite solar cells by making them more durable for practical use while also enhancing performance. The researchers' solar cells reached 16.2% efficiency — among the state-of-the-art for tin perovskite cells — and remained remarkably stable during extended testing.
"Tin perovskites are very promising materials for solar energy, but they are extremely sensitive to air and moisture," says Song Jin, a UW–Madison professor of Chemistry and a corresponding author of the study. "We wanted to find a way to protect these materials while preserving the properties that make them attractive for solar cells."
Perovskite solar cells have attracted intense interest as a next-generation photovoltaic technology. They can efficiently convert sunlight into electricity and potentially offer advantages in how solar cells are manufactured and used. Researchers and startup companies around the world have been working on improving their efficiency and stability and scaling up the manufacturing for commercial deployment.
However, most of the highest-performing perovskite solar cells contain lead, raising significant environmental concerns. Tin-based perovskites are considered the most promising lead-free alternatives because of their excellent light-absorbing and electronic properties.
The problem is durability. Tin perovskites are especially vulnerable to oxygen and moisture, which can rapidly degrade the material and reduce its performance.
Jin and his collaborators tackled the problem by understanding the way the components of these hybrid materials are assembled and designing new tin perovskite materials that can impede the damage from oxygen and water. The researchers made and compared tin perovskites containing a family of closely related organic components substituted with different halogen atoms (fluorine, chlorine, and bromine). They discovered that the chlorinated version allowed the tin perovskite material's crystals to pack most tightly together.
The difference was striking, landing in the sweet spot for both durability and efficiency. Conventional tin perovskite materials began degrading within hours or days in ambient air. The newly designed material maintained its structure and bright luminescence in ambient air for several months. Even sitting for days immersed in water, the new tin perovskite remained undissolved — unlike typical perovskite materials.
Theoretical calculations performed by postdoctoral researcher Jiahao Xie and Professor Yanfa Yan from the University of Toledo further showed that the tight packing of the new materials makes it much more difficult for oxygen and water to penetrate and damage the sensitive tin-containing perovskite material.
"What is exciting is that a relatively small change in the material's design produces such a large improvement in stability," says Christopher T. Triggs , the first author of the study who recently earned his doctorate in materials chemistry at UW–Madison. "It shows how designing the organic components and controlling the way perovskite structures pack together can provide powerful protection of the resulting perovskite materials from the surrounding environment."
The researchers put their material design to a tougher and practical test by incorporating it into working solar cells, working in collaboration with Lei Chen and Kai Zhu, solar cell researchers at the National Laboratory of the Rockies.
After optimization, beyond the 16.2% power-conversion efficiency, the best of the new solar cell devices also retained more than 95% of their initial efficiency after 1,600 hours in dry air. Even when operated continuously under simulated sunlight at 55 degrees Celsius (131 degrees Fahrenheit), the cells retained 80% of their initial efficiency after 1,000 hours. These are among the best stability and device performance reported for tin-based perovskite solar cells to date.
The results also show that improving durability can go hand-in-hand with improving solar-cell performance.
"This gives us a new design strategy for making tin perovskite solar cells both efficient and much more robust," Zhu says. "By understanding how the different parts of these hybrid materials interact, we can design and further improve the stability of these promising non-toxic solar materials to enable future applications."
The researchers say the work opens up new strategies for designing more stable tin-based perovskites and helps move lead-free perovskite photovoltaics closer to practical applications. The National Laboratory of the Rockies and the Wisconsin Alumni Research Foundation have jointly filed for a patent related to the new material.
This research was supported in part by grants from the Department of Energy (DE-SC0002162, BES-ERCAP0032847 and Perovskite Enabled Tandems award 52776) and a National Science Foundation graduate fellowship (DGE-2137424).
