There’s an old expression: you can’t see the forest for the trees. It feels particularly apt when talking about the transition to green energy.
Electric cars may eliminate tailpipe emissions, but we still have to ask where their electricity comes from and what was mined and manufactured to make their batteries. Renewable energy has a similar inconvenient truth. Solar panels, wind turbines, batteries, inverters and the infrastructure that collects and moves electricity are all hardware, and making hardware has an environmental footprint.
Solar technology has its own materials problem. Many of the most efficient next-generation perovskite solar cells contain hazardous lead. Tin perovskites are considered one of the most promising non-toxic, lead-free alternatives, but there has been a catch: tin-based materials are extremely vulnerable to oxygen and moisture.
Researchers at the University of Wisconsin–Madison, the National Laboratory of the Rockies and collaborating institutions may have found a way around that problem. They have developed a tin-based perovskite with what amounts to a built-in raincoat, protecting the material against air and water while maintaining its ability to turn sunlight into electricity.
The research, published in Nature Materials, produced solar cells with a power-conversion efficiency of 16.2%, among the state-of-the-art efficiencies reported for tin perovskite cells.
“Tin perovskites are very promising materials for solar energy, but they are extremely sensitive to air and moisture,” says Song Jin, a professor of chemistry at UW–Madison and 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.”
Finding the sweet spot in the sun
Perovskites have attracted considerable attention because they can convert sunlight efficiently and could offer advantages in how solar cells are manufactured and used. Researchers and companies are working to improve their efficiency, stability and manufacturing at commercial scale, but lead in many of the best-performing versions remains an environmental concern.
The Wisconsin-led team approached the tin problem by looking at how the components of the hybrid material assemble. They created tin perovskites containing related organic components substituted with fluorine, chlorine and bromine, then compared them.
The chlorinated version allowed the crystals to pack together most tightly. That tighter structure made it considerably more difficult for oxygen and water to penetrate the material, according to theoretical calculations by Jiahao Xie and Professor Yanfa Yan at the University of Toledo.
The difference was significant. Conventional tin perovskite materials can begin degrading within hours or days when exposed to ambient air. The new material maintained its structure and bright luminescence for several months. It even remained undissolved after sitting immersed in water for days, unlike typical perovskites.
“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, 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.”
Can it survive outside the laboratory?
The researchers then incorporated the material into working solar cells in collaboration with Lei Chen and Kai Zhu at the National Laboratory of the Rockies.
The best devices retained more than 95% of their original efficiency after 1,600 hours in dry air. Under the more punishing test of continuous simulated sunlight at 55°C, or 131°F, they retained 80% of their initial efficiency after 1,000 hours.
The results are among the best stability and performance reported so far for tin-based perovskite solar cells, suggesting that making the material more durable doesn’t necessarily mean sacrificing 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.”
It doesn’t make solar hardware impact-free, and a laboratory advance isn’t the same thing as a commercially viable solar panel. But if solar power is going to become cleaner, looking beyond the electricity produced to the materials used to produce it is part of seeing the forest and the trees.
