According to a recent study by University researchers, a new material called Cesium Titanium (IV) Bromide can replace lead in a specific type of solar panel. The study’s goal is not necessarily to replace commonly used silicon solar cells, but to create environmentally stable and conscious solar cells that are more affordable and have broader applicability.
The new solar cells are about “1,000 times thinner” than silicon solar cells, said Nitin Padture, co-author of the study, professor of engineering and director of Brown’s Institute for Molecular and Nanoscale Innovation. “These thin films can be made flexible. If you try to bend a silicon wafer, you’ll shatter it. But when the film is so thin, it can be flexible, so you can put them on a camping tent, or a backpack, or clothes or umbrellas,” Padture said.
The researchers focused on solar panels based on perovskite, a material with a crystal structure allowing for increased conductivity, lower cost and better solar cell properties, according to the American Ceramics Society. The study found that lead-based perovskite panels contributed to toxicity in the environment and increased volatility, but this new material provides a less toxic alternative.
“The amounts of lead in solar cells are quite small,” said Robert Hurt, professor of engineering and head of the Laboratory for Environmental and Health Nanoscience. “We have lead in a lot of old products, we have lead pipes that our drinking water flows through, but those are all legacy things,” he added. “But to put new products on the market that contain lead is always going to be of some concern.”
“(Perovskite solar cells) also contain an organic molecule in the crystal structure that actually helps it be a good absorber,” Padture said. The presence of this molecule and the lead make a “good” solar cell, but the molecule is volatile outside the lab and lead is toxic, so the goal was to circumvent these issues, Padture added.
“Typically, (solar panels) are made of silicon,” but due to the complex formation process, “there is a lot of cost involved,” Padture said. “So there’s always been a motivation for reducing the cost in the last 25 to 30 years.”
“In 2009, this new technology of perovskite solar cells took a foothold,” and while the efficiency was just 4 percent in the beginning of development, the cells recently reached a new record of about 23 percent efficiency at a reduced cost, Padture said. “The reason that these solar cells are so efficient is because they contain lead, and lead helps in absorbing light.”
“Our goal is not to replace the silicon solar cells. We want to boost the efficiency,” said Min Chen GS, co-author of the study.
Tandem cells can be formed by combining perovskite and silicon technologies, Padture said. This process boosts the overall efficiency of the solar cell, as each type only absorbs certain spectrums of sunlight, Padture said. “Perovskite you can actually tune to absorb a complementary part of the solar spectrum that silicon doesn’t absorb,” he added.
The project is in collaboration with the University of Nebraska at Lincoln, where they run “computational modeling, because there are thousands of compounds that are lead-free, but we cannot test each of those in the lab,” Padture said. “They complete experiments on computers screening thousands of possible compounds” based on the desired properties for the solar cells, he added.
“If you just do experiment after random experiment, there are too many, and you’re not going to be able to find the perfect combination,” Padture said. “Integration of computation and experiment is really key for making advances, and that is where the future is for research.”
Researchers at the University took the suggested compositions from modeling for further testing, Padture said. This showed that computational modeling predictions could be actualized in a lab environment, and led to the use of this new substance..
“Silicon needs to be melted at 1500 degrees Celsius,” while the new materials only need to be heated from around 100 to 200 degrees Celsius, so there’s a huge energy savings cost, Padture said.
“It’s not just the device itself, but someone has to make the device, and people have to mine the lead, and there’s a whole chain behind it. So if you can get rid of the toxic substance in the beginning, then you’ve certainly done a good thing,” Hurt said.
“There are a lot of other costs with using cheap toxic materials, so you have to take special steps to protect the workers and the environment,” Padture said. “In general, cheap toxic materials do not make good economic sense.”