Why are people so hot on perovskite solar cells? For one thing, they are lighter and cheaper to manufacture than conventional solar cells made from silicon. For another, they can be printed, they are flexible, and can be mounted just about anywhere, making them ideal for building integrated solar applications. The downside is, they aren’t very efficient, don’t last very long, and are adversely affected by heat and humidity. Some would say perovskite technology today is at about the same point where traditional solar panel technology was a decade ago.
One of the techniques being explored by researchers is a hybrid solar cell, one that deposits a perovskite layer on top of a silicon wafer. The coating can be as thin as a millionth of an inch and the hybrid cells have an efficiency rating of 25%. But heat is an issue. Mounted outdoors in a sunny area, a solar panel can reach temperatures of 200º F — much too high for the perovskite layer to survive.
At Iowa State University, a research team is investigating hybrid perovskites that use inorganic materials like cesium in place of organics, a change that makes the cells stable at high temperatures. They also developed a fabrication technique that deposits the many thin layers of perovskite material just a few billionths of an inch thick on top of a silicon wafer. This vapor deposition technique is consistent, leaves no contaminants, and is already used in other industries so it can be scaled up for commercial production, according to Iowa State University. The research has been published recently in the journal Applied Energy Materials.
“Our perovskite solar cells show no thermal degradation even at 200 degrees Celsius (390 degrees Fahrenheit) for over three days, temperatures far more than what the solar cell would have to endure in real-world environments,” says Harshavardhan Gaonkar, the paper’s first author. “That’s far better than the organic-inorganic perovskite cells, which would have decomposed totally at this temperature. So this is a major advance in the field,” adds professor Vikram Dalal, who led the research team.
The team replaced the iodine commonly used in perovskite cells with bromine, which made them much less sensitive to moisture — a common problem with hybrid perovskites. But that lowered the efficiency of the cells to 11.8%. “We are now trying to optimize this cell — we want to make it more efficient at converting solar energy into electricity,” Dalal says. “We still have a lot of research to do, but we think we can get there by using new combinations of materials.”
Why bother, some might say. Who cares about solar cells that are less than 12% efficient? An analogy could be made to electric transportation or other forms of renewable energy. No one technology is best for every situation. Choices are good and all contribute to the overall goal — getting rid of fossil fuels forever so humans can continue to thrive here on Earth.
“This study demonstrates a more robust thermal stability of inorganic perovskite materials and solar cells at higher temperatures and over extended periods of time than reported elsewhere,” the researchers say. “(These are) promising results in pursuit of the commercialization of perovskite solar cell materials.” When will perovskites be ready for prime time? Patience, grasshopper.
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