Thin films of crystalline materials called perovskites provide a promising new way of making inexpensive and efficient solar cells. Now, an international team of researchers has shown a way of flipping a chemical switch that converts one type of perovskite into another — a type that has better thermal stability and is a better light absorber. Credit: Padture Lab / Brown University
Thin films of crystalline materials, perovskites provide a promising new way of making inexpensive and efficient solar cells. An international team has shown a way of flipping a chemical switch that converts one type of perovskite into another – a type that has better thermal stability and is a better light absorber. The technique is simple and has the potential to be scaled up, which overcomes a real bottleneck in perovskite research at the moment.
Perovskites have emerged in recent years as a hot topic in the solar energy world. The efficiency with which they convert sunlight into electricity rivals that of traditional silicon solar cells, but perovskites are potentially much cheaper to produce. These new solar cells can also be made partially transparent for use in windows and skylights that can produce electricity, or to boost the efficiency of silicon solar cells by using the two in tandem.
Despite this, perovskite technology has several hurdles to clear — one of which deals with thermal stability. Most of the perovskite solar cells produced today are made with of a type of perovskite called methylammonium lead triiodide (MAPbI3). The problem is that MAPbI3 tends to degrade at moderate temperatures. “Solar cells need to operate at temperatures up to 85 degrees Celsius,” said Yuanyuan Zhou. “MAPbI3 degrades quite easily at those temperatures.”
So there’s a growing interest in solar cells that use a type of perovskite called formamidinium lead triiodide (FAPbI3) instead which can be efficient and more thermally stable than MAPbI3. However, thin films of FAPbI3 perovskites are harder to make than MAPbI3 even at laboratory scale let alone making them large enough for commercial applications. Formamidinium has a different molecular shape than methylammonium. So as FAPbI3 crystals grow, they often lose the perovskite structure that is critical to absorbing light efficiently.
This latest research shows a simple way around that problem. The team started by making high-quality MAPbI3 thin films using techniques they had developed previously. They then exposed those MAPbI3 thin films to formamidine gas at 150C. The material instantly converted from MAPbI3 to FAPbI3 while preserving the all-important microstructure and morphology of the original thin film.
“It’s like flipping a switch,” Padture said. “The gas pulls out the methylammonium from the crystal structure and stuffs in the formamidinium, and it does so without changing the morphology. We’re taking advantage of a lot of experience in making excellent quality MAPbI3 thin films and simply converting them to FAPbI3 thin films while maintaining that excellent quality.”
This latest research builds on the work this international team of researchers has been doing over the past year using gas-based techniques to make perovskites. The gas-based methods have the potential of improving the quality of the solar cells when scaled up to commercial proportions. The ability to switch from MAPbI3 to FAPbI3 marks another potentially useful step toward commercialization, the researchers say.
Laboratory scale perovskite solar cells made using this new method showed efficiency of around 18% – not far off the 20 to 25% achieved by silicon solar cells. “We plan to continue to work with the method in order to further improve the efficiency of the cells,” said Kai Zhu, senior scientist at NREL and co-author of the new paper. “But this initial work demonstrates a promising new fabrication route.” https://news.brown.edu/articles/2016/04/perovskite
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