Exposure to formamidine gas converts perovskite into a more stable form without sacrificing quality.
AsianScientist (May 6, 2016) – 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 how flipping a chemical switch converts one type of perovskite into another—a type that has better thermal stability and is a better light absorber. Their work was published in Journal of the American Chemical Society. The study, by researchers from Brown University in collaboration with the Chinese Academy of Sciences’ Qingdao Institute of Bioenergy and Bioprocess Technology, could be one more step toward bringing perovskite solar cells to the mass market. 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. Yet, despite perovskites’ potential, perovskite technology has several hurdles to clear, one of which is thermal stability. Most perovskite solar cells produced today are made with a type of perovskite called methylammonium lead triiodide (MAPbI3), which tends to degrade at moderate temperatures. That’s not ideal for solar panels that must last for many years. As a result, there’s a growing interest in solar cells that use a type of perovskite called formamidinium lead triiodide (FAPbI3) instead. Research suggests that solar cells based on FAPbI3 can be more 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. Part of the problem is that formamidinium has a different molecular shape than methylammonium. So as FAPbI3 crystals grow, they often lose the perovskite structure that is critical for 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 150°C. The material instantly converted from MAPbI3 to FAPbI3 while preserving the all-important microstructure and morphology of the original thin film.
Perovskite solar cells made using this new method. Credit: Padture Lab/Brown University “It’s like flipping a switch,” said study co-author Professor Nitin Padture. “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.” The gas-based method has 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. “The simplicity and the potential scalability of this method was inspired by our previous work on gas-based processing of MAPbI3 thin films, and now we can make high-efficiency FAPbI3-based perovskite solar cells that can be thermally more stable,” said Mr. Zhou Yuanyuan, a graduate student at Brown. “That’s important for bringing perovskite solar cells to the market.” Laboratory scale perovskite solar cells made using this new method showed efficiency of around 18 percent—not far off from the 20 to 25 percent achieved by silicon solar cells. “The technique is simple and has the potential to be scaled up, which overcomes a real bottleneck in perovskite research at the moment,” said Padture.