Among solar energy researchers, there's nothing new about "perovskites" -- crystals that absorb both visible and infrared light and convert sunlight into electricity.
They've been around since 2009. But though they're inexpensive to produce, scientists have been unable to manufacture uniform crystals that will provide consistent performance in commercial applications.
Perovskites can be manufactured – printed, actually – at much lower temperatures than are needed for conventional silicon films, which makes them cost efficient.
Timothy Kelly, a chemist at Canada's University of Saskatchewan, tells the MIT Technology Review that they already can convert 17.9 percent of solar energy into electricity, nearly as good as existing commercial films made from silicon and cadmium telluride.
But the crystals made so far are showing wildly inconsistent performance. "When you make 10 different perovskite cells, you get 10 different efficiencies," says Prashant Kamat, a chemist at the University of Notre Dame. "It's frustrating."
The source of the problem is the variety of sizes of the crystals in different solar cells. Kamat explains that the boundaries between crystals of different sizes are like walls blocking the flow of electricity.
Now researchers in Switzerland and South Korea say they've solved that problem.
Michael Graetzel, a chemist at Switzerland's École Polytechnique Fédérale de Lausanne, and Nam-Gyu Park, a chemist at Sungkyunkwan University in Korea, say they've brought uniformity to the manufacturing process. They've reported the results in the Aug. 31 issue of the journal Nature Nanotechnology.
Graetzel and Park says perovskite crystals for use in solar cells are made first by coating a surface with lead iodide, which dries, then coating it with methyl ammonium iodide. When this second layer dries, the compounds from both layers form perovskite crystals.
The Swiss and Korean scientists say their research has led to a method to control various steps in the process, including the concentrations of the two solutions, thus leading to consistently larger perovskites that are necessary for an efficient solar cell. In fact, the efficiency of the cells they've created is 16.4 percent, nearly as high as perovskites' previous peak efficiency.
But their work isn't done. Park says humidity causes the materials in perovskites to leak methyl ammonium, so they either have to coat perovskite solar cells with a moisture-proof sealant or find substitutes for the crystals' ingredients.
Further, toxic lead is used in manufacturing the perovskites, and the researchers would prefer to find a substitute for that. One promising candidate is tin.
Finally, Graetzel says he's not satisfied with the perovskites' current efficiency of 16.4 percent. He believes that its performance can be boosted, even beyond the previous ceiling of 17.9 percent. "I think 20 percent efficiency should be possible in the near term," he says.
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