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Aug. 28 (UPI) -- Scientists have developed new chemical engineering techniques for producing high-performing solar cell materials. The recipe for perovskites is seemingly foolproof. Combine the three main ingredients -- lead, iodide and methylammonium -- any number of ways, and you get the same basic material.
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However, slight tweaks at various stages of the perovskite production process can alter the material's qualities. Now, scientists have found a way to make perovskites with qualities ideal for the material's use in solar cells.
"Our study builds on work by other groups of researchers at Oxford, Cornell and Stanford that showed using chlorine in the processing can lead to high-quality perovskite films with impressive performance," Aryeh Gold-Parker, PhD student in Stanford University's chemistry department, said in a news release.
The perovskite production process begins by dissolving the basic ingredients in a solvent. The solution is deposited and dried, creating a film. The initial crystalized film is known as the precursor. Finally, the film is heated and cooled, reorganizing the film's structure and yielding a perovskite.
A perovskite is any material that takes on same cubic structure of the eponymous mineral. Perovskite, the calcium titanium oxide mineral, was first discovered in Russia's Ural Mountains by Gustav Rose in 1839. Rose named the mineral after Russian mineralogist Lev Perovski.
Though the basic recipe and ingredients are simple, slight chemical manipulations at each stage of the production process can alter the material's physical properties.
"There are dozens of different methods for depositing perovskite films, for example," Gold-Parker said. "And these methods lead to differences in thickness, texture, grain size and crystallinity of the films."
During previous experiments, scientists realized large amounts of chlorine are lost as the film crystalizes and is transformed into a perovskite.
"In this latest study we wanted to know: Where does the chlorine go and what purpose does it serve? Why chlorine in the first place?" said Kevin Stone, staff scientist at the Stanford Synchrotron Radiation Lightsource. "What does the precursor consist of, and how is it influencing this transformation?"
Scientists were able to answer these questions using X-ray scattering and X-ray spectroscopy, which provided high-definition images of the perovskite production process. The images revealed the atomic structure of the precursor and detailed the escape of the gaseous salt of chlorine called methylammonium chloride, or MACI.
"We were also able to show that the transformation into the final perovskite is limited by the gradual evaporation of MACl, and that this slow transformation might actually lead to a higher quality perovskite material," Gold-Parker said.
While the breakthrough, detailed this week in the journal Nature Communications, could pave the way for improved solar cell materials, the research also has broader implications for material science. Often, material scientists don't fully understand the synthesis process. The latest findings offer material scientists a solution.
"In the paper we lay out a clear pathway for anyone who wants to study the processes involved in making this or other materials," said SSRL scientist Christopher Tassone. "This is an important step in perovskites research but also in the broader field of synthesis science."
