Scientists at the Stanford Synchrotron Radiation Lightsource (SSRL) and Stanford University 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.
The perovskite production process begins by dissolving the basic ingredients in a solvent. The solution is deposited and dried, creating a film. The initial crystallized film is known as the precursor. Finally, the film is heated and cooled, reorganizing the film's structure and yielding a perovskite. 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 crystallizes 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?"
The 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 this breakthrough could pave the way for improved solar cell materials, the research also has broader implications for material science, namely in better understanding the synthesis process.
"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."