Friday, 20 October 2017 14:25

Testing perovskite materials

Summer Scholar Grace Noel characterizes new crystal semiconductors with solar cell potential.

 

2017 Materials Processing Center – Center for Materials Science and Engineering [MPC-CMSE] Summer Scholar Grace Noel explored the process of making and characterizing perovskite crystal materials for possible solar cell use in the lab of William A. Tisdale, ARCO Career Development Professor of chemical engineering. Noel synthesizes these lead bromide perovskite materials with different cations, including methyl ammonium, cesium and formamidinium.

“By changing the cation, you can change the properties of the perovskite,” Noel explains. “The word perovskite refers to a class of semiconductors that have a specific crystal structure, and they're an interesting area of research with applications in photovoltaics.”

MIT chemical engineering graduate student Nabeel Dahod, who studies thermal transport in nanostructured materials for his thesis project, is supervising Noel’s work in the Tisdale Lab. “Perovskites are a particularly new and exciting material with at this point undiscovered thermal transport properties, and this is where Grace's project this summer comes in,” Dahod says.

During a visit to the lab, Dahod and Noel demonstrate how these crystals are dried with a vacuum after wet synthesis. Noel explains that she caps her wet solution with tinfoil, perforated with a small hole in it, to slow the diffusion process. Dahod cautions Noel to set up a trap along with the vacuum so solvent vapors don’t harm the pump. Dahod prompts Noel to make sure the vacuum tube is firmly attached in order to be able pull vacuum through into the funnel.

Noel tests the vacuum process to make sure it is pulling out solvent using a pipette, which extracts a small volume. Noting that not all of the solid crystals made it into the filter, Dahod suggests scraping out the rest. Noel asks whether she should be worried about breaking any of the crystals. “No it's okay. They're pretty robust,” Dahod reassures. “Make sure to get as many of the orange ones as you can.”

At a separate workbench, Noel displays formamidinium single crystals and methylammonium single crystals, which have crystallized for about four days. Noel observes that the methylammonium single crystals are slightly larger and that there is a color difference between the two. “The formamidinium are more red in color,” says Noel, who is a rising senior at Penn State University, where she majors in chemical engineering.

“My project is synthesizing these different perovskites in the two different forms, which are single crystals and microplates,” Noel explains. “Basically the single crystals are crystals that are millimeters in size, whereas the microplates are a lot smaller, so they're more like microns in size. But they should exhibit similar properties to the single crystals. This is advantageous because the single crystals have properties that aren't disturbed by things like defects in the material or grain boundaries. So we have the three different types of microplates with the different cations, and we want to see how the thermal properties of them change based on their composition.”

In CMSE’s Shared Experimental Facilities, Noel analyzes scanning electron microscope [SEM] images of the microplates. Speaking about images on a computer monitor, she notes, “These ones are formamidinium lead bromide perovskites, and they form these little plates that are about 1 to 3 microns. So I'm looking at the microplates to see the different sizes that they are, and looking to see if there are any defects or impurities.”

Noel’s internship is supported in part by NSF’s Materials Research Science and Engineering Centers program [grant DMR-14-19807]. Participants in the Research Experience for Undergraduates, co-sponsored by the Materials Processing Center and the Center for Materials Science and Engineering, presented their results at a poster session during the last week of the program. The program ran from June 15, 2017, to August 5, 2017, on the MIT campus in Cambridge, Mass.

 

Denis Paiste, Materials Processing Center
September 25, 2017