New discoveries made on a promising solar cell material, thanks to a new microscope

A team of scientists from the Department of Energy’s Ames National Laboratory have developed a new characterization tool that has given them unique insight into a possible alternative material for solar cells. Led by Ames Lab senior scientist Jigang Wang, the team developed a microscope that uses terahertz waves to collect data on material samples. The team then used their microscope to explore methylammonium lead iodide (MAPbI₃) perovskite, a material that could potentially replace silicon in solar cells.

Richard Kim, a scientist from Ames Lab, explained the two features that make the new scanning probe microscope unique. First, the microscope uses the terahertz range of electromagnetic frequencies to collect data on materials. This range is well below the visible light spectrum, lying between the infrared and microwave frequencies. Second, terahertz light is projected through a sharp metal tip which enhances the microscope’s capabilities to nanometer length scales.

“Normally, if you have a light wave, you can’t see things smaller than the wavelength of the light you’re using. And for this terahertz light, the wavelength is about a millimeter, so it’s pretty big,” Kim explained. “But here we used this sharp metal tip with a sharpened top at a radius bend of 20 nanometers, and it acts like our antenna to see things smaller than the wavelength we were using.”

Using this new microscope, the team studied a perovskite material, MAPbI₃, which has recently attracted scientific interest as an alternative to silicon in solar cells. Perovskites are a special type of semiconductor that carry electrical charge when exposed to visible light. The main challenge of using MAPbI₃ in solar cells is that it degrades easily when exposed to elements like heat and humidity.

According to Wang and Kim, the team expected MAPbI₃ behave like an insulator when exposed to terahertz light. Since the data collected on a sample is a reading of how light scatters when the material is exposed to terahertz waves, they expected a constant low level of light scattering throughout the material. What they found, however, was that there was a lot of variation in light scattering along the grain boundary.

Kim explained that conductive materials, like metals, would have a high level of light scattering, while less conductive materials like insulators wouldn’t have as much. The large variation in light scattering detected along grain boundaries in MAPbI₃ highlights the problem of material degradation.

Over the course of a week, the team continued to collect data on the material, and data collected during this time showed the degradation process through changes in light scattering levels. This information can be useful for improving and manipulating the material in the future.

“We believe the present study demonstrates a powerful microscopy tool for visualizing, understanding, and potentially mitigating grain boundary degradation, defect traps, and material degradation,” Wang said. “A better understanding of these issues could lead to the development of highly efficient perovskite-based photovoltaic devices for many years to come.”

MAPbI samples₃ were provided by the University of Toledo. This research is further discussed in the article “Terahertz Nanoimaging of Perovskite Solar Cell Materials”, written by Richard HJ Kim, Zhaoyu Liu, Chuankun Huang, Joong-Mok Park, Samuel J. Haeuser, Zhaoning Song, Yanfa Yan, Yongxin Yao, Liang Luo, and Jigang Wang, and published in the ACS Photonics.