A team of scientists at the Department of Energy’s Ames National Laboratory has developed a new characterization tool that allows them to gain unique insight into a possible alternative material for solar cells. Led by Jigang Wang, chief scientist at the Ames Lab, the team developed a microscope that uses terahertz waves to collect data on material samples. The team then used their microscope to image methylammonium lead iodide (MAPbI3) perovskite, a material that could potentially replace silicon in solar cells.
Richard Kim, an Ames Lab scientist, explained the two features that make the new scanning probe microscope unique. First, the microscope uses the terahertz range of electromagnetic frequencies to collect material data. This range is well below the visible light spectrum, falling between the infrared and microwave frequencies. Second, the terahertz light is emitted through a sharp metal tip that extends the microscope’s capabilities to nanometer length scales.
“If you have a light wave, you usually 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 tip sharpened to a 20 nanometer radius of curvature, and this acts as our antenna to see things smaller than the wavelength we’re using.”
Using this new microscope, the team studied a perovskite material, MAPbI3, which has recently become of interest to scientists as an alternative to silicon in solar cells. Perovskites are a special type of semiconductor that transport an electrical charge when exposed to visible light. The biggest challenge when using MAPbI3 in solar cells is that it degrades easily when exposed to elements such as heat and humidity.
According to Wang and Kim, the team expected MAPbI3 to behave like an insulator when exposed to terahertz light. Since the data collected on a sample is a measure of how light is scattered when the material is exposed to terahertz waves, they expected a uniformly low level of light scattering throughout the material. What they did find, however, was that light scattering varied greatly along the boundary between grains.
Kim explained that conductive materials like metals have high levels of light scattering, while less conductive materials like insulators don’t have as much. The large variation in light scattering along grain boundaries in MAPbI3 sheds light on the degradation problem of the material.
Over the course of a week, the team continued to collect data on the material, and the data collected over that time showed the degradation process through changes in light scattering. This information can be useful to improve and manipulate the material in the future.
“We believe the present study represents a powerful microscopy tool to visualize, understand, and potentially mitigate grain boundary degradation, defect traps, and material degradation,” Wang said. “A better understanding of these issues could enable the development of highly efficient perovskite-based photovoltaic devices for many years to come.”
The samples of MAPbI3 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 was written by Luo and Jigang Wang, and published in the ACS photonics.
Richard HJ Kim et al, Terahertz Nanoimaging of Perovskite Solar Cell Materials, ACS photonics (2022). DOI: 10.1021/acsphotonics.2c00861
Provided by Ames Laboratory
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