A team of scientists at the U.S. Department of Energy’s Ames National Laboratory has developed a new characterization tool that allows them to gain a unique insight into a possible alternative material for solar cells. Led by Jigang Wang, a senior research scientist at Ames Lab, the team developed a microscope that uses terahertz waves to collect data on material samples. The team then used their microscope to study methylammonium lead iodide (MAPbI).₃) perovskite, a material that could potentially replace silicon in solar cells.

Richard Kim, a scientist at Ames Laboratory, explained two features that make the new scanning probe microscope unique. First, the microscope uses the terahertz electromagnetic frequency range to collect data about materials. This range is well below the visible light spectrum, which lies between the infrared and microwave frequencies. Second, light in the terahertz range is passed through a sharp metal tip that expands the scope of the microscope to the nanometer scale.

“Normally, if you have light with a wavelength, 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,” explained Kim. “But here we used this sharp metal tip, the tip of which has been sharpened to a radius of curvature of 20 nanometers, and it acts as an antenna to see objects smaller than the wavelength we use.”

The team investigated the perovskite material MAPbI using this new microscope.₃which has recently attracted the attention of scientists as an alternative to silicon in solar cells. Perovskites are a special type of semiconductor that carries an electric charge when exposed to visible light. The main problem of using MAPbI ₃ It is easily destroyed by elements such as heat and moisture in solar cells.

According to Wang and Kim, the team believes that MAPbI’s₃ When exposed to terahertz light, they act as insulators. They expected a consistently low level of light scattering throughout the material, as the data collected on the sample is an indication of how light is scattered when the material is exposed to terahertz waves. However, they found that the scattering of light along the grain boundary varies greatly.

Kim explained that conductive materials such as metals will have high levels of light scattering, whereas less conductive materials such as insulators will not. Large variation in light scattering along grain boundaries in MAPbI₃sheds light on the problem of material degradation.

The team continued to collect data on the material for a week, during which the data collected showed a process of degradation due to changes in light scattering levels. This information may be useful for developing and manipulating the material in the future.

“We believe this work demonstrates a powerful microscopy tool for visualizing, understanding and potentially mitigating grain boundary distortion, defect trapping, and material degradation,” Wang said. “A better understanding of these issues could enable the development of high-efficiency perovskite-based photovoltaic devices in the years to come.”