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

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

Visualization of the tip of the microscope exposing material to terahertz light. The colors on the material represent the light scattering data and the red and blue lines represent the terahertz waves. Credit: US Department of Energy Ames National Lab

A team of scientists at the Department of Energy’s Ames National Laboratory has developed a new characterization tool that allowed them to gain unique insight into a possible alternative solar cell material. 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 measure 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 data about materials. This range is well below the visible light spectrum and falls between the infrared and microwave frequencies. Second, the terahertz light is shined through a sharp metal tip that extends the microscope’s capabilities down to nanometer length scales.

“When you have a light wave, you normally can’t see things that are smaller than the wavelength of the light you’re using. And for this terahertz light, the wavelength is about a millimeter, so it’s quite large,” Kim explains . “But here we’ve used this sharp metal tip with a tip sharpened to a curvature with a radius of 20 nanometers, and this acts as our antenna to see things smaller than the wavelength we used.”

Using this new microscope, the team examined 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 carries an electrical charge when exposed to visible light. The main challenge in using MAPbI3 in solar cells is that it breaks down easily when exposed to elements such as heat and moisture.

According to Wang and Kim, the team was expecting MAPbI3 to act like an insulator when they exposed it to the terahertz light. Since the data collected on a sample is a reading of how the light is scattered when the material is exposed to the terahertz waves, they expected a consistently low level of light scattering through the material. What they found, however, was that there was a lot of variation in light scattering along the boundary between the grains.

Kim explained that conductive materials, such as metals, would have a high degree of light scattering, while less conductive materials such as insulators would not. The wide variation of light scattering detected along the 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 data collected over that time showed the degradation process through changes in the levels of light scattering. This information can be useful to improve and manipulate the material in the future.

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

The samples from MAPbI3 were provided by the University of Toledo. This research is further discussed in the paper “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.

More information:
Richard HJ Kim et al, Terahertz Nanoimaging of perovskite solar cell materials, ACS Photonics (2022). DOI: 10.1021/acsphotonics.2c00861

Supplied by Ames Laboratory

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