New microscope sheds novel light on perovskite degradation

A team of scientists from the US Department of Energy’s Ames National Laboratory has developed a new characterization tool that allowed them to gain unique insight into a possible alternative material for solar cells.

Under the leadership of Jigang Wang, senior scientist at Ames Lab, the team developed a scanning probe microscope that uses terahertz waves to collect data on material samples. The team then used their microscope to study methylammonium lead iodide (MAPbI₃) perovskite, a material that could potentially replace silicon in solar cells. The scientists report their work in a paper in ACS Photonics.

There are two features that make this new scanning probe microscope particularly unique. The first is its use of the terahertz range of electromagnetic frequencies to collect data on materials; this range is far below the visible light spectrum, falling between the infrared and microwave frequencies. The second is that this terahertz light is shone through a sharp metallic tip that enhances the microscope’s capabilities toward nanometer length scales.

“Normally if you have a light wave, you cannot see things smaller than the wavelength of the light you're using,” said Richard Kim, a scientist at Ames Lab. “And for this terahertz light, the wavelength is about a millimeter, so it’s quite large. But here we used this sharp metallic tip with an apex that is sharpened to a 20nm-radius curvature, and this acts as our antenna to see things smaller than the wavelength that we were using.”

Using this new microscope, the team investigated the perovskite material MAPbI₃, which has recently become of interest to scientists as a possible alternative for silicon in solar cells. Perovskites are a special type of semiconductor that can transport an electric charge when it is exposed to visible light. The main challenge to using MAPbI₃ in solar cells is that it degrades easily when exposed to elements like heat and moisture.

According to Wang and Kim, the team expected MAPbI₃ to behave like an insulator when they exposed it to the terahertz light. Since the data collected on a sample is a reading of how the terahertz light is scattered by the material, they expected a consistent low-level of light-scatter throughout 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, like metals, would have a high-level of light scattering while less-conductive materials, like insulators, would have a lower level. The wide variation of light scattering detected along the grain boundaries in MAPbI₃ sheds light on the material’s degradation problem.

Over the course of a week, the team continued to collect data on the material, showing the degradation process through changes in the levels of light scatterings. This information could be useful for improving and manipulating 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 materials degradation,” said Wang. “Better understanding of these issues may enable developing highly efficient perovskite-based photovoltaic devices for many years to come."

This story is adapted from material from Ames National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.