A potential solar cell material is discovered using a new type of microscope

A new characterization technique created by a team of researchers at the Department of Energy's Ames National Laboratory gave them a rare window into a potential replacement material for solar cells.

Ames Lab's senior scientist Jigang Wang and his team created a microscope that uses terahertz waves to gather information on material samples. Using their microscope, the team then identified methylammonium lead iodide (MAPbI3) perovskite, as a substance that could eventually replace silicon in solar cells.

The results were published in the journal ACS Photonics.

The power of terahertz

A scientist from Ames Lab, Richard Kim, described the two characteristics that set the new scanning probe microscope apart.

The microscope first gathers information on materials using electromagnetic frequencies in the terahertz region. This frequency range, which is between infrared and microwave frequencies, is significantly below the visible light spectrum.

Second, a pointed metallic tip that receives the terahertz light boosts the microscope's abilities to observe objects at nanoscale length scales.

“Normally, if you have a light wave, you cannot 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 quite large,” explained Kim.

“But here we used this sharp metallic tip with an apex that is sharpened to a 20-nanometer radius curvature, and this acts as our antenna to see things smaller than the wavelength that we were using.”

Perovskites are a unique class of semiconductors that, when exposed to visible light, carries an electric charge. The biggest difficulty with employing MAPbI3 in solar cells is how quickly it deteriorates when exposed to heat and moisture.

The team used their microscope to study MAPbI3, which has lately caught the attention of researchers as a potential replacement for silicon in solar cells.

A powerful microscopy tool for highly efficient perovskite-based photovoltaic devices

Wang and Kim claim that the scientists anticipated MAPbI3 to act as an insulator when it was subjected to terahertz light. They anticipated a consistently low level of light scattering across the material because the data obtained from a sample is a reading of how the light scatters when the material is exposed to terahertz vibrations.

But what they discovered was that there was a great deal of diversity in the light scattering along the grain boundaries.

“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."

Study Abstract:

Direct visualization and quantitative evaluation of charge filling in grain boundary (GB) traps of hybrid metal halide perovskites require dynamic conductivity imaging simultaneously at the terahertz (THz) frequency and nanometer (nm) spatial scales not accessible by conventional transport and imaging methods used thus far. Here, we apply a THz near-field nanoconductivity mapping to the archetypal metal halide perovskite photovoltaic films and demonstrate that it is a powerful tool to reveal distinct dielectric heterogeneity due to charge trapping and degradation at the single GB level. Our approach visualizes the filled defect ion traps by local THz charge conductivity and allows for extracting a quantitative profile of trapping density in the vicinity of GBs with sub-20 nm resolution. Furthermore, imaging material degradation by tracking local nanodefect distributions overtime identifies a distinct degradation pathway that starts from the GBs and propagates inside the grains over time. The single GB, nano-THz conductivity imaging demonstrated here can be extended to benchmark various perovskite materials and devices for their global photoenergy conversion performance and local charge transfer proprieties of absorbers and interfaces.