Solar cells and semiconductors–with a twist

University of Tennessee Assistant Professor Mahshid Ahmadi and Assistant Professor Jonghee Yang at Yonsei University, Korea—one of Ahmadi’s former postdoctoral fellows—contributed to two significant research projects published in Nature journals this spring.

Both papers represent major breakthroughs for the study and creation of perovskites, a type of material with outstanding potential for use in advanced technologies like next-generation solar panels, light emitting diodes (LEDs), and lasers. Both were also international efforts, involving researchers from three institutions in the Republic of Korea and three in the United States.

“It feels amazing that we can generate such high-quality science,” said Ahmadi, a professor in the University of Tennessee’s Department of Materials Science and Engineering (MSE). “Publishing in this type of journal takes hard work from all the co-authors. It feels really good, and it makes our group shine in the scientific community.”

Ahmadi specializes in promoting materials discovery by integrating traditional methods with machine learning and automated synthesis. Her expertise, combined with unique equipment that allows for high-throughput creation and characterization of new material samples, makes her lab a desirable destination for visiting scholars.

Ahmadi was also recently named an associate editor of APL Machine Learning.

Finding a Solar Energy Groove

This February, a team led by Ahmadi and Hong Kong University of Science and Technology (HKUST) Professor Yuanyuan Zhou published the results of their work to stabilize perovskite solar cells (PSCs) in Nature Nanotechnology, which publishes research of high significance in nanoscience and technology.

PSCs use less expensive materials, have more sustainable manufacturing processes, and have a much higher theoretical power conversion efficiency than silicon panels. However, unexplained difficulties with cation distribution within the materials makes them susceptible to uneven degradation, holding them back from overtaking silicon panels.

“Perovskites can include more than five different elements, and it is very challenging to distribute them uniformly,” Ahmadi explained. “If the material is not homogeneous, then some parts of the solar panel will fail or degrade more quickly.”

The lead author on the Nature Nanotechnology paper, HKUST postdoctoral fellow Mingwei Hao, spent five months in Ahmadi’s laboratory in 2023, working with Ahmadi’s team to characterize perovskites for more efficient and stable solar cells.

“We worked on every significant step towards the stability and commercialization of PSCs,” Ahmadi said. “Being part of the team that can advance this technology is an amazing privilege.”

By using an advanced characterization technique called cathodoluminescence imaging to investigate the nanoscale structure of PSCs, Ahmadi and the other members of Hao’s team discovered the key to the cation distribution problem. Junctions between groups of PSC grains create grooves that can trap cations, preventing them from diffusing homogenously throughout the material.

The team was able to significantly reduce the depth of these grooves with a simple chemical additive step in PSC production, leading to a nearly 26 percent solar conversion efficiency—close to the theoretical limit for PSCs—and a considerable improvement in durability.

“Throughout our experiments, we found notable structural features in perovskite thin films that are remarkably different from those in conventional materials,” Hao said. “We are making every effort to elucidate the related mechanisms to promote the commercial viability of PSCs, pushing forward the development of the renewable energy (market.)”

Other coauthors on the study include Yang; ORNL Senior Research Scientist Benjamin Lawrie; and additional collaborators from HKUST, Hong Kong Baptist University, and Yale University.

Self-Twisting Semiconductors

In March, a team led by Yang published research concerning the self-assembly of two-dimensional halide perovskites in Nature Synthesis. This journal focuses on important technological advances in material science, such as the development of new approaches to material creation or shifts in the fundamental understanding of chemical and material systems.

Two-dimensional halide perovskites (2D HPs) are excellent semiconductors made by placing a layer of spacer molecules between two sheets of halide perovskites. By using different spacers between those sheets, engineers can tune the materials’ optical and electronic properties, customizing the perovskites for different applications.

Using large organic molecules would also help protect the perovskite sheets from damage due to environmental factors like humidity, but engineers have had limited success with creating 2D HPs with bulky spacers.

Yang, now an assistant professor at Yonsei University in Seoul, Republic of Korea, led the team in creating a high-throughput, autonomous program to create and characterize a large number of 2D HPs using bulky spacers. Surprisingly, the team found that adding small, three-dimensional crystals of the HPs stabilized the 2D sheets—and even led to some of the 2D HPs self-assembling with a desirable moiré (twisted) structure.

“While other twisted structures have been studied for many years, it’s been much more difficult to fabricate them,” said Ahmadi. “We’ve discovered a very cheap and easy way to synthesize them using 2D perovskites. I think this is going to open up a lot of research in the exotic physics of these 2D perovskites.”

Other authors on the paper include MSE Professor Sergei Kalinin; Bin Hu from UT’s Institute for Advanced Materials and Manufacturing (IAMM); computational scientists Addis Fuhr, Kevin Roccapriore, Bogdan Dryzhakov, and Bobby Sumpter from the Oak Ridge National Laboratory (ORNL) Center for Nanophase Materials Sciences; and additional researchers from Yonsei University and the Pohang Accelerator Laboratory in Pohang, Republic of Korea.