LEXINGTON, KY -- Two University of Kentucky Department of Chemistry professors in the College of Arts & Sciences. several current UK graduate students and a former grad student  contributed to an article reporting a major advance in increasing the stability of perovskite solar cells, which was published recently in the journal Science.

Co-authors at UK are Kenneth Graham, associate professor of chemistry; Chad Risko, John C. Hubbard Professor of Chemistry; and graduate students Harindi R. Atapattu, Keerthan R. Rao, and Zhuoyun Cai. An article on the discovery issued by the University of Toronto can be found here.

Currently, perovskite solar cells show power conversion efficiencies that are on par with commercially available solar cells. They fall short, however, of the 25-year stability needed for commercialization. The researchers report a ligand design that can greatly increase the stability of these perovskite solar cells, taking the devices one-step closer to commercialization.

The article, titled "Engineering ligand reactivity enables high-temperature operation of stable perovskite solar cells," reports a strategy to overcome a major hurdle for perovskite solar cell development through applying novel surface ligands to increase device stability. The article is led by So Min Park, a previous graduate student with the Graham group who is working as a post-doctoral researcher in the Sargent group at Northwestern University. 

"Park began the project while working on a grant from the Department of Energy at UK, where she focused on the fundamentals of surface ligand binding and intercalation in organic metal halide perovskites." Graham said. "She continued to drive the work forward as a post-doctoral researcher in the Sargent group, where she applied and expanded this fundamental understanding to develop high-performing perovskite solar cells with high stability."

According to the U.S. Department of Energy Solar Cells Technology Office, perovskites are a family of materials that have shown potential for high performance and low production costs in solar cells. Ligands are ions or molecules that are attached to a metal atom by coordinate bonding, and are often used to improve the performance of perovskite solar cells. In the article by Park, et al., the researchers demonstrate that key considerations in ligand design include surface coordination, surface defect passivation, and minimal intercalation of the ligand into the bulk of the perovskite. Using their ligand design strategy, the researchers created a perovskite solar cell "that can stand up to high temperatures for more than 1,500 hours," according to the University of Toronto article.  

"Perovskite solar cells ...  have enabled rapid progress toward the goal of uniting performance with stability," the abstract for the article states. "However, as the field continues to seek ever-higher durability, additional tools that avoid progressive ligand intercalation (the reversible inclusion or insertion of a molecule or ion into layered materials with layered structures) are needed to minimize degradation at high temperatures. We used ammonium ligands that are nonreactive with the bulk of perovskites and investigated a library that varies ligand molecular structure systematically. We found that fluorinated aniliniums offer interfacial passivation and simultaneously minimize reactivity with perovskites."

The abstract goes on to note that the solar cell in the experiment operated at 185 degrees Fahrenheit (85 degrees Celsius) for 1,560 hours in sunlight while maintaining 85% of its original performance. The article in Science can be found here.

Research reported in this publication was supported by the National Science Foundation under Award Number 1849213. The opinions, findings, and conclusions or recommendations expressed are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. This material is based upon work supported by the Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0018208.