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An Innovative Chemical Treatment Improves Efficiency of a Promising Solar Technology

The innovative use of chemical treatments can improve the performance of perovskite solar cells under natural conditions, according to the paper, “Strong-Bonding Hole-Transport Layers Reduce Ultraviolet Degradation of Perovskite Solar Cells,” published in Science.

Researchers are making significant strides in addressing the durability of perovskite solar cells, an advanced type of solar cell known for its high potential for significant power conversion efficiencies. Perovskite solar cells have shown great promise in laboratory tests under LED lights; however, these tests do not fully replicate the conditions of actual sunlight, which includes significant UV radiation.

“UV radiation can weaken certain chemical bonds within the cells, leading to faster degradation and reduced efficiency,” said Dr. Jinsong Huang, a leading author of the paper and Louis D. Rubin Jr. Distinguished Professor in UNC-Chapel Hill’s Department of Applied Physical Sciences. “Specifically, weak bonds between the perovskite material and other parts of the cell cause the material to migrate over time, undermining the cell’s stability.”

To tackle this issue, researchers have introduced a chemical called EtCz3EPA, a molecule synthesized by Professor Alan Sellinger’s group at Colorado School of Mines, which forms stronger bonds within the solar cells, enhancing the connection between various parts of the cells. By doing so, it increases the cell’s stability and efficiency even when exposed to UV radiation and tested outdoors.

Dr. Laura Schelhas, deputy director of the Perovskite PV Accelerator for Commercializing Technologies (PACT) center and manager of the Hybrid and Nanoscale Materials Chemistry group at the National Renewable Energy Laboratory (NREL), said, “The modules, installed in September 2023 and reported in this study, are the first tested in PACT to demonstrate a greater than 16% efficiency after 35 weeks of outdoor testing and remain operational in the field. There is still work to be done as modules need to perform for years in the field; however, this is an encouraging step in the development of these materials.”

Dr. Huang collaborated with researchers from the Department of Chemistry at UNC-Chapel Hill, Colorado School of Mines, National Renewable Energy Laboratory, University of Toledo and University of California, San Diego.

Some perovskite cells can maintain 90% of their power conversion efficiency after 10,000 hours of indoor light exposure or 1,000 hours at 85 to 90 degrees Celsius. UV light can cause much faster degradation in outdoor conditions due to UV-induced chemical changes in the solar cell materials. A quick degradation was observed after only two weeks of outdoor testing using the same devices.

“The study underscores a critical difference in how perovskite solar cells behave in indoor tests compared to outdoor use,” said Dr. Huang. “Perovskite solar cells often perform well under LED lights, which lack UV radiation. This discrepancy can lead to overly optimistic predictions about their outdoor durability.”

Researchers are exploring various approaches to mitigate UV damage, such as combining EtCz3EPA with PTAA, or Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], a type of organic polymer commonly used in electronic applications including solar cells, in a hybrid structure, which showed remarkable stability and performance. Devices with a combination of EtCz3EPA and PTAA exhibited nine-times-slower degradation rate.

Dr. Huang said hole transport materials (HTMs), which are substances used in solar cells and other electronic devices to facilitate the movement of positively charged carriers, known as “holes,” are critical components for helping to improve the efficiency, stability and longevity of perovskite solar modules. The movement of these holes, along with electrons, is vital for electrical conduction in devices like solar cells.

“Hybrid hole transport material devices are particularly important for advancing the commercialization of perovskite solar cells,” said Dr. Huang. “By addressing the stability and performance challenges of perovskite solar cells, hybrid HTM structures enable the development of more reliable and efficient solar technologies suitable for real-world applications, including large-scale solar power generation.”

June 6, 2024

An outdoor stability testing system for perovskite modules.