Previous research primarily examined how radiation affected the energy conversion efficiency of organic solar cells. This new study delved into molecular-level changes that degrade performance.
"Silicon semiconductors aren't stable in space because of proton irradiation coming from the sun," explained Yongxi Li, first author of the study and a former U-M associate research scientist in electrical and computer engineering. "We tested organic photovoltaics with protons because they are considered the most damaging particles in space for electronic materials."
While gallium arsenide is favored for its efficiency and resilience against proton damage, its weight, inflexibility, and high cost pose challenges. Organic solar cells, on the other hand, are lightweight, flexible, and potentially more affordable. This study is part of ongoing efforts to assess the reliability of organic materials for critical space missions.
Organic solar cells built from small molecules demonstrated strong resistance to proton radiation, showing no performance degradation after simulated exposure equivalent to three years in space. In contrast, cells made with polymer-based materials lost half their efficiency over the same period.
"We found that protons cleave some of the side chains, and that leaves an electron trap that degrades solar cell performance," said Stephen Forrest, the Peter A. Franken Distinguished University Professor of Engineering at U-M and lead corresponding author. These traps capture electrons generated by light, reducing the flow of electricity to the electrodes.
Forrest noted that heating the solar cells, a process known as thermal annealing, can repair the damage by restoring broken molecular bonds. "You can heal this by thermal annealing, or heating the solar cell. But we might find ways to fill the traps with other atoms, eliminating this problem," he added.
The study suggests that solar cells exposed to sunlight in space could self-repair at temperatures around 100C. However, questions about the effectiveness of this self-healing in a vacuum and its reliability for extended missions remain. The team is also exploring ways to design materials that prevent the formation of electron traps altogether.
Li, now an incoming associate professor of advanced materials and manufacturing at Nanjing University in China, plans to further investigate these avenues.
The research received funding from Universal Display Corp and the U.S. Office of Naval Research. The devices were fabricated at the Lurie Nanofabrication Facility, tested with proton beams at the Michigan Ion Beam Laboratory, and analyzed at the Michigan Center for Materials Characterization.
U-M Innovation Partnerships is assisting the team in patent applications, with Universal Display licensing the technology and filing additional patents. Forrest holds a financial interest in Universal Display Corp.
Research Report:Radiation hardness of organic photovoltaics
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Michigan Center for Materials Characterization.
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