Researchers from the US Department of Energy National Renewable Energy Laboratory (NREL) and Colorado School of Mines apply a new technique to identify defects in silicon solar cells that cause a drop in efficiency. The lessons learned at the atomic level could lead to improvements in how manufacturers fortify their products against what is known as light-induced degradation.
Light-induced degradation, or LID, reduces the efficiency of silicon solar cells by about 2%, representing a significant drop in power output over the 30 to 40-year lifetime of the technology used in the field. Solar cells made of silicon account for more than 96% of the global market and the most common semiconductor used in these cells is made of boron doped silicon. But boron-doped silicon is sensitive to LID, so manufacturers have developed methods to stabilize the solar modules.
Without understanding the defects at the atomic level, the researchers said it’s impossible to predict the stability of those modules.
“Some modules are fully stabilized. Some of them are only half stabilized,” said Abigail Meyer, a Ph.D. candidate at Mines and researcher at NREL. She is the lead author of a new paper detailing efforts to identify the source of the LID phenomenon. The article, “Atomic Structure of Light-Induced Efficiency-Degrading Defect in Boron-Doped Czochralski Silicon Solar Cells,” appears in the journal Energy & Environmental Science.
Her co-authors are Vincenzo LaSalvia, William Nemeth, Matthew Page, David Young, Paul Stradins, all from NREL; Sumit Agarwal, Michael Venuti and Serena Eley, who are from Mines; and P. Craig Taylor, a retired mining professor who advised on the study.
Stradins, a chief scientist and a project leader in silicon photovoltaic research at NREL, said the problem of LID has been studied for decades, but the exact microscopic nature of the cause of the degradation has not been established. Researchers have concluded through indirect experimentation and theory that the problem gets smaller if less boron is used or if there is less oxygen in the silicon.
The collaboration between NREL and Mines researchers relied on electron paramagnetic resonance (EPR) to identify defects responsible for the LID. For the first time, the microscopic examination revealed a clear defect signature as the sample’s solar cells were degraded more by light.
The defect signature disappeared when the scientists applied the “regeneration” process to cure the LID that the industry has adopted. To their surprise, the researchers also found a second, “broad” EPR signature affected by light exposure, involving many more dopant atoms than there are LID defects. They hypothesized that not all atomic changes caused by light lead to the LID.
The techniques developed to study LID could be expanded to reveal other types of degrading defects in silicon solar cells and in other semiconductor materials used in photovoltaics, including cadmium telluride and perovskites, the scientists noted. The Solar Energy Technologies Office within the Department of Energy funded the research.
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