A US research team has investigated metastable dark degradation and light recovery in commercial TOPCon solar cells exposed to ultraviolet-induced degradation (UVID), finding significant variation among manufacturers.
The scientists said previous studies had identified significant UVID in TOPCon devices, with reported degradation rates comparable to or higher than those observed in earlier commercial technologies such as PERC. However, they noted that light soaking or current injection can reverse some of the resulting damage.
Their technical analysis focused on unencapsulated M10 bifacial TOPCon solar cells supplied by three undisclosed manufacturers, designated Vendor 1, Vendor 2 and Vendor 3.
“All cells are commercially available and intended for module production,” the researchers said, without providing further details about the manufacturers.
The team examined metastable recovery under illumination and subsequent degradation in darkness using localized external quantum efficiency (EQE) measurements. It used controlled light soaking from the EQE system’s lamp to determine how exposure dose and wavelength affect recovery. The researchers then compared recovery trends among the three vendors to assess variations between commercial cells with nominally identical structures.
The analysis showed that differences in materials and manufacturing caused vendor-specific degradation responses, despite the cells sharing the same nominal architecture. Variations in layer thickness, composition, compositional gradients and microstructure may have little effect on initial performance but become pronounced during long-term UV exposure, the researchers said.
UV exposure produced clear differences among the manufacturers. Cells from Vendors 2 and 3 lost about 4% of their short-circuit current density, while those from Vendor 1 lost less than 1%. UV-range quantum efficiency losses reached approximately 40% for Vendor 2, compared with about 10% for Vendors 1 and 3.
The assessment also showed that UV light could partially restore performance following accelerated UV aging and dark storage. Recovery was strongest at wavelengths between 320 nm and 340 nm and generally occurred within seconds under sunlight or suitable UV illumination.
Light soaking produced the strongest recovery in cells with the greatest UV-induced degradation, making the procedure essential for accurate measurements. Although the extent of recovery varied among vendors, the required exposure dose was similar. The cells typically returned to a dark-degraded state within approximately 55 hours, suggesting that they could degrade overnight and recover rapidly in morning sunlight under field conditions.
“Light soaking immediately prior to measurement is recommended to ensure full recovery,” the researchers said. “In addition, when multiple light-based measurements are conducted on the same sample, the UV dose history should be carefully controlled to maintain consistency.”
The findings are presented in “Metastable dark degradation and light recovery in UV exposed commercial TOPCon,” published in Solar Energy Materials and Solar Cells.
“These findings show that light-induced metastability operates on the same timescale as the natural day–night cycle of fielded modules and must be considered when assessing long-term degradation mechanisms,” the scientists concluded. “Future studies correlating this effect with detailed material properties – such as layer thickness, composition, and microstructure – as well as fielded module studies, will be critical for understanding and improving the predictive capacity of TOPCon module degradation models.”
The research group was formed by scientists from Case Western Reserve University, University of Central Florida, Arizona State University and US-based manufacturer GAF Energy.
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