NREL researchers warn of unexpectedly high UV-induced degradation in n-type solar modules

A research team from the U.S. National Renewable Energy Laboratory has analyzed n-type modules deployed in a 3 MW commercial PV project in an effort to assess the high degradation rate reported by the plant owner. The scientists found that ultraviolet-induced degradation (UVID) was a major factor affecting the facility’s performance.

September 17, 2025 Emiliano Bellini

Ultraviolet (UV) light hits solar energy systems most intensely at
solar noon, generally between 12 p.m. and 1 p.m., when the sun is at its highest point in the sky

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Researchers from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have investigated how ultraviolet-induced degradation (UVID) can affect the performance of PV systems using n-type solar modules and have found that current IEC standards designed to screen for early module failures may not be suitable for assessing all the risks of potential performance losses.

The scientists conducted their analysis on a 3 MW commercial rooftop PV system deployed at an undisclosed location in a temperate zone of the United States. “The site was identified as underperforming by the owner, and field current–voltage (IV) curves indicated modules had degraded approximately 2.4%/year relative to nameplate power,” they explained. “After about 6 years of deployment, four fielded modules and two unfielded spares were set aside for our study.”

The system relies on n-type modules based on passivated emitter rear totally diffused (PERT) modules from an unspecified manufacturer, but the scientists warned that their findings could be extended to other module technologies like tunnel oxide passivated contact (TOPCon) or heterojunction (HJT). “There is evidence via lab studies that some of these cells are more susceptible to UVID, yet there is a lack of confirmation that such degradation occurs in the field,” the group stressed.

For their analysis, the academics used current–voltage (IV) and Suns-Voc scanning, electroluminescence (EL) imaging, photoluminescence (PL) imaging, external quantum efficiency (EQE) Measurements, dark lock-in thermography (DLIT), scanning Electron Microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS).

Furthermore, they conducted damp heat (DH) and UV stress tests combined with scanning spreading resistance microscopy (SSRM), optical microscopy, the Fourier transform infrared spectroscopy (FTIR), high-performance liquid chromatography (HPLC), and X-ray photoluminescence spectroscopy (XPS) to assess the extent of module degradation.

Through EQE measurements, the scientists found that the analyzed modules suffered UVID losses caused by cell surface recombination losses. Furthermore, they identified a second cause of UVID loss in the lack of zinc (Zn) in the metallization paste used for the analyzed modules.

“UV exposure alone, but beyond the IEC 61215-2 MQT10 standard (15 kWh/m², 280–400 nm), reproduced the surface degradation, but not the increased series resistance,” they explained. “UV exposure of 67.5 kWh/m² (200–400 nm) was needed before there was measurable power loss. Subsequent damp heat stress (1000 h 85/85) caused a severe increase in series resistance of the UV-exposed cells, but not the non-UV-stressed cells. This suggests an important interplay between these stress factors.”

Moreover, the research group exposed the modules to a 1,000 h DH testing that showed further “severe” series resistance degradation even in those panels with a metallization paste containing Zn. “We attribute this to higher concentrations of acetic acid generated on the UV-exposed area of the module, leading to degradation of the gridline/cell interface and high series resistance,” it said.

The academics concluded that the current IEC standards require only minimal UV exposure, which could be detrimental to accurate UVID loss assessments.

“The UV stress test currently included in the IEC 61215-2 standard is 15 kWh/m² (MQT 10), which typically amounts to around 2–3 months in the field, depending on location (1.8 months at the specific utility site studied herein), implying that longer-term UV effects may go undetected,” they emphasized.

Their findings were presented in the study “UV + Damp Heat Induced Power Losses in Fielded Utility N-Type Si PV Modules,” published in Progress in Photovoltaics.

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