Shedding light on ultraviolet-induced degradation

Reports first began to emerge of ultraviolet-induced degradation (UVID) as early as 2022, as a potential field reliability issue for some newer high-efficiency PV cell technologies – most notably, n-type tunnel oxide passivated contact (TOPCon) devices. RETC’s “2024 PV Module Index Report” noted that roughly 40% of UVID test samples degraded by 5% or more, but also documented double-digit degradation rates in some mass-produced PV modules. While more recent UVID test results show signs of improvement, the 2026 annual report continues to document red flag results in roughly one in 12 PV module test samples.

Since market analysts predict that TOPCon devices could account for 85% of total solar cell shipments in 2026, it is imperative that the ­technical due diligence community understand UVID causes and effects, especially the extent to which laboratory observations correlate with in-field degradation. With this in mind, RETC embarked on a series of experiments to identify, characterize, and mitigate UVID risk in commercial PV products.

History lesson

The solar industry has long managed the harmful effects of UV radiation on PV modules in the field. Industry veterans are undoubtedly familiar with visible module packaging problems caused by UV exposure, such as encapsulant discoloration, internal delamination, or cracking of the polymer backsheet. Module manufacturers and encapsulant suppliers have largely eradicated the UV degradation problems that once contributed to teething issues in legacy modules, such as ethylene vinyl acetate (EVA) browning. UV light can also contribute to the formation of acetic acid on the encapsulant, which can degrade electrical performance by corroding metalized contacts.

UV exposure can also damage surface passivation and the dielectric layers of antireflective coatings, causing degradation in solar cell performance. Although the root causes of these types of UVID appear to vary by cell type and passivation stack, testing laboratories, researchers, and materials scientists worldwide have noted that modern solar cell technologies frequently exhibit comparatively high degradation rates under accelerated UV exposure. Whereas legacy aluminum back surface field (Al-BSF) PV cells consistently exhibited excellent UV stability, laboratory testing generally indicates that n-type TOPCon cells are the commercial technology most vulnerable to UVID.

To test the potential for UV degradation, RETC worked with technical due diligence experts to develop a rigorous, UVID testing sequence that goes beyond certification. For more than five years, the lab has publicly reported that newer PV cell technologies appeared susceptible to UVID. Other testing laboratories have reported similar results, warning of larger-than-expected efficiency and voltage losses.

Lessons learned

Laboratory-reported levels of UVID susceptibility not only merit industry-wide discussion and dialogue but also demand immediate and collective action to ensure that UVID does not become a major reliability issue in the field. RETC began inviting manufacturers to participate in a series of experiments to address common questions about UV degradation modes and test results. Although simulated long-term exposure testing is ongoing, there is sufficient data to make several preliminary observations.

UVID is not unique to TOPCon devices. Passivated emitter rear cell (PERC) and silicon heterojunction (HJT) are also potentially susceptible to high (more than 2%) UVID rates after 110 kWh/m² UV exposure. In our sample set, the median UVID rate for PERC devices was approximately 2%, as compared to nearly 4% for HJT and TOPCon. The highest degradation rates were evident in TOPCon devices, with over 12% degradation in outlier cases and a cluster of results in the 5%-10% range.

Light soaking reverses some UVID effects. Performance recovers in virtually all test samples after exposure to a full-spectrum light source for 1 to 2 hours. While UV recovery is most significant in TOPCon devices, it is also evident in PERC and HJT. However, full-spectrum light soaking never recovers the full effects of UVID. As UV recovery in the laboratory is always partial, some UV damage could be permanent.

UVID is preventable in TOPCon devices. Even before the UV recovery stage, some TOPCon test samples effectively register 0% degradation during 110 kWh/m² UV exposure, which is roughly equivalent to one year of in-field exposure on a single-axis tracker. Even after multiple cycles of UV exposure, some bills of materials do not exhibit significant degradation. In other words, some manufacturers have succeeded in designing and manufacturing UVID-resistant TOPCon modules.

RETC has seen this firsthand in its work supporting manufacturers’ iterative product design and continuous improvement efforts. After our testing identified high UVID in a particular test sample, one of our customers tried modifying its bill of materials (BOM) by adding a UV blocker that shifts UV wavelengths to the blue region. When RETC tested the subsequent product samples, we were happy to report that the BOM alteration had eliminated the UVID problem. Additional testing is required to assess the overall reliability of this design approach under prolonged exposure to heat, humidity, and thermal cycling.

Real-world applications

Though researchers have thoroughly documented high UV degradation rates in newer high-efficiency PV cell technologies, it remains unclear whether a correlation exists between these laboratory observations and real-world degradation modes. In an outdoor environment, PV modules are not subject only to isolated UV irradiance but to full-spectrum light, which can reverse UVID and induce a variety of metastability effects.

If full-spectrum light exposure ameliorates or prevents UV damage in advanced PV modules, we may one day learn that UVID is a laboratory artifact that has little to no effect in fielded projects. Given the current scale of field deployments, however, RETC recommends that project stakeholders err on the side of caution. After all, developers routinely report perceived instances of solar asset underperformance in real-world operating conditions. If a correlation exists between laboratory-observed UVID and real-world production losses, industry stakeholders must address this problem as soon as possible.

About the author

Cherif Kedir is president and CEO of VDE Group’s North American business units, RETC and VDE Americas. In addition to expertise in product development, failure analysis, and performance enhancement, he has an extensive background in solar and semiconductor durability, reliability, and certification testing.

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