What is UV-induced degradation?

UV-induced degradation (UVID) describes a reduction in the performance of solar cells or solar modules caused by exposure to ultraviolet radiation. While visible light is primarily used for electricity generation, the UV portion of sunlight has higher photon energy and can trigger material changes in polymers, passivation layers, and interfaces. UVID is therefore clearly distinct from other well-known effects such as LID or PID.

In laboratory environments, UV radiation is often used in accelerated tests to simulate aging processes, particularly with a focus on encapsulation materials. However, it has been observed that certain test procedures can make the real performance degradation in the field appear more severe than it actually is under natural operating conditions. For this reason, the relevance and interpretation of conventional UV tests are increasingly being discussed, especially for modern high-efficiency cell technologies.

In which modules can UVID occur?

UVID has mainly been observed in modern crystalline high-efficiency solar cells. This particularly includes TOPCon and heterojunction (HJT) cells. A common characteristic of these technologies is the use of complex passivation layers and sensitive interface structures.

Depending on the material combination, process quality, and layer design, the sensitivity to UV radiation can vary significantly. Older cell concepts with simpler rear-side structures are generally considered less susceptible.

How did UVID become known?

UVID became known through noticeable performance losses observed in accelerated UV tests as well as through individual feedback from operating PV systems in the field. In laboratory testing, certain module types showed unexpectedly high performance losses after intensive UV exposure. At the same time, occasional discrepancies between predicted and measured performance were observed in real installations.

This combination of laboratory findings and practical observations led to further investigations and a broader technical discussion about how UV test results should be interpreted.

When did UVID become an industry-wide topic?

Starting in 2024, UVID increasingly moved into the focus of the solar industry. New research results, discussions about testing methodologies, and uncertainties in project evaluations and financing led operators, investors, and insurers to examine the topic more closely.

In addition to the performance loss observed after UV exposure, so-called metastable behavior was identified in some module types, particularly in TOPCon modules. After UV exposure followed by dark storage, further significant performance losses could be measured. When the modules were subsequently exposed again to natural sunlight, partial recovery of the performance was observed. This dynamic helped put earlier alarming laboratory results into perspective and highlighted the need to distinguish between temporary effects and permanent degradation.

Which factors influence UVID?

The extent of UVID depends on several parameters. These include the specific cell technology, the quality and composition of passivation layers, the stability of tunnel oxides or dielectrics, as well as the intensity and spectrum of UV radiation.

Temperature conditions during exposure and storage conditions after UV irradiation can also play an important role. Studies indicate that modules may initially become destabilized after strong UV exposure. In many cases, a partial stabilization or recovery effect occurs only after renewed exposure to sunlight.

What happens inside the module during UVID?

At the cell level, UV radiation can generate defect states in passivation layers and activate additional charge carrier traps. Interfaces between silicon and adjacent dielectric layers may also be damaged, increasing recombination losses.

Electrically, this often manifests itself as a reduction in open-circuit voltage (Voc) and an overall loss of module power. In TOPCon cells, this mechanism particularly affects the ultra-thin tunnel oxide and the rear-side passivation layer. Some of these changes appear to be reversible, as partial performance recovery can occur after renewed light exposure, although not always back to the original level.

What is the industry doing about UVID?

Manufacturers and testing institutions are responding by further developing UV test protocols and introducing defined stabilization phases after UV exposure. The goal is to obtain more realistic statements about long-term module behavior in the field and to clearly distinguish temporary destabilization effects from permanent performance losses.

At the same time, passivation layers and material systems are being further optimized to improve the UV stability of modern solar cell technologies. Increased field validation of new products is complementing laboratory testing.

How can installers prevent UVID?

Installers cannot directly prevent UVID, as the root cause lies in the cell architecture. However, risks can be significantly reduced through structured quality assurance. This includes documented commissioning measurements, string-level IV curve recordings, and consistent monitoring during the first years of operation.

A goods-incoming test before installation—ideally even before purchasing the modules—is highly recommended. At the PV Test Center of SecondSol, modules can be tested and technically evaluated using flash tests, electroluminescence imaging, and IV curve measurements. This creates a reliable reference for the later operation of the system and provides a solid basis for potential warranty or insurance cases.

If further analysis is required, specialized testing institutes such as the Fraunhofer Center for Silicon Photovoltaics CSP can perform advanced material and cell investigations, including dedicated UVID tests, to clearly distinguish temporary effects from permanent degradation mechanisms.

What to do if a system might be affected?

If a PV system shows unexpected performance deviations during the first year of operation or unusual behavior in open-circuit voltage, a systematic analysis should be carried out. The first step is usually to compare irradiation data with expected yield forecasts. This can be followed by string analysis, IV curve measurements, or targeted module testing.

It is particularly important to consider potential recovery effects caused by renewed sunlight exposure in order to avoid misinterpreting temporary destabilization effects as permanent damage.

Tip if UVID is suspected

If there is a concrete suspicion of UVID, professional module testing should be carried out, taking into account the latest findings regarding stabilization after UV exposure. Proper technical documentation can significantly simplify discussions with manufacturers, insurers, and investors.

If modules need to be replaced, suitable replacement modules can be sourced through the SecondSol online marketplace, helping to minimize yield losses and keep system downtime as short as possible.