Combined indoor, outdoor testing shows sensitivity to moisture in TOPCon PV modules

Researchers at the Fraunhofer ISE have combined indoor and outdoor testing to understand and quantify degradation processes in TOPCon solar panels. The novel methodology is intended to provide data for validating and improving stress laboratory protocols.

July 23, 2025 Emiliano Bellini

Image: Fraunhofer ISE, RRL Solar, CC BY 4.0

A research team at Germany’s Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) has combined accelerated laboratory aging testing with outdoor exposure to improve the assessment of TOPCon solar module reliability.

The novel methodology is intended to provide data for validating and improving stress laboratory protocols. “By comparing the evolution of lab-induced damage under real outdoor conditions with known field degradation phenomena, it could validate that these individual lab tests effectively target field-relevant failure modes and accurately reflect their potential severity,” the scientists stressed.

In the paper “Combination of Indoor and Outdoor Measurements for the Identification of Degradation Trends in PV Modules,” published in RRL Solar, the researchers explained that one of the main goals of their work was to understand if outdoor conditions accelerate or decelerate specific degradation phenomena observed in indoor environments.

With the proposed approach, they tested six commercially available photovoltaic modules, of which three were based on TOPCon technology, two were designed with a heterojunction (HJT) architecture, and one was based on PERC cells. Their power output ranged from 391. W to 571.5 W. The names of the manufacturers were not disclosed.

The testing was performed for around eight months, with the IV curves being constantly monitored and kept at maximum power point (MPP). Indoor characterization measurements were taken for all modules before and after the accelerated aging.

Different module types after 5 months of outdoor exposure
*Image: Fraunhofer ISE, RRL Solar, CC BY 4.0*

The outdoor testing was conducted at Fraunhofer ISE’s monitoring station in Merdingen, southern Germany, with all modules being tilted at 30° with a south-facing orientation. “IV curve measurements are filtered at irradiance ranging between 775 and 825 W m⁻², with irradiance and temperature fluctuations during curve acquisition kept below 1% for steady state conditions,” the research group specified.

The analysis showed that both TOPCon and HJT modules are prone to moisture ingress, with short-circuit current being affected, which is attributed by the scientists to be due to damage to the encapsulant layer or changes in the optical properties.

Furthermore, they revealed that the highest degradation rates were observed in a TOPCon module encapsulated with polyolefin elastomers (POE), namely 3% in the laboratory and 9.75% after 8 months of exposure at the testing field. “This suggests that even in the absence of acetic acid for POE, other factors, such as non-optimal lamination processing that releases curing residues or reactive species, polymer additives, or solder flux, could still contribute to moisture sensitivity and corrosion,” they further explained.

The outdoor testing also showed that the TOPCon modules are vulnerable to UV-induced degradation, although this was found to be in line with laboratory observations.

“The results suggest that standard IEC tests, while valuable, may not fully capture the complexity of field degradation,” the scientists concluded. “A combination of indoor and outdoor evaluations could further determine degradation risks, helping to improve the overall reliability of these advanced technologies and will be further developed.”

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