Researchers compared UV-blocking, UV-transmitting, and UV-downshifting encapsulants in lightweight SHJ modules under accelerated UV exposure
Modules with conventional UV-transmitting encapsulants lost over 9% efficiency after testing, while the proposed dual-layer design retained more than 98% of initial performance
The study links degradation mainly to passivation losses caused by UV radiation and suggests dual-layer encapsulation as a potential reliability solution for lightweight PV modules
Lightweight photovoltaic (PV) modules are gaining attention for applications such as vehicle-integrated PV (VIPV), portable power systems, and low-load rooftops. These modules use lightweight polymer materials instead of conventional glass, reducing weight and enabling deployment in applications where standard modules are unsuitable. However, the use of polymer-based front sheets increases UV exposure of the solar cells. This also introduces reliability challenges for advanced silicon cell architectures, particularly silicon heterojunction (SHJ) cells, which are known to be highly sensitive to short-wavelength radiation.
Researchers from the Jülich Aachen Research Alliance (JARA), a partnership between Forschungszentrum Jülich and RWTH Aachen University, evaluated the ultraviolet-induced degradation (UVID) of lightweight SHJ solar modules incorporating encapsulants with different UV-transmission properties. The work compared several encapsulation approaches and assessed their effectiveness in improving UV stability in lightweight module designs.
The study used modules fabricated with bifacial n-type rear-junction SHJ cells. Multiple encapsulation materials with different UV-filtering properties were evaluated, including thermoplastic polyolefin (TPO) encapsulants blocking wavelengths below the 350-375 nm range, a standard UV-transmitting EVA encapsulant, and a UV-downshifting EVA encapsulant containing luminescent particles designed to convert part of the UV spectrum into visible blue light. The module structures also incorporated ETFE front sheets and polyolefin-based aluminum backsheets. Compared with conventional glass-based module designs, these lightweight configurations provide less shielding against UV radiation, allowing a greater fraction of UV photons to reach the solar cell and passivation layers.
The modules were subjected to accelerated UV aging in a chamber equipped with mercury UV lamps emitting a spectrum centered around 353 nm. The test exposed the modules to 120 kWh/m² of UV radiation, equivalent to roughly 30 months of outdoor exposure in Jülich, Germany.
The study showed that encapsulation design had a significant impact on UV stability. Among the tested configurations, the standard UV-transmitting EVA module exhibited the highest relative efficiency degradation of 9.25%. The UV-blocking encapsulant configuration showed the lowest degradation at 2.17%, while the UV-downshifting encapsulant reduced degradation to 6.15%. The best performance was achieved with the dual-layer UV-downshifting and UV-blocking configuration, which limited efficiency degradation to less than 2% after testing.
The performance losses were mainly driven by reductions in fill factor (FF) and open-circuit voltage (Voc). After 120 kWh/m² of UV exposure, the UV-transmitting EVA module recorded an FF loss of 4.72 percentage points and a pseudo FF loss of 3.72 percentage points. The corresponding losses for the UV-downshifting module were 3.92 and 2.68 percentage points, respectively. In contrast, modules using UV-blocking encapsulants showed much smaller reductions, with the 375 nm UV-blocking material exhibiting almost no measurable pseudo FF degradation. The researchers attributed this behavior to the deterioration of surface passivation caused by UV photons below approximately 365 nm. These high-energy photons can break Si-H bonds at the SHJ interface, creating dangling-bond defects that increase carrier recombination and reduce both FF and Voc. Additional FF losses were attributed to increasing series resistance within the module structure.
Photoluminescence imaging and external quantum efficiency analysis further confirmed stronger passivation degradation in the UV-transmitting module structures. The downshifting encapsulant reduced direct exposure of the SHJ passivation layers to high-energy photons by converting part of the UV spectrum into visible blue light.
However, the UV-downshifting effect itself gradually deteriorated during prolonged UV exposure in the lightweight modules. The researchers suggested that oxygen and moisture ingress through the polymer front sheet may have contributed to photooxidation of the luminescent downshifting materials, leading to a gradual reduction in the downshifting effect. As the conversion effect weakened, a larger fraction of UV radiation reached the solar cell, increasing passivation-related degradation.
In addition to passivation losses, the researchers observed increasing series-resistance-related FF losses in lightweight module structures using interconnection foils. The study suggested that moisture ingress affecting the interconnection foil contributed to this degradation mechanism. This indicates that reliability challenges in lightweight module designs are not limited to the solar cell itself, but also extend to module materials and interconnection systems.
To improve long-term UV stability, the team proposed a dual-layer encapsulation structure combining a UV-downshifting layer with an additional UV-blocking encapsulant. In this configuration, the upper downshifting layer converts part of the incoming UV radiation into usable visible light, while the secondary UV-filtering layer absorbs residual short-wavelength photons before they reach the SHJ passivation stack.
The combined encapsulation structure demonstrated substantially improved stability during accelerated UV aging. The configuration combining UV-downshifting EVA with the stronger UV-blocking encapsulant retained more than 98% of its initial performance after exposure to 120 kWh/m² of UV radiation. According to the researchers, the dual-layer module also maintained significantly higher post-aging efficiency than modules using conventional UV-transmitting encapsulation.
The findings were published in the paper titled Mitigation of UV-Induced Degradation in Lightweight SHJ Solar Modules via a UV-Downshifting Encapsulation Strategy in Progress in Photovoltaics: Research and Applications.
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