A German research team has investigated the UV-induced degradation (UVID) behavior of lightweight silicon heterojunction (HJT) solar modules utilizing encapsulants with different UV-transmission. Based on their findings, they have proposed a novel encapsulation architecture that combines UV-downshifting and UV-blocking encapsulants to ensure UV utilization and stability in lightweight SHJ solar modules.

“Solar modules featuring this innovative dual-layer structure preserved over 98% of their initial performance after 120 kWh/m2 of UV exposure, demonstrating a promising new approach for enhancing UV stability,” said corresponding author Kai Zhang to pv magazine. “In-depth investigation of the degradation mechanism of the downshifting effect and optimizing strategy will be investigated in our follow-up research.”

The investigation started with the fabrication of lightweight HJT solar modules using several encapsulation materials with different ultraviolet transmission properties. All module configurations primarily employed 156.75 mm × 156.75 mm zero-busbar (0BB) bifacial monocrystalline n-type rear-junction HJT cells, with the encapsulant type as the main experimental variable. The modules also incorporated an ethylene tetrafluoroethylene (ETFE) front sheet and a polyolefin (PO)-based aluminum backsheet.

Four encapsulant types were investigated. Thermoplastic polyolefin (TPO) served as a UV-blocking encapsulant, blocking wavelengths below 350 nm (UV-B350), while PO served as a stronger UV-blocking encapsulant, blocking wavelengths below 375 nm (UV-B375). Ethylene-vinyl acetate (EVA) was used as a UV-transmitting encapsulant (UV-T), allowing most UV radiation to reach the solar cell. In addition, EVA containing downshifting particles that absorb UV photons and re-emit them as blue visible light was tested (UV-DS).

 The fabricated modules were subjected to accelerated UV aging tests in a UV chamber equipped with mercury light tubes producing a UVA spectrum peaking at 353 nm. The modules received a cumulative UV dose of 120 kWh/m², corresponding to approximately 30 months of outdoor exposure in Jülich, Germany, and were characterized after every 15 kWh/m² increment. Based on the results, the UV-B, UV-T, and UV-DS modules exhibited relative efficiency losses of 2.17%, 9.25%, and 6.15%, respectively. The decrease in efficiency was mainly attributed to a reduction in the fill factor of the solar modules, accompanied by a diminished pseudo fill factor.

“The results revealed that lightweight HJT solar modules with UV-downshifting encapsulants effectively mitigated UV-induced degradation,” Zhang explained. “However, the UV-downshifting encapsulants cannot completely shift UV photons, and a reduction in the downshifting effect was observed, supposedly caused by the photo-oxidation.”

To address this issue, the scientists have developed an architecture that combines UV-downshifting and UV-blocking encapsulants. In the new structure, a UV-downshifting EVA layer is placed above an additional UV-blocking encapsulant layer. The UV-downshifting layer converts incoming UV photons into visible blue light that can be more efficiently used by the HJT cell, while the UV-blocking layer absorbs UV photons that were not converted. The layers are then combined above the solar cell together with interconnection foils and edge sealing.

“Solar modules featuring this innovative dual-layer structure preserved over 98% of their initial performance after UV exposure, demonstrating a promising new approach for enhancing UV stability,” the researchers said. “The efficiency of this dual-layer solar module after UV exposure was 8.92% higher than that of the solar module with UV-transmitting encapsulant. The dual-layer structure provides more initial power, which remains high even after UV degradation, showing promising energy yield.”

The novel technique was presented in “Mitigation of UV-Induced Degradation in Lightweight SHJ Solar Modules via a UV-Downshifting Encapsulation Strategy,” published in Progress in Photovoltaics: Research and Applications. Scientists from Germany’s Forschungszentrum Jülich, Jülich Aachen Research Alliance, and RWTH Aachen University have participated in the study.