Finnish scientists have recently developed a new bio-based material that could extend the life of solar cells and reduce reliance on petroleum-based plastics. A team led by the University of Turku in collaboration with Aalto University and Wageningen University has demonstrated that nanocellulose films incorporating red onion skin extract are superior to traditional polyethylene terephthalate (PET) protective films in terms of UV protection.
Solar cells gradually deteriorate after exposure to ultraviolet radiation, damaging the electrolyte layer such as in dye-sensitized cells. To this end, manufacturers often use polymer coatings such as polyvinyl fluoride (PVF) or PET for protection, but these materials are derived from fossil fuels and do not degrade easily. The goal of this research is to explore whether renewable, cellulose-based materials can achieve the same protective effect.
The research team chose nanocellulose as the basis, made a film by extracting nanoscale fibers, and treated it with natural UV absorbers (including lignin, iron ions and red onion peel extract). All three have previously had the potential to block UV rays, but this study is the first to conduct a long-term systematic comparison.
The results showed that films incorporating red onion skin extract performed best. Tests have shown that the film can block 99.9% of ultraviolet rays below 400 nanometers, while retaining more than 80% of light transmittance in the 650 to 1100 nanometer band. Maintaining high transmittance of visible light and near-infrared light is crucial for solar power generation.
The researchers placed the dye-sensitized solar cell under the film and exposed it to artificial sunlight for 1,000 hours (approximately equivalent to one year of outdoor sunshine in Central Europe), and continued to observe the performance and appearance of the film and battery. As a result, the red onion skin film showed only slight discoloration and basically maintained its optical properties, effectively protecting the battery at all times. In contrast, the membrane treated with iron ions can transmit light well in the early stage, but then degrades rapidly. Although the lignin membrane has strong UV blocking ability, it has a darker color, which significantly affects the visible light transmittance.
Rustem Nizamov, a doctoral researcher at the University of Turku, said that long-term testing revealed significant differences in the stability of various bio-based membranes. "This study highlights the importance of long-term testing of UV filters. Other bio-based filters have obvious changes in UV protection and light transmittance over time."
The research focuses on dye-sensitized solar cells, which are particularly sensitive to UV degradation. The team pointed out that the results are applicable to perovskite and organic photovoltaic technologies. These cutting-edge solar cells also require reliable UV protection and currently rely on non-degradable plastics. Protective films made from plant waste, such as onion skins, are not only durable but also offer sustainability benefits.
The research is part of the Finnish forest and materials industry's push to develop value-added products from natural resources. Kati Miettunen, professor of materials engineering at the University of Turku, pointed out that the forest industry hopes to develop new high value-added products, which are also expected to become key components in the electronic field such as solar cells.
The team envisions that in the future this fully biodegradable material could be used in products where recycling is not feasible, such as disposable sensors or smart packaging. Replacing the petroleum-based protective layer with nanocellulose enhanced with natural pigments not only promotes the sustainable development of solar energy technology, but also helps expand application scenarios.
The research was supported by the BioEST project funded by the Finnish Research Council, and the results have been published in the journal ACS Applied Optical Materials.