Forward-looking: Scientists in Finland have identified a new bio-based material that could help extend the lifespan of solar cells while reducing reliance on petroleum-based plastics. A team led by the University of Turku, working with Aalto University and Wageningen University, has shown that nanocellulose films infused with red onion skin extract provide durable ultraviolet protection for solar cells, outperforming conventional protective films made from polyethylene terephthalate.
Solar cells face gradual deterioration when exposed to ultraviolet radiation, which can damage sensitive layers such as electrolytes in dye-sensitized cells. To mitigate this, manufacturers typically use polymer coatings such as PVF or PET, which shield against UV rays but are derived from fossil fuels and do not biodegrade easily. Researchers in Turku set out to explore whether renewable, cellulose-based materials could be engineered to provide the same level of protection.
Their solution involved nanocellulose, a material created by breaking cellulose into nanoscale fibers. Thin films of nanocellulose were treated with natural UV-absorbing agents, including lignin, iron ions, and an extract from red onion skins. All three materials were previously noted for their UV-blocking potential, but the study marked the first systematic comparison over extended exposure periods.
The film that incorporated red onion skin extract proved to be the most effective. Tests showed it blocked 99.9 percent of UV radiation up to 400 nanometers and transmitted more than 80 percent of light in the 650 to 1,100 nanometer range. Maintaining high transmission of visible and near-infrared light is critical because this spectrum contributes directly to solar electricity generation.
In order to evaluate how these bio-based films perform over time, the team subjected the materials to 1,000 hours of artificial solar irradiation – about the equivalent of one year of exposure in central European outdoor conditions. Researchers monitored both the protective films and the solar cells underneath, observing visual changes and measuring light transmittance.
The nanocellulose film with red onion extract preserved its optical qualities remarkably well, showing only minor discoloration yet continuing to protect the solar cells. By comparison, materials treated with iron ions initially allowed good light transmission but degraded significantly during the test. Lignin-based films also absorbed UV light effectively but were naturally darker in color, limiting their ability to transmit visible light needed for cell efficiency.
Doctoral researcher Rustem Nizamov from the University of Turku noted that the long-term testing revealed significant differences in stability among the bio-based films. "The study emphasized the importance of long-term testing for UV filters, as the UV protection and light transmittance of the other bio-based filters changed significantly over time," he said.
The researchers carried out their experiments on dye-sensitized solar cells, a type especially vulnerable to UV-related degradation. Still, they argue the results have broader implications. Perovskite and organic photovoltaic technologies also require effective UV shielding but often depend on non-biodegradable plastics. Using films derived from renewable sources such as plant waste could provide alternatives that are both durable and sustainable.
The project forms part of a wider push within Finland's forest and materials industries to develop new products that add value to natural resources. "The forest industry is interested in developing new high-grade products. In the field of electronics, these may also be components for solar cells," said Kati Miettunen, professor of materials engineering at the University of Turku.
The team behind the study envisions a future where fully biodegradable solar technologies can be integrated into products where recovery is not feasible, such as disposable sensors or smart packaging. By replacing petroleum-based protective films with renewable nanocellulose enhanced by natural pigments, researchers believe solar technology could become both more sustainable and more adaptable to new applications.
The work was carried out within the BioEST project, funded by the Research Council of Finland, and the findings were recently published in ACS Applied Optical Materials.