Sci
ence & eoogy · Renewable Energy
Finnish Scientists Turn Red Onion Skins Into a Natural UV Shield
A University of Turku led study shows that a biodegradable film dyed with red onion skin extract blocks 99.9% of UV radiation, opening the door to a compostable alternative to the plastic backsheets protecting the world's solar panels.
Solar power has become the cheapest new source of electricity in most of the world, and panels now blanket rooftops, deserts and reservoirs from Rajasthan to Rotterdam. But the same sunlight that solar cells convert into electricity is quietly working against them. Ultraviolet radiation degrades the plastic layers that protect a panel's delicate photovoltaic materials, and over twenty or thirty years that degradation chips away at the energy output utilities and homeowners were promised. A team of researchers in Finland has now proposed an unlikely fix, a UV shielding film made from nanocellulose dyed with the skins of red onions, and early results suggest it could outperform the petroleum based films the industry has used for half a century.
How Sunlight Slowly Damages Solar Panels
A solar panel is a sandwich of glass, encapsulant, photovoltaic cells and a rear backsheet, and every one of those layers is engineered to survive decades outdoors. In practice, ultraviolet light below roughly 400 nanometres is the harshest part of that outdoor exposure. It breaks chemical bonds in the encapsulant, usually ethylene vinyl acetate, causing the yellowing and browning that solar technicians recognise on older panels. It embrittles plastic backsheets, leading to micro cracking and eventually to delamination, where layers separate and moisture creeps in. For dye sensitised and organic solar cells, which use light sensitive dyes and organic semiconductors instead of pure silicon, UV exposure is even more damaging because it directly attacks the electrolyte and dye molecules that generate current.
This is not a theoretical risk. A 2025 field study by the U.S. Department of Energy's National Renewable Energy Laboratory on an operating rooftop system found power output falling by roughly 2.4% a year, well above typical crystalline silicon degradation rates, with researchers tracing much of the loss directly to ultraviolet induced degradation the current international testing standards were not designed to catch.
To manage this, manufacturers laminate panels with protective films made from polyethylene terephthalate, commonly known as PET, or polyvinyl fluoride, sold under trade names such as Tedlar. These petroleum derived films work well, but they do not biodegrade, they are energy intensive to produce, and they still degrade under prolonged UV exposure, gradually losing the very protective qualities they were designed to provide. With global solar capacity expected to keep climbing for decades, according to the International Energy Agency's renewables outlook, the volume of plastic backsheet material reaching end of life is becoming a genuine waste management question, not just an engineering one. A joint IRENA and IEA-PVPS assessment projects that cumulative solar panel waste, most of it glass, backsheet plastic and encapsulant, could reach 78 million tonnes worldwide by 2050.
🧅 Key Facts at a Glance
- Researchers from the University of Turku, Aalto University and Wageningen University developed a nanocellulose film dyed with red onion skin extract.
- The film blocked 99.9% of UV radiation up to 400 nanometres, beating a commercial PET based filter used as the industry benchmark.
- It let through more than 80% of light between 650 and 1,100 nanometres, the range solar cells need to generate electricity.
- Modelling based on early degradation trends suggested the onion dyed film could stretch a cell's working life to about 8,500 hours, against roughly 1,500 hours for the PET filter.
- The active ingredients are anthocyanins along with flavonol glycosides and phenolic acids, natural pigments and compounds concentrated in the papery outer skins of red onions.
- The research was published in the journal ACS Applied Optical Materials and funded through Finland's BioEST project, backed by the Research Council of Finland.
- Findings were tested on dye sensitised solar cells but researchers say the approach is also relevant to perovskite and organic photovoltaics.
Finland's Answer: Turning Kitchen Waste Into a Solar Shield
The project began with a fairly ordinary materials science question, how do you replace fossil fuel plastics in an application where sunlight is both the enemy and the resource. The team at the University of Turku, working with colleagues at Aalto University in Finland and Wageningen University & Research in the Netherlands, tested four different bio based coatings built on nanocellulose, a material made by breaking cellulose fibres down to the nanoscale. Cellulose nanofibres on their own let too much light through unfiltered, so the team treated them with different natural additives known to absorb UV light, including lignin, iron ions and dye extracted from red onion skins.
Doctoral researcher Rustem Nizamov, who led the laboratory work, said the results marked out the onion treated film as the standout performer among the bio based options tested. He described the treated nanocellulose as "a promising option in applications where the protective material should be bio-based," a modest way of describing a film that beat the market standard plastic outright in blocking harmful radiation.
What makes the discovery notable is not simply that onion skins can filter UV light. Various plant pigments have shown UV blocking properties before. It is that the red onion film managed the much harder balancing act that has held back earlier bio based attempts, filtering out damaging short wavelength light while still letting through the visible and near infrared light a solar cell needs to actually generate power. Lignin, for example, absorbs UV effectively but its dark brown colour also blocks too much of the useful visible spectrum, making it impractical as a transparent protective coating.
The Chemistry of the Peel: Anthocyanins at Work
Red onions owe their deep purple red colour to anthocyanins, water soluble pigments that also occur in blueberries, red cabbage and grape skins. Anthocyanins absorb strongly in the ultraviolet and blue regions of the light spectrum, which is part of why they evolved in plants in the first place, as a natural sunscreen protecting delicate tissue from radiation damage. Red onion skins also carry flavonol glycosides and phenolic acids, compounds known for antioxidant activity that appear to add further UV stability to the film over time.
When researchers extracted these compounds from discarded onion skins and used them to dye nanocellulose sheets, the resulting film behaved less like a simple coloured filter and more like an engineered optical material, absorbing sharply below 400 nanometres while remaining largely transparent to the wavelengths solar cells rely on. Because the base material, nanocellulose, is itself derived from wood pulp fibres and is fully biodegradable, the entire film can in principle be composted at the end of a panel's life, unlike PET or PVF backsheets that persist in landfill for decades.
Nizamov and his colleagues note the findings extend well beyond dye sensitised solar cells, adding that the same UV filtering approach is relevant to perovskite and organic photovoltaics, two of the fastest growing next generation solar technologies.
The Numbers That Matter: Testing Durability, Not Just Performance
What separates this study from earlier lab curiosities is its emphasis on long term testing rather than a single snapshot measurement. The team exposed all four film types, along with a commercial PET filter, to 1,000 hours of artificial solar irradiation, roughly equivalent to a year of outdoor sunlight in a central European climate. Over that period they tracked both visual changes and light transmittance, since a film that starts strong but degrades quickly is of limited practical use on a panel expected to last a quarter century.
The iron ion treated film illustrated exactly that risk, starting with good transmittance that fell noticeably as testing continued. The red onion dyed film, by contrast, held its UV blocking performance and transparency steady across the full test period. Extrapolating from these degradation curves, the researchers estimated the onion treated nanocellulose film could extend a solar cell's operational lifetime to around 8,500 hours under equivalent conditions, compared with roughly 1,500 hours projected for the PET based benchmark film, a more than fivefold improvement reported by outlets covering the peer reviewed findings. The full study methodology and supporting data are available via the National Institutes of Health's PubMed Central archive, which hosts related open-access nanocellulose UV filter research from the same lab group.
It is worth being precise about what that figure means. These are accelerated laboratory hours under artificial irradiation designed to compress years of outdoor UV exposure into a manageable test window, not a literal guarantee of real world panel lifespan. Translating lab durability data into field performance still requires further outdoor pilot testing, something the research team has signalled as a next step. Even with that caveat, a fivefold improvement over the current commercial standard is a striking result for a first generation bio based material.
Why Finland: Forests, Circular Economy and the BioEST Project
The research did not emerge in isolation. It forms part of Finland's broader strategy to find higher value uses for its enormous forestry sector, which has traditionally supplied pulp, paper and timber to global markets. Nanocellulose is one of several materials Finnish researchers and companies are investing in as a way to diversify beyond commodity paper products into advanced materials for electronics, packaging and energy applications. The onion skin study was carried out under the BioEST project, funded by the Research Council of Finland, which supports work at the intersection of sustainable materials and applied technology.
Professor Kati Miettunen, who leads the Solar Energy Materials and Systems research group at the University of Turku, framed the work as part of a wider ambition for Finland's forest industry to supply components for electronics, not just raw pulp. The long term vision the team has described includes fully biodegradable solar cells suited to applications where recovery and recycling are impractical, such as disposable sensors, smart food packaging and wearable devices, uses where a solar cell only needs to function for a limited period before being safely composted.
💡 Key Takeaways
- UV radiation is a genuine long term threat to solar panel performance, degrading encapsulants and backsheets and reducing power output over a panel's service life.
- A nanocellulose film dyed with red onion skin extract outperformed a commercial PET UV filter in both UV blocking and durability over 1,000 hours of accelerated testing.
- The onion film's 99.9% UV blocking came without sacrificing transparency to the visible and near infrared light solar cells need to generate power.
- The material is biodegradable and made from food industry waste, aligning with circular economy goals as global solar deployment scales up.
- The technology is still at an early, lab tested stage and needs outdoor field trials and manufacturing scale up before it reaches commercial panels.
- Applications could extend beyond conventional silicon panels to perovskite and organic photovoltaics, and eventually to fully biodegradable, disposable solar cells for sensors and packaging.
What This Means for the Global Solar Industry
Global solar capacity has grown at a pace few analysts predicted even five years ago, and with that scale comes a second, less discussed challenge, what happens to the materials in a panel once it degrades or reaches the end of its warranty. Backsheet failure and UV related degradation are already recognised causes of reduced energy yield in ageing solar fleets, and the disposal question for plastic laminates is becoming more pressing as the first large wave of utility scale solar installations approaches retirement in the coming decade.
A biodegradable, plant derived UV filter that matches or beats current plastic films would not immediately replace every backsheet on the market. Manufacturing costs, large scale production of onion derived dye, and long term outdoor validation all need to be worked out before the material could be integrated into commercial panel production lines. But the direction of the research fits a pattern seen elsewhere in materials science, where researchers are increasingly mining agricultural and food processing waste streams for functional compounds, turning what was once discarded into an input for advanced manufacturing. Red onion skins, typically stripped away and thrown out before onions reach a kitchen or a supermarket shelf, represent exactly that kind of low cost, abundant waste stream.
For an industry under growing pressure to reduce its own environmental footprint even as it displaces fossil fuels elsewhere in the energy system, a film like this addresses two problems simultaneously, cutting reliance on petroleum based plastics while potentially extending the working life of the panels themselves.
Conclusion
The idea that a vegetable peel most people scrape into a compost bin could help protect one of the defining technologies of the clean energy transition is the kind of finding that tends to travel fast, and it deserves the attention, but the underlying science is what makes it credible rather than merely charming. Anthocyanins and related compounds in red onion skins genuinely absorb UV light in the range that damages solar cells, and when bound into a nanocellulose film they held that protective performance steady across sustained testing in a way that other bio based alternatives did not. The result is not yet a finished commercial product, and researchers themselves are careful to frame it as a promising early stage material rather than a ready replacement for PET or PVF backsheets. Still, as solar power continues its rapid global expansion, the search for durable, non plastic protective materials is only going to matter more, and Finland's red onion breakthrough offers a genuinely useful data point for where that search might lead.
Frequently Asked Questions
How does sunlight actually damage a solar panel?
Ultraviolet radiation below about 400 nanometres breaks down chemical bonds in a panel's encapsulant and backsheet, causing yellowing, embrittlement, micro cracking and eventually delamination, all of which reduce how much sunlight reaches the photovoltaic cells and lower power output over time.
What did the Finnish researchers actually discover?
A team from the University of Turku, Aalto University and Wageningen University found that a biodegradable nanocellulose film dyed with red onion skin extract blocked 99.9% of UV radiation up to 400 nanometres, outperforming a commercial PET based UV filter in both protection and long term durability.
Why red onion skin specifically, and not another plant material?
Red onion skins are rich in anthocyanins, along with flavonol glycosides and phenolic acids, natural pigments and compounds that absorb UV light strongly while remaining largely transparent to visible and near infrared light, a balance other bio based options tested, such as lignin, could not match.
Is this technology available in solar panels today?
No. The findings come from laboratory testing on dye sensitised solar cells under accelerated artificial irradiation. Outdoor field validation, cost analysis and manufacturing scale up are still needed before the film could appear in commercially sold panels.
Could this material work on other types of solar cells?
Researchers involved in the study say the approach is also relevant to perovskite and organic photovoltaics, two rapidly developing next generation solar technologies, as well as to any application requiring a bio based UV filter.
Where was the research published?
The study appeared in the peer reviewed journal ACS Applied Optical Materials and was funded through Finland's BioEST project, supported by the Research Council of Finland.
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Sources
University of Turku press release · ACS Applied Optical Materials, DOI: 10.1021/acsaom.4c00484 · PubMed Central open-access archive · Aalto University research portal · Research Council of Finland · IEA Renewables 2025 report · IRENA / IEA-PVPS end-of-life PV report · NREL / pv magazine on UV-induced degradation · NIH Office of Dietary Supplements on anthocyanins · Live Science · Slashdot / ZME Science
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