In a lab in Turku, Finland, scientists have found a surprising ally in the fight for sustainable solar energy: the papery red skin of an onion.

Researchers from the University of Turku, in collaboration with Aalto University and Wageningen University, have developed a bio-based UV protection film for solar cells that not only blocks nearly all harmful ultraviolet light but also outperforms commercial plastic films. The key ingredient is a water extract made from red onion skins.

“Nanocellulose films treated with red onion dye are a promising option in applications where the protective material should be bio-based,” said Rustem Nizamov, a doctoral researcher at the University of Turku.

A Biodegradable Barrier

Solar cells convert light into electricity. But the same sunlight that powers them can also degrade their delicate components—particularly the electrolyte inside dye-sensitized solar cells (DSSCs), a type known for their flexibility and low-light performance. To mitigate this, manufacturers typically wrap cells in UV-protective films made from petroleum-based plastics like polyethylene terephthalate (PET). But these plastics degrade over time and are difficult to recycle.

Seeking a greener alternative, the team turned to nanocellulose, a renewable material derived from wood pulp. Nanocellulose can be processed into thin, transparent films that serve as the perfect substrate for UV-blocking compounds.

Their breakthrough came when they dyed these films using an extract from red onion skins, a common kitchen waste. The result was a filter that blocked 99.9% of UV radiation up to 400 nanometers, a feat that outstripped even the PET-based commercial filters chosen for comparison.

In solar cells, preserving visible and near-infrared light is crucial. That’s the part of the spectrum that powers electricity generation. And here, too, the onion-treated filter excelled: it let through over 80% of light in the 650–1,100 nm range—an ideal sweet spot for energy absorption.

What 1,000 Hours of Sunlight Revealed

But how long can the material last?

The team put their filters through a rigorous test: 1,000 hours of artificial sunlight exposure, which is equivalent to about one year of outdoor use in central Europe. They applied the filters to DSSCs and monitored the degradation of both the film and the solar cells beneath. The loss of color, particularly a yellowing or “bleaching” of the electrolyte, signals the degradation of the solar cell’s core chemistry.

“The study emphasised 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,” Nizamov explained. “For example, the films treated with iron ions had good initial transmittance which reduced after aging.”

The CNF-ROE film—short for cellulose nanofiber with red onion extract—held up remarkably well. It exhibited only minor discoloration and preserved the yellow hue of the electrolyte far better than any other filter. Even predictive modeling based on early degradation trends suggested the CNF-ROE filter could extend a solar cell’s lifetime to roughly 8,500 hours. The PET-based filter? Just 1,500 hours.

Onion Seasoned Solar Power

The researchers tested three other bio-based filters: one containing iron ions, and another using lignin nanoparticles—an industrial byproduct of papermaking. While both showed promise in early tests, they degraded more rapidly under UV light. In particular, films treated with iron (TOCNF-Fe³⁺) showed decent UV blocking at first, but their transmittance and structural integrity diminished noticeably with time.

By contrast, the red onion extract offered a rare combination of longevity, transparency, and sustainability.

That’s partly due to anthocyanins, the pigment molecules that give red onions their deep color. These compounds are known to absorb UV radiation. But red onion skins also contain flavonol glycosides and phenolic acids, which may contribute additional stability.

The team envisions biodegradable solar cells for smart packaging, remote sensors, or wearable devices—especially in applications where recovery and recycling are not feasible.

Their work is part of the BioEST project, funded by the Research Council of Finland, which supports sustainable innovation across electronics and materials science.

Why Does This Matter?

This achievement taps into a broader movement to decarbonize every step of solar energy production. Plastic packaging is one of the overlooked sources of emissions in clean technology. Swapping out fossil-based plastics for biodegradable alternatives helps close that loop.

But the stakes go beyond packaging. As solar panels proliferate the demand for sustainable, long-lasting materials will only grow.

Right now, most commercial solar modules still rely on protective films derived from petroleum. Even newer solar technologies, like perovskite cells, remain highly sensitive to UV light and urgently need better barriers.

This is where a biodegradable material that outperforms the current standard—made from what would otherwise be food waste—could make a quiet revolution.

And for that, we may have to thank the humble red onion.

The findings appeared in the journal Applied Optical Materials.