Researchers at the University of Turku, Finland, investigated how to use bio-based materials to produce effective UV protection films for solar cells. The study was the first to compare how the properties of different bio-based UV filters change over time.

Solar cells are prone to UV-induced degradation and are most often protected against it by petroleum-based films, such as those made of polyvinyl fluoride (PVF) and polyethylene terephthalate (PET).

Research in materials engineering seeks to find alternatives to oil-based plastics from bio-based materials, one of which is nanocellulose. Nanocellulose is made by breaking down cellulose into nanoscale fibres, which can then be treated in different ways to gain UV protection.

A recent study by the University of Turku and Aalto University in Finland and Wageningen University in the Netherlands found that nanocellulose dyed with red onion skin extract provides very effective UV protection. The nanocellulose film protected 99.9% of UV radiation up to 400 nanometres. This UV filter outperformed even the commercial PET-based UV filter, which was selected to the study to represent the market standard.

“Nanocellulose films treated with red onion dye are a promising option in applications where the protective material should be bio-based,” says Doctoral Researcher Rustem Nizamov from the University of Turku.

The study compared the durability and properties of four types of protective films made from cellulose nanofibers. The nanocellulose films were treated with red onion extract, lignin, and iron ions respectively, all of which have been found to have good UV-blocking abilities in previous research. The film treated with the red onion extract proved to be the most efficient in blocking UV radiation.

Preserving transmission of visible light

UV radiation (below 400nm) is harmful to solar cells, but the transmission of visible light and partly infrared light as well (particularly between 700‒1,200 nm) is important, as the solar cells turn this radiation into electricity.

Developing bio-based materials often involves a trade-off between UV protection and light transmission in visible region. For instance, lignin, a natural polymer known for its UV-absorbing properties, has a dark brown colour that limits its use in transparent films.

The film treated with red onion dye proved to be an interesting solution, exceeding 80% light transmission at longer wavelengths (650‒1,100 nanometres). The film also maintained its performance throughout the long testing period.

The durability and performance of the filters were tested under artificial light for 1,000 hours, which is equivalent to about a year of sunlight in the open air in central European climate. Visual changes in the filter materials and solar cells were monitored using digital photography.

"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. For example, the films treated with iron ions had good initial transmittance which reduced after aging,” tells Nizamov.

The UV filter films were tested on dye-sensitised solar cells, as they are particularly vulnerable to wear induced by UV radiation.

"These results are also relevant for the UV protection of other types of solar cells, including perovskite and organic photovoltaics, as well as any application where the use of a bio-based UV filter is paramount,” Nizamov says.

In the future, the researchers' vision is to develop solar cell types that are biodegradable and can be used as power sources for sensors, for example, in food packaging.

"The forest industry is interested in developing new high-grade products. In the field of electronics, these may also be components for solar cells,” says Professor in Materials Engineering Kati Miettunen.

The Solar Energy Materials and Systems (SEMS) research group at the University of Turku is studying the integration of solar energy and solar energy systems into the energy system. This study was part of the BioEST project funded by the Research Council of Finland.