Cleaning glass facades and solar installations is expensive and time-consuming, but it’s necessary, as dirt reduces the yield of solar modules. Researchers from the Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP say they have succeeded in making the cleaning process automatic with their European Union-funded “NewSkin project.” The project means applying crystalline titanium oxide to ultra-thin glass using a roll-to-roll process, thus achieving hydrophobic surfaces that become “superhydrophilic” under UV light.

In 2021, photovoltaics covered 8.9% of gross electricity consumption in Germany with electricity generation of 50 TWh. Of course, this should and must be increased in the interests of a sustainable energy transition. Dirt-repellent, easy-to-clean surfaces ensure transparency and cleanliness for facades and more efficient and consistent energy production for solar, with less maintenance costs.

“We are focusing on photoinduced hydrophilicity [water repellency] on surfaces here,” explains graduate student Valentin Heiser from Fraunhofer FEP. “To upscale this effect, we apply crystalline titanium oxide to ultrathin glass in a roll-to-roll process for the first time. This is very efficient. The ultrathin and lightweight glass can be applied subsequently to facades or directly incorporated into solar modules as a composite material – and even onto curved surfaces.”

Titanium dioxide changes its hydrophilicity when exposed to UV radiation (such as activation by sunlight). Unirradiated, it is hydrophobic [forms water drops]. After irradiation, it is superhydrophilic [completely moistening]. In the case of photoinduced hydrophilicity, the surface changes from hydrophobic to superhydrophilic after approx. 30 minutes of irradiation with sun-like UV light.

On surfaces with this kind of titanium dioxide coating, this effect means that no or only very little dirt can be deposited. If, for example, traffic dust, sand or other dirt deposits on glass facades or solar panels, it is washed off by the nightly hydrophobicity of the surface via beading raindrops. In addition, the cyclic alternation of hydrophobic and superhydrophilic properties means that the dirt does not adhere to the surface during the day.

Titanium oxide activated with UV light also decomposes organic molecules on the surface. This produces antibacterial and sterile surfaces.

The researchers at Fraunhofer FEP have now developed the first coatings: Specifically, a 30 cm wide and 20 m long roll of thin glass, with a glass thickness of 100 micrometers, was coated with 30 –150 nanometers of titanium oxide, in a roll-to-roll system. This pilot plant for roll-to-roll coating of thin glass (FOSA LabX 330 Glass from VON ARDENNE) is located at Fraunhofer FEP.

Challenges for this application

One challenge for this showcase project is that thin glass is a new substrate with significant handling requirements, as it breaks very easily and reacts sensitively to thermal and mechanical stresses. Secondly, titanium dioxide achieves its special properties of hydrophobicity and hydrophilicity only when it is crystalline. For this, it requires high temperatures during production. Sputter coatings with these requirements could not be implemented in roll-to-roll technology until now because common substrates, such as films, could not withstand high temperatures. This is where thin glass provides an alternative.

Thanks to this work through NewSkin, Fraunhofer FEP scientists say they are now working on combining the properties of titanium dioxide and thin glass in an optimal and cost-efficient way. Researchers from the Newskin partner Uppsala University are working on transferring the results even to polymer films.

In the future, work will also be done by Fraunhofer FEP on layer systems that can be activated not only with UV light but also with visible light. The production and embedding of nanoparticles or doping with nitrogen, for example, are also being considered.