Superhydrophilic Coatings Increase Efficiency of Solar Cells

Researchers at the Fraunhofer Institute for Organic Electronics, Electron Beam, and Plasma Technology FEP (Fraunhofer FEP) produced hydrophobic, or water-repelling, surfaces that become superhydrophilic, or superwetting, under ultraviolet (UV) light. For solar modules, the creation of a superhydrophilic surface will better protect panels and glass facades from expensive and time-consuming cleanings to remove dirt and other debris that affect performance.

In 2021, photovoltaics covered 8.9% of gross electricity consumption in Germany, with electricity generation of 50 TWh. 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.

Fraunhofer FEP researchers fabricated a coating mechanism that keeps solar module surfaces free from performance-affecting debris. A surface coated with titanium dioxide is shown in its initial, hydrophobic state with a water drop contacting at an angle of approximately 95°. Courtesy of Fraunhofer FEP.

The Fraunhofer FEP researchers’ focus is on photoinduced hydrophilicity on surfaces. To upscale this effect, the researchers applied crystalline titanium oxide to ultrathin glass using a roll-to-roll process.

In addition to the efficiency of this process, the ultrathin and lightweight glass can be applied subsequently to facades (to retrofit facades and solar panels) or incorporated directly into the solar modules as a composite material, said Valentin Heiser, a graduate student from Fraunhofer FEP. Further, the glass can be applied onto curved surfaces, Heiser said.

Though unirradiated titanium dioxide is hydrophobic, in the case of the photoinduced hydrophilicity of a titanium dioxide-coated surface, the researchers reported that the surface changes from hydrophobic to superhydrophilic after approximately 30 minutes of irradiation with UV light. On surfaces with such a coating, this effect means that zero or very little dirt can be deposited.

If, for example, traffic dust, sand, or other dirt is deposited 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.

The researchers have developed the first coatings and have coated a 30-cm-wide and 20-m-long roll of thin glass, with a glass thickness of 100 μm, with 30 to 150 nm of titanium oxide in a roll-to-roll system at a pilot plant. Still, challenges remain: Thin glass is a very new substrate with significant handling requirements since it breaks very easily and reacts sensitively to thermal and mechanical stresses. Also, titanium dioxide achieves hydrophobicity and hydrophilicity only when it is crystalline. High temperatures during production are required to make the material crystalline. 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 the high temperatures.

This is where thin glass provides an alternative.

The coated surface after irradiation with UV light for 30 minutes is superhydrophilic. The water droplet contact angle is now less than 5°. Courtesy of Fraunhofer FEP.

The researchers, working within the EU-funded NewSkin project, now target bringing products to market together with industry. Researchers from the Newskin partner Uppsala University are working on transferring the results even to polymer films.

Beyond implications for solar, titanium oxide activated with UV light also decomposes organic molecules on the surface by photocatalysis. This produces antibacterial and sterile surfaces that are of particular interest in medical technology or in connection with flexible displays.

In the future, Fraunhofer FEP researchers will work to develop layer systems that can be activated with visible light. The production and embedding of nanoparticles or doping with nitrogen, for example, are also being considered.

Fraunhofer FEP researchers plan to present initial results of these coatings at BAU 2023, April 17-22, in Munich.