Fraunhofer ISE and Freiburg Researchers Develop Scalable Semitransparent Solar Modules for Smart Windows

Researchers at the University of Freiburg and the Fraunhofer Institute for Solar Energy Systems ISE have demonstrated scalable, semitransparent organic photovoltaic (PV) modules measuring 14.5 by 14.5 centimeters. The 210.25-square-centimeter units achieved energy conversion efficiency of up to 9.26 percent alongside an average visible-light transmission of 43.2 percent, yielding a light utilization efficiency (LUE) of up to 4.0 percent. The research was published in the journal Joule in early August, as first reported by TechXplore.

Organic photovoltaics offer key advantages over conventional silicon panels, including mechanical flexibility, optical transparency, and favorable environmental footprints due to wavelength-dependent light absorption. However, converting small-scale, handmade laboratory cells into standardized industrial manufacturing formats has historically posed a significant technical hurdle. To solve this, the German research team designed a fabrication workflow combining sputtering and slot-die coating—two established industrial processes capable of uniform deposition over large surface areas.

Each module contains more than 100 individual solar cells interconnected through laser structuring. The multilayer architecture features a sputtered back electrode on a glass substrate that reflects near-infrared light, an active absorber layer of organic semiconductors, and a metal-free top electrode made of the conductive polymer PEDOT:PSS applied in multiple layers via slot die. Chemical supplier Heraeus Epurio developed a new PEDOT:PSS formulation specifically for the top electrode, helping the modules maintain higher visual clarity.

Designing semitransparent solar tech requires balancing optical transparency with power conversion output. Dr. Uli Würfel, head of the Organic and Perovskite Photovoltaics Department at Fraunhofer ISE, underscored the importance of transferring these processes to industrial tooling. "Scaling up to larger areas is one of the major challenges in organic photovoltaics," Würfel said. "The fact that we have now, for the first time, successfully applied all the layers of the solar cells using the slot-die process with virtually no loss is a major breakthrough for us."

Following the successful production trials, the team plans to push transparency metrics further without sacrificing electrical performance. "Now that we have the manufacturing process under control, we are optimistic that we can significantly increase transparency without compromising efficiency," Würfel added. Solar modules with light transmission levels exceeding 50 percent could eventually replace standard window panes in commercial real estate and agricultural greenhouses. Conversely, lower-transparency variants remain suitable for applications where tinted glass is preferred, such as sunroofs in automobiles or decorative building facades.

Because slot-die coating integrates easily into high-throughput roll-to-roll production lines, the fabrication technique also translates directly to flexible polymer substrates. Working alongside industrial partner ROWO Coating, which produced specialized film materials, the research team constructed flexible organic solar modules designed to withstand physical deformation. "As part of the project, we have already produced the first flexible, organic PV modules that retain 100% of their original efficiency after 1,274 bending cycles over a rod with a diameter of 15 millimeters (0.6 inches)," explained Dr. Mathias List, a research associate for organic and perovskite photovoltaics at Fraunhofer ISE.

List noted that the project's next phase will focus on scaling these flexible film-based solar modules to larger surface dimensions. The full study, titled "Toward scalable semitransparent organic photovoltaics: Slot-die-coated 210-cm2 modules with visible transmission of up to 50% and LUE up to 4%," was authored by Leonie Pap and her research colleagues, marking a critical milestone toward commercializing building-integrated photovoltaics.