To achieve this, the researchers used a combination of sputtering and slot-die coating. “Scaling up to large areas is one of the major challenges in organic photovoltaics,” explains Dr. Uli Würfel, head of the Organic and Perovskite Photovoltaics Department at ISE. Laboratory results from small, manually fabricated solar cells often cannot be transferred to industrial production because their fabrication process does not work on larger areas. “The fact that we were now able, for the first time, to deposit virtually all layers of the solar cells with virtually no loss using an industrially viable slot-die process is a major breakthrough for us.” The back electrodes of the solar cells are deposited in the preceding process step using a sputtering process.
Solar modules can replace window glass
In the development of semi-transparent organic photovoltaics, there is always a trade-off between light transmittance and the associated loss of efficiency, expressed as light utilization efficiency (LUE). For the 210.25 cm² semi-transparent organic photovoltaic modules, the research team achieved an efficiency of up to 9.26% with an average visible light transmittance of 43.2%. This corresponds to an LUE of up to 4.0%.
Organic solar modules with light transmittance well above 50% could be used in place of window glass in building facades or greenhouses. If tinted glass is desired—for example, in car roofs and facade elements—lower transparency can even be an advantage.
In each module, more than 100 solar cells were interconnected using laser structuring. The solar cells consist of a back electrode that reflects near-infrared light, which was deposited onto a glass substrate via sputtering; an absorber layer made of organic semiconductors; and a metal-free top electrode made of the polymer Pedot:PSS, which was applied in multiple layers using a slot die. Heraeus Epurio developed a new Pedot:PSS formulation for this top electrode, which helped the PV modules achieve higher transparency.
Manufacturing on film is also possible
Slot-die coating is compatible with roll-to-roll processes and is therefore also suitable for manufacturing solar modules on film. “In the project, we have already produced the first flexible, organic PV modules that retained 100% of their original efficiency after 1,274 bending cycles over a rod with a diameter of 15 mm,” says Dr. Mathias List, research associate for organic and perovskite photovoltaics at Fraunhofer ISE.
The research results are part of the project “Durchblick-PV – Development of Organic Solar Modules with High Visual Transparency,” supported by the Federal Ministry for Economic Affairs and Climate Action (BMWK, now BMWE). Project partners included Heraeus Epurio, Rowo Coating, Asca, and the University of Freiburg.