Researchers at the University of Freiburg and Fraunhofer ISE have built semi-transparent organic solar modules using a manufacturing process they say is ready for industrial scale-up, combining sputtering and slot-die coating, two techniques that are already proven in other industries, to produce 14.5 by 14.5 centimetre modules with 43.2 percent average transparency and efficiency up to 9.26 percent.
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The result addresses what the team calls one of the most persistent obstacles in organic photovoltaics. "Scaling up to larger areas is one of the major challenges in the field," saysDr Uli Würfel, head of the Organic and Perovskite Photovoltaics Department at Fraunhofer ISE. Small, hand-made cells routinely produce strong lab results that fail to translate once production moves to larger areas, since the processes used to build them don't scale. In this case, slot-die coating applied every layer of the solar cell with what the team describes as virtually no loss, a result Würfel called a major breakthrough for the group. The back electrodes were deposited in a preceding step using sputtering, itself an established industrial process.
Balancing light and power
Semi-transparent organic photovoltaics involve a persistent trade-off between how much light a module lets through and how much electricity it generates, a relationship the field expresses as light utilisation efficiency, or LUE. At 210.25 square centimetres, the team's semi-transparent modules reached up to 9.26 percent efficiency at 43.2 percent average visible-light transmittance, working out to a LUE of up to 4.0 percent. The results were published in the journal Joule in early August. "Now that we have the manufacturing process under control, we are optimistic that we can significantly increase transparency without compromising efficiency," says Würfel.
Higher transparency would open different applications than lower transparency. Organic modules transmitting more than 50 percent of visible light could substitute for window glass in building facades or greenhouses, while lower transparency suits contexts where tinted glass is already desirable, car roofs and certain facade elements among them.
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Each module links more than 100 solar cells through laser structuring. The cells themselves consist of a back electrode, deposited onto a glass substrate by sputtering, that reflects near-infrared light back into the cell; an absorber layer of organic semiconductors; and a metal-free top electrode built from the polymer PEDOT:PSS, applied in multiple layers via slot die. Heraeus Epurio developed a new PEDOT:PSS formulation specifically for that top electrode, contributing to the modules' higher transparency.
From flat glass to flexible film
Because slot-die coating is compatible with roll-to-roll manufacturing, it also suits production of solar modules on film. "As part of the project, we have already produced the first flexible, organic PV modules that retain 100 percent of their original efficiency after 1,274 bending cycles over a rod with a diameter of 15 millimeters," says Dr Mathias List, a research associate for organic and perovskite photovoltaics at Fraunhofer ISE. ROWO Coating manufactured the films used for this test. "The next step is to achieve larger module areas here as well.“
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Fraunhofer ISE
The work builds directly on an earlier project, Durchblick-PV (See-Through PV), which ran from April 2021 to March 2023 with the same institutional backbone, Fraunhofer ISE, the University of Freiburg, Heraeus Epurio and ROWO Coating, plus ASCA, and the same funding source, the Federal Ministry for Economic Affairs and Climate. That earlier project set out the basic physics this release now reports as manufactured hardware: certain organic semiconductors absorb infrared radiation strongly while transmitting visible light almost unimpeded, a property that, combined with carefully engineered electrodes, allows a transparent solar module to look continuous rather than gappy. Durchblick-PV's own published benchmark, around 30 percent transmittance at roughly 10 percent efficiency, gives useful context for how far the manufacturing side has now moved: broadly comparable efficiency, but transparency up more than ten points, achieved through a production method the earlier project was still under development.
Two electrodes, two jobs
Getting to that point required work on both electrodes simultaneously. Thereby, the front electrode needed very high, broadband transmission so both visible and near-infrared light could pass through. The back electrode needed the opposite behaviour in one wavelength band and the same in another: transmitting visible light while reflecting near-infrared light back into the cell for absorption, a job that was handled by sputtered, silver-based multilayer systems developed at Fraunhofer ISE. Modelling and experimental optimisation balanced competing requirements, cutoff wavelength, edge steepness, electrical contact, sheet resistance and manufacturing cost, before laser structuring turned individual cells into scalable modules.
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The research sits within the wider See-Through PV project, backed by the Federal Ministry for Economic Affairs and Climate, with Heraeus Epurio, ROWO Coating, ASCA and the University of Freiburg as partners. (TF)
