Rising raw material prices and a lack of availability, especially of silver, are causing the production costs of photovoltaic modules to rise. Researchers at the Fraunhofer Institute have developed an electroplating process in which the expensive precious metal silver is substituted by readily available copper. The researchers were able to replace the polymers that are usually produced during electroplating, which are costly to dispose of. Instead, they use aluminum for masking, which can be easily recycled. In order to bring the technology to market more quickly, the spin-off PV2+ was founded.
Photovoltaics is a mainstay of electricity generation from renewable energies. Modern heterojunction solar cells have a particularly lowcarbon footprint due to their low silicon consumption and achieve the highest efficiencies in industrial production. Accordingly, there is a good chance that this technology will become the standard in production. The figures prove the increasing importance of photovoltaics. According to the International Renewable Energy Agency (IRENA), global electricity generation from photovoltaic systems was more than 96 TWh in 2012, rising to nearly 831 TWh by 2020. According to the Federal Environment Agency, electricity generation from photovoltaics in Germany rose from just under 27 TWh to just under 50 TWh in the same period.
The potential of photovoltaics is far from exhausted. However, valuable silver is used in the manufacture of solar cells for the conductive tracks and contacts. They dissipate the current generated in the silicon layer by solar radiation. Prices for the precious metal are rising; silver already accounts for around 10 percent of the manufacturing price of a photovoltaic cell. Furthermore, silver is only available in limited quantities on earth. The solar industry processes 15 percent of the silver mined. Due to the strong growth, this share would have to rise sharply. However, this is hardly feasible, as other industries such as electromobility or 5G technology are also announcing higher silver consumption for the future. Therefore, the solar industry will not be able to realize its full power without breakthrough technological innovations.
Copper for the conductors
Researchers at the Fraunhofer Institute for Solar Energy Systems ISE have taken up this challenge. With around 1400 employees, the Fraunhofer Institute in Freiburg is the largest solar research institute in Europe. A team of researchers led by Dr. Markus Glatthaar, an expert in metallization and patterning, has developed an electroplating process that replaces silver with copper in the promising heterojunction technology. Copper is many times cheaper and more readily available.
To prevent the entire electrically conductive surface of the solar cell from being electroplated with copper, the areas that are not to be coated must be masked beforehand. These areas receive an electrically insulating coating that prevents galvanic deposition. Thus, the copper layer grows only in the non-insulating coated areas.
The researchers have made a second significant advance here: To mask the silicon wafer in the electrolyte bath, the industry previously used expensive polymer-based coatings or films. Proper disposal of the polymers is costly and causes a lot of waste. Glatthaar and his team succeeded in substituting the polymers with aluminum. Like copper, aluminum can be fully recycled. The double switch – from silver to copper and from polymer to aluminum – also brings a double advantage: the production of solar cells becomes more sustainable and at the same time significantly cheaper.
Breakthrough with innovative electroplating and improved electrolyte
How did the researchers manage to replace the expensive precious metal silver? “We have developed a special electroplating process that makes it possible to use copper for the conductive tracks instead of silver,” explains Glatthaar. Even the performance is improved as a result. This is because the copper conductors are particularly narrow thanks to laser structuring. Due to the extremely small diameter of only 19 micrometers, the shadowing of the light-receiving silicon layer is less than with silver tracks. This improves the performance and ultimately the electricity yield.
The second technological achievement of the Fraunhofer team is the use of aluminum as a masking layer. One difficulty is that aluminum is electrically conductive and therefore not suitable for masking. The Fraunhofer researchers take advantage of the fact that aluminum forms an insulating oxide layer on its surface by itself. But this is only a few nanometers thick. “We succeeded in adapting the process parameters and developing a special electrolyte so that the extremely thin, native oxide layer of the aluminum can reliably fulfill its insulating function. This was an important milestone for the success of our research project,” says Glatthaar.
The recyclable materials copper and aluminum bring photovoltaic production one step closer to the circular economy. This also improves environmental and social standards. “Since we have enough copper in Germany, the supply chains are shorter, and dependence on the price on the international raw material markets or on foreign suppliers is reduced,” adds Glatthaar.
Spin-off PV2+ brings solar technology to market
In order to bring this promising technology to market more quickly, Fraunhofer ISE has founded the spin-off PV2+. The letters P and V stand for photovoltaics, 2+ for two positively charged copper ions in the electroplating bath. The company is also based in Freiburg. Fraunhofer researcher Markus Glatthaar serves as CEO of the spin-off. He wants to set up pilot production together with industrial partners as early as the beginning of 2023.
Prof. Dr. Andreas Bett, Institute Director at Fraunhofer ISE, explains: “The innovative solar cells are an important building block for the future power supply from renewable energies and will give the photovoltaic industry a much needed boost. The spin-off has the best chances to establish itself quickly and successfully on the market. Of course, we are all the more pleased that these technologies were created at our institute.” In addition, the German Federal Ministry of Economics and Climate Protection (BMWK) is funding the project as part of its “Exist” start-up program.
