In addition to wind power, photovoltaics is currently the second mainstay of electricity generation from renewable energies. In particular, modern heterojunction solar cells, i.e. the combination of crystalline photovoltaics with thin-film technology, achieve promisingly high efficiencies in industrial production while at the same time consuming little silicon - which also means that these modules have a comparatively low CO2 footprint. Accordingly, there are good prospects that this technology will become the standard in production. In principle, it can be assumed that the importance of photovoltaics will increase, fueled not least by the shortage of fossil fuels as a result of the war in Ukraine. But even before that, global electricity generation from photovoltaic systems rose from around 96 terrawatt hours in 2012 to just under 831 terrawatt hours in 2020, according to the International Renewable Energy Agency (IRENA). By comparison, according to the Federal Environment Agency, electricity generation from photovoltaics in Germany rose from around 27 terrawatt hours to just under 50 terrawatt hours in the same period.

Push for silver, everything depends on silver

But these figures also show that the potential of photovoltaics is far from exhausted. But where there is, hopefully, a lot of light, there is usually also some shadow. For example, the production of solar cells currently consumes a lot of valuable silver for conductors and contacts. And as with so many raw materials, the prices for the precious metal have also risen sharply recently. Today, silver already accounts for around 10 percent of the cost of manufacturing photovoltaic cells. In addition, silver, like all precious metals, is only available in limited quantities. The solar industry already consumes 15 percent of the silver mined. Since other industries such as electromobility or mobile communications are also announcing higher silver consumption for the future, the expected production growth in photovoltaics could lead to a bottleneck effect for silver. Or to put it another way: without sweeping technological innovations, the solar industry will hardly be able to fulfill the hopes placed in it.

And what if we use copper instead?

Researchers at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg also recognized this problem at an early stage and subsequently devoted themselves to this challenge. And that means a certain amount of momentum, because with around 1400 employees, Fraunhofer ISE is the largest solar research institute in Europe. So research was carried out - and a breakthrough was achieved. Actually, two breakthroughs, but more on that in a moment. A team of researchers led by Dr. Markus Glatthaar, an expert in metallization and patterning, has developed an electroplating process in which silver can be replaced by copper, with two rather obvious advantages: Copper is many times cheaper than silver and a lot more readily available. And there's also a Cossack's corner in the form of a slight but demonstrable increase in yield, because the copper conductors measure just 19 micrometers on average thanks to laser structuring, which means there's less shadowing of the light-receiving silicon layer than with silver tracks.

And while we're on the subject of substitution...

In order to prevent the entire electrically conductive surface of the solar cell from being coated with copper during the aforementioned electroplating process, the areas that are not to be coated must first be masked with an electrical insulator. And in this context, the researchers have made a second remarkable advance: Until now, expensive polymer-based paints or films have been used to mask the silicon wafer in the electrolyte bath. Proper disposal of the polymers, however, is costly and also generates a lot of waste. Glatthaar and his team have now succeeded in replacing the polymers with aluminum. This is a material that can be completely recycled, just like copper. This double substitution - from silver to copper and from polymer to aluminum - also provides a double advantage: The production of the solar cells becomes more sustainable and at the same time significantly cheaper.

But aluminum also conducts, doesn't it?

In fact, one difficulty was that aluminum is basically electrically conductive and thus not at all suitable for masking. On the other hand, aluminum tends to form an insulating oxide layer on its own, albeit only a few nanometers thick. "We succeeded in adapting the process parameters and developing a special electrolyte in the process so that, as a result, 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 circle is beginning to close

By using the recyclable materials copper and aluminum, photovoltaic production also comes a significant step closer to the circular economy. And the availability of raw materials also improves environmental and social standards. "Since we have enough copper in Germany, supply chains are shorter, and dependence on the price on international raw material markets or on foreign suppliers is reduced," explains Glatthaar.

Spin-off PV2+ to bring optimized solar technology to market

Unfortunately, it sometimes takes time for breakthroughs in research to reach everyday production. In order to be able to launch the promising technology on the market more quickly, Fraunhofer ISE itself became active and founded the spin-off PV2+. The young company, headed by Fraunhofer researcher Dr. Markus Glatthaar, plans to set up pilot production in Freiburg 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 photovoltaics industry a much-needed boost. The spin-off has the best chances to establish itself quickly and successfully on the market. We are all the more pleased, of course, that these technologies were created at our institute." The project has received additional support from the Federal Ministry of Economics and Climate Protection (BMWK) as part of its "Exist" start-up program.

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