Making Cheaper and More Efficient Solar Cells with Copper

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There are two ways to lower the cost of producing energy through photovoltaics – more efficient solar cells (more power per square foot) and lower production costs (lower cost per square foot). The ETAlab of the Fraunhofer ISE has devised a way to do both simultaneously. The research team in the laboratory for new solar cell structures and processing steps has not only made the solar cell contacts out of 100% cost-effective materials (replacing expensive silver with cheaper copper and nickel), but the process also increases the efficiency of the solar cell to a very respectable 21.4%.

In order to achieve a high level of efficiency, the front contacts in the solar cell not only have to exhibit a low level of loss with light-generated current but also cover the least amount of the cell surface possible. Technologically speaking, materials with high conductivity work best. Traditionally speaking, silver has been the metal of choice. In the current industry standard, the relatively wide and porous contacts are produced by screen-printing silver pastes. The ETAlab team’s switch from silver to copper reduced the cost by around 10%, due to the significant difference in price. The electro-plating processes necessary to generate the contacts with copper were also comparatively inexpensive.

Of course, copper and silver have different chemical characteristics despite conducting about the same amount of electricity, so the contacts aren’t quite built the same way. The challenge in constructing a copper contact was that the copper tended to leak into the semiconductor (which then means the solar cell slowly stops working altogether!). To stop the leakage, the ETAlab research team bound the copper to a silicon layer with nickel – which also conducts electricity and can be cheaply electro-plated, so double win there.

While the nickel/silicon/copper combination can be used with the silver screen-print, it can also be used without it. The anti-reflective coating (ACR) can be locally removed from the combination with processes like laser ablation, so that the line width is no more than 20 microns. A narrower line width means that less of the solar panel is shaded by its own contact. The opened range of the ARC nickel (reinforced with copper and tin or silver so that it can be soldered) is then selectively deposited.

The new process had one more advantage – one that the team wasn’t necessarily expecting. The copper-using cells not were not only as efficient as the more expensive standard cells, but they were more stable in the heat – the research team put it under a thermic load of 1600 hours at 392°F and the copper cells still worked just as well as they had in the beginning.

The next challenge is adapting the process for a larger solar array, as the prototype nickel/silicon/copper solar cell was smaller than 1”x1”. The Fraunhofer ISE will introduce the theme of long-term stable copper metallization and its results so far at the 26th European Photovoltaic Solar Energy Conference and Exhibition this week in Hamburg.

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