How the cracks are forming

Scientists in the United Kingdom have investigated the formation of cracks in PV cells, confirming that the high temperatures cells are exposed to during soldering for interconnection are a leading cause of cracking. Their findings will aid future research into module reliability and predictions of how cracks are likely to form.

Scientists at the University of Wolverhampton have published new research into the formation of cracks in silicon PV cells, and the influence of different cell interconnection strategies.

That cracks can form as a result of damage caused to cells during soldering has long been known in the industry. Sensitive cell layers are exposed to temperatures in excess of 200 C. And as it cools, the metals contract more than the silicon, causing it to crack.

Strategies are in place to limit this damage, but cell cracking is still a major quality concern for the whole PV industry. Cracks that form during manufacturing are often almost invisible in the beginning, but can grow larger and begin to affect module performance later on in the field.

New interconnections

Module manufacturers today are increasingly adopting different cell interconnection strategies. This is part of a push to minimize or eliminate the use of problematic materials such as lead and silver from modules, and also to allow the use of new cell technologies such as heterojunction, which cannot be exposed to temperatures above 180 C in production without causing major damage. And this once again creates some uncertainty around cell cracking.

The scientists at University of Wolverhampton investigated how changing various parameters in the interconnection process affected crack formation. These included different solder thicknesses, ribbon widths, silver and copper thicknesses.

Their results are presented in “Crack initiation and growth in PV module interconnection“, recently published in Solar Energy. The group found that cracks on the frontside (silver) grow faster than the rear (copper) and that thicker coatings of solder, silver and copper, as well as wider cell interconnections, all lead to higher rates of crack growth.

The group says that its results will help to develop methods for accurate prediction of a module’s long-term reliability, and also demonstrate a need for revision of the manufacturing processes used in cell interconnection.

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