Detection and Impact of Cracks Hidden Near Interconnect Wires in Silicon Solar Cells

The thermal stresses associated with the soldering of interconnect wires onto the busbars of solar cells is one of the leading causes of cracks in silicon solar cells. Cracks will often branch outward from the busbar region so that they are easily seen in an electroluminescence (EL) image. However, since the wires are often wider than the busbar metallization, cracks can be located underneath or close to the wires and be "hidden" within the EL image. If the cracks remain beneath the busbar metallization, they may cause no reliability problems. However, if they propagate along the side of busbars and remain hidden under or next to the wires, they can prevent continuity of the gridlines to the busbars. The cracks may cause minimal problems in a new solar panel, but over time they can open up with thermal cycling and cyclic loading in the field. We demonstrate how these hidden cracks may be detected with the technique of UV Fluorescence, and we show examples of their signature in EL images. It is our observation that many groups are not familiar with these EL signatures, and do not consider that hidden cracks may be the cause of many gridline interruptions. We also show how gridline corrosion is strongly linked to hidden cracks where moisture can penetrate through the cracks. We hope that these techniques and improved understanding can lead to improved testing and feedback that accelerate product development and improve panel reliability.