Thermal processes driving laser-welding for module interconnection

Laser welding of thin Al layers offers a silver-free and highly flexible option for the interconnection of Al metallized solar cells. Welding requires the melting of the Al layers in order to form a reliable electrical contact. Here, we investigate the process driving the melt front of the Al towards the interface between the two Al layer. In experiments we observe two different mechanisms depending on the thickness of the irradiated layer. In the case of Al layers thinner than 5 μm a melt-through of the Al-layer is observed, whereas for thicker layers thermal expansion causes a breakage of the surface and ejection of molten Al, which enables the contact formation. Using simulations based on finite element method we instigate the mechanisms leading to the different behavior. The simulations match the experimental results with the experimental measurement uncertainty. In case of thin layers, the simulation show that the process is limited by thermal diffusion. For thicker Al layers the onset of melting on the front side initiates the breakage of the surface and the ejection of the aluminum.