Laser soldering - A technology for better contacts?

Solar cells have to be connected electrically to each other in a solar module. In most cases this is done by soldering tined copper taps onto the cells. Nearly all manufacturers use an iron or infrared soldering method. For thinner solar cells both methods have disadvantages. By using hot irons there has to be a mechanical contact between iron and tab. There is a big risk that the solar cell breaks due to this contact. When infrared irradiation is used for connection the whole cell is heated up to high temperatures and the length dimensions of the copper strip and the solar cell is fixed at these high temperatures. Due to the different temperature coefficients mechanical stresses are induced in the solar cell. In [1] it has been shown, that the use of spot soldering reduces the curvature to an acceptable level. In the SOLPRO project [2], we have developed a new method to solder solar cells with a laser. First measurements of the fill factor losses are compared with the losses of state of the art soldering methods. Microscopic pictures also show that the diffusion zone between solar cell contact and the tined copper band is better formed than in the standard soldering technology. There seem to be no differences in the electrical resistance between cells from different manufacturers if the laser solder process is adapted to the different cells. For making the contact a laser beam with a diameter of 0.4 mm is moved on a squared shape with an side length of 0.3 mm. From this area of energy feed in the melting of the solder runs radial outward. The resulting length of the solder point is 1.8 mm.