Wave-Shaped Wires Soldered on the Finger Grid of Solar Cells: Solder Joint Stability under Thermal Cycling
In this work the reliability of silicon solar cells interconnected by wires soldered directly on the contact fingers of the front side grid is analyzed in detail. The interconnection of busbarless solar cells enables significant silver reduction. We use solder coated wave-shaped wires to reduce thermomechanical stress in the solder joints, which results in minimized cell bowing. Consequently, this interconnection concept is especially suitable for the interconnection of back-contact solar cells. We analyze the mechanical and electrical properties of wave-shaped wires with different amplitudes by measuring length and electrical resistance, and by performing standard tensile tests. Furthermore, we manufacture 8 one-cell modules, 4 with full scale 156 x 156 mm2 solar cells and 4 with half cells, as well as a module including a string of three half cells using semi-automatic infrared soldering and a conventional lamination process. Subsequently, the one-cell module samples undergo thermal cycling and are characterized by EL imaging and IV measurements to detect solder joint defects. After temperature cycling we determine additional solder joint failures, mainly in areas with initial defects and at the solar cell edges. This confirms our assumption that most defects occur at the cell edges, where the thermomechanical stress maxima are located. However, all module samples show a maximum relative power loss of <3% after 200 temperature cycles and except one module all samples show a power loss of <5% after 400 temperature cycles. This demonstrates the feasibility of our interconnection approach based on soldering wave-shaped wires on the finger grid or small contact pads of silicon solar cells.