Analysis of EL images on Si solar module under thermal cycling

Thermal cycling (TC) induces defects in solar modules. The electroluminescence technique has been used to characterize the defects of solar modules, which are represented by a rectangular dark area (RDA) on the cell. In this study, the physical meaning of the RDA phenomenon on a solar module was investigated. It is proven that the RDA indicates cracks in the solder layer of the ribbon wire rather than broken electrode fingers in the solar module. For the proof, a test of TC was conducted on Si solar modules while monitoring the RDAs by EL during TC. Next, a failure analysis was performed on the RDAs, proving that the locations of the RDA coincided with the locations of the cracks in the solder layer between the Cu layer and Si wafer. Quantitative time analysis revealed that the RDA incidence in cells increased from 0 to 18% on average over TC 1000. In addition, the larger RDA incidence at the ends of the ribbon wires of the cell was detected and explained based on the previous study result of the high increases in shear strains of the solder layer at all ends of the ribbon wire after TC. Finally, a structural numerical simulation was performed to show a low probability of cracking at the electrode finger during TC.