Thermomechanical stress in solar cells: Contact pad modeling and reliability analysis

This study analyses thermomechanical stresses in silicon solar cells after the soldering process by finite element modeling. An experimentally validated model shows compressive and tensile stresses, longitudinal and transversal to a busbar or a pad row on the surface of a silicon solar cell. The impact of the interconnector segments at and in between two solder pads was investigated and characteristic locations of maximum stress were identified. In addition, the influence of the layout of the contact metallization on the thermomechanical stress was identified by geometry variations to reveal design guidelines that lead to reduced thermomechanical stress in a solar cell after the soldering process. The model results reveal maxima of the tensile stress located at the outermost contacts. Furthermore, a significant influence of the distance between the outermost contact areas and the solar cell edge was determined; with decreasing distance, the compressive stress maxima are higher, but in contrast the more critical tensile stress maxima decrease. For connected pad rows tensile stress maxima are larger compared to single pad connection, which shows the influence of the interconnector segments in between the solder joints of a pad row. After several stages of thermal cycling, electroluminescence measurements showed, in compliance with the model results, contact damages, mainly at the outermost contacts. Furthermore, connected pad rows revealed a steadily growing amount of damaged contacts, whereas single solder joints showed no defects up to 400 thermal cycles.