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

• Published in: Solar energy materials and solar cells 2019-07, Vol.196, p.167-177
• Main Authors: Rendler, L.C., Romer, P., Beinert, A.J., Walter, J., Stecklum, S., Kraft, A., Eitner, U., Wiese, S.
• Format: Article
• Language: English
• Subjects: Busbars; Compressive properties; Contact stresses; Electroluminescence; Finite element method; Mathematical models; Metallizing; Modelling; Photovoltaic cells; Reliability analysis; Segments; Silicon; Solar cells; Soldered joints; Soldering; Tensile stress; Thermal cycling; Thermomechanical analysis
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2019.03.041

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. •Experimentally validated model that shows stresses after the interconnection of silicon solar cell.•Locations of maximum stress after the interconnection process identified.•Influence of the layout of the contact metallization on the thermomechanical stress determined.•Design guidelines that lead to reduced thermomechanical stress.