Microcharacterization of Interface Oxide Layer on Laser-Structured Silicon Surfaces of Plated Ni-Cu Solar Cells
Light-induced plating on laser-structured passivation layer openings is an alternative method to silver screen printing for fabrication of the front-side metal grid on silicon solar cells. Fundamental contact properties, such as adhesion and contact resistance, are determined at the highly inhomogeneous interface of laser-structured silicon and plated nickel. Usage of laser structuring and plating holds the risk that an interfacial oxide layer is incorporated to the contact stack. It is shown within this article that both native and laser-induced oxide formation influence contact properties. Although, a native oxide layer does not impede metal deposition during plating, the dynamics of the layer formation is shown to be changed. It is known that interface oxides harm the contact resistance, and it is reported that postplating annealing decreases the contact resistance. In this article, the microstructural basis of the temperature-induced changes is examined. By transmission electron microscopy and energy dispersive X-ray spectroscopy, no temperature-induced modification of the interface microstructure, such as silicide formation or metal-induced crystallization of the amorphous surface, is observed. Results suggest that the decrease is attributed to a temperature-induced change of the electrical properties of the interface oxide. Considering reports that partially closed interface oxide layers are an inherent feature of laser-structured plated contacts, the results presented motivate reconsidering the objective of postplating annealing. The results indicate that the beneficial impact of silicide formation is overrated for the case of laser-structured plated contacts, whereas the temperature-induced modifications on residual interface oxide layers are neglected in the scientific discussions on the given contact stack.