Understanding the Origin of Recombination Losses after Co-Plating of Bifacial Solar Cells: In-Depth Microstructure Study

Contactless plating with electroless solutions can provide self-aligned high-efficiency contacts with very low silver content, using simple and inexpensive equipment. With prior surface activation, it can even be used to metallize both sides of bifacial silicon solar cells simultaneously. However, we observe in such a coplating process with nickel, where surface activation is achieved by immersion plating and thickening by electroless plating, that Voc and fill factor can sometimes significantly decrease with immersion-plating time. To understand the reason for this electrical degradation, we studied the impact of immersion plating on the microstructure of the plated silicon surface. The evolution of the Si-Ni interface was studied by scanning and transmission electron microscopies, energy-dispersive X-ray analysis, secondary ion mass spectrometry, and scanning spreading resistance microscopy. Our attention focused on metal in-diffusion, silicon roughening and etching as the origin for increased recombination. Etching was found to have a significant impact on Voc. The thickness of N+ Si etched during Ni deposition can in fact suppress in a few locations most of the field-effect passivation underneath the contacts. This means that a thicker surface field with doping beyond 1019/cm3 must be foreseen under the plated areas, or that the amount of Si lost in the reaction must be reduced. Our observations also confirm that immersion plating can hinder silicide formation and allow Ni in-diffusion, which may be a concern for reliability.