**6. Summary and outlook**

A detailed overview of research activities in the field of Ni/Cu-based metallization for crystalline silicon solar cells has been presented. Many research groups across the globe have taken on the challenge of working on crystalline silicon solar cells with Ni/Cu contacts. The Ni/Cu plating, which was the topic of this section, has enormous potential in realizing improved solar cell efficiency and low cell and module costs. The generic built-up process for such metallization starts from the deposition of the Ni seed layer, which offers lower contact resistance when sintered at specific temperatures. A Cu metal is electroplated on top of this stack and after confining this layer by a top Ag or Sn capping layer. The Ni seed layer acts as a potential barrier to block Cu diffusion into the silicon and the capping layer prevents the Cu from being oxidized.

Progress has been made with various deposition options available for Ni in promoting adhesion and a diffusion barrier to the Cu. Electroless chemical baths containing Ni salts and a reducing agent of NaH2PO2 H2O offer a cost-effective deposition process. The inclusion of light source in the electroless plating process results in uniform and faster deposition by utilizing the photovoltaic effect of the solar cell. Various new patterning techniques are available, including the use of etching pastes, aerosol jet and laser-based chemical metal deposition. Laser-assisted chemical metal deposition provides an opportuni‐ ty to pattern the front contact grid and to deposit the Ni seed layer together. Ni deposi‐ tion and ARC patterning in a single step is the solution to minimizing the processing steps and reducing the process complexity. It can provide the opportunity for cell production on the industrial scale.

Promising results in the form of solar cells with a higher FF and improved efficiency have been reported recent years. FFs of the range approaching 80% and efficiencies above 20% have already been reached at various research institutes. More recently, at IMEC, an industryfeasible Ni/Cu plating scheme for i-PERC-type solar cells with a best cell efficiency of 20.5% has been presented.

If we consider material cost and cell performance, Cu as an electrode seems to be the best alternative to the existing screen-printed Ag contacts. However, its implementation on the industrial scale is still limited by a number of obstacles. Recent reports suggest that there has been progress in finding a solution to the hurdles of background plating and adhesion. Surface pre-treatment prior to passivation layer deposition and the low stress handling of the samples yielded an ARC surface with decreased background plating. A uniform Ni seed layer covering the contact area resulted in good adhesion strengths of the Cu lines to n-type and p-type emitters. However, the challenge of finding sustainable and reliable contacts is vital in creating viable Ni/Cu contacts for solar cells on the industrial scale.
