**Acknowledgements**

1,000 hours damp-heat exposure at 85 °C with 85% relative humidity or a ≤5% *pmax* loss after

Limited progress in terms of reliability for the Ni/Cu contacts has been made, with few reports made so far [81-83]. Fraunhofer ISE and IMEC have tested solar modules according to the criteria defined by the IEC 61215 reliability test. Both Fraunhofer ISE and IMEC have come up with satisfactory results after damp heat exposure and thermal cycling tests [82, 83]. In another report, reliable Ni/Cu contacts for a heterojunction solar cell were presented which successfully passed the damp heat test when conducted for 3,000 hours [81]. In a report from RENA GmbH, Germany, modules comprising Ni/Cu/Sn metal stacks successfully passed the module

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

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

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%

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

reliability test, confirming that no Cu diffusion had occurred in the silicon [104].

200 thermal cycles between-40 °C and 85 °C.

292 Solar Cells - New Approaches and Reviews

**6. Summary and outlook**

from being oxidized.

the industrial scale.

has been presented.

The authors would like to thank all the members of the Green Strategic Energy Research Institute, Sejong University, Seoul, Korea. We gratefully acknowledge the financial support of the Ministry of Trade, Industry and Energy, the New and Renewable Energy Core Technology Programme of the Korea Institute of Energy Technology, Evaluation and Plating (KETEP) (No. 20133010011780) and the Korea Evaluation Institute of Industrial Technology (KEIT) (No. 10043793), Republic of Korea.
