**5. Challenges and open research issues**

Higher efficiencies for Ni/Cu contacts have been achieved, with a reported cell efficiency as high as 23.5% for a heterojunction solar cell on CZ wafers [80]. The potential high efficiency and lower processing costs of Ni/Cu-based metallization compared to its screen printing counterpart is striking; however, there are certain factors that limit its implementation on the industrial scale. It is assumed that replacing Ag-based metallization by Ni/Cu contacts could reduce the overall cost, yet the cost of ownership for Ni/Cu contacts needs to be reviewed. If it only concerns material costs, Ni/Cu may be beneficial. However, the additional process complexity and the replacement of an Ag screen printer/dryer with equipment comprising a laser, plating arrangements and a sintering furnace should be kept in mind. Apart from the issue of process complexity, in addition contact adhesion, background plating and-more importantly – long-term reliability need to be addressed.
