**5.2. Contact adhesion**

Heavy Industries and Shinsung Solar, respectively [86, 92]. A 19.33% efficient LDSE-type solar cell with Ni/Cu contacts was presented at the UNSW [93]. More recently, a laser-doped Cuplated bifacial silicon solar cell exceeding 19% efficiency was developed at UNSW [94]. Solar cells consisting of Cu-plated contacts and exceeding 20% efficiency are presented in Table 2.

Kaneka Hetero-junction 737 39.97 79.77 23.5 [80]

Roth & Rau Hetero-junction 734 38.1 79.9 22.3 [81]

Fraunhofer ISE PERC 679 38.8 81.5 21.4 [82]

Schott Solar PERC 665 39.9 80.5 21.3 [91]

IMEC PERC 649.8 39.3 78.3 20.79 [83]

IMEC Rear Junction 676 38.4 79.2 20.5 [78]

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

To realize Ni/Cu metal contact for solar cell applications, a number of processing steps are required. Steps such as ARC opening, Ni deposition/sintering and Cu electroplating are required. These steps make the process more complex and difficult to implement in mass production. To realize such a metallization scheme on the industrial scale, the processing steps need to be minimized. The use of lasers for depositing a Ni seed layer could help to decrease

Fraunhofer ISE - 646.4 38.86 80.8 20.3 [42]

**) Fill Factor (%) Efficiency (%) References**

**Research Centre Type Voc (mV) Jsc (mA/cm2**

290 Solar Cells - New Approaches and Reviews

**Table 2.** Solar cells consisting of Cu-plated contacts exceeding 20% efficiency.

importantly – long-term reliability need to be addressed.

**5.1. Process complexity**

**5. Challenges and open research issues**

Cu exhibits poor adhesion to silicon and requires a seed layer to form an effective and adhesive metal contact. However, Ni has shown some promising results in providing ohmic contact with good adherence to silicon. Further improvements are required to satisfy the standard heat-quenching and peel force tests. Bath-ageing, acidity and residues on the substrate surface have been found to affect contact adhesion adversely [96, 97]. Various research institutes are currently working to improve the adhesion between Cu and silicon in order to resolve this issue completely. So far, IMEC has presented Ni/Cu metals stacks with adhesions within the range of 1~2.7 N/mm [89]. A research team at Fraunhofer ISE has also presented a two-stage process for etching the back of unreacted Ni and re-plating it [98]. Adhesion strengths of up to 2.5 N/mm were achieved by adopting just such a two-stage process at Fraunhofer ISE.
