**3.1. Ni seed layer deposition**

**3. Ni/Cu contacting schemes**

282 Solar Cells - New Approaches and Reviews

involves two steps:

**i.** Ni seed layer deposition.

However, the schematics for these steps are shown in Fig. 3.

**Figure 2.** Processing steps involved for depositing Ni/Cu/Ag or Sn metal stacks.

**ii.** Cu deposition by LIP.

Cu metallization offers greater resistance against electron migration and has been implement‐ ed widely for ultra-large-scale integrated circuits (ULSIs). However, it has a major weakness as being a deep-level impurity in silicon and can disturb the electrical performance of the device. These impurities tend to generate traps which act as generation/recombination centres and degrade the minority carrier lifetimes in the substrates [31, 32]. To avoid the Cu from being diffused in the silicon, Ni, as a diffusion barrier, has been employed successfully. Ni not only acts as a diffusion barrier but also promotes adhesion between Cu and silicon [9, 33]. Cu along with a Ni seed layer has given some promising results in terms of the efficiency of the crystalline silicon solar cells. The metallization technique using Ni/Cu metal stacks mainly

The two step process (seed and plate) for the metallization of solar cells increases the efficiency potential considerably [34]. After depositing Ni/Cu metal stacks, a thin capping layer of silver (Ag) or tin (Sn) is usually electroplated above the Cu. The purpose of this capping layer is to prevent the Cu metal lines from being oxidized. Moreover, these capping layers help to solder the interconnecting tabs and also prevent the Cu interacting with the EVA encapsulant. The processing steps involved in metalizing the Ni/Cu/Ag or Sn metal stacks are shown in Fig. 2.

**Figure 3.** Schematic structures of the steps involved in the formation of Ni/Cu/Ag or Sn-based metallization schemes.

Ni as a seed layer has to be deposited to prevent the diffusion of the Cu in the silicon. There are a number of ways to effect Ni deposition by adopting the mechanism of oxidation reduction reactions. The diffusion barriers used as alternatives to Ni include titanium (Ti) and tungsten (W) [35]. However, Ni has the ability to provide a lower contact resistance to doped silicon and it works well as a diffusion barrier [28, 36]. Low resistivity ohmic contacts can be made by initiating a low temperature sintering process after Ni deposition [8, 11, 36-38]. The basic requirement of the Ni seeding layer formation is a uniform and adequate thickness over the entire front grid. The effectiveness of the Ni barrier layer can be defined by its role in blocking Cu diffusion. In this section, various methods adopted to form a Ni diffusion barrier at various research institutes will be discussed.
