**3.3. Copper electroplating**

Subsequent to the Ni seed layer's formation with the sintering process, the deposition of a suitable metal is required in order to further reduce the line resistance of the front grid. Materials such as Ag and Cu have been deposited by using electroplating arrangements. Cu is under consideration, since it is relatively cheap in comparison to Ag and it can reduce the overall cell costs drastically. To deposit Cu, an electrolytic metal deposition method can be used. Here, a sample composed of a np-junction and a Cu anode is immersed into an admixed solution of copper sulphate (CuSO4[H2O]5) and sulphuric acid (H2SO4) [56]. The CuSO4[H2O]5) supplies, the Cu (positively charged) and SO4 (negatively charged) ions for the electroplating process. A positive potential is applied at the anode while the sample is kept at the negative terminal. This applied potential attracts the positively charged Cu ions towards the cathode and the negatively charged SO4 ions towards the anode. According to the reactions shown in Eq. 4, the Cu deposition occurs at the cathode due to the reduction of Cu ions [39].

$$\text{Cu}^{2+} + \text{ } 2\text{e} \rightarrow \text{Cu} \tag{4}$$

**Figure 5.** Schematic arrangements of Cu-based light-induced electroplating systems.

The Cu electroplating is mostly done by the approach implementing LIP arrangements as shown in Fig. 5. The LIP process works on the same principle as the conventional electroplating process; however, the inclusion of a light source enables the utilization of the photo-generation property of the solar cell. This process, developed at Fraunhofer ISE, includes the immersion of a patterned cell into the electrolyte plating with the light source arrangements. A protective potential is applied at the sample in order to make the rear-side of the cell more cathodic, which helps in minimizing the corrosion of the aluminium back-electrode. Furthermore, the plating can also be done relatively uniformly, as this potential operates the cell closer to its short-circuit conditions [43]. As to having a uniform voltage distribution across the grid pattern, the LIP process also provides stable baths with no reducing agents. LIP is an encouraging method, particularly for the solar cell metallization since it helps to deposit metal with higher aspect ratios and higher deposition rates.
