**6.3 Plating-based front-side metallization**

The costing of various factors in solar cell processing have decreased over the years, while the contribution of front Ag is still the most significant [42]. Significant amount of work has been done to replace Ag by alternate metal like copper (Cu) which has a conductivity value of very close to that of Ag and also offers a potential significant cost advantage [43, 44]. Cu has high diffusivity and solubility in Si and hence a barrier-layer like nickel (Ni) is deposited on Si prior to Cu plating [42]. Light-induced plating (LIP) which is derived from conventional plating utilizes the photovoltaic effect of light to plate the desired metal and has many advantages compared to conventional plating [43, 44].

**Figure 18.**

*(a) Example of a firing furnace for sintering metal contacts and (b) illustrative temperature profile of a firing furnace. Source: centrotherm GmbH.*

#### **Figure 19.**

*Illustration of the firing process. (a) Burning out of the organic binders, (b) melting of glass frit which etches the SiNx:H and (c) Ag crystallite formation at the emitter interface.*

#### **Figure 20.** *(a) FS of a complete solar cell and (b) RS of a complete solar cell.*

Ni-Cu-based front-side metallization requires an additional front-side ARC patterning step unlike the Ag paste-based metallization and in most cases also an additional Ni sintering step to reduce the contact resistance and have good adhesion of the metal stack [42]. Commercial DWS cut mc-Si solar cells based on Ni-Cu-Ag plated stack have been demonstrated with finger width of 22 μm, aspect-ratio of close to 0.5 and similar efficiency as that of reference screen-printed Ag-based solar cells [45].

Continuous improvement in the Ag FS pastes along with simplicity, reliability and high throughput of the screen-printing process has made it difficult for Ni-Cubased metallization to compete with Ag-based FS metallization. However, high solar cell efficiency concepts like bifacial heterojunction solar cells, where Cu can be directly plated onto the transparent conducting oxide, the plating process is simplified and requires only a single tool [39]. Similarly, high efficiency concepts which require reduced amount of metal can achieve the same using plating-based metallization [42, 46].
