**6. Metallization and solar cell characterization**

#### **6.1 Screen-printing-based metallization**

The last processing step for solar cell fabrication is the FS and RS metallization to draw out the power with minimum resistive losses. Ag is a good contact material for the n-type emitter, while Al makes a very good contact with the p-type substrate. A combination of Ag/Al paste is used to print pads on the RS to facilitate interconnection of solar cells in a module. Screen-printing is a simple, fast and continuously evolving process for solar cell metallization.

A schematic representation of the screen-printing process is shown in **Figure 16**. The screens have an emulsion coated stainless steel mesh with openings as per the desired metallization pattern as illustrated in **Figure 17(a)**.

*Industrial Silicon Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.84817*

**Figure 16.**

*Illustration of the screen-printing process for solar cell metallization.*

**Figure 17.**

*(a) Mesh-emulsion screen with finger opening for FS Ag printing [40] and (b) representative FS metallization pattern.*

The metal paste is spread over the screen via the flood and the squeegee movement that deposits the paste on the solar cell based on the screen-pattern. Snapoff is the distance the screen and the solar cell. The squeegee pressure and the snap-off distance are the critical parameters that determine the paste lay down and geometry of the Ag FS fingers.

Typical paste lay down for Ag/Al RS pads, RS Al and FS Ag are 35–45 mg, 1.1–1.4 g and 100–120 mg, respectively for a 6 inch Al-BSF multi-crystalline solar cell. An illustrative Ag FS metallization pattern is shown in **Figure 17(b)**. The Ag finger opening has reduced to below 30 μm, while application of 5 bus-bar is being increasingly adopted now. With such screen parameter and good paste lay down, consistent FF of >80% should be obtained for the Al-BSF solar cells with an optical shading loss of <6%.

#### **6.2 Drying and fast firing of metallization pastes**

The metallization pastes consist of metal powder, solvents and organic binders. In case of FS Ag paste, the paste also contains glass-frit while etches the SiNx:H layer and makes contact with the n-type emitter [41]. The metal pastes

are dried after printing and finally they are sent through a fast-firing furnace for sintering and form the RS Al-BSF and FS Ag contact. An example of such a fast-firing furnace with the temperature profile is shown in **Figure 18**. The FS Ag finger sintering process is illustrated in **Figure 19**. When the solar cell passes through the fast-firing furnace, the organic binders are burnt, followed by melting of the glass frit and finally formation of Ag crystallites contacting the n-type emitter. The firing profile needs to be tuned based on the specific types of metallization pastes and emitter diffusion profile. As an example, the firing peak temperature could be low to not form a good ohmic contact on the FS, while a too high temperature can lead to diffusion of Ag through the junction and shunting of the p-n junction. Image of a complete multi-crystalline Al-BSF solar cell is shown in **Figure 20**.
