*2.3.1 Influence of process parameters on local-BSF formation*

To form a high-quality localized contact, a deep Al-BSF is required for Al▬Si contact interface to minimize the rear surface combination along with shunt free rear surface passivation. Urrejola [58] reported a shallow BSF or the presence of Kirkendall voids at the Al▬Si interface as shown in **Figure 4**. **Figure 4(a)** shows the uniform BSF with a thickness of (4 μm). Void formation is due to sub-optimal conditions (**Figure 4(b)**) or inadequate BSF depth (**Figure 4(c** and **d)**). These voids reduce the FF as well as act as high recombination centers which affects Jsc and Voc. Though the electrical contact is not affected with inadequate BSF depth, a very high contact recombination is expected. To minimize Rs with low contact resistivity, a very narrow local opening is required, hence high recombination beneath the metal contact is reduced. Urejola [56] obtained a lowest contact resistivity of 8 mΩ cm<sup>2</sup> for a shallow dielectric opening which lead to the FF loss minimization. However, narrow Al▬Si alloy formation increases the dielectric passivated area under the contact, thus reducing Jsc and Voc. The influence of contact size and finger spacing was investigated by Urejola [58]. The decrease in contact spacing reduced the overlap of Al on each side of the local opening leading to high quality BSF, thus lowering the presence of voids. For a contact spacing of 100 μm, the BSF thickness around 6–7 μm with less void (8%) was obtained. Similarly, Rauer et al. [38] concluded that the thickness of the local BSF depends on the contact spacing and obtained a BSF thickness up to 4 μm for a contact spacing of 400 μm. Further to increase the local BSF thickness and to avoid the void formation, the authors added more Si powder to the Al paste. This prevents the contact penetration into the Si bulk with enhanced Al-BSF thickness. Moreover, this increase in Si powder diminishes the emitter saturation current density (J0e).

**Figure 4.**

*The Al-Si interface showing (a) a well formed contact with deep BSF and no void formation, (b) a Kirkendall void, and (c) & (d) contact with shallow BSF [58].*
