**5.3. Background plating**

Background plating-also known as 'ghost plating' or 'parasitic plating'-is an unwated copper plating that occurs at the passivation layers. Inhomogeneities at the SiNx surface, silicon residuals and cracks are mainly responsible for background plating [99, 100]. The phenomenon may affect cell performance by creating undesirable shading and can divest the solar cell aesthetically. The shading affects cell performance by causing a reduction in the short-circuit current density and can also lead to junction shunting due to metal diffusion [101]. Further‐ more, the localized metal silicon contact due to this undesired plating increases the recombi‐ nation velocity at the surface. The point-like effects and cracks on the samples are the two main types of background plating [99].

An appropriate cleaning step is required before applying the passivation layers to remove forms of silicon residuals. Cleaning in a chemical solution, such as piranha (an admixed solution of H2O2 and H2SO2) is one way to clean the sample surface properly [99]. Another way to reduce pinholes on the SiNx surface might be the deposition of thin (10~15 nm) silicon dioxide on diffused silicon before SiNx deposition [101]. Mechanical stress during the ultra‐ sonic step should be minimized and wafers should be handled carefully to avoid the microcracks which cause the undesired metal plating [99, 102].
