## 3 Dielectric-Based Passivation Layers
### Approaches Used to Add the Passivation Layer  
In this sub-section, the type of materials that are used as passivation layers and the deposition techniques of these layers will be discussed. As shown in Table 1, in most of the studies, aluminum oxide (Al\({}_{2}\)O\({}_{3}\)) was used as a passivation layer. The popularity of this material comes from the Si PV community. Al\({}_{2}\)O\({}_{3}\) is commonly used and is proven to be a successful passivation layer for Si solar cells in many studies like [26]. Al\({}_{2}\)O\({}_{3}\) is also proven to be functional as a passivation layer for CIGS solar cells by reducing recombination losses at the rear metal contact of CIGS thin-film solar cells [27]. Unfortunately, it also acts as an electron and diffusion barrier layer, and thus prevents current flow and sodium (Na) diffusion from the SLG substrate. To trick the natural behavior of alumina layers, i.e., insulator (or barrier) layer, different techniques are used to make contact openings in this layer, which will be discussed in the following section. In the studies that did not use Al\({}_{2}\)O\({}_{3}\) as a passivation layer, silicon (SiO\({}_{2}\)) nanoparticles (NPs) or nano-meshes (NM), magnesium fluoride (MgF\({}_{2}\)) layers, or titanium oxide (TiO\({}_{2}\)) layers were implemented with various techniques to act as a passivation layer in combination with contact opening approaches (Table 1). Using SiO\({}_{2}\) NPs [19], SiO\({}_{2}\) NM [22], and MgF\({}_{2}\)[11] as passivation layers was successful. However, using TiO\({}_{2}\) as a passivation layer caused a reduction in \(V_{oc}\), fill factor (_FF_), and efficiency [5]. Hence, it was not a sufficient passivation layer yet. Furthermore, in some of these studies, additional Na supply (only important for the studies using SLG as substrates), Ga grading, or additional light management techniques were used (Table 1). In order to deposit the passivation layers, various techniques were used, such as atomic layer deposition (ALD), direct current (DC) sputtering, thermal evaporation, spin coating, e-beam evaporation, and chemical vapor deposition (CVD). The most commonly used technique is ALD. There are many advantages of ALD, such as creating a conformal layer with easily controllable layer  
Figure 2: Transmission electron microscopy (TEM) image of Al\({}_{2}\)O\({}_{3}\)-passivated CIGS solar cell that has nano-sized local point contacts in passivation layer [14]. thickness and a variety of materials that can be used. Spin coating is another easy and fast technique to create the passivation layer, yet this technique is not applicable to large scale materials. There is also a more technologically viable technique to create these layers, which is DC sputtering. DC sputtering is a highly versatile vacuum coating system used for the deposition of a variety of materials at any preferred scale, i.e., a technologically viable way. Among all other studies that used Al\({}_{2}\)O\({}_{3}\) as a passivation layer, only one study used this passivation layer alone, without using an additional contacting approach. In that study, it was proposed that using a very thin layer of Al\({}_{2}\)O\({}_{3}\) rear surface passivation in combination with NaF deposition enhances the electrical characteristics of CIGS solar cells. In that study this surface passivation was claimed to allow tunneling [24] (Table 1). In order to investigate the effects and influence of back surface passivation in CIGS solar cells, there are studies that used bi-facial thin CIGS solar cells [28; 29; 30]. For bi-facial solar cells, all characterizations are made from both the front and back side of the solar cell. In [30], atomic-layer-deposited Al\({}_{2}\)O\({}_{3}\) was used for surface passivation, and lift-off processed CdS NPs were used to create nano-sized contact openings in this layer.