**2. Silver** *vs***. copper metallization**

Currently, the photovoltaic industry is primarily occupied by metallization schemes composed of silver-based (Ag-based) screen-printed contacts. The key attribution is a simple process control with an attainable mass production on the industrial scale. However, the efficiencies of cells with screen-printed electrodes are degraded by factors such as the finger width and fill-factor (FF). Ag-based screen-printed contacts usually offer high contact resistance and have lower metal conductivity, and junction shunting can occur during the contact firing process. Such issues degrade the FF of the cell, which is why screen-printed contacts typically offer FFs within the range of 75-78%. On the other hand, the FFs of the contacts patterned with the photolithography process lies within the range of 81-82% [24, 25]. Moreover, screen-printed contacts offer higher shading losses because of larger finger widths (usually ≥ 100 μm wide).

The metallization of solar cell has a vital role in contributed to cell performance and it also determines a considerable number of cell processing costs. On average, about 40% of expenses are associated with the pastes used for front-and rear-side metallization [1]. This shows that the overall cost of the solar modules can be reduced drastically if the expensive Ag paste is replaced by relatively cheaper materials, such as copper. The schematics of both the metalli‐ zation concepts are shown in Figure 1.

Considering the higher cost of Ag and tarnished cell parameters due to shading losses and lower FFs, screen-printed contacts offer an opportunity for replacement by an alternate metallization. Recently, metal contacts composed of Ni/Cu metal stacks have emerged and are

front metals stack is usually formed by an electro-plating process, which is a well-developed plating technique and can only be applied to conductors. Cu has also been deposited by an electroless plating process, but it also requires a seed layer at the silicon surface [13, 14]. An auto-catalytic method of depositing Ni-phosphorus or Ni-boron by electroless plating bath compositions can be adopted to contact the semiconducting surface of silicon [15-17]. The use of a seed layer (i.e., Ni) can help to plate metal stacks on semiconducting or even nonconducting materials. Apart from Ni deposition from a conventional Ni plating bath, it can be plated under illumination (light-induced electroless plating) [18, 19] or by laser-assisted electroless plating [20]. A galvonic deposition method was also adopted to realize thin Ni layers on a textured silicon surface uniformly [21]. The Cu metal is usually electroplated by a light-induced plating (LIP) process developed at the Fraunhofer Institute for Solar Energy (ISE) [22, 23]. The LIP process involves the immersion of a sample composed of an np-junction with aluminium (Al) screen-printed on the back into an illuminated plating bath. The details of the

This chapter deals with issues regarding contact formations using Ni/Cu metal stacks for crystalline silicon solar cells. The key issues for the discussed technology are: (i) an effective Ni seed layer formation, and (ii) the development of Cu metal contacts by the LIP process. In this section, we will give an overview of the process conditions for such metallization schemes and of the current research work as well as the challenges and issues that have emerged. The

Currently, the photovoltaic industry is primarily occupied by metallization schemes composed of silver-based (Ag-based) screen-printed contacts. The key attribution is a simple process control with an attainable mass production on the industrial scale. However, the efficiencies of cells with screen-printed electrodes are degraded by factors such as the finger width and fill-factor (FF). Ag-based screen-printed contacts usually offer high contact resistance and have lower metal conductivity, and junction shunting can occur during the contact firing process. Such issues degrade the FF of the cell, which is why screen-printed contacts typically offer FFs within the range of 75-78%. On the other hand, the FFs of the contacts patterned with the photolithography process lies within the range of 81-82% [24, 25]. Moreover, screen-printed contacts offer higher shading losses because of larger finger widths (usually ≥ 100 μm wide). The metallization of solar cell has a vital role in contributed to cell performance and it also determines a considerable number of cell processing costs. On average, about 40% of expenses are associated with the pastes used for front-and rear-side metallization [1]. This shows that the overall cost of the solar modules can be reduced drastically if the expensive Ag paste is replaced by relatively cheaper materials, such as copper. The schematics of both the metalli‐

Considering the higher cost of Ag and tarnished cell parameters due to shading losses and lower FFs, screen-printed contacts offer an opportunity for replacement by an alternate metallization. Recently, metal contacts composed of Ni/Cu metal stacks have emerged and are

LIP process will be discussed in the following sections.

**2. Silver** *vs***. copper metallization**

280 Solar Cells - New Approaches and Reviews

zation concepts are shown in Figure 1.

sections are dedicated to addressing the most important topics in detail.

**Figure 1.** Schematic structures of solar cells composed of metal contacts formed with screen-printed Ag paste (left) and Ni/Cu plating (right).

considered to be a potential candidate for future crystalline silicon photovoltaic cells. Ni/Cu has shown some good results for solar cell performance in terms of improved FFs and higher efficiencies. The overall cost per watt-peak (Wp) can also be decreased as the copper is cheaper than Ag (about 100 times) and is almost equally conductive [26, 27]. It has been reported that Ni/Cu-in comparison to Ag paste-has higher electrical conductivity and lower contact resistance, even at lower doping concentrations (Ns) [28]. Cu is usually deposited by a selfaligned electro-plating process which helps to plate electrodes with finer line widths. The electrodes are plated and processed at lower temperatures (250-450 °C), considerably lower than the temperatures (700-850 °C) required for Ag pastes. The low temperature process is favourable for cell processing, as the degradation of the passivation layers can occur at higher temperatures. Furthermore, the rear side Al contact may be melted as well if Ag pastes are fired at higher temperatures. Although Cu offers certain advantages, it can create highly active generation/recombination centres if diffused in silicon [29]. In order to prevent Cu diffusion in silicon, a barrier layer is required. It has been reported that Ni plays a vital role in reducing the contact resistance as well acting as an effective diffusion barrier between Cu and silicon [30]. In Table 1, both the metallization schemes are compared in terms of the advantages and issues that they offer to crystalline silicon solar cells.


**Table 1.** Comparison of the solar cells fabricated with screen-printed Ag contacts and Ni/Cu metallized solar cells.
