**2.2 Ni/Cu metallization-based solar cell**

The limitations of screen printing and chance of considering alternate materials for front contact led many researchers [26] to look for the Ni/Cu metallizationbased cell process. It is observed that it is important to optimize the Ni/Cu metallization to compete with screen printing technology in terms of reliability, cost competitiveness, and high throughput production. Ni plating on solar cells started in 1959, and the process has been developed in subsequent years and came into the present process flow in early 1980s. Interdigitated back contact solar cells by Sun Power Corporation integrated metallization scheme of patterned Al followed by plated Ni▬Cu▬Ag which was further annealed to realize the contact [27] and its world record efficiency is 24% [28].

**Table 1** shows the cost and properties comparison of copper (Cu), silver (Ag), and nickel (Ni). It can be seen that the resistivity of copper is only more by 3.7% as compared to Ag, while the cost being less by around a 100 times, which process to be an important factor for cost reduction. Moreover, Cu is widely used as interconnects in ultra-large-scale integrated circuits owing to its low resistivity and good resistance to electro migration and has a proven track record in the microelectronics industry. Hence, Cu is a possible choice for metallization of solar cells. The main drawback of Cu is its high mobility and being a highly reactive recombination center in silicon. This necessitates a diffusion barrier like Ni to prevent its diffusion in Si. The nickel silicide formed at the interface reduces the contact resistance, which will ensure minimum power loss due to series resistance (RS) in a solar cell. Many groups have demonstrated cells based on Ni▬Cu front side metallization with improved fill factor (FF) and higher efficiency (ƞ) compared to Ag-based cells. The laser grooved buried contact (LGBC) technology utilizes Ni-Cu-based front-side metallization and has been successfully commercialized by BP Solar. The process flow of silicon solar cells with Ni/Cu front contact is shown in **Figure 2**.

One of the crucial steps in Ni/Cu metallization is opening of ARC to make selective contact with an emitter. Literatures reported for patterning ARC and subsequent metal deposition to make the front contact; however, it is important to choose the process which can compete with screen printing technology both in cost and performance. It has been found that one such a technique is Ni/Cu metallization which can be commercialized with few additional process steps. But in Ni/Cu metallization, it is


#### **Table 1.**

*Cost and properties comparison of copper (Cu), silver (Ag), and nickel (Ni).*

**Figure 2.** *Ni-Cu process flow.*


#### **Table 2.**

*Solar cell performance data for front contacts with electroplated metal layer(s).*

important to standardize and optimize few critical parameters such as Ni thickness, annealing temperature for silicide formation, and Ni/Cu deposition parameters. The 80-μm thick stencil printed grid lines were thickened by electroplating of Ni▬Cu▬Sn stack with a commercial plating tool, improving the efficiency of the solar cells by 0.4% abs [29]. The platform had single side wafer processing and hence no chemical attack on the back side Al. Complete solar cell metallization based on electrochemical deposition of Ni and Cu has also been demonstrated [30]. Pulsed plating was used as compared to direct plating in the work to ensure homogeneous and well adhered contacts. LIP-based thickening of screen-printed contacts was first reported by Mette et al. [31]. An absolute *ƞ* gain of 0.4% was obtained for large area cells based on standard production process, while an improvement of more than 1% absolute was possible for fine line 70-μm printed contacts. For an optimized grid design, an absolute *ƞ* gain of 0.7% was achieved for large area cells as shown in **Table 2**. With an optimized SiNx:H, *ƞ* of 19.2% has been demonstrated on industrial grade Si with Ni and Cu layers deposited by LIP on LDSE cells [32]. pFF nearing 80% were possible with Ni▬Cu▬Sn-based contacts deposited by LIP, with no observable degradation in the pFF after a stress test at 200°C for 1000 hours.

After plating the conducting Cu layer, an Ag or Sn capping layer is deposited to protect the Cu conducting lines from oxidation and facilitate soldering of the interconnection tabs. Another important function of the capping layer is to prevent interaction of Cu with the EVA encapsulant. An impulse voltage test failure indicated by development of discharge sites was observed due to the presence of Cu particles at the Al/EVA interface on the back side [33]. Though the Ni▬Cu contact-based LGBC cells were commercialized, low throughput rates and increased processing costs led to screen printed contacts becoming the standard for solar cell metallization [34]. Other major concern is regarding the chemical waste due to the metal baths which can lead to serious environment contamination issues [35]. Steady research and advances in plating techniques have enabled transition of solar cell with Ni▬Cu-based metallization from labs to commercial scale production. Economic factors play vital role when considering an alternative technology with the introduction of new equipment in the fabrication line. As per the ITRPV roadmap, direct plating and plating on the seed layer are expected to have a share of around 15% in 2028 for the front-side metallization [36].

*Review on Metallization in Crystalline Silicon Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.84820*

The Pluto series from Suntech Power is based on Ni▬Cu metallization with stabilized efficiencies of 19.0% on large area mono-Si solar cells. There was an improvement of over 6% as compared to the screen printed contacts due to reduced shading and improvement in VOC. IMEC has demonstrated conversion efficiencies of 20.3% on large area i-PERL cells with plated contacts. Using a PERC structure, Schott Solar along with Schmid Group demonstrated 20.9% efficient 6″ cells. Schott Solar has also demonstrated a median *ƞ* of 20.8% with a best *ƞ* of 21.3% on an industrial production line with electroplated contacts. Rena has recently demonstrated solar cells based on PERC technology reaching 20.8% with Ni▬Cu metallization. Modules made with these cells successfully passed IEC 61215 test three times and adhesion of >1 N/mm. The technology can lead to a reduction in the cell production cost by \$ 0.06.

Ni▬Cu metallization yields the better efficiency compared to the conventional screen printing solar cells; however, due to the low throughput rates and increased processing costs, standard solar cell metallization dominates the PV industry. Also the chemical wastes of the metal baths in Ni▬Cu metallization lead to environment disputes.

#### **2.3 Passivated emitter rear contact solar cell**

The conventional screen-printed Al-BSF cells suffer from the optical losses (front reflectance), transmission losses, and the recombination (rear side). The major limitation arises from the rear surface recombination which is due to the low solubility of Al in Si (doping concentration 7 × 1018 cm3 ) during the very short firing process employed for alloying of screen printed Al paste. Though the boron codoping with Al-BSF improves the doping concentration of Al-BSF [37, 38] owing to the higher solubility of boron, only 65% of the internally reflected longer wavelength light reaches the rear side, and hence the rear surface recombination is still being high [39]. One way to overcome the drawback of Al-BSF is the introduction of a dielectric rear side passivation with local contact points, which improves the optical properties with less surface recombination. One such cell architecture is the passivated emitter and rear cell [40]. With this structure, low rear surface recombination velocity of 60–200 cm<sup>−</sup><sup>1</sup> and internal reflectance over 95% have been realized so far. The dielectric passivation layer is locally opened for contact formation [40–47] by laser [48–50] or by printing etching pastes [51]. About 1% of the total rear surface is covered by the local point contacts. The local point contact is realized by photolithography in the laboratory level, and in mass production, the contacts are formed either by screen printed Al [38, 46, 47, 51–60] or by physical vapor deposition (PVD) of Al [40, 47]. The process flow for the PERC cell is shown in **Figure 3**. The key challenge in the local aluminum contact formation is that the Al should not be penetrated into the dielectric passivation layer [45]. The local Al-BSFs produced during the alloying process create voids below the Al contacts. These voids result in incomplete BSF formation and hence the rear surface recombination and contact resistances

**Figure 3.** *PERC process flow.*

are increased [38, 46, 47, 51–60]. An effective local Al-BSF is formed [61, 62] in the laser-fired contact process [63]. In this process, the deposition of the passivation layer followed by rear metallization (screen printed or evaporated contact) is carried out and finally with laser local contacts were formed. As a novel route, the rear Al electrode is formed by using commercial Al foil, thus complicated equipment such as evaporation or screen printing systems are avoided [64].

The best commercial PERC cell with 20–21% (mc-Si) and 21–22% (mono c-Si) has been achieved in the commercial scale [36]. A detailed investigation on the various factors involved in the formation of local Al contact formation and the influences of process steps have been studied by various authors and are described in the following sections. Meemongkolkiat et al. [52] observed that voids are created beneath the local Al contacts during the alloying process. Rauer et al. [38] avoided these voids by adding Si powder to the Al paste. The various factors that influence the contact formation are: (a) dielectric opening method, (b) rear-side contact geometry, (c) the amount of Al in the metallization paste, and (d) firing process.
