*2.3.2 Impact of dielectric opening method*

A detailed investigation on the influence of formation of BSF using industrially screen printable local BSF Al paste and laser processing for removing the dielectric barrier was carried by Fang et al. and Bahr et al. [51, 53]. The laser ablation was carried out using nanosecond [wavelength (λ) = 1064 nm, pulse duration = 300ns, and pulse energy = <1.6 mJ] and femtosecond [wavelength (λ) = 1025 nm, pulse duration = 300fs and pulse energy = <36 μJ] laser. The ns laser has a strong influence on the local removing of the dielectric stack AlOx/SiNx:H, whereas fs showed a moderate influence. The strong and moderate influence is attributed to the interaction time between the laser pulse and the silicon substrate. With ns laser, few ten micrometer etching depth is achieved depending on the laser power, whereas in fs laser with very short interaction, few-micrometer depth is obtained. With screen printable local BSF Al etching paste, the passivation stack with 105 nm thick is etched off. After firing in an IR furnace with optimum belt speed, the local BSF formed with etching paste was more thicker (around 5 μm) and homogeneous with less voids. In the case of ns and fs, laser showed more voids with inhomogeneous thinner BSF (1–2 μm) due to the increased surface roughness.

### *2.3.3 Impact of contact resistivity*

In recent days, the aluminum pastes are improved in such a way that even for thin laser contact opening (LCO), very low surface recombination is achievable. In future, decrease in the fraction of metallized area at the rear might is expected, hence Rs plays a vital role in the contact resistance of the Al▬Si interface which is given by

$$\mathbf{R}\_{c,max} = \rho\_c / f\_{max} \tag{1}$$

where *frear* is the rear metallization fraction and ρc is the specific contact resistivity. However, ρc is independent of the contact size [65]. Similarly, Rohatgi et al. [66], on 2.3 Ω-cm wafers, obtained a ρc = 10 mΩ cm2 . Urrejola et al. [56] carried out the contact measurements with a PERC structure. The Al paste is printed on the top of the locally opened dielectric, and the transmission line model revealed the dependence of the ρc on the contact area. They obtained the ρc of 9–17 mΩ cm2 for the dielectric opening width of 80–170 μm. Gatz et al. [67], to determine ρc, varied the rear contact pitch of PERC solar cells and obtained a ρc of 40–55 mΩ cm2 . The contribution of the bulk to the series resistance Rb is acquired either by calculation or numerical simulation. Kranz et al. [68] processed PERC-like TLM samples and measured the ρc of 3 mΩ cm2 , whereas the fit to the solar cell data resulted in ρc of 0.2–2 mΩ cm2 and is shown in **Figure 5**.

### *2.3.4 PVD metallization*

In most of the high efficiency solar cell concepts, the metallization is carried out using three different physical vapor deposition (PVD) techniques: sputtering, electron gun, and thermal evaporation. During the deposition of aluminum layer (2 μm), the substrate temperature increases to ~350°C, which mainly arises from the recrystallization heat of the aluminum. Comparing with the screen printing process, the mechanical and thermal impact on the wafer is substantially reduced. After the deposition of PVD aluminum layers, the contacts can be formed using laser pulses with different laser parameters which results in a much shallower profile. Hoffmann et al., on a 0.5 Ω cm p-type silicon, demonstrated a solar cell efficiencies up to 21.7% [69]. Reinwand et.al. [70] investigated PERC cells with

**Figure 5.** *Rs-Rb vs. inverse metallization fraction 1/f. Reproduced with permission from [68].*

#### **Figure 6.**

*Measurement of the internal reflection R at the rear side, after foil attachment and laser fired contacts dependent on the SiNx capping layer thickness. The reflection varies by < 0.5% in the IR regime [74].*

sputtered aluminum on the rear side and a Ti▬Ag (50/100 nm) seed layer on top prior to the silver plating. With the optimized annealing temperature, the highest efficiency η = 21.1 and 19.4% for FZ and CZ wafers, respectively, was determined with the lowest contact resistivity ρc = 0.36 mΩ cm2 .

#### *2.3.5 Foil metallization*

In 2007, researchers from F-ISE introduced the laser-based foil metallization technology called "FolMet." With this technology, the conventional aluminum foil is attached to the silicon wafer [71], and thus the laser fired contact process forms both the electrical contact at the rear side of PERC cell as well as the mechanical contact by locally melting the aluminum through the passivation layer into the bulk silicon [72]. The key advantages of this process is its enhanced internal optical


**Table 3.**

*Advantages and disadvantages of different printing mechanisms [75].*

#### **Figure 7.**

properties obtained due to the air gap between foil and passivation layer [73], cost reduction potential by decreasing the capping layer thickness, and ease of cell production process [74].

**Figure 6** shows the internal reflection R at the rear side, after foil attachment and laser fired contacts. Nekarda et al., [73] by using the thick passivation layer optimized for the screen printed Al-paste, obtained an efficiency of 20.5%. In order to further reduce the cost, Graf et al. [74] adapted the rear side passivation layer with thinner capping layer and demonstrated an efficiency of 21.3% with a high Jsc due to the improved internal reflectance. Moreover, a low series resistance of 9 mΩ cm<sup>2</sup> of Al foil improved the FF to 80%. Pros and cons of various metallization schemes such as screen printing (SP), physical vapor deposition (PVD), and foil are tabulated in **Table 3**.

### *2.3.6 Metal wrap through PERC*

MWT cell (**Figure 7**) is similar to the conventional solar cell design, and the external front contact busbars for interconnection are located at the rear side which increases Jsc due to the reduced shading loss. Lohmüller et al. [76], from FhG-ISE, combined the MWT concept (Jsc improvement) and passivated emitter rear contact (PERC) concept (reduced rear SRV) and reported a conversion efficiency of 18.7% with Jsc of 39.9 mA/cm2 , Voc of 638 mV, and FF of 80.9% on a boron-doped p-type Cz grown silicon. The higher FF is due to the successful implementation of seed and plate technology [76]. Thaidigsmann from the same group introduced a simplified MWT-PERC cell called HIP-MWT (high performance metal wrap) to improve the efficiency by reducing the process complexity. In HIP-MWT structure, the formation of rear emitter is neglected hence no need of structuring steps. On a p-type FZ wafer with 0.5 Ω cm, a substrate thickness of 160 μm, on a cell area of 149 cm<sup>2</sup> resulted in an efficiency of 20.1% with Jsc of 39.1 mA/cm<sup>2</sup> , Voc of 659 mV, and FF of 77.8%, was obtained. The HIP-MWT cell demonstrated an efficiency of 19.6%

with Jsc of 40.2 mA/cm2 , Voc of 649 mV, and FF of 75.1% on Cz grown wafer with 2.6 Ω cm for the same substrate thickness and cell area [77].

Passivated emitter rear contact solar cell with dielectric layer at the rear side and locally rear aluminum contacts reduces the recombination losses which increases the open circuit voltage. Also the rear dielectric layer increases the internal reflection and thus increases the current of the solar cell. Though the performance of PERC cell is better, the efficiency of PERC cell decreases after light-induced degradation which is around 0.5–1.0% absolute.
