**4.2. Current industrial trends**

based etching solutions. Furthermore, lower expenses are involved and the method produces

Laser-based front-contact openings and metal deposition are considered to constitute the most appropriate method for fabricating cells with Ni/Cu contacts on the industrial scale. In 1990, laser-based contact patterning was done for EFG polycrystalline silicon solar cells [63]. The laser process was used to fabricate laser-grooved buried contact (LGBC) solar cells at the University of New South Wales (UNSW), Australia [64]. Later on, a BP Solar was involved to

Laser-based ARC ablation can not only help in patterning the grid more easily with Ni deposition, but can also assist in producing cells with higher efficiencies. Thin-width laser grooves for the LGBC cells and selective emitter doping can be performed with the laser-based process for solar cell applications. Thinner finger lines and selective emitter doping ensure that a high open-circuit voltage (Voc) and higher efficiencies can be realized. Moreover, the heavy diffusion step for selective emitter solar cells can be conducted relatively easily [66, 67]. At Fraunhofer ISE, laser chemical processing (LCP) was demonstrated involving the performance of local etching by the use of phosphorus doping and laser grooving with damage free silicon microstructuring [68]. LCMD and LTC methods were also adopted to pattern the front-contact

grid and Ni seed layer deposition simultaneously for solar cell applications [51, 52].

silicon with a record 17.5% efficiency was also reported [71].

**4. Potential and commercial aspects**

The front and rear contacts of the solar cells can be patterned by a process called 'mechanical scribing'. Here, an artificial diamond tip is used to scribe a uniform and shallow depth over a given surface area. The method has the advantage of process simplicity and it can be helpful in patterning the solar cell surface with high throughputs. A mechanical scribing method was used to make grooves for buried contact solar cells [69]. Passivated emitter rear-cell (PERC) solar cells with a back-contact formed by a mechanical scribing process were also reported [11]. Solar cell efficiencies of more than 20% were achieved for such PERC solar cells. An artificial diamond tip of about 10 μm was used to pattern SiOx and SiNx passivation layers, while Sienriched SiNx layers were found to be etched away easily. Solarex Corporation, USA, used a diamond blade to form deep grooves with widths within the range of 25-45 μm for solar cell applications [70]. Mechanical V-texturing for buried contact solar cells on multi-crystalline

Copper can be the best alternative to silver in the front-electrode formation of crystalline silicon solar cells. The main motivation derives from the fact that it exhibits conductivity almost equal to silver, while its cost is about 100 times lower. The higher efficiency potential of the copper in terms of lower shading losses and higher FFs also provides additional benefits compared

less waste.

*3.4.3. Laser ablation*

288 Solar Cells - New Approaches and Reviews

*3.4.4. Mechanical scribing*

produce LGBC solar modules commercially [65].

Additional steps for front-contact patterning and Ni seed layer formation with a sintering step followed by Cu electroplating makes the process more complex for Ni/Cu metallized silicon solar cells. At present, Ni/Cu contacts have seen limited implementation on the industrial scale. Modules based on laser-grooved buried grid (LDBG) cells from BP Solar were only produced commercially [79]. However, there has been a lot of progress in terms of increases in solar cell efficiencies. More than 20% efficiencies have already been achieved at Fraunhofer ISE, Interuniversity Microelectronics Centre (IMEC), Kaneka, Roth & Rau Research and Schott Solar [78, 80-83]. Many research institutes have been involved in investigating metallization schemes composed of Ni/Cu metal stacks [8, 10, 13, 42, 84-90].

Thus far, efficiencies of up to 23.5% for a Cu-plated heterojunction-type solar cell have been achieved by Kaneka [80]. Roth & Rau Research also came up with a 22.3% efficient hetero‐ junction cell with Ni/Cu contacts [81]. For a passivated emitter rear cell (PERC), solar cell structure efficiencies above 20% have already been demonstrated by Fraunhofer ISE, Schott Solar and IMEC. Fraunhofer ISE and Schott Solar presented efficiencies of 21.4% [82] and 21.3% [91], respectively. However, at IMEC, an industrially applicable Ni/Cu-plated i-PERC-type solar cell with an average efficiency of 20.5% (more than 100 cells) and a best cell efficiency of 20.79% has been fabricated [83]. IMEC also investigated the application of Ni/Cu contacts for a rear-junction solar cell on a n-type substrate and achieved a best cell efficiency of 20.5% [78]. Ni/Cu metal stacks were also applied to contact laser-doped selective emitter (LDSE) solar cells. LDSE-type solar cells with efficiencies of 19.8% and 19.64% were developed at Hyundai Heavy Industries and Shinsung Solar, respectively [86, 92]. A 19.33% efficient LDSE-type solar cell with Ni/Cu contacts was presented at the UNSW [93]. More recently, a laser-doped Cuplated bifacial silicon solar cell exceeding 19% efficiency was developed at UNSW [94]. Solar cells consisting of Cu-plated contacts and exceeding 20% efficiency are presented in Table 2.


**Table 2.** Solar cells consisting of Cu-plated contacts exceeding 20% efficiency.
