**3.2. Nickel sintering**

**•** A reducing agent of sodium hypophosphite (NaH2PO2 H2O).

Step 1: H2PO2

Sum: 2H2PO2

simultaneous steps [27]:

284 Solar Cells - New Approaches and Reviews

*3.1.2. Light-induced deposition*

reported [44].

*3.1.3. Laser-assisted deposition*

production on the industrial scale.

**•** A buffer or mild complex agent of triammonium citrate [(NH4)3C6H5O7)].

Step 2: 2H- + Ni


Additionally, a small amount of ammonium hydroxide (NH4OH) is also added to elevate the pH of the solution. The pH of the plating bath should be maintained at around 8 ~ 10 for bath stability and uniform deposition rates [10, 16, 41]. The chemical reactions based on catalytic oxidation-reduction between hypophosphite ions and Ni can be described as the sum of two


Light source along with an electroless deposition process were used to deposit a Ni barrier layer. The phenomenon of a chemical reaction taking place is the same (catalytic oxidationreduction) as described in the previous section. However, the light source provided here helps in adjusting the electrochemical potential of the front are rear of the cell and enhances the plating rates [42]. The electron migration at the surface is controlled by the photo-voltage generated from the np-junction and the electronegativity of the substrates. Moreover, higher plating rates can be achieved as these photo-generated electrons enhance the reduction of the Ni2+ions on the silicon surface [15]. The increase in the plating rates due to light inclusion relives in the form of operating the bath at lower temperatures. Although a uniform Ni layer at higher plating rates can be deposited, the process of light-induced electroless plating involves complexity related to the process's characterization. Furthermore, the light-induced current in the LIP process helps to transport electrons only to the n-type surface, which limits the technique in metallizing n-type surfaces [43]. The light-induced nickel plating (LINP) process was also investigated by Yu-Han et al., and uniform Ni surfaces of high intrinsic quality were

Laser-assisted deposition has also been used to employ the Ni deposition process on a silicon surface. The process is considered to be feasible for industrial applications, as the anti-reflection coating (ARC) layer can be ablated along with the Ni deposition. This can reduce the number of steps involved in cell processing and can help the development of solar cells with mass

<sup>2</sup>- <sup>+</sup> 2H<sup>+</sup> <sup>+</sup> <sup>H</sup>- (1)

<sup>2</sup>- (3)

2+<sup>→</sup> Ni <sup>+</sup> H2 (2)

2+<sup>→</sup> Ni <sup>+</sup> H2 <sup>+</sup> 4H<sup>+</sup> <sup>+</sup> 2HPO3

In order to form a contact between the deposited Ni and silicon, a sintering step is required to form Ni silicide [38, 53, 54]. This sintering process helps to form an alloy of Ni and silicon and it act as a seed layer for the Cu. Furthermore, the sintering process reduces the contact resistance between the metals and the silicon interface [33]. The process involves heat treatment in the ambiance of N2 gas, and Ni is known to form various phases at different temperatures [55]. Three different phases of Ni2Si (~300 °C), NiSi (300~700 °C) and NiSi2 (~700 °C) can be realized after the sintering process. The NiSi phase offers the lowest resistivity (~14 μΩ.cm<sup>2</sup> ) among all three phases and is suitable for solar cell applications [10].
