Developing a metallization that has low cost and high efficiency is essential in solar-cell industries, to replace expensive silver-based metallization. Ni/Cu two-step metallization is one way to reduce the cost of solar cells, because the price of copper is about 100 times less than that of silver. Alkaline electroless plating was used for depositing nickel seed layers on the front electrode area. Prior to the nickel deposition process, 2% HF solution was used to remove native oxide, which disturbs uniform nickel plating. In the subsequent step, a nickel sintering process was carried out in N2 gas atmosphere; however, copper was plated by light-induced plating (LIP). Plated nickel has different properties under different bath conditions because nickel electroless plating is a completely chemical process. In this paper, plating bath conditions such as pH and temperature were varied, and the metal layer's structure was analyzed to investigate the adhesion of Ni/Cu metallization. Average adhesion values in the range of 0.2-0.49 N/mm were achieved for samples with no nickel sintering process.

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In this research, we investigated selective emitter formation with a single-step photolithography process having a metallization scheme composed of nickel/copper metal stacks. The nickel seed layers were deposited by applying the electroless deposition process while copper was formed by light induced electro-plating arrangements as the main conducting electrode. The electroless deposition of nickel, along with a sintering process, was employed to create a diffusion barrier between copper and silicon. The nickel metal stack below the copper-conducting electrode also helped in lowering the sheet resistance and improving the contact adhesion. The nickel used as a seed layer was successfully demonstrated in the fabrication of a homogeneous 60 Ω/ emitter and selective emitter cells. Lower series resistances of 0.165 Ω and 0.253 Ω were achieved for the selective emitter and the homogeneous emitter cells, respectively. The best cell efficiency of 18.37% for the selective emitter solar cell was achieved, with average cell efficiencies of 18.17% and 17.3% for the selective emitter and the homogeneous emitter cells, respectively. An approximate efficiency increase of about 0.8% was recorded for the selective emitter solar cells.

The adhesion of the front electrodes to silicon substrate is the most important parameters to be optimized. Nickel silicide which is formed by sintering process using a silicon substrate improves the mechanical and electrical properties as well as act as diffusion barrier for copper. In this experiment p-type mono-crystalline czochralski (CZ) silicon wafers having resistivity of 1.5 Ω·cm were used to study one step and two step nickel electroless plating process. POCl3 diffusion process was performed to form the emitter with the sheet resistance of 70 ohm/sq. The Six, layer was set down as an antireflection coating (ARC) layer at emitter surface by plasma enhanced chemical vapor deposition (PECVD) process. Laser ablation process was used to open SiNx passivation layer locally for the formation of the front electrodes. Nickel was deposited by electroless plating process by one step and two step nickel electroless deposition process. The two step nickel plating was performed by applying a second nickel deposition step subsequent to the first sintering process. Furthermore, the adhesion analysis for both one step and two steps process was conducted using peel force tester (universal testing machine, H5KT) after depositing Cu contact by light induced plating (LIP).