**1. Introduction**

In silicon solar cell technology, metallization plays an integral part in outlining the cost and efficiency of solar cells. Indeed, the development of better techniques for the metallization of silicon solar cells is vital for achieving higher efficiencies. Presently, screen-printed contacts are primarily employed in photovoltaic industry as they can be realized easily with high throughputs. However, screen-printing technology has the draw-backs of higher contact resistance and lower aspect ratios, degrading the solar cells' performance. In recent years, an overall decrease in the photovoltaic module price index has occurred, while the higher cost of silver has disturbed this decrease adversely [1]. The future of solar cell technology will involve decreasing the wafer's thickness as well as decreasing the use of silver according to the standards set out by the international technology roadmap for photovoltaics (ITRPV) [2]. Metal contacts with superior electrical performance and lower production costs are vital for photo‐ voltaic industrial production. In short, the pressing process of screen printing technology for thinner silicon wafers, along with expensive silver pastes, needs to be replaced by a fresh metallization technology. A series of workshops have been dedicated to the metallization of crystalline silicon solar cells, where various metallization techniques have already been inquired [3-6]. All of these meetings have helped in understanding and reforming present-day advancements in solar cell metallization techniques.

Among the capable metallization techniques, contacts composed of nickel/copper (Ni/Cu) metal stacks are considered to be one of the most feasible candidates for future metallization technologies. The use of such metal stacks offers precise and low contact resistance and also helps in improving solar cells' efficiency [7]. The most important feature is that the technique can be realized with lower material costs. Ni/Cu contact formations involve two major steps: a Ni seed layer formation followed by a Cu metal deposition as a front electrode. The Cu metal stack over the Ni layer plays the role of the main contact to the front of the cell [7-12]. The Cu

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

front metals stack is usually formed by an electro-plating process, which is a well-developed plating technique and can only be applied to conductors. Cu has also been deposited by an electroless plating process, but it also requires a seed layer at the silicon surface [13, 14]. An auto-catalytic method of depositing Ni-phosphorus or Ni-boron by electroless plating bath compositions can be adopted to contact the semiconducting surface of silicon [15-17]. The use of a seed layer (i.e., Ni) can help to plate metal stacks on semiconducting or even nonconducting materials. Apart from Ni deposition from a conventional Ni plating bath, it can be plated under illumination (light-induced electroless plating) [18, 19] or by laser-assisted electroless plating [20]. A galvonic deposition method was also adopted to realize thin Ni layers on a textured silicon surface uniformly [21]. The Cu metal is usually electroplated by a light-induced plating (LIP) process developed at the Fraunhofer Institute for Solar Energy (ISE) [22, 23]. The LIP process involves the immersion of a sample composed of an np-junction with aluminium (Al) screen-printed on the back into an illuminated plating bath. The details of the LIP process will be discussed in the following sections.

This chapter deals with issues regarding contact formations using Ni/Cu metal stacks for crystalline silicon solar cells. The key issues for the discussed technology are: (i) an effective Ni seed layer formation, and (ii) the development of Cu metal contacts by the LIP process. In this section, we will give an overview of the process conditions for such metallization schemes and of the current research work as well as the challenges and issues that have emerged. The sections are dedicated to addressing the most important topics in detail.
