Nickel silicide contacts formed by excimer laser annealing for high efficiency solar cells

Abstract

Driven by the relatively high cost of silver (Ag), interest has grown in the photovoltaic (PV) industry to substitute conventional screen printed (SP) Ag front contacts with copper (Cu) plated contacts. The approach chosen here applies selective laser ablation of the front anti‐reflection coating (ARC), then forming self‐aligned nickel silicides (NiSix) contacts, and thickening the lines by Cu plating to achieve the desired line conductivity. A successful implementation of this scheme requires annealing to form NiSix with low contact resistance. However, it has been shown that industrial shallow emitters can be damaged severely upon conventional annealing of nickel. In this paper, we show that by using large area excimer laser annealing (ELA), NiSix contacts can be formed on industrial shallow emitters without the associated junction degradation. On the basis of sheet resistance, transmission electron microscopy, and lifetime measurements, we demonstrate that NiSix formation by ELA can be achieved in narrow contact openings without damaging the passivation and reflectance properties of the neighboring ARC. In addition, the thresholds for NiSix formation for different Ni thicknesses are quantified by rigorous finite element simulations and compared with experimental data. Finally, high efficiency passivated emitter and rear cell type solar cells featuring a shallow 85 Ω/sq emitter have been processed on large area CZ–Si using laser ablation of the ARC and subsequent NiSix formation by ELA. These cells show an average efficiency gain of 0.4%abs compared with cells processed with reference SP contacts. In this work, the best performing cell with the ELA process reached 20.0% energy conversion efficiency. Copyright © 2013 John Wiley & Sons, Ltd.

Improved adhesion of Ni/Cu/Ag plated contacts with thermally formed nickel silicon interface for C-SI solar cells

“…1(c), confirmed using EDS analysis. This NiSi x layer increases the contact adhesion between Ni and Si and is already been reported [8][9][10][11]. To deposit Cu in the subsequent step the Ni surface was activated by 2 minutes SPM etching at 40 °C.…”

“…The formation of Ni silicides, by heating the stacks of Ni over Si has been a useful way to increase the contact adhesion [8]. There have been reports showing promise to the adhesion promotion by silicide formation through the sintering process [9][10][11]. Si surface pretreatment was also used to achieve excellent adhesion for Ni/Cu plated contacts [11].…”

Challenges facing copper‐plated metallisation for silicon photovoltaics: Insights from integrated circuit technology development

“…This permitted a long sintering duration that would be expected to effectively reduce any surface oxides which may have formed at the Si interface during the electroless Ni plating process . Consequently, a uniform Ni silicide most likely formed across the entire contact surface and increased contact adhesion …”

Crystalline Silicon Solar Cells with Nickel/Copper Contacts

“…More than 20% efficiencies have already been achieved at Fraunhofer ISE, Interuniversity Microelectronics Centre (IMEC), Kaneka, Roth & Rau Research and Schott Solar [78,[80][81][82][83]. Many research institutes have been involved in investigating metallization schemes composed of Ni/Cu metal stacks [8,10,13,42,[84][85][86][87][88][89][90].…”