Introduction
Screen-printed silver (Ag) metal contacts have long been favored in the production of silicon (Si) solar cells due to their simplicity, maturity, and high throughput. Their dominance in the photovoltaic (PV) market is largely due to their excellent conductivity and solderability. [1]–[4]. However, despite its advantages, the use of screen-printed Ag contacts has significant downsides. One of the most notable is its high cost, contributing up to 40% of the total cell production expense, posing a major barrier to scaling and achieving cost-effective solar cells [5]–[7]. Consequently, there is a pressing need to investigate alternative metals that have the potential to form ohmic contacts with Si substrate while reducing overall production costs. Metals such as copper (Cu) and nickel (Ni) have been extensively explored due to their similar conductivity and significantly lower cost compared to Ag [8]–[13]. Techniques such as plating, including electroplating and light induced plating, have emerged as promising methods for precise contact formation with these metals. However, the complexity of this process, including additional steps such as photoresist application, laser opening of Ni or Cu deposition, and the use of large amounts of consumables, adds to the fabrication cost and deviates from current manufacturing practices.
To maintain efficiency without increasing costs, it is crucial to adopt a metallization process that aligns with the state-of-the-art in its simplicity and high throughput. Printable metal pastes derived from Cu or Ni offer a potential solution, provided they can be adapted for fire-through applications. Prior studies have experimented with low-temperature curable Cu alloys or floating busbar designs [14]–[16]. However, these methods necessitate additional laser contact openings at the front surface and are often only applicable to certain grid designs such as floating busbars. In addition, the use of high temperature Cu pastes has been largely avoided due to Cu’s aggressive diffusion even at room temperature, except for recent studies of the Cu fire-through pastes, though these have yet to match the efficiency of their Ag counterparts [17]–[19]. Furthermore, directly contacting alternative metals such as Ni and Cu onto the Si emitter can potentially limit a solar cell’s power conversion efficiency due to recombination losses and contact resistance. To mitigate these issues, a seed layer capable of forming an ohmic-like contact with Si, is often placed beneath the metal bulk [10], [20], [21]. Ag is an ideal candidate for this seed layer, owing to its established role in the industry and the compatibility of its metal work function with the Si substrate.
Despite the extensive exploration of Cu and Ni as alternatives Ag in Si solar cells, this study introduces a novel approach by employing a screen printable fire-through technique for these metals, both with and without the inclusion of glass frit. This work further distinguishes itself by providing a detailed investigation into the composition of in-house metal pastes and their performance relative to commercial Ag paste. Additionally, we uniquely examine the influence of peak temperature on the fill factor for different contact designs, offering a comprehensive evaluation through an array of electrical assessments, thereby contributing a significant advancement to the field. Here in this study, we explore the fabrication of passivated emitter and rear contact (PERC) Si solar cells with stack contacts comprising of Ag seed layer topped with Ni and Cu bulk using screen printing. The use of three different in-house prepared fire-through pastes of Ag, Cu, and Ni is demonstrated here. These pastes are screen printed on the front surface of PERC cells to form the front contacts. Employing bi-layer contact structures of Ag/Ni and Ag/Cu not only presents a potentially effective alternative to Ag counterparts for highly efficient solar cells, but also results in significant cost reduction.