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.