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\begin{document}

\title{Implementation of Nickel and Copper as Cost-Effective Alternative
Contacts in Silicon Solar Cells}



\author[1]{Veysel Unsur}%
\affil[1]{Necmettin Erbakan University}%


\vspace{-1em}



  
  \date{May 21, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
Efficient metal contact formation is pivotal for the production of
cost-effective, high-performance crystalline silicon (Si) solar cells.
Traditionally, screen-printed silver (Ag) contacts on the front surface
have dominated the industry, owing to their simplicity, high throughput,
and significant electrical benefits. However, the high cost associated
with using over 13-20mg/Wp of Ag can impede the development of truly
cost-effective solar cells. Therefore, there is an urgent need to
explore alternative, economically viable metals compatible with silicon
substrates. This study reports on the application of a contact stack
consisting of Ag, nickel (Ni), and copper (Cu) in Si solar cells. To
prevent Schottky contact formation, Ag is implemented as a seed layer,
while Ni and Cu form the metal bulk layer. The fabricated bi-layer stack
without selective emitter exhibits a maximum efficiency of
\textasciitilde{}21.5\%, a fill factor of 81.5\%, and an average contact
resistance of 5.88m\selectlanguage{greek}Ω\selectlanguage{ngerman}·cm \textsuperscript{2} for a monofacial PERC cell.
Microstructure analysis demonstrate that the metals within the contacts
remain distinct, and Cu diffusion into the silicon during the firing
process is absent. Consequently, printed bi-layer contacts emerge as a
promising alternative to Ag contacts, reducing the Ag consumption to
below 2.5mg/Wp per cell without compromising overall efficiency.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


Veysel Unsur \textsuperscript{1, 2, *}

\textsuperscript{1} Department of Fundamental Sciences, Necmettin
Erbakan University, Konya, Turkiye

\textsuperscript{2} Center of Solar Energy Research and Applications,
ODT\selectlanguage{ngerman}Ü-GÜNAM, Ankara, Turkiye

*Corresponding author: vunsur@erbakan.edu.tr ,
veysel.unsur@odtugunam.org

\textbf{Abstract}

Efficient metal contact formation is pivotal for the production of
cost-effective, high-performance crystalline silicon (Si) solar cells.
Traditionally, screen-printed silver (Ag) contacts on the front surface
have dominated the industry, owing to their simplicity, high throughput,
and significant electrical benefits. However, the high cost associated
with using over 13-20mg/Wp of Ag can impede the development of truly
cost-effective solar cells. Therefore, there is an urgent need to
explore alternative, economically viable metals compatible with silicon
substrates. This study reports on the application of a contact stack
consisting of Ag, nickel (Ni), and copper (Cu) in Si solar cells. To
prevent Schottky contact formation, Ag is implemented as a seed layer,
while Ni and Cu form the metal bulk layer. The fabricated bi-layer stack
without selective emitter exhibits a maximum efficiency of
\textasciitilde{}21.5\%, a fill factor of 81.5\%, and an average contact
resistance of 5.88m\selectlanguage{greek}Ω\selectlanguage{ngerman}·cm\textsuperscript{2} for a monofacial PERC cell.
Microstructure analysis demonstrate that the metals within the contacts
remain distinct, and Cu diffusion into the silicon during the firing
process is absent. Consequently, printed bi-layer contacts emerge as a
promising alternative to Ag contacts, reducing the Ag consumption to
below 2.5mg/Wp per cell without compromising overall efficiency.

\textbf{Keywords: PERC, screen printing, alternative metallization, Ni
contacts, Cu contacts, solar cells}

\textbf{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.

\textbf{Experimental Details}

In the present study, passivated emitter and rear contact (PERC) solar
cells were fabricated from silicon wafers possessing a resistivity of 2
\selectlanguage{greek}Ω-\selectlanguage{english}cm. A silicon nitride (SiNx:H) layer with a thickness of 70 nm was
deposited as an antireflection coating layer (ARC) layer on the front
surface, and a 5 nm thick aluminum oxide (AlOx) layer along with a 75 nm
thick SiNx:H layer were deposited on the rear side through
plasma-enhanced chemical vapor deposition (PECVD). An aluminum (Al)
contact was screen-printed on the back surface and subsequently dried at
200\selectlanguage{ngerman}ºC, followed by the application of front gridlines using three
distinct metal pastes.

The first of these was a commercially available Ag paste, used as a
reference for subsequent comparisons. The second paste, an in-house Ag
paste, was composed of Ag metal powder (\textasciitilde{}3\selectlanguage{greek}µ\selectlanguage{english}m particle
size), a glass frit, and an organic vehicle. The glass frit, prepared
using the melt quenching method was constituted of 30-40\% lead oxide
(PbO), 25-35\% tellurium oxide (Te\textsubscript{2}O), 22-26\% bismuth
oxide (Bi\textsubscript{2}O\textsubscript{3}), 1-5\% silicon dioxide
(SiO\textsubscript{2}), 1-2\% zinc oxide (ZnO), 1-2\% tungsten oxide
(WO\textsubscript{3}) and 1-2\% magnesium oxide (MgO). The organic
vehicle contained terpineol (C\textsubscript{10}H\textsubscript{18}O)
and texanol (C\textsubscript{12}H\textsubscript{24}O\textsubscript{3})
as solvents, ethyl cellulose and polyvinylpyrrolidone (PVP) as binders,
hydrogenated castor oil as a thixotropic agent, and sorbitan-triolate as
a surfactant. These components were blended in a weight ratio of
82.5:3.5:14 respectively, and the resulting mixture was subjected to
three-roll milling for 60 minutes for proper dispersion.

The third paste employed was a Nickel (Ni) paste, prepared both with and
without the aforementioned glass frit. The Ni paste, which included the
glass frit, was prepared using the same glass frit and organic vehicle
as the in-house Ag paste, but with a different weight ratio of
components (77:3.5:19.5 for the Ni metal powder, glass frit, and organic
vehicle, respectively). The Ni paste without the glass frit was prepared
by combining Ni metal powder (4\selectlanguage{greek}µ\selectlanguage{english}m particle size) with the same organic
vehicle in a weight ratio of 85:15.

Finally, a copper (Cu) paste was prepared without glass frit. It
contained the same organic vehicle and was mixed in a weight ratio of
88:12 with Cu metal powder (2\selectlanguage{greek}µ\selectlanguage{english}m particle size).

Upon fabrication, the commercial Ag paste (Heraeus SOL9661) was applied
to identical M2 wafers (with 90 \selectlanguage{greek}Ω/\selectlanguage{english}sq resistivity) to serve as a
reference contacting mechanism. The printing was arranged according to
the conventional H-pattern screen which has 40\selectlanguage{greek}µ\selectlanguage{english}m openings. Other wafers
of the same type were printed with the in-house Ag paste (single layer),
Ni paste (single layer), as well as stacks of Ag paste + Ni paste and Ag
paste + Cu paste. To measure the saturation current density and one of
its components, J\textsubscript{0metal}, according to {[}22{]}, a
special grid pattern containing different metal fractions is printed on
a symmetrical sample divided into 4 cm\textsuperscript{2}-area cell
size.

Following the preparation and printing of the different contact
materials onto the wafers, a co-firing process was executed using a
six-zone conveyor infrared (IR) belt furnace, adhering to the firing
profiles outlined in Fig. 1. An exploration of various peak temperatures
was conducted at a constant belt speed of 230 inches per minute (ipm) to
understand the influence of peak temperature on the fill factor (FF) for
different contact designs. Rapid rates of temperature increase and
decrease were employed, as detailed in the inset graph of Fig. 1, in
order to ensure uniform formation of the back surface field (BSF) on the
rear side, and to enhance metal crystallite formation beneath the front
contacts.

Upon completion of the firing process, the resulting solar cells
underwent an array of electrical and optical assessments. Electrically,
Suns-V\textsubscript{OC} measurements were performed to investigate
resistive effects, ideality factor (n), and saturation current density
(J\textsubscript{0}). Light current-voltage (I-V) measurements were
carried out to ascertain the maximum power, as well as the open circuit
voltage (V\textsubscript{OC}) and fill factor (FF). Optically, the
fabricated cells were examined via a scanning electron microscope (SEM)
integrated with energy-dispersive X-ray (EDX) spectroscopy to visualize
the cross section between the metal contacts and the emitter region of
the solar cells.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Fig. 1} The contact co-firing temperature profiles with
different peak temperatures. The inset graph shows the temperature
velocity of the conveyor belt furnace for the profiles.

\textbf{Results and Discussions}

Prior to the contact co-firing process, the average finger widths, as
presented in Table I, fall within the range of 51\selectlanguage{greek}µ\selectlanguage{english}m to 55\selectlanguage{greek}µ\selectlanguage{english}m.
Post-firing, the width of the Ag+Cu bi-layer stack notably expands to
56\selectlanguage{greek}µ\selectlanguage{english}m due to most likely ductile behavior of copper, while concurrently
attaining a height of 10\selectlanguage{greek}µ\selectlanguage{english}m. Although the widths of the fingers shrink
for each contact types, there is no ghost finger formation on the
surface after firing process as shown in Fig. 2. In comparison, the
width of the conventional screen-printed Ag contacts is slightly
narrower. However, the metal fractions of the contacts, which are
related to glass frit used in this study, are similar, measuring 5.1\%
for Ag, 5\% for in-house Ag, 5.4\% for the Ag+Ni bi-layer, and 5.7\% for
the Ag+Cu bi-layer, respectively. These are closely aligned with the
approximated 5\% metal fraction of commercial Ag contacts {[}23{]},
{[}24{]}. As tabulated also in Table I, the width of the fingers varies
depending on the metal used, a phenomenon potentially attributable to
disparities in wetting and concentration within each paste. The Ni
contact exhibits the narrowest average width of 47\selectlanguage{greek}µ\selectlanguage{english}m post-firing, having
commenced at 55\selectlanguage{greek}µ\selectlanguage{english}m prior to the firing process. This highlights the
increased tendency for Ni fingers to sinter and compact, forming into a
continuous line. For the finger height of the printed fingers, contacts
with in-house prepared Ag paste gives the tallest finger which suggests
that the surfactant and thixotropic agent used in the paste, although
the same type is used in each paste, correlate with Ag particles better
than the others.

\textbf{Table I} The average finger widths and height before and after
contact co-firing process\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Contacts & Finger Width Before Firing (\selectlanguage{greek}μ\selectlanguage{english}m) & Finger Width After Firing
(\selectlanguage{greek}μ\selectlanguage{english}m) & Finger Height After Firing(\selectlanguage{greek}µ\selectlanguage{english}m)\tabularnewline
\midrule
\endhead
Ag (reference) & 53 & 49 & 9\tabularnewline
Ag (in-house) & 52 & 50 & 14\tabularnewline
Ni & 55 & 47 & 9\tabularnewline
Ag + Ni & 51 & 51 & 13\tabularnewline
Ag + Cu & 55 & 56 & 10\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Fig. 2} Top view SEM images of the two bi-layer stack contacts
after firing

Supporting the above-mentioned premise, the results of finger resistance
(R\textsubscript{F}) for the single layer Ni contact compared to the
single layer Ag contact across different firing profiles, as depicted in
Fig. 3a, demonstrate promising outcomes. Particularly, screen-printed Ni
contacts at the industry-standard peak temperature of around 770-800\selectlanguage{ngerman}°C
(on the sample) exhibit favorable results in finger resistance compared
to Ag counterparts. The R\textsubscript{F} shows limited deviation with
varying contact co-firing peak temperature, suggesting that metal
particle sintering occurs properly for both Ni and Ag contacts This can
potentially be attributed to softening of Ni metals at the intersection
of Ni/Si predominantly occur at elevated temperatures. Comparing the
contact resistance of two single layer contacts, on the other hand,
proves that Ag makes a better contact with Si than Ni as seen in Fig.
3b. This advantage stems from Ag's lower metal work function compared to
Ni. The closer alignment of Ag's work function with Si's electron
affinity (\textasciitilde{}4.05 eV) facilitates efficient charge carrier
transfer across the metal-semiconductor interface, thereby reducing
contact resistivity. Additionally, the relatively low contact resistance
observed for both contact types suggests that the in-house prepared
glass frit is effectively etching the ARC layer without damaging the
emitter. This is further supported with power conversion efficiency and
fill factor (FF) results of both single layer contacts, as shown in
Figure 4. In line with the result of resistivity measurements of the
contacts, the FF and the efficiency for Ni contacts suffer due to high
series resistance thereby underscoring the potential of Ag as a viable
seed layer.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Fig.3} The finger and contact resistances measurements according
to different peak temperature of contact co firing for Ag and Ni
contacts\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Fig 4.} The efficiency and fill factor measurements according to
different peak temperature of contact co firing for Ag and Ni contacts

To overcome the high contact resistivity of Ni and reduce the high cost
of Ag contacts, Ag is used as a seed layer with Ni and Cu forming the
bulk of the metal in a bi-layer contact type. The average electrical
output parameters and the Suns-V\textsubscript{OC} measurement results
for the single layer along with bi-layer contact types are presented in
detail with respect to their optimized firing temperature in Table II.
For reference, a screen-printed Ag contact solar cell is presented in
Table III. The in-house prepared Ag paste performs the best among the
other types with an average efficiency of 21.7\%. The second best
performing structure is Ag/Cu bi-layer contact stack which achieves
21.32\% efficiency and FF of 81.4\%. This result implies that even
though the samples are fired at a peak temperature of 770\selectlanguage{ngerman}°C, there is no
Cu contamination in the cell, as supported by high shunt resistance
(R\textsubscript{SH}) values of 150000. The J\textsubscript{0,
metal}value of 48fA/cm\textsuperscript{2} and the R\textsubscript{C} of
5.88m\selectlanguage{greek}Ω\selectlanguage{english}[?]\selectlanguage{english}cm\textsuperscript{2} for the Ag/Cu contact, compared to
reference Ag contact of 41fA/cm\textsuperscript{2} and
4.63\selectlanguage{greek}Ω\selectlanguage{english}[?]\selectlanguage{english}cm\textsuperscript{2}, also strongly indicate that Ag is
successfully blocking the Cu diffusion into the junction. The total
series resistance (R\textsubscript{S}) of the same contact, obtained by
the light IV measurement, on the other hand, shows an additional
resistive effect that may be due to the line resistance of the stack
contact as a whole. This conclusion is further substantiated by the
microstructure analysis depicted in Fig. 5. The elemental examination
reveals that the Cu remains on top of the Ag layer, indicating that Cu,
despite its aggressive diffusion properties, has not permeated
underneath the Ag. The Ag/Ni contacts also result in promising
electrical performance. The FF of 81.2\% indicates that the contact
formation is complete and there is no shunt in the cell considering the
contact stack of Ag/Ni yielded close to maximum shunt resistance.
However, the contact resistance and total series resistance is slightly
higher than the others, possibly due to some delamination between Ag and
Ni. For the Ni as a single contact, in addition to above-mentioned
analysis, the cell is clearly shunted and degraded due to use of same
glass frit in each paste and the peak firing temperature of 800\selectlanguage{ngerman}°C, which
is clearly pushing the contact proximity closer to the junction. This is
supported by the low shunt resistance of 12050 \selectlanguage{greek}Ω\selectlanguage{english}[?]\selectlanguage{english}cm\textsuperscript{2}.

\textbf{Table II.} The average electrical output parameters of best
performed cells with respect to peak temperature\selectlanguage{english}
\begin{longtable}[]{@{}lllllllll@{}}
\toprule
\begin{minipage}[b]{0.11\columnwidth}\raggedright\strut
Metal Stack\strut
\end{minipage} & \begin{minipage}[b]{0.11\columnwidth}\raggedright\strut
V\textsubscript{OC} (mV)\strut
\end{minipage} & \begin{minipage}[b]{0.11\columnwidth}\raggedright\strut
FF (\%)\strut
\end{minipage} & \begin{minipage}[b]{0.11\columnwidth}\raggedright\strut
\selectlanguage{greek}η \selectlanguage{english}(\%)\strut
\end{minipage} & \begin{minipage}[b]{0.11\columnwidth}\raggedright\strut
Rs (\selectlanguage{greek}Ω\selectlanguage{english}[?]\selectlanguage{english}cm\textsuperscript{2})\strut
\end{minipage} & \begin{minipage}[b]{0.11\columnwidth}\raggedright\strut
Rc (m\selectlanguage{greek}Ω\selectlanguage{english}[?]\selectlanguage{english}cm\textsuperscript{2})\strut
\end{minipage} & \begin{minipage}[b]{0.11\columnwidth}\raggedright\strut
J\textsubscript{0, metal} (fA/cm\textsuperscript{2})\strut
\end{minipage} & \begin{minipage}[b]{0.11\columnwidth}\raggedright\strut
R\textsubscript{SH} (k\selectlanguage{greek}Ω\selectlanguage{english}[?]\selectlanguage{english}cm\textsuperscript{2})\strut
\end{minipage} & \begin{minipage}[b]{0.11\columnwidth}\raggedright\strut
Peak (\selectlanguage{ngerman}°C)\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
Ag\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
665\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
81.7\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
21.72\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.54\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
4.63\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
51\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
150\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
790\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
Ag / Ni\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
661\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
81.2\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
21.14\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.76\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
6.41\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
53\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
131\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
780\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
Ag / Cu\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
663\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
81.4\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
21.32\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.65\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
5.88\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
48\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
150\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
770\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
Ni\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
654\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
80.1\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
20.44\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.84\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
7.42\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
98\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
12\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
800\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table III.} The electrical output parameters of a screen printed
Ag contacted reference cell\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
\begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
Type\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
V\textsubscript{OC} (mV)\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
FF (\%)\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\selectlanguage{greek}η \selectlanguage{english}(\%)\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
Rs (\selectlanguage{greek}Ω\selectlanguage{english}[?]\selectlanguage{english}cm\textsuperscript{2})\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
R\textsubscript{C} (m\selectlanguage{greek}Ω\selectlanguage{english}[?]\selectlanguage{english}cm\textsuperscript{2})\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
J\textsubscript{0, metal} (fA/cm\textsuperscript{2})\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
Reference Ag Contacts\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
667\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
81.6\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
21.9\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
0.51\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
4.11\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
41\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

Fig. 5 Cross section elemental x-ray analysis of Ag/Cu contacts

Challenges in the printing process manifests themselves in two distinct
forms; lift-off of Ag/Ni stack from the front surface and delamination
of Ag/Cu contacts between the Ag and Cu. As illustrated by SEM images,
these issues not only compromised the mechanical stability of the
contacts but also influenced their electrical performance. Figure 6a
provides a cross-sectional SEM image of an Ag/Cu contact after
undergoing a contact co-firing process at 780\selectlanguage{ngerman}°C. The clear separation
between the Ag and Cu layers might be attributable to the differential
thermal expansion of these metals during the firing process. However,
this delamination issue is not apparent in contacts co-fired at peak
temperatures below 780°C, underlining the significance of optimal firing
conditions for the stability of these bilayer contacts. The SEM image in
Fig. 6b exhibits a lift-off scenario with the Ag/Ni stack, where the
contact stack detaches from the Si substrate in the middle, while
remaining intact at the edges. This occurrence could stem from multiple
factors, including differential thermal expansion, interfacial
reactions, and inadequate surface preparation, among others. These
observations align with previous study {[}25{]}, which reported similar
lift-off and delamination issues in aerosol printed bi and tri stack
layers of Ag frit/Ni and Ag frit/Ni/Cu when fired above 790°C peak
firing temperature.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{Fig 6. (a)} SEM image of the Ag/Cu bi layer stack contacts after
contact co-firing, the metals detached from each other created high line
resistance and \textbf{(b)} SEM image of the Ag/Ni contact lifts off
from the Si substrate

The efficiency and series resistance parameters, plotted based on finger
width associated with lift-off and delamination in stacked contacts, are
presented in Fig. 7. The results show that bi-layer contacts perform
similarly to Ag counterparts, provide no delamination or a lift-off
occur in the structure. Lift-off from the center damages the performance
to the extent that the efficiency of Ag/Ni contacted cell may drop to as
low as 16.62\% as shown by the highlighted datapoints in Fig. 7 (left).
The series resistance of those cells, shown in Fig 7 (right), may exceed
1\selectlanguage{greek}Ω\selectlanguage{ngerman}·cm\textsuperscript{2} due to high contact resistance while
delamination present itself as a finger resistance and increases the
R\textsubscript{S} for the Ag/Cu contacts. For those cells where the
contact layers are intact, the contact resistance is in the range of the
Ag reference, implying that the performance can reach industry standards
and lower the Ag usage per watt. Following this data, it can be
concluded that the performance of both Ag/Cu and Ag/Ni contacts closely
mirrors that of their Ag counterparts. These findings suggest a high
potential for these bi-layer configurations to serve as effective
alternatives to Ag, especially considering their performance, which is
commensurate with industry standards. By leveraging these alternatives,
a considerable reduction in Ag consumption per watt can be achieved
without compromising the efficiency of solar cells.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\textbf{Fig. 7} The efficiency and series resistance for different
contacts with different finger width. Highlighting on the data point
shows if the contact has a lift off or delamination, green highlight
means lift off from substrate and pink means interlayer delamination.

One of the key determinants of the viability and scalability of the
proposed alternative metallization technique is its cost-effectiveness
and potential impact on the Levelized Cost of Electricity (LCOE). Prior
research by Zhang {[}26{]} demonstrated that in traditional solar cell
manufacturing, Ag consumption is around 13-20 milligrams per watt-peak
(mg/Wp). This represents a significant cost due to the high market price
of Ag, thereby posing a potential barrier to terawatt-scale production.
In order to calculate the reduction in silver usage achieved through the
employment of a bi-layer structure, using Ag as a seed layer and either
Ni or Cu as bulk metal, all cells were weighed before printing and after
co-firing the contacts, ensuring the complete removal of solvents and
binders. The metallization patterns for each contact type were
maintained consistent with 110 fingers and 5 busbars. The cells weighed
for metal consumption analysis were carefully selected based on the
average finger width, to minimize discrepancies between samples. The
bi-layer contacts for consumption analysis underwent a two-step firing
process: once after Ag printing, and once after Ni or Cu printing. This
allowed for accurate determination of Ni and Cu consumption. As Table IV
illustrates, the average values of Ag, Ni, and Cu consumption for the
completed cells demonstrate that Ag consumption for bi-layer contacts is
below 2.5mg/Wp. This represents nearly a sevenfold reduction when
compared to the Ag contacted reference cell. Considering the Ag price of
\$808, Ni price of \$24 and Cu price of \$8, the metallization cost per
wafer drops 80.75\% lower for Ag/Ni contacts and 82.15\% for Ag/Cu
contacts. The reflection of this cost drop on the LCOE is calculated
according to Lazard's calculation {[}27{]} with System Advisor Model
(SAM) by NREL in a scenario in which it is assumed that M2 sized solar
cells are used for 1GW solar farm in south Turkiye where the solar
insolation is 5.1kWh/m\textsuperscript{2}/day. The results show that for
the reference Ag contacted solar cells yields an LCOE of 8.9\selectlanguage{ngerman}¢/kWh. To
analyze the effect of the bi-layer contacted cells on the LCOE, the
sensitivity analysis is carried out in which the inputs are varied
according to their electrical output parameters and metal consumption
given in Table II and Table IV, respectively. Fig. 8 shows the
sensitivity analysis results where negative correlation (to the left
from the centerline) means lower LCOE and positive correlation (to the
right from the centerline) results in higher LCOE. It is illustrated in
Fig. 7 that although the loss in power conversion efficiency due to high
series resistance has an impact to increase the LCOE around 0.35¢/kWh,
the cost drop of metal consumptions has a higher impact to lower the
LCOE of around 0.65¢/kWh. The increase in thermal budget for the contact
firing of bi-layer contacts has a limited effect on the LCOE. Therefore,
the financial implications of adopting this bi-layer metallization
approach are significant. The drastically reduced silver consumption
leads to substantial cost savings, which are directly reflected in the
LCOE.

\textbf{Table IV} Ag Ni and Cu consumption of the finished cells after
contact co-firing\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
\begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\textbf{Ag reference cell}\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\textbf{Ag/Ni contacted cell}\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\textbf{Ag/Cu contacted cell}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\textbf{Ag consumption}\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
91.5mg -- 13.07mg/Wp\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
16.4mg -- 2.41mg/Wp\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
15.9mg -- 2.31mg/Wp\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
\textbf{Ni consumption}\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
41.15mg -- 6.04mg/Wp\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
\textbf{Cu consumption}\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
44.9mg -- 6.51mg/Wp\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

\textbf{Fig. 8} Sensitivity analysis of the LCOE for solar cells with
bi-layer contact structure

\textbf{Conclusion}

In conclusion, the adoption of the proposed alternative metallization
technique, employing a screen printed bi-layer configuration with either
Ag/Ni or Ag/Cu, presents significant advantages in terms of
cost-effectiveness. These configurations, while maintaining a similar
level of performance as traditional Ag contacts, substantially reduce
the silver consumption and hence the overall metallization cost per
wafer. Importantly, these cost savings are reflected in the Levelized
Cost of Electricity (LCOE), showcasing the potential of this alternative
technique to bring about notable cost efficiencies in the solar power
industry. Although the efficiency loss due to higher series resistance
and the slight increase in thermal budget for contact firing of bi-layer
contacts are to be taken into account, the reduction in metal costs
substantially outweighs these factors, resulting in a lower LCOE. This
innovative approach, combining enhanced performance with economic
viability, has the potential to significantly advance the manufacturing
process of solar cells and bolster the broader transition towards more
sustainable and cost-effective energy solutions.

\textbf{Acknowledgments}

The authors would like to express their profound gratitude for the
support provided by the Scientific and Technological Research Council of
Turkiye (T\selectlanguage{ngerman}ÜB\selectlanguage{polish}İ\selectlanguage{english}TAK). This research has been made possible through grant,
under reference number 120E474.

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\selectlanguage{english}
\FloatBarrier
\end{document}

