Experimental Details
In the present study, passivated emitter and rear contact (PERC) solar
cells were fabricated from silicon wafers possessing a resistivity of 2
Ω-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
200ºC, 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 (~3µm 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 (Te2O), 22-26% bismuth
oxide (Bi2O3), 1-5% silicon dioxide
(SiO2), 1-2% zinc oxide (ZnO), 1-2% tungsten oxide
(WO3) and 1-2% magnesium oxide (MgO). The organic
vehicle contained terpineol (C10H18O)
and texanol (C12H24O3)
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 (4µm 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 (2µm particle size).
Upon fabrication, the commercial Ag paste (Heraeus SOL9661) was applied
to identical M2 wafers (with 90 Ω/sq resistivity) to serve as a
reference contacting mechanism. The printing was arranged according to
the conventional H-pattern screen which has 40µm 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, J0metal, according to [22], a
special grid pattern containing different metal fractions is printed on
a symmetrical sample divided into 4 cm2-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-VOC measurements were performed to investigate
resistive effects, ideality factor (n), and saturation current density
(J0). Light current-voltage (I-V) measurements were
carried out to ascertain the maximum power, as well as the open circuit
voltage (VOC) 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.