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.