## 3 Results and Discussion  
In the currently exemplified Ag paste for rear-side metallization of c-Si PERC solar cells, the content of the glass frit is 1.2% of the total mass, while the Ag powder accounts for nearly 60%. For comparison, two Ag pastes were prepared using different glass frits labeled as G-1 and G-2, respectively. Both the two glass frits can meet the requirement of the rear Ag paste for PERC  
Figure 1: The contact pattern between the Ag and Al electrode. Ag paste was screen-printed into a discontinuous line on the SiN\({}_{x}\)/Al\({}_{2}\)O\({}_{3}\) passivated Si wafer, and then Al paste was printed into 5 x 5 square pads to interconnect the Ag paste. In order to magnify the contact effect, this model contains 48 Ag/Al joints with an overlap of 4.5 square millimeters for each contact. solar cells, making a firm and non-corrosive interconnection between the fired Ag electrode and the rear SiN\({}_{\rm x}\)/Al\({}_{2}\)O\({}_{3}\) passivation layer, see Supporting Information Figure S1. When used with PERC solar cells, the two pastes resulted in significantly different performances. Table 2 presents the I-V cell characterization results. It is shown that the two cells had similar open circuit voltages (V\({}_{\rm oc}\)) and short circuit currents (I\({}_{\rm sc}\)) but differed remarkably in Fill Factor (FF). Clearly, the lower FF gained with the G-1 glass frit was attributed to the greater series resistance (R\({}_{\rm s}\)), which was 98.68% larger than when the G-2 glass frit was used. As a result, the cell efficiency with the G-1 glass frit had a 0.33 percentage point decrease compared to what was achieved with the G-2 glass frit. To ascertain the reason for the difference in R\({}_{\rm s}\) between the two cells, the line resistance of the fired Ag electrode and the contact resistance between the rear Ag and Al electrode were studied. The two Ag paste electrodes had almost the same line resistivity, see Supporting Information Figures S2 and S3, implying that the difference in R\({}_{\rm s}\) between the two cells resulted from the difference in the contact resistance between the rear Ag and Al electrode. As presented in Figure 2, the Ag/Al electrode contact resistance was tested to be 36 \(\Omega\) for P-1 paste and 2 \(\Omega\) for P-2 paste, respectively. This remarkable difference can be attributed to the significant influence of the glass frit on the Ag and Al paste metallization because P-1 and P-2 paste differed only in the glass composition. The Ag/Al electrode contact interface was observed by using a FE-SEM. Figure 3a1,a2 show Ag/Al electrode cross-sectional images between P-1 paste and Al paste. The surface of Al particles was relativity smooth, as shown in Figure 3a1, and the FE-SEM image at higher magnification shows that there was an inter-miscible zone between the Ag electrode and the Al electrode, see Figure 3a2. We observed different results when P-2 paste was printed and sintered with Al paste, see Figure 3b1,b2. The surface of Al particles was quite rough, as shown in Figure 3b1. In addition, from the higher magnification FE-SEM image, shown in Figure 3b2, we observed that no inter-miscible zone formed at the Ag/Al electrode contact interface and the surface of Al particles was covered with some large aspect ratio metal precipitate films which resulted from the inter-diffusion between Ag and Al during the sintering process [9; 10]. Our previous research revealed that the inter-miscible zone formed between the Ag electrode and Al electrode was glass layer dissolved metal particles [11]. Therefore, \begin{table}
\begin{tabular}{c c c c c c} \hline
**Glass Frit** & **V\({}_{\rm oc}\) (mV)** & **I\({}_{\rm sc}\) (mA)** & **R\({}_{\rm s}\) (m\(\Omega\))** & **FF (\%)** & **Efficiency (\%)** \\ \hline G-1 & 664 & 9.72 & 3.02 & 79.48 & 20.96 \\ G-2 & 667 & 9.71 & 1.52 & 80.45 & 21.29 \\ \hline \end{tabular}
\end{table} 
Table 2: I-V cell characterization results. Figure 2: The chart of Ag/Al electrode contact resistance.