## 3 Results and Discussion
the high resistance and non-conductive glass layer could hinder the current transmission between the Ag electrode and Al electrode, causing high Ag/Al contact resistance. The above results indicate that a significant difference in the Ag/Al contact resistance is caused by the different Ag/Al contact interfaces. Therefore, the different Ag pastes, especially, the contained glass frit, have a significant effect on the Ag/Al contact resistance and interfacial morphology. To reveal the interaction between glass frit and Al in co-sintering, we investigated the reaction of glass with Al by firing a glass frit coated in Al foil in an open electric furnace at 700 \({}^{\circ}\)C for 1 min, as shown in Figure 4. The interaction between G-1 glass frit and Al was evident, and Al foil changed into a quasi-transparent thin film after sintering, see Figure 4a. However, different results occurred when using the G-2 glass frit and results showed that the G-2 glass did not react with Al, see Figure 4b. The different reaction degree between glass frit and Al foil indicated that the glass frit in Ag paste leads to a different Ag/Al electrode contact interface by reacting with Al paste during sintering metallization, thus causing a remarkable difference in the contact resistance. Figure 3: FE-SEM images showing contact interface between Al electrode and Ag electrode. (**a1**) and (**a2**): Low and high magnification images for P-1 paste. (**b1**) and (**b2**): Low and high magnification images for P-2 paste. To further reveal the interaction between glass and Al, the mixture of glass frit and Al powder was heated at 700 \({}^{\circ}\)C for 1 min on the Si wafer after mixing them in an agate mortar at the mass ratio of 1:1. Figure 5a shows a FE-SEM cross-sectional image obtained when using the G-1 glass frit. It is clearly seen that the molten glass phase is totally coated in the Al particles, forming a non-conductive layer. However, when using the G-2 glass, we observed that some areas of the Al particles were not coated with glass frit, see Figure 5b. Glass, as a non-conductive phase, can hinder the current transmission. Therefore, the glass layer coated on Al particles forms a barrier between the Ag electrode and Al electrode, obstructing the current transmission between Ag and Al and causing a high Ag/Al contact resistance. Figure 6 shows the DSC curve of glass frit powder prepared by the traditional melting route. It was noticed that the glass transition temperatures T\({}_{\text{g}}\) of G-1 and G-2 glass frit powder are 259.89 \({}^{\circ}\)C and 513.31 \({}^{\circ}\)C, respectively. The T\({}_{\text{g}}\) of glass frit plays an important role in the physical and electrical properties of thick-film Ag contacts [12]. For the contact between the front-side Ag electrode and Si emitter surface, increasing the current transmission path at the Ag/Si interface is beneficial for achieving low Ag/Si contact resistance and high cell efficiency. It was reported that the glass frit with low T\({}_{\text{g}}\) in front-side Ag paste flows easily during the sintering process, allowing the glass to react with Ag for a long time and, consequently, dissolves more Ag particles, resulting in a great number of Ag precipitates distributed on the Si emitter surface and improving the possibility of tunneling, eventually achieving a low Ag/Si contact resistance and a high cell efficiency [13; 14]. Many previous  
Figure 4: Photographs showing the interaction of glass with Al by firing a glass frit coated in Al foil in an open electric furnace at 700 \({}^{\circ}\)C for 1 min. (**a**) G-1 glass frit; (**b**) G-2 glass frit. Figure 5: SEM images showing the interaction between glass and Al by heating the mixer of glass frit and Al powder at 700 \({}^{\circ}\)C for 1 min. (**a**) G-1 glass frit; (**b**) G-2 glass frit. studies [12; 13; 14] focused on the effect of the \(\rm T_{g}\) of the glass frit on the Ag/Si contact formation but ignored the \(\rm T_{g}\) of the glass frit on the Ag/Al contact resistance.