## 3 Results
### Optimization Experiment and Curing Mechanism  
The variables (operating conditions) can be optimized within the range of the experimental design using quadratic models to obtain higher Y values. Three validation experiments were conducted with the optimized predicted operating condition to obtain the maximum value of Y in the range of the experimental design. It is shown in Table 4 that the deviations between the results of three validation experiments and the predicted values were less than 2.2%. To further investigate the curing mechanism of the E51-MeTHPA curing system, a FTIR spectrometer was used to observe the characteristic absorption band transformation at different time intervals. The curing reaction was carried out at 199 \({}^{\circ}\)C with the curing agent dosage of 41 wt% (the optimization experiment condition of No. 3 in Table 4). The FTIR spectrogram at different curing times are shown in Figure 5. As the curing reaction proceeded, the characteristic absorption band of epoxy group (about 915 cm\({}^{-1}\)) and anhydride carbonyl group (1750-1824 cm\({}^{-1}\)) became weaker. This means that these functional groups were gradually reacted to form other materials during the curing process. The decreased intensity of the band at 976 and 1257 cm\({}^{-1}\) (cyclic anhydride) indicated the ring-open process of anhydride during the reaction. At the same time, the characteristic absorption peak of the ester (1740 cm\({}^{-1}\)) and hydroxyl group (nearby 3500 cm\({}^{-1}\)) became stronger. This indicated that there were both addition and esterification processes in the reaction of E51 and the ring-open MeTHPA [25]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**t (\({}^{\circ}\)C)**} & \multirow{2}{*}{**T (Min)**} & \multirow{2}{*}{**M (wt\%)**} & \multicolumn{2}{c}{**Experimental Value**} & \multicolumn{2}{c}{**Predicted Value**} \\ \cline{5-6}  & & & & & **Y (KPa)** & **Y (KPa)** \\ \hline
1 & 207 & 38 & 41 & 1145 & 1131 \\
2 & 198 & 42 & 39 & 1141 & 1131 \\
3 & 199 & 44 & 41 & 1156 & 1131 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Experimental and predicted values for adhesion strength (Y) in the validation experiments. Figure 4: Response surface showing Y as a function of curing agent dosage (m) and treatment temperature (T). A suitable curing time can also be determined by calculating the conversion rates of the epoxy group with the decreasing absorption band of the epoxy group at 915 cm\({}^{-1}\). The band at 1609 cm\({}^{-1}\) was attributed to the stretching vibration of C = C in the benzene ring, which is invariable during the curing process. Therefore, it can be used as an internal standard peak. The conversion rate of the epoxy group is calculated by Equation (6):  
\[\delta_{\mathrm{t}}=\left[1-\frac{\left(\mathrm{A}_{915}/\mathrm{A}_{1609} \right)_{\mathrm{t}}}{\left(\mathrm{A}_{915}/\mathrm{A}_{1609}\right)_{ \mathrm{t}0}}\right]\times 100\% \tag{6}\]  
where A is the peak area of corresponding wave number and t\({}_{0}\) means the starting time. After 30 min, the conversion rate was 84.3%. The value became 93.6% at 45 min. When the curing time reached 60 min, the conversion rate was 95.3%. The system was almost fully cured in 45 min. To clearly show the microstructure of the silver paste, the SEM images of the silver paste before and after curing have been taken and shown in Figure 6. The curing agent dosage of the paste was 41 wt%. Figure 6A shows the surface image of the HJT silver paste before curing, and Figure 6B shows the surface image of the same silver paste after curing at 199 \({}^{\circ}\)C for 44 min. It can be seen from Figure 6A that a dense interconnected network was formed by Ag flakes, implying good contact between the flakes. From Figure 6B, it can be seen that the Ag flakes coalesced with each other and the neck formation was observed. This means that the silver paste after curing can provide a continuous pathway for current carrier transmission for solar cell.