# Research Progress of Zero-Busbar Technology Based on Heterojunction Photovoltaic Modules
## 3 Results and Analysis
### Reliability Analysis
#### 3.2.2 Metal Paste Selection  
In order to form a reliable connection between the solder ribbons and the paste on the cell, it is also necessary to inspect different types of paste. Ag@Cu paste can reduce the silver consumption of solar cells without significantly reducing their electrical performance [39], and it can also greatly reduce their costs, thus making it one of the mainstream conductive pastes. Thus, two types of pastes were compared in SRCAs, namely, pure silver paste and Ag@Cu paste. Firstly, soldering-based 0 BB technology was investigated. Cells were fabricated using pattern 1, and the suitable Bi14 solder was selected as described in Section 3.2.1. The changes in the modules' electrical parameters and the EL images after TC200 are shown in Table 2 and Figure 12, respectively. The module using the Ag@Cu paste shows a larger area of subpar soldering in the initial EL image (Figure 12a), with de-soldering and finger breakages on almost all cells, and more de-soldering is shown after TC200. As the silver content in the Ag@Cu paste is reduced to 50%, it may cause the weak bonding strength. This shows that the Bi14 solder and Ag@Cu paste are not suitable for soldering. As the de-soldering area increases, the _FF_ of the module obviously decreases, leading to a significant power loss. The module with the silver paste shows stable parameters, such as \(P_{max}\) and _FF_, after the TC200 test. Additionally, the EL images of the module with the silver paste show no defects before (Figure 12c) or after TC200 (Figure 12d). This shows that silver paste in SRCAs can better bond with the solder and ensure the module's reliability after TC tests. Figure 11: EL images of the Bi21 sample: (**a**) initially and (**b**) after TC200. EL images of the Bi26 sample: (**c**) initially and (**d**) after TC200. Figure 10: Cross-section of finger–solder-ribbon contact in IFC-based process: (**a**) Bi21 and (**b**) Bi26. Cross-section of TCO–solder-ribbon contact in IFC-based process: (**c**) Bi21 and (**d**) Bi26. (**e**) The ratio of contact between TCO and Bi21/Bi26 solder ribbons. EL images of the Bi21 sample: (**f**) initially and (**g**) after baking. EL images of the Bi26 sample: (**h**) initially and (**i**) after baking. According to the solderability test results (Figure 12e), there is a significant difference in the solderability between the silver and silver-coated copper pastes. After the solderability test, the retention rate of the silver paste reached 80%, while the Ag@Cu paste showed almost no retention after the test. This is primarily because Cu accelerates the dissolution rate of Ag into molten Sn. When Cu reacts with Sn to form IMC, the chemical potential of Cu and Ag in Sn would be lowered, facilitating Ag's diffusion into molten Sn. The lattice structure of Sn is also altered by the dissolution of Cu, leading to more Ag diffusion pathways. Additionally, the presence of Cu induces the formation of a Cu-Sn-Ag composite IMC rather than just a Ag-Sn IMC, which enhances Ag's dissolution and diffusion. This indicates that, during soldering, the thickness of the metallurgical bond formed between the silver paste and the solder is small, allowing for the majority of the paste to remain on the surface of the solar cell. In contrast, the Ag@Cu paste reacts with the solder at a much greater thickness, with nearly all of the paste participating in the metallurgical bond. Moreover, the bonding system of the paste is compromised, leading to a significant detachment of the paste from the surface of the solar cell. This explains the extensive breakage observed in the silver-coated copper fingers after soldering. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & \(\Delta\)\(V_{od}\)/**\%** & \(\Delta\)\(I_{sel}\)/**\%** & \(\Delta\)\(FF\)/**\%** & \(\Delta\)\(P_{max}\)/**\%** \\ \hline Ag@Cu & 0.06 & 0.24 & \(-\)7.27 & \(-\)6.97 \\ Silver & 0.63 & \(-\)0.49 & 0.26 & 0.41 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Changes in electrical parameters for different soldering-based modules after TC200.