# Research Progress of Zero-Busbar Technology Based on Heterojunction Photovoltaic Modules
## 3 Results and Analysis
### Reliability Analysis
#### 3.2.1 Solder Selection  
As the cell connectors, the solder ribbons affect the power output and other parameters of the module. The position of the interconnected points can often put PV modules at a high risk of degradation, which has a serious impact on the long-term working of these modules. Therefore, we need to identify the appropriate solder for achieving high reliability in practical applications. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & \(\mathbf{V_{\infty}/V}\) & \(\mathbf{I_{sc}/A}\) & \(\mathbf{FF/\%}\) & \(\mathbf{P_{max}/W}\) \\ \hline IFC & 49.5 & 17.41 & 82.69 & 712.62 \\ Soldering & 49.5 & 17.38 & 82.42 & 709.07 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrical parameters of soldering- and IFC-based 0 BB modules. As for soldering-based 0 BB technology, at present, low-temperature solders (with a lower melting point than Sn60 Pb40 solder) are used, because HJT solar cells are not resistant to high temperatures [29]. Bi14 and Bi26 solders were chosen for comparison. A ribbon peeling test was conducted to compare the bonding strength with cells of the Bi14 and Bi26 solders, and the results are shown in Figure 9a. The peeling force of the Bi26 solder was only 0.52 N, and that of the Bi14 solder joint was 0.74 N, which is 0.2 N higher. As shown in the microscopy images (Figure 9b), both solder materials spread across large areas on the silver paste surface, indicating good wetting in both cases, which could be related to the reduced surface tension enabled by Pb and Bi in the solder materials [30, 31, 32]. After the solder ribbons were peeled off, the Bi14 sample showed peel-off of the silver paste, whereas the Bi26 sample showed visible solder residue, indicating solder fracture. Previous works have investigated the effect of Bi content on the mechanical performance of alloy solders, showing that an increase in Bi content leads to a lower tensile strength of the soldering material, and that an overly high Bi content lowers the solder's impact absorption energy (often used to assess the toughness of materials) [33, 34]. Therefore, the usage of a low-Bi-content solder aids in the formation of soldering joints with a higher peeling strength and a lower risk of solder fracture. As differences in the thermal expansion coefficient of the various materials in modules can affect device performance during the TC process, small modules with different solders were used for comparison. The Bi14 and Bi26 samples did not show obvious de-soldering (Figure 9d,f). The IMC thickness of the Bi14 and Bi26 solder joints after welding and TC200 is shown in Figure 9g,h. After TC200, the average IMC growth thickness at the Bi26 joint reached 4.77 um, while the IMC thickness at the Bi14 joint was only 2.43 um. After solder joint formation, IMC growth at the joint depends on the atomic interdiffusion rate, where the diffusion coefficient D affects the growth rate of the IMC. The diffusion coefficient D can be described as follows:  
\[\text{D}=D_{0}\cdot exp\left(-\frac{E_{a}}{RT}\right)\]  
For high-melting-point solders like Bi14 (melting range 136-167 \({}^{\circ}\)C), atoms have stronger binding energy, which means higher diffusion activation energy \(\text{E}\), resulting in a lower diffusion coefficient D and lower mobility. In contrast, low-melting-point solders like Bi26 (136-143 \({}^{\circ}\)C) have lower atomic binding energy, facilitating easier atomic movement. Consequently, during the thermal process of TC testing, atoms in Bi26 solders can diffuse faster to form a thicker IMC layer than Bi14 solder. Due to the brittleness of IMC, the IMC layer with a larger thickness has a lower tensile strength, putting it at greater risk of brittle fracture. During TC testing, cracks in the IMC layer can easily propagate to the adjacent solder/IMC interface and solder layer, leading to long-term fatigue failure. To ensure the reliability of the solder joints, researchers suggest maintaining an IMC thickness within the range of 1-3 um [35, 36].