# Research Progress of Zero-Busbar Technology Based on Heterojunction Photovoltaic Modules
## 3 Results and Analysis
### Reliability Analysis
#### 3.2.1 Solder Selection
The IMC thickness at the Bi26 joint exceeded this range, indicating a risk of detachment. In contrast, the IMC thickness at the Bi14 joint fell within the recommended range, presenting a significantly lower risk than Bi26. Therefore, it can be concluded that Bi14 solder is more suitable for a soldering-based 0 BB design. For the IFC-based 0 BB design, Bi21 and Bi26 solders with a similar melting range were chosen for comparison. The Bi14 solder was excluded because its upper melting point is 167 \({}^{\circ}\)C, which is close to the temperature limit of the lamination machine (165 \({}^{\circ}\)C). Cross-section images of the soldering joint with Bi21 show that a large amount of the solder material remains on the solder ribbons (Figure 10a), which is due to the partial mismatch between the lamination temperature and Bi21 solder's melting range. Conversely, the Bi26 solder can melt at the lamination temperature and flow onto the cell surface under the stress of the carrier film (Figure 10b), and the tin in the solder ribbons can form metallurgical bonds [37, 38] with silver to develop an intermetallic compound (IMC) layer. As displayed in Figure 10c,d, the ribbon with Bi21 detaches from the TCO, while the ribbon with Bi26 is in good contact with the TCO. Therefore, the statistical probability of contact between the ribbons and TCO is achieved, which comes from the observation of a random cross-section. The probability shown in Figure 10e indicates that the Bi21 solder is unable to form good contact with the cell. As hot spots can result in a high temperature, solders with a low melting point would be at high risk of de-soldering between the solar cells and soldering ribbons. Therefore, a baking test was conducted to simulate the process wherein hot spots appear, for a quick assessment of the high-temperature stability of the interconnection. According to the EL images obtained after baking at 180 \({}^{\circ}\)C for 2 h (Figure 10g,i), the small module with Bi21 shows severe de-soldering and some finger breakages, while the Bi26 sample shows slight de-soldering and finger breakages. This may be attributed to the stronger bonding between the Bi26 solder and silver paste. In addition, the Bi26 solder is concentrated on the cell surface, the tin in the solder can easily enter the interstices in the silver paste to form a mixture, and the further movement of the re-melted Bi26 solder is limited to some degree at the hot-spot temperature. However, when re-melting at the hot-spot temperature, the Bi21 solder can move on the cell surface, as well as along the solder ribbon, creating de-soldering points due to insufficient solder material. Figure 9: (**a**) Peeling force of Bi14 and Bi26 soldering points. (**b**) Microscopy of Bi14 and Bi26 soldering points. Bi14 sample EL images: (**c**) initially and (**d**) after TC200. Bi26 sample EL images: (**e**) initially and (**f**) after TC200. (**g**) The thickness of IMC. (**h**) The micrographs of IMC growth. Furthermore, both samples show partial finger breakages, which may be due to thermal shock causing the different materials to crack. The TC200 results also show more de-soldering in the Bi21 sample (Figure 11b). This may be due to the lower bonding strength, which causes the cells and solder ribbons to more easily disconnect during the TC process. Therefore, for different solders to create interconnections during the lamination process, the lamination temperature plays a pivotal role. For a lamination temperature of 140-150 \({}^{\circ}\)C, the Bi26 solder is more suitable.