# Research Progress of Zero-Busbar Technology Based on Heterojunction Photovoltaic Modules
## 3 Results and Analysis
### Reliability Analysis
#### 3.2.3 Results and Analysis of Reliability Tests  
According to the results in Section 3.2.2, the soldering-based 0 BB technology was optimized for use with the Bi14 solder and silver paste for the SRCAs, and the IFC-based technology with the Bi26 solder and Ag@Cu paste was chosen. As both designs underwent tests from the IEC standards, such as TC200 and DH1000, the reliability of both 0 BB modules was explored, including hot-spot, T-DML, and T-SML tests, with a soldering-based SMBB module used as a benchmark for comparison. _Hot-Spot Test_  
Hot spots on PV modules refer to local areas shadowed by grime, leaves, dust, or obstacles, which can result in high temperatures and severely hinder the power output of the module. Therefore, a hot-spot test was performed on IFC-based 0 BB modules, soldering-based 0 BB modules, and soldering-based SMBB modules. As shown in Figure 16a, the hot-spot temperature of the three types of modules was within the range of 140-180 \({}^{\circ}\)C. The EL images show no discernable change after the test (Figure 16b-d). There were no changes in the appearance of any of the samples after the tests. As shown in Table 6, none of the three modules showed any obvious reduction in electrical performance, and they all met the IEC standards. _T-DML test_  
With the removal of busbars from the SMBB design, the connection of any 0 BB design depends on the contact between the solder ribbons and fingers in the SRCAs. In 0 BB designs, the connection areas with paste are highly reduced. Thus, it is reasonable to assume that the 0 BB design presents risks of de-soldering and finger breakages. Stability \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & \(\Delta\mathbf{V}_{\textit{od}}\)\% & \(\Delta\mathbf{I}_{\textit{sed}}\)\% & \(\Delta\mathbf{F}\)\% & \(\Delta\mathbf{P}_{\textit{max}}\)\% \\ \hline IFC & \(0.31\) & \(-0.42\) & \(0.43\) & \(0.30\) \\ Soldering-0 BB & \(-0.12\) & \(-0.25\) & \(0.40\) & \(0.03\) \\ Soldering-SMBB & \(-0.10\) & \(-0.15\) & \(0.30\) & \(0.05\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Electrical parameter changes in PV modules after hot-spot test. Figure 16: (**a**) Hot-spot temperature distribution in three types of modules. EL images after hot-spot test: (**b**) IFC-based 0 BB modules, (**c**) soldering-based 0 BB modules, and (**d**) soldering-based SMBB modules. was further investigated by conducting T-DML tests at 85 \({}^{\circ}\)C, and T-DML1000\(\times\)10 was conducted, where T-DML1000 is a cycling basis as detailed in the IEC standard. The defects shown in Figure 17 were primarily caused by finger breakage or ribbon de-soldering. Finger breakage always results in a longer current path and decreased efficiency of current collection, while ribbon de-soldering can additionally cause an increased contact resistance. Both defects contribute to a higher series resistance, thus lowering the output power. Over time, these issues can worsen under continuous testing, ultimately leading to module failure. As shown in Table 7, the continuously decreasing _Isc_ and _FF_ further confirm this. The IFC-based 0 BB module showed slight de-soldering after 6 cycles of T-DML1000 (Figure 17c), and it showed a power loss of 1.47% after 10 cycles. The soldering-based 0 BB module had some subpar soldering, as shown in the initial EI image (Figure 17e). After the T-DML1000 test, slight finger breakages were observed in the module (Figure 17f); after 10 cycles, the finger breakages became severe (Figure 17h), and a power loss of 1.69% was observed (Figure 17i). The difference in power loss could be attributed to the degradation of electrical interconnections at the soldering joints. Soldering-based modules are subjected to a higher upper temperature during the manufacturing process, resulting in a thicker initial IMC, which also accounts for the difference in EL defects [40]. Additionally, the difference in the silver content has an effect on metallurgical bonding, leading to differences in the EL defects.