# Research Progress of Zero-Busbar Technology Based on Heterojunction Photovoltaic Modules
## 3 Results and Analysis
### Electrical Performance Analysis
#### 3.1.1 Contact Resistance  
For this test, cells with pattern 1 and \(\varphi\)0.25 mm solder ribbons with Bi14 were applied in the soldering-based samples, while cells with pattern 2 and \(\varphi\)0.25 mm solder ribbons with Bi26 were applied in the IFC-based samples. Both the soldering-based 0 BB sample and IFC-based 0 BB sample were tested for their contact resistances using the method above, and the fitting lines are shown in Figure 7a. The two lines have similar slopes, indicating similar finger resistance \(R_{fig}\) values. Furthermore, the contact resistances of the laminated/soldered sample were tested. The \(R_{con}\) value in test area (1) was calculated by inserting the values of \(R_{fig}\), \(R_{1}\), and \(R_{line}\) into Equation (1), and the \(R_{con}\) values in test areas (2), (3), (4), and (5) were similarly obtained. The IFC-based samples showed a lower \(R_{con}\) than the soldering-based samples. The resistance differences between the soldering-based samples and IFC-based samples are displayed in Figure 7b, which shows that the average \(R_{con}\) values of the soldering-based samples were 161 m\(\Omega\) (front surface) and 444 m\(\Omega\) (near surface) higher than those of the IFC-based samples. Cross-sectional images of the TCO-solder-ribbon contact (Figure 7c-f) reveal both complete and incomplete contacts in the IFC and soldering-based samples. Statistical analysis (Figure 7g) indicates a higher probability of incomplete contact between solder and TCO in soldering-based samples, explaining the higher contact resistance. This may be because alloying occurs during the lamination process under 1 bar of pressure for IFC-based modules, resulting in improved wetting and contact between the solder and the TCO surface. In contrast, ribbons in soldering-based modules rely only on fixture pressure, likely resulting in incomplete contact with the TCO. The higher contact resistance of soldering-based samples will further lead to the higher Rs of modules. The IFC-based 0 BB fabrication method creates an interconnection with a lower contact resistance; this reduces the power loss of the photovoltaic modules due to resistance, leading to a higher \(P_{max}\) and \(FF\). Therefore, a theoretical calculation was performed to assess the effect of contact resistance on PV modules containing 132 solar cells with a size of 210 mm \(\times\) 105 mm. For 0 BB cells, the contact resistances between the solder ribbons and fingers are in parallel, and the current is collected by nearby fingers and converges into perpendicular solder ribbons. The current through each soldering point is denoted as \(I_{1}\), \(I_{2}\), \(I_{3}\), \(I_{4}\), \(\ldots\)\(I_{x}\), as shown in Figure 8.  
Assuming that the current through each finger is the same, the current through each soldering point I\({}_{x}\) is calculated as follows:  
\[I_{1}=I_{2}=I_{3}=I_{x}=\frac{I_{pm}}{N_{1}N_{2}} \tag{3}\]  
where \(N_{1}\) is the number of SRCAs, and \(N_{2}\) is the number of fingers in contact with a given solder ribbon. Here, \(N_{1}\) is 24, and \(N_{2}\) is 52 (front)/106 (rear). Figure 8: Schematic of contact resistance between solder ribbon and solar cell. Figure 7: (**a**) Finger resistances of HJT-0 BB solar cells. (**b**) Resistance differences between soldering-based samples and IFC-based samples. Cross-section of TCO–solder-ribbon contact in IFC-based modules with (**c**) complete contact (1 refers to the cell, 2 refers to the Cu ribbon, 3 refers to the solder) and (**d**) incomplete contact. Cross-section of TCO–solder-ribbon contact in soldering-based modules with (**e**) complete contact and (**f**) incomplete contact. (**g**) The ratio of contact between TCO and solder ribbons for soldering- and IFC-based modules.