# Research Progress of Zero-Busbar Technology Based on Heterojunction Photovoltaic Modules
## 2 Materials and Methods
### Test Methods  
Contact resistance test: As shown in Figure 5, probes need to be fixed onto two SRCAs; thus, by taking the 2 nd SRCA as the fixed probe and connecting the other probe to the 5 th, 8 th, 11 th, 14 th, and 17 th SRCAs, five resistance values were obtained. The five resistance values were plotted against the corresponding SRCA numbers between the probes, linear fitting was performed, and the finger resistance \(R_{fig}\) was obtained from the slope of the fitting line. After soldering or IFC, the resistance was obtained by clamping probes onto the solder ribbons. A probe was fixed at the 2 nd solder ribbon, whereas the other probe was connected to the 5 th, 8 th, 11 th, 14 th, and 17 th solder ribbons, and the resistance of area (1) was obtained as \(R_{1}\), the resistance of area (2) was obtained as \(R_{2}\), etc. Taking the tested area (1) as an example, the equivalent circuit for \(R_{1}\) is shown in Figure 6, and it is obtained using Equation (1). \[R_{1}=\frac{1}{\sum\limits_{1}^{n}{\frac{1}{(n-1)R_{line}*2+R_{con}*2+R_{fig}*3}}} \tag{1}\]  
where \(R_{con}\) is the contact resistance between the solder ribbon and finger; \(R_{line}\) is the solder ribbon resistance, which is obtained using Equation (2); and n is the number of fingers. \(R_{1}\), \(R_{line}\), and \(R_{fig}\) are obtained using our test method, and \(R_{con}\) is obtained using Equation (1). \[R=\frac{\rho L}{S} \tag{2}\]  
where \(\rho\) is the electric resistivity of the solder ribbon, \(L\) is the distance between two adjacent fingers, and \(S\) is the cross-section of the solder ribbon. Figure 5: Schematic of contact resistance test. Solder ribbon peeling test (180\({}^{\circ}\)): The tested solar cell was fixed onto a metal plate perpendicular to the ground, and the solder ribbon was clamped onto a moving apparatus's front tip, with the apparatus being parallel to the solar cell. The apparatus stripped the solder strip from the solar cell at 180\({}^{\circ}\) degrees and a moving rate of 10 mm/min, and a force-displacement curve was plotted. Paste solderability test: According to GB/T 17473.7-2022 [26], a silver electrode was printed onto a silicon wafer and, after applying flux to the surface of the silver electrode, the wafer was immersed in liquid tin for 10 s and then removed. A microscope was used to measure the change in the silver electrode area before and after tin immersion. Metallography microscopy analysis: Samples of the soldering point (the spot where the solder strip and solar cell make contact) were cut out using a laser, solidified in a mold using the cold mounting method, and polished with 120-2500-grit sandpapers and diamond suspensions. After the polished samples were inspected and determined to have no apparent scratch marks, they were washed with anhydrous ethanol. The samples were characterized and photographed with a Zeta-3D microscope. EL analysis: By applying forward bias to small samples or PV modules, the hidden cracks, non-wetting solder points, and broken solder points were characterized. Additionally, the quality and performance of the PV modules were analyzed. Hot-spot test: According to IEC 61215-2:2021 [27], 3 solar cells (1 on the edge of the module) with the highest leak current and 1 solar cell with the lowest leak current were chosen from the module. Using an irradiance of 1250 \(\pm\) 100 W/m\({}^{2}\), the worst occlusion area was determined, the PV module was shorted, and the module was subjected to a 1 h hot-spot stress test. The hottest spot of the modules was detected with a thermocouple, and an IR image was taken after the temperature of the modules was stabilized. According to IEC 61215-2:2021, the maximum power output drop should be less than 5%, referencing the module's initial output power \(P_{max}\). Thermal cycling (TC) test: The PV modules were tested in a temperature cycle with a range of (\(-\)40 \(\pm\) 2) \({}^{\circ}\)C and (85 \(\pm\) 2) \({}^{\circ}\)C and a temperature change rate below 100 \({}^{\circ}\)C/h.