来自美国的研究团队Barry Hartweg等人在Solar Energy Materials and Solar Cells国际杂志上发表文章Qualification of laser-weld interconnection of aluminum foil to back-contact silicon solar cells。
01
Guide
Laser welding can be used to connect high-efficiency back-contact silicon solar cells with low-cost aluminum foil. This welding method is relatively new, so its reliability needs to be scrutinized in detail before it can be adopted commercially. In this study, the researchers welded a 50 μm thick aluminum foil to a Sunpower back-contact cell and observed a high correlation between laser weld adhesion, module fill factor, and thermal cycling reliability. The JMP statistical model based on adhesion data showed that the statistically significant parameters for improving the adhesion of laser welding were laser pulse energy, pulse density and pattern.
Increasing the laser pulse energy and density can improve the adhesion of aluminum foil to battery metallization, which may be due to the improved melting of the Sn cap layer on the battery's Cu electrode identified by cross-sectional microscopy. 94.4% of modules manufactured using laser welding with an average adhesion greater than 0.8 mJ lost less than 5% of their initial maximum power after 200 thermal cycles, which is the IEC 61215 standard for any single accelerated stress test. In addition, 90.0% of the modules with a series resistance of less than 1.9 Ω cm passed the thermal cycle. Therefore, laser welding adhesion and series resistance for fabrication modules can be used for the further development of new laser welding setups and as quality control parameters for the manufacture of these modules.
Laser & Electron Beam Processing
02
Thesis Overview
The study demonstrates a robust nanosecond laser welding process that interconnects copper-metallized BC cells with aluminum foil. A wide range of laser parameters were explored to find settings that produce measurable adhesion without damaging the solar cell. Peel tests are performed on various laser welding setups, and the results are correlated with the performance of the modules in the TC to determine the mechanical requirements required for the laser weld to withstand the TC test.
Laser & Electron Beam Processing
03
Graphic analysis
The miniature components in the study were manufactured at Arizona State University and each module uses a single Sunpower E60 BC battery; These batteries are readily available from online suppliers. The manufacturing process requires a two-step lamination, after the first lamination, the rear aluminum foil is exposed for laser welding; A schematic diagram of the module stack during welding is shown in Figure 1.
Figure 1. Schematic diagram of a stack of micromodules or peel-off test samples during laser welding. The micromodules are laminated at 95 °C and the peeled samples are laminated at 110 °C
Figure 2.Top-down microscope images of the different laser patterns used in this study. The images were taken "welded" while the foil was still in place
When testing the laser setup in the initial screening, the researchers observed that laser welding fell into three broad categories: unwelded, properly welded, and overwelded. Figure 3 shows each example. These samples are prepared like peel test samples and imaged after stripping off the aluminum foil to show the effect of laser welding on the battery electrode surface.
Figure 3. Confocal microscope image of the battery electrode surface after laser welding and foil removal. Three different welds were prepared using the same pulse density (500 pulses/mm) and pattern (three 0.5 mm lines). Only the pulse energy is varied
The model selected for discussion in this study achieves the highest F-ratio (lowest signal-to-noise ratio) and predicts that most of the factors in the model are statistically significant. Note that a factor is considered statistically significant if its p-value on the two-tailed t-test (Prob > |t|) is less than 0.05. Figure 4 shows the split data, predicted values and prediction equations for the JMP model.
Figure 4. The black dots represent the measured peel energy values and the values predicted by the linear JMP model, including three factors: pulse energy, pulse density and laser pattern. The red line is the prediction equation, and the shaded areas in red represent the 95% confidence interval around the mean
Figure 5. (a) Micromodule FF, (b) Micromodule Rs, and (c) Peel energy of the sister sample as a function of laser energy density, which is the product of laser pulse energy and density in Table 2. All samples were made with SoS laser patterns. The black line in (a) is the guide line of the eye, and the blue dotted line represents the range of the two welded reference micromodules. The black line in (b) is a linear regression of the mean, and the error bars represent the standard deviation of the mean
Figure 6.(a) Low-magnification microscope image of SoS-3 cross-section. The red line marked on the "outer aluminum" image of the SoS-3 shows an estimate of the position of this cross-sectional "slice" on the sample (b) Battery electrode: Top-down microscope image after laser welding with aluminum foil peeled off to expose battery electrodes. Outer aluminum: Top-down microscope image after laser welding, with the aluminium left in place. Cross-section: SEM image of a sample of the "outer aluminum" column after potting and polishing in epoxy. Cracks and defects in silicon are the result of cell lysis in sample preparation. EDS Map: Image element diagram from the Cross-Section column. The legend in the upper right corner indicates the element color code used.
Figure 6.(a) Low-magnification microscope image of SoS-3 cross-section. The red line marked on the "outer aluminum" image of the SoS-3 shows an estimate of the position of this cross-sectional "slice" on the sample (b) Battery electrode: Top-down microscope image after laser welding with aluminum foil peeled off to expose battery electrodes. Outer aluminum: Top-down microscope image after laser welding, with the aluminium left in place. Cross-section: SEM image of a sample of the "outer aluminum" column after potting and polishing in epoxy. Cracks and defects in silicon are the result of cell lysis in sample preparation. EDS Map: Image element diagram from the Cross-Section column. The legend in the upper right corner indicates the element color code used
In Figure 7, the initial Rs is plotted using the corresponding average peel energy values for each microblock manufactured with the laser settings listed in Table 2 and its sister peel samples. Also included are the range of two reference welded micromodules FF and Rs and the average ± standard deviation values of the four unwelded foil interconnect micromodules.
Figure 7. The average ± standard development peel energy values correspond to their corresponding micro-modules (a) FF and (b) R values manufactured prior to any TC stress testing. The blue dotted line represents the range of the two welded reference micromodules, and the red shaded area represents the average ± standard development value of the four aluminum foil interconnect micromodules that have not undergone any laser welding
As can be seen from Figure 8, there are five laser welding setups, resulting in a module with an initial Rs that is equivalent to or lower than the value achieved by the weld, and subsequently passes the TC. These are the 12s-2, SoS-5, SoS-6, 6x7-2, and Sp-7 setups, and interestingly, they use four different laser patterns and four different combinations of laser pulse energies and densities, suggesting that it is possible to achieve adequate laser welding in a variety of laser directions if the appropriate corresponding laser parameters are used.
Figure 8. The relative change of the MPP was plotted according to (a) the laser weld peel energy and (b) the initial FF of the micro module after 200 TC. The blue dotted line indicates the range of the two welded reference mini-modules. See Table 2 for laser welding legends. There is a horizontal reference line at the −5% mark that indicates the threshold for passing IEC 61215. Vertical reference lines are recommended cut-offs to identify laser welding setups and prefabricated modules that require further testing
Laser & Electron Beam Processing
04
Key conclusions:
In this work, laser pulse energy, density, and pattern were found to be statistically significant parameters for adhesion between the aluminum foil and the BC battery electrodes. Cross-sectional microscopy revealed that the higher adhesion and Rs appear to be related to the melting and redistribution of the Sn coating on the Cu electrodes of the BC cell during welding. Laser welding adhesion and module initial Rs were found to be highly correlated with module performance in TC testing. Of the 18 modules fabricated with laser welds, the average peel energy was 0.8 mJ or higher, of which 17 (94.4%) passed the TC test with a loss of less than 5% of their initial MPP.Of the 20 modules manufactured with laser welds, the initial Rs were 1.90 Ω cm2 or higher, of which 18 (i.e., 90.0%) passed the TC.
Therefore, this adhesion reference value can be used as an indicator for laser welding development before laser welding is applied to the module, and this initial RS reference value can be used as a simple quality control indicator for passing or rejecting modules in the production line. While the focus of this work is on TC testing, which has been observed to accelerate prominent failure mechanisms in improperly welded modules, follow-up studies should emphasize interconnections with other relevant tests, such as current loading, humidity freezing, and mechanical loading through TC.
Source: High Energy Beam Processing Technology
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