The phenomenon of brightness difference during EL testing of solar panels

　　In response to the phenomenon of light and dark differences during the EL testing of solar modules, the reasons for the abnormalities are analyzed through the principles of the EL tester and the factors affecting light and dark, and the quality risks of such photovoltaic modules are evaluated. Analysis shows that the light and dark differences found in the EL testing of solar modules are due to both battery efficiency mismatch and current mismatch. From the perspective of component power attenuation, efficiency mismatch has a greater impact on component power attenuation, while current mismatch has a smaller impact on component power attenuation.

　　1、 Principle of EL tester

　　The mechanism of EL testing is electroluminescence imaging, which utilizes the electroluminescent recombination of minority carriers to collect and image the fluorescence emitted by photovoltaic modules under applied bias, and can quickly detect composite defects in solar cells that cannot be recognized by the naked eye. The difference in brightness in EL images can reflect the situation of composite luminescence, where black spots are caused by the absence of 1150nm infrared light emitted at that position in the battery when powered on. The black spots and brightness difference in the battery are also related to the concentration of minority carriers.

　　2、 Analysis of the Reasons for Light and Dark Differences in EL Testing

　　Through methods such as retesting battery efficiency after rework and disassembly, and reassembling experimental components after battery classification, it has been found that the main reasons for the large difference in brightness and darkness in EL testing are as follows.

　　1. Differences in brightness caused by differences in battery efficiency

　　Perform EL testing on the component, and the test result is a mixed gear component. Remove the battery (with solder tape) from the mixed gear component, and select 2 bright and 2 dark chips from them to test their efficiency. The efficiency of bright films is 20.25% and 20.29%, while the efficiency of dark films is 19.50% and 19.20%, respectively. For components with high differences in brightness due to different efficiencies, they are identified as efficiency mixing.

　　2. The brightness difference of a single battery in EL testing

　　3. EL testing brightness differences of batteries with different efficiencies

　　Select batteries with efficiencies of 20.3% and 19.8% for EL testing, and observe the differences in brightness. It was found that there was no difference in brightness between high efficiency batteries during EL testing, while a 10% difference in brightness was observed in low efficiency batteries, indicating a gray scale issue. Therefore, it can be considered that the difference in brightness only occurs in inefficient batteries.

　　4. EL test brightness difference for batteries with the same efficiency

　　Select 200 batteries with an efficiency of 19.8% for EL testing, and then select 12 batteries with brighter EL images (referred to as "bright chips"), numbered as 1 # to 12 #; Select 12 batteries with darker EL image display (referred to as "darker chips") and number them as 13 # to 24 #.

　　The selected 1 # to 12 # bright cells and 13 # to 24 # dark cells were sequentially soldered and stacked with 48 conventional batteries of the same efficiency to form experimental components, and then subjected to EL testing. The results showed that the blackened batteries were all dark cells when the batteries were selected. This indicates that at the same low efficiency level, components made from batteries with significant differences in brightness and darkness will exhibit a similar phenomenon of "mixing" in EL testing.

　　Randomly select 7 polarizing chips and 4 polarizing chips from the 24 selected batteries, and test them with an electrical performance tester. The current of the polarizing chips is not less than that of the polarizing chips. This indicates that even with a reasonable current grading scheme, it cannot be distinguished. From this, it can be seen that although batteries with the same efficiency all pass the EL test (the dimming image is uniformly distributed throughout the entire panel, which belongs to the qualified category), there will still be differences in brightness when the components made of them undergo EL testing, and such problems cannot be solved through current grading.

　　5. Differences in brightness caused by high and low current mismatch

　　Prepare 116 high current (>9.00A) batteries with an efficiency of 20.5% and 100 low current (<8.95A) batteries with the same efficiency. 72 high current batteries were selected to make component 1, 28 low current batteries and 44 high current batteries were selected to make component 2, and 72 low current batteries were selected to make component 3. EL testing was conducted on three components, and no mixed gear phenomenon was observed in all three components, but there were differences in power. Compared to components 2 and 3, component 1 has a power advantage of at least 0.5W. Dividing the current of the batteries in the component production line and selecting high current batteries has a significant gain in component power.

　　3、 Efficiency and current mismatch in power loss assessment

　　The efficiency mismatch problem is mainly reflected in the attenuation of component power, and current mismatch requires a long outdoor empirical evaluation. Therefore, the following tests were conducted using a simulation approach.

　　1) Select batteries with efficiencies of 19.7% and 19.6%, and observe the efficiency degradation of the batteries after being left for 105 days (approximately 3.5 months)

　　Although the battery efficiency in the experiment differs by one gear, there is a significant difference in efficiency attenuation during the placement process. After making the components, there may be significant power loss due to differences in battery efficiency degradation.

　　2) Select batteries with different current levels and an efficiency of 20.3% for a 60kWh/m2 light attenuation test. The efficiency attenuation of batteries with the same efficiency but different current levels is not significantly different. Therefore, it can be considered that when batteries with the same efficiency but different current levels are made into components, the power loss of the components is relatively small due to the different efficiency decay rates of the batteries.

　　4、 Conclusion

　　This article analyzes and studies the phenomenon of light and dark differences that occur during EL testing of photovoltaic modules, and draws the following conclusions:

　　1) The issue of brightness difference discovered during component EL testing is due to both battery efficiency mismatch and current mismatch.

　　2) After assembling batteries with the same efficiency but different current levels, no significant differences in brightness were found in the EL test chart; However, in component production, current grading and selecting high current batteries to make components can increase component power. Experimental data shows that selecting components made from high current batteries will have a power advantage of at least 0.5W.

　　3) Some batteries with the same efficiency tend to be dark, but the current is not low, so the issue of brightness differences in component EL testing cannot be solved through current grading. For batteries with low efficiency that may cause unqualified products in production, a gray scale classification should be established, and the specific classification requirements should be combined with the needs of the production line.

　　4) For components with efficiency mismatch and current mismatch, the reliability risk is relatively low, mainly due to power loss issues. From the data of battery efficiency degradation, it can be seen that the power loss of efficiency mismatched components is relatively large, while the power loss of current mismatched components is relatively small.