The DPL image shown in Fig. 2(a) shows a module half with 60 half-cells, exhibiting significant PL intensity variations between half cells, indicative of voltage variations and a strong indication of module degradation. The image also clearly shows a several centimetre-long microcrack (red circle), various point-like defects and the front bus bars, demonstrating the excellent image quality that was achieved with inverter-based DPL imaging.
Fig.2(b) shows a section of another module from the same rooftop system, containing various examples of a defect that we have named the Yin Yang defect, and which we first reported recently . These defects appear in pairs of half-cells, whereby the patterns match when one half cell is de-rotated computationally. Tens of examples of this defect were observed across different modules within the Wylie’s Baths rooftop system.
We observed the same defect during earlier field work, using the optical string modulation technique in an Australian utility-scale solar farm, where tens of thousands of glass-glass half-cell PERC modules exhibited this defect after only approximately 1.5 years of operation in the field. Importantly, the modules in that solar farm were from a different brand compared to the residential system discussed above. Fig.3 shows a DPL image from that earlier study of a module that exhibits several examples of the Yin Yang defect. The assembly of half-cell modules in mass production involves breaking a full cell into halves and rotating one half-cell by 180 degrees prior to layup in the module. As shown in Fig.3 (three examples are highlighted in the original image with white squares), the computational de-rotation of one half-cell in each pair results in perfect matching of the defect patterns, demonstrating that the defect was already present in the full cell. DPL images of non-fielded modules of the same module series did not show these defects, the presence of the defects is thus a strong sign of module degradation due to field exposure. We conclude that this defect is present in the full cell as a latent defect that only becomes activated by field exposure. Interestingly, we find that the luminescence intensity from the higher-intensity regions within these defects is comparable to that of modules from the same series that have not been exposed to sunlight and which do not show these defects. This suggests that the bright regions have degraded less strongly due to field exposure, compared to the surrounding dark regions and that prevention or mitigation of these defects could result in significant improvements in system power output. The nature of the underlying degradation mechanism is currently unclear and subject to ongoing investigations, which are not the focus of this publication.