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.