As known, there are several system-, module-, cell- and environmental-related factors that are linked with the development (or not) of PID in PV modules. Before and after recovering (regeneration) from PID, the PV modules present characteristic electroluminescence (EL) patterns that seem random to me. I suppose that different anti-reflective cover (ARC) materials might give different EL patterns. But can we assume that these patterns might be also indicative of possibly permanent (non-reversible) degradation of the PV module? In the case of IR thermal imaging, should we expect similar differences in the resultant thermal patterns before and after regeneration?
Assuming we're discussing the shunt-type PID, the degraded cells in the module will give no EL signal, as all current is flowing through the shunt. This will create hot spots on those cells, visible as hot cells in IR imaging, the spots are best identified by dark lock-in thermography (DLIT).
After recovery, the EL pattern will depend on the completeness of the recovery (per cell). A totally recovered cell should show its original pattern. A partly recovered cell with some remaining "shunt characteristics" will show a dark area around the remaining shunted pathway, but not all current can/will flow through that shunt, thus parts of the cell will give their normal EL signal. In IR, this will result in a small(er) hot spots. If the shunt(s) are not recovered, I still expect a fully dark cell and similar hot spot IR image. Again DLIT might be a more accurate measurement, to separate regular heating of a cell/module from localised shunts and/or bad contacts that dissipate a lot of heat.
PID has a polarization type or permanently degraded type signature. It is just similiar to flash memory read or write damage. The ARC (SiN, SiOx/SiN, or AlOx) will play quite different degradation signature. But usually there is ARC film change itself or surface Dit increase. The majority of PID related damage is reversible in outdoor environment.
I agree with the colleagues above but i will speak from the conceptual point of view. Electrolumescence EL results from the photon emission activity of forward biased solar cells where the injection of electrons and holes across the junction leads to the formation of excess electrons and holes. The intensity of such radiation is proportional to the product of electron concentration n by the hole concentration p.
So, the EL image is an indication of the current density distribution in the cell since the cell current is proportional to the excess electron concentration in the p-substrate assuming the cell is an n+-p cell.
So, if you have an EL image of the cell before the degradation, after the degradation and after the regeneration you can judge on the degree of the degradation and how far your regeneration is successful.
wish you success
Thank you all for your valuable answers and explanations.
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