Dear Ralf,

To answer this question one has to observe the the subjected modules in the first working period in the field. Visual and electrical inspection is required, especially the visual inspection which may be very helpful in the initial phase of the defect.

However i will try to give an interpretation of the phenomenon.

The most important notice is that it is formed on part of the silver fingers, and the silver fingers changed their color to brown which means that the silver in these locations is oxidized. In addition the fingers in these defect areas are no longer contacting the underlying silicon. Also there is some bubbles seen.

All these changes require local excessive heating to be formed. So, there is some excessive heating effects in these locations.

Let us from the other side see the silver fingers  their appearance is so that they are of non homogeneous thickness such that in some area the cross thicness of the finger gets very small.

So, when the solar cell is illuminated and the silver fingers collect the photo generated current they will get hotter in leading to local heating and oxidation of the silver. As the fingers get partially oxidized they will be more hotter as their conducting cross section decreases and their resistance increases. this process continues till the path is ultimately interrupted.

I think as the silver metallisation is relatively expensive, one tries to save cost by making the silver  fingers too thin.

I think this is the bottle neck of these types of solar cells.

Best wishes

Thank you Abdelhalim for your opinion and explanation.

These bubbles are lined up along a snail trail respectively cell crack. With an uneven thickness of the grid fingers I would expect such bubbles also apart of the cell cracks.

Further more the bubbles appear along the full length between two busbars (see file attached, zooming in). Half way between on could expect almost zero current in the grid finger. Where shall the power come from to create the bubbles?

Dear Ralf ,

What i proposed may be a possible explanation. The burning is clear in your first picture irrespective of the bubble formation. The bubbles far from bus bars may be formed during not optimized  lamination process. It is so that one has to inspect the solar cells thoroughly before staring the  investigation of the formation of snail tails. 

The presence of the bubbles far from the metal paths may point also out deficient lamination process.

Best wishes

Hi Ralf,

I have been watching the conversation between Abdelhalim and yourself with interest.

I would tend to agree with his last comment that there has been some initial heterogeneity within the manufactured solar panel, making it open to initial local overheating, followed by partial oxidation, followed by further local overheating, leading to the bubbles.

In a nutshell: an initial heterogeneity with other phenomena contributing to progressive degradation. The mid-finger current will be really low, as you rightly pointed out, and is probably not much of a contributor on its own !

Cheers !

-Sanjay

Hi Ralf,

BTW, you may consider checking the study presented in this Energy Procedia paper:

http://ac.els-cdn.com/S1876610213013957/1-s2.0-S1876610213013957-main.pdf?_tid=49d81bc6-14c3-11e6-adc5-00000aacb360&acdnat=1462674147_bcaab6a8009aae59b96e33e38d836b95

Cheers !

-Sanjay

Abdelhalim, absolutely right. Up to now I only have this visual inpression. And other modules in that plant show similar an also other impressions. There was delamination along the busbards - most likely from flux on the cell surface and so on. Maybe I get the chance to do further investigation....

Sanjay, thank you for the hint to that paper. I saw that already.

Overall these modules seem to be B-quality if not worse....

But why it goes along a line like that? Lamination process cannot create this kind of specific line, I think. I would think there must be a micro-crack or cell crack a long the line and induced the higher temperature said. Eventually, the EVA bubbles. 

Sure Boon, the line is along a cell crack. But normally appearing snail trails are just discoloration and not such bubbles. Is it really boiling EVA ore something else?

My friend's pv modules also shows this kind of snail trails. But normally we don't have budget to test for EL.

Dear friends,

I have known this kind of defect and found its explanation. I discovered it in a photovoltaic park in Romania build with European panels. I was also technical manager 5 years at Altius Fotovoltaic, PV panels maker. I attached a photo with . His explanation is the following. In the PV panel production process, tabber-stringer machines, which produce PV cell strings, sometimes do not stick the ribbon on the cell. In this case, an operator repeir the soldering with manual tools. In this process the operator start to splashes the flux, manually from a bottle. If the quantity of flux is exaggerated, bubbles will remain between the cells and the EVA film. The flux bubbles will emigate on the panel surface becouse vacum process, like water on the car parbrize. The encapsulated fluxant, being acidic, will corrode the fingers in time and create a hotspot.

Hello,

I found this defect at IRESEN-GEP after two years of outdoor exposure. This defect appears at a crystalline PV module.

Generally this defect is due to density of Ag nanoparticles located within the encapsulating foil directly above the grid finger surface. The Ag nanoparticles lead to chemical reaction, which result in some bubble like tracks of the snail.