Traditionally they use a sheet of glass (iron free tempered if you can get it), a sheet of EVA, the cells themselves, another sheet of EVA, and whatever your having yourself. After the last EVA sheet, you only really need to put something there that will not wick, and will give you something for the very very sticky EVA to bond to that is not your "jig" It can be painted later if you want more protection. Usually they use tedlar, but really any inert permanently waterproof material will do for protection.|
The simple theory of what we are trying to achieve is this:
We need to get the cells fully encapsulated in a UV stable envelope, that resists water and air from denaturing the cells. It must allow for differential expansion rates to occur between the silicon wafers, the glass and the medium itself....... and no bubbles.
Ethelyene vinyl acetate is a plastic sheet .5mm thick. It is soft plastic in texture and seems innocuous enough....... however, when it gets to only 65 degres C, it melts. This means that we need not get to very high temperatures to get something to happen.
I'll bet there are plenty of hopefuls ( like me for instance) that grabbed some EVA, some glass, and the wifes oven, and made a prototype cell......but it is filled with bubbles!..... It does stick to the glass and, it envelops the cells well, but it is useless because of the bubble population.
It is at this point that you start to get a bit anxious about where to get a laminator...... but you don't really need one. We just need to emulate what it does.
Next thing we look at is what happens when it melts. To get the EVA to be useful, we need to get enough energy into the stuff to cross link the molecules. It changes the way the material behaves, and increases the melt point considerably as well. At low temp (65C-80C) it melts to itself very well, but is only slowly converted into the final product, so we have to keep it at these temperatures for long periods..... but as we increase temp, things happen a whole lot faster, and 5 mins may be enough at 145C to get it to cross link, and stick very strongly to the glass and the cell itself.
It also needs some encouragement from pressure..... or a vacuum perhaps, and use the air pressure to do the job of supplying the pressing force.
Now we know it works perfectly well without vacuum, but you will have myriads of fine bubbles.... blocking out the sunlight.... well, not so perfect after all.
So we need a vacuum pump. Ideally one that can draw a near perfect vacuum. The vacuum you can achieve sort of directly relates to the bubbles you will get left with. High vacuum.... no bubbles. It's that simple. The vacuum will also press the cells very firmly (200 plus pounds per cell for the 6x3 ones) against the glass, and this will help the glass bond, and make for a very flat cell, with no voids in the plastic envelope.............as always there is a but:... you must have NO leaks at all, or you will get uneven vacuum in the sheets, and you will get bubbles forming.... probably in the inter cell space, but bubbles all the same.
The heating profile I will use is like this:
15 mins at around 50C with full vacuum
15 mins at 65C with full vacuum
70-80 mins at 100C-120C (or hopefully more) with full vacuum
This last bit is dependent on the EVA you get. Low temp EVA is used for window lamination, where electronic components may be incorporated into the glass matrix....
For “normal” EVA if we can get the temp to rise some more.... then good. Even if the containment bag (ok garbage bag) fails at this point, it's work is done for all practical purposes... better if it does'nt.......but we get what we can.