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Hello folks.
In the last video we talked about emitter wrap through, or
this EWT cell, and we con, contrasted that to a traditional
design and we realized that you can reduce the amount of shading
quite substantially by eliminating these bus bars and these fingers.
And moving that towards the back of the cell.
But at the same, we also realize that
this is a, you know, this is a, a hard nut to crack.
There are many challenges, there are, you need a
very high density of these holes, first of all, then
you need emitter doping to wrap through and you know,
emitter doping to diffuse on all sides of the via.
So, it's good to scale back our ambition a little bit.
And take baby steps at imes. So the design which is commonly used
currently in the undustry, is the metal wrap through the lines.
So, shown here are these two examples of this metal wrap through the line or
MWT. And as compared to the, as compared to
this EWT design, these designs have still have a more,
slightly larger, more of metal present on the top surface.
So you can see that these fingers are still
present on the top on the top of the cell.
But there are no bus bars.
The bus bars are, moved behind towards the back of the cell.
And you see these holes which are connecting these
fingers to the bus bars at the, at the back.
And these fingers are in fact many
times they are arranged in a very nice and beautiful looking pattern, so shown
here is one such pattern where you see this these fingers at the, at the top, are
[INAUDIBLE]
this floor kind of a design.
And then these holes are present which are connecting these fa,
fingers to the bus bar which are present at the back.
So as as compared to the emitter wrap through, design, I still have these
fingers present here, so I still get two to three percent, around three
to four percent area and the top is covered by this metal lines
which give me shading, but it's still less than the seven to
eight percent area coverage that you have in a, in a conventional cell.
And also it's a, it's a much easier process flow.
And you don't require as high a density of these holes, so the
density of these holes is reduced as compared to emitter wrap through design.
And so you again, one might one wonder how,
how do I realize this kind of a design. So it's good to take a peek under the
hood and take a look at the at the cross section of this cell.
So it's it's starts like a conventional cell, so you have this In this case, this
P type wafer and then you diffuse the,you
found this M plus emitter diffusion at the top.
So this would be our M plus diffusion over here.
And then you drill holes into this and you, fill
it with matter which is contacting the fingers at the top.
And the, now, this this fingers are now routed to towards the
back towards this P type bus bar which is located at the back.
Similarly these N type bus bars are also located at the back.
So, there can be many, again, similar to the, similar to the emitter wrap
through design, there could be many process
flow for realizing this metal wrap through scheme.
I I, I, I'm, I'm going to give you one example but this might not be the
only way to do it, or even might not be the most efficient way to do it.
But one way to do that is to,
you know, start with a conventional cell process flow,
so you reform this texturing. by acidic
[INAUDIBLE]
etch.
And then we form this diffusion similar to
the way we would do in a conventional cell.
So we have this Nplus diffusion on the top, and
we started with a, let's say started with a P wafer.
And now what is done is you drill holes.
So you drill holes now, and you drill this hole towards the
in the, in the cell.
And you essentially form this hole and these vias over here.
And now the next thing to do is again revert back to the conventional process
flow, so this would be again covered with covered with anti-reflection coating.
So let's say this green is my anti-reflection
coating and this will essentially block out this
well.
And then I will deposit my metal finger
like normal screen printing kind of a flow.
And then when I fire it, I, what I expect is that this metal will diffuse into
this via, during that firing process, and it will fill up this
via, and I can connect these to form these P plus bus bars at the back.
So this is one such
process flow which is closely I'm trying
to closely assemble the conventional process flow.
I'm only adding this extra strip for drilling this via.
And you know the metal diffuses through this
via ultimately hoping ultimately during the firing process.
So this is as you can see it's it's
much more closer to the conventional process flow used
for a traditional solar cell.
So the metal wrap through design is what is commercially available
or is being manufactured by many of these solar cell companies.
For example, JA Solar is one of the companies that manufacture those.
And, similarly, Canadian Solar is another company which manufactures these.
So these metal wrap
through cells, they are in high-volume production currently.
And you can easily recognize them by looking
at looking at the top surface of these cells.
So you see these kind of patterns as shown over here,
so these dots over here, they resemble these they resemble the vias.
And then you have these these fingers which could be
printed in many fashions.
For example, over here you see a floral kind
of a design, which is used to collect all
the current from the emitter and route it through
this via towards the bus bar at the back.
Similarly over here, you see again, these we see
these many of these holes which are drilled over here.
And at the same time you see these fingers in this this kind of a design.
So there are many kind of a design
you could imagine off printing these fingers and holes on this on this cell.
So now I'm in a great position to compare these different kind of cell designs.
So we talked about the conventional cell which
has a front contact and has a back contact.
And then we talked about this emitter wrap through and metal wrap through scheme,
which minimize the area at the front
surface, which is colored by these contacts.
And these bus bars, which are essentially you know, occupy a large percentage
of the area, they are now moved towards the back of the cell.
And then you, from the perspective, the shading the base design is this all back
contact cell, where both your N and P kind of contacts are at the back.
So, in, in terms of efficiency, this this this
metal wrap through, emitter wrap through cell, it has
higher efficiency as as compared to the, conventional cell,
but it's lower as compared to the back contact scheme.
So this efficiency could be more.
You could get up to one percentage point
increase in your cell efficient by using this,
metal wrap through, emitter wrap through design.
And most of that increase, it comes from the
increase in the short circuit current because now you
have less shading so you can collect, more of
these more of these photons coming from the sunlight.
Another increase that people do observe in the cell is increase
in the fill factor, and that usually comes from the fact that
now you have, since you have moved your bus bar towards the back of your cell.
Now you can make it much wider so instead there's no restriction
on the width of this bus bar, because it's not causing any shadowing.
So you can reduce the I square R loss, you know, related to this bus bar, and
that improves your fill factor. Another advantage of these these back
contacted schemes is that they give much better aesthetics.
Both these IBC and this EWT, MWT cell, they give better
aesthetics as compared to the as compared to the conventional cell.
Another major advantage that comes handy when, especially when you are
making modules out of these back contacted cell, is that your module
assembly becomes much more easier.
So in a conventional cell, you have this N contact on the, on the front,
and then you have this P contact on the, on the back of the cell.
So whenever you are forming whenever you are forming a module you need to
connect these N and P contacts, and
you need to form these inter-digitated inter-digitated bus
bars, going between these two cells.
Whether if you, if you have all the contacts located at the back, you could
use a module assembly very similar to what is used in the chip industry, where
you just the take these cells and mount them on top of this on
top of this printed circuit, and you just take this cell and mount it over here.
There's no need to simply take wires from one cell and
connect it to another cell.
So it makes the whole module assembly much easier, right?
And that's another advantage you get both from these emitter wrapped
and milliwrapped through and also this all back contact kind of scheme.