Emitter Wrap Through (EWT) and Metal Wrap Through (MWT) schemes for solar cells: P2


Hello folks. In the last video we talked about emitter
wrap through, or this EWT cell, and we 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 realized 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 times.
So this design which is commonly used currently in the industry, is this metal
wrap through design. So, shown here are these two examples of
this metal wrap through design or MWT.
And as compared to the, as compared to this EWT design, these designs have still
have a more, slightly larger amount 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, arranged this floor kind of a design. And then there are these holes present which are
connecting these 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 on 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 the 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 form this n+ emitter diffusion at the
top. So this would be our n+ diffusion over
here. And then you drill holes into this and
you, fill it with metal 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 flows 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 wet etch And then we form this diffusion similar to the way we would do in a conventional
cell. So we have this n+ 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 via as well. 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+ bus bars at the back. So this is one such process flow which is closely, I’m trying to closely resemble the conventional
process flow. I’m only adding this extra step for
drilling this via. And you know the metal diffuses through
this via ultimately, I’m 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 these. 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 kinds of design you could imagine of printing these
fingers and holes on this on this cell. So now I’m in a good 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 covered 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 of the
shading, the best 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 all 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 squared 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
cells, 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 interdigitated
interdigitated bus bars going between these two cells. Whereas 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 essentially 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 wrap through and metal wrap through and also this all
back contact kind of scheme.

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