Sunpower IBC cell: unique features

Sunpower IBC cell: unique features


Hello folks. In these couple of videos I want to
present to you this fascinating story of SunPower
and their cell. And this cell is one of the most efficient crystalline silicon-based solar cells that
you can buy. And you can, you know, it’s not just a
laboratory cell, you can buy these panels from Home Depot and
install them on your house. And these solar cells achieve efficiencies, the latest
generation of them, achieves an efficiency of solar conversion of more than 24%, and
the cell design what SunPower calls them the Maxeon, third generation of these Maxeon cells. And on a module level, they achieve
efficiencies of more than 21%. And these modules, they have very nice
characteristics. They have a low temperature coefficient of
efficiency. They have a better immunity to humidity,
and you know, better thermocycling and so on. All of those nice characteristics that
come along on the module level results from this very unique cell
design that SunPower has. And they are leveraged into all these nice
characteristics that the module gives. So a SunPower cell, it looks very
different from a, from a conventional crystalline silicon
cell that you must have seen. So if you look at the front side of it, it
looks you know, very elegant.
It looks all black. So there are no fingers or grid lines, you
know these metallic grid lines which are usually present in in a
conventional crystalline silicon cell. So none of these are present on the, on the SunPower cell and it looks completely
black. The contacts in the, you know, the real
magic of these device is located in the back so
both the p and n contacts, they are located in the back and they are located in this interdigitated
fashion such that let’s say these lines I’m drawing, they
represent the n-type contacts, so they’re all collected
like this. And then the p-type contacts are
interdigitated. You know these p-type contacts are
essentially interdigitated between these n-type fingers, and
this cell design is also called as a Interdigitated back-contact cell or IBC
cell design, the way, as a result of the way these contacts are
are formed in this interdigitated fashion. So let’s, let’s take a deeper dive and look in
what actually goes inside this sun power cell and some
of the unique features. So I’m, I’m plotting over here, I’m drawing over here this cross-section
of the cell and inverted the cell so essentially the light is incident
on, from, from this side and all the metal contacts are
located now on as shown over here on the, on the top.
So these, these contacts now since you know, they’re they’re all
located on the side which is not facing the sun, I can now essentially make these contacts
as as tall as possible without worrying about
that they will interfere with my light when it’s
incident at oblique angles, or I can even make them very wide without worrying about the shape, the
shaping losses. As a result of that, I can minimize the
resistive losses which are usually associated with
these fingers and these bus bars because now I can make these
contacts, I can make them wide and I can also make
them tall. And also since these p and n bus bars over here,
these are interdigitated, so these, since we have this
interdigitated design, these electrons and holes which are generated,
inside, inside the, inside the silicon, they can be now collected close
to each other, so none of them have to travel very far as would have been the case if I
don’t use this interdigitated designs. So it’s kind of helps in minimizing the
resistive losses which are associated with these
contact scheme design. Sometimes, and actually now the latest
generation of cells that SunPower cells, these things are
actually made pretty thick, and they’re made using
copper electroplating. And they offer the additional advantage
that, you know, they are, they provide a mechanical
strength to the cell. So even though your cell can crack, so
these you know, these these copper plating of these
interdigitated contacts at the back. It, you know, the cell can still, deliver, decent efficiencies and, good
power conversion if, if you have these, metal grids at the back which provides, mechanical
strength, to the cell as well. As compared to a conventional
cell, where you know, the cell is held together just by the mechanical strength of this
silicon. So some of the other unique nice features
of of this cell are illustrated in this figure.
So the cell, it uses this anti-reflection, coating and
texturing on the tops to minimize, to minimize light reflection
and maximize light trapping. Just like a conventional solar cell does. So the unique unique thing that it has
that the cell uses n-type silicon. As compared to most of the conventional
cells which use the p-type silicon or p-type wafers for making
these cells. So one might you know, wonder why this is
the case. Because in n-type silicon the minority
carriers are, the minority carriers are holes, and versus in p-type
silicon, the minority carriers are electrons.
So usually you tend to minimize, you, you tend to maximize the collection
of your minority carriers. And since holes have a lower mobility as compared to what you get for electrons,
this at a, at a first glance, it might seem a counterintuitive way of making
these cell design. But there’re a lot of other technological
considerations. One is that these n-type silicon wafers, they are less susceptible to a phenomenon
called light induced degradation so whenever you shine a lot of
light on these cells, the efficiency of these cells
degrades and that especially happens in cells made out of p-type wafer, because this p-type wafers have these boron and
then they combine with these oxygen present in the wafer as
well to form these boron oxygen cluster. Which can cause up to you know, 2-3% decrease in the efficiency of the cell in
the first few weeks that this cell is being used. But those things, light induced degradation
effects, they don’t occur in the n-type silicon. Also this n-type wafers, they’re very
immune to impurities such as iron and so on, which are present in the feedstock of, of what is used to
make these, these wafers. So because of both of these things,
impure, its immunity to impurities and no presence of these light
induced degradation. In fact, the n-type silicon can give
higher diffusion lengths as compared to, even for the minority carriers,
as compared to p-type silicon. And that’s why this n-type silicon is used
in the SunPower cell. And in fact if you look at some
other companies which are making these high efficiency silicon
cells, such as Sanyo, which makes the HIT cell. Or more recently Yingli, it makes a series call the Panda cell, They also use n-type silicon because
you can get, higher efficiencies as a result of these,
better diffusion lengths that you get, in, n-type silicon.
So that is a unique feature number one. Another thing, that you notice, in this
design is that both your p+ and n+ diffusions are
located at the back. So, this p+ diffusion helps in forming a selective contact for
holes. This n+ diffusion similarly helps in
forming a selective contact for electrons, and they are, you know,
located at the back. Also, they are localized, so they, all of
the carriers which are generated inside these
different regions, they are not collected with a very high
efficiency, so now these things are localized and
located at the back. So they minimize the generation in this p+ and n+ region. At the same time they help in forming these selective contacts for
electrons and holes. So that’s another unique feature, product unique feature number two. I already talked about these backside
contacts with another unique feature. Then also there’s this mirror located at
the back. So, this mirror especially, it reflects
back these red photons, these high you know, these high wavelength
photons which might not be absorbed during the first or the second reflection.
And it reflects it back into this silicon with a very high reflectivity.
So that’s another feature of this cell. The other feature that I would point out
is that the use of this lightly doped diffusion in
the front. So let me illustrate the need for this lightly doped diffusion using a 2D
cross-section over here. So let me compare the 2D cross-section of
conventional cells. In a conventional cell you have a p-type, a
p-type wafer. And then you have an n+ emitter at the
top. So you have this n+ emitter, and you
have this emitter contact at the top. And at the back you form this p+
backside, back surface field using this aluminum
line. So this electric fields which is
present inside this conventional solar cell which helps in
separating this carriers. So it helps in electrons being collected
on the emitter side and it drives these holes which are
generated inside this depletion over here to the back surface. But at the same time, we have this contact
which is present at the top, so it, this one
causes shading. Also a lot of the carriers which are
generated inside these emitter regions, the electrons and holes which
are generated inside this emitter region, they get recombined very easily because
this region is heavily doped. Especially the lifetime for these minority carriers, these holes which are generated
in this emitter region, is very small. So versus in, in comparison to that, what
has been done in this IBC design that SunPower is using is that you have moved both your p and n contacts are
towards the back, so you form these localized p+ and n+ diffusions, and then you form these
interdigitated contact at the back, so, it helps in minimizing this shading loss,
because there’s nothing facing the. Incoming light from the top. But, at the same time there’s a, this
electric field which was helping me separate the carriers is no
longer present in this design. So as a result of that, you need a very good high quality, you need a very high quality of the semiconductor
material. So that the diffusion length for these
carriers which are generated, these electrons and holes which are
generated, are long enough such that they can be collected at these p+ and n+
contacts without getting recombined. So that’s one of the reason you used this
n-type substrate, because it gives better quality, better immunity, better immunity to these impurities in
your silicon. So this is fine. So now why do I need a surface field at
the top? The reason is that even though I passivate these top surface using silicon nitride
or a combination of silicon oxide and silicon
nitride, the surface recombination at these
surface is at best you know something around 100 to 200 to 300 kind of a range. So even though I passivate this surface, the surface recombination is still a
dominant cause of, dominant loss of mechanism, especially in this case,
since you know, there’s light coming in especially
this blue light it will generate a lot of these electrons
and hole pairs very close to the surface and
they can now essentially go to the surface and get recombined without getting recollected
at this back. So what is done over here, is that you
form a very lightly doped junction at the top, so this wafer was
n-type to start with, so you Dope it with a very light p diffusion at
the top. And it creates this surface field, surface
field at the top which repels these, repels these holes from getting, entering into
this region, and you know, essentially pushes them back. Just like, you know, a back surface field
over here does. So, this helps in minimizing the surface
recombination. At the same time, you keep it you know, you
don’t dope it very heavily, so that the carriers which are generated in
this region can still be collected without
getting recombined. So that’s the reason why you still keep a lightly doped diffusion at the top, to minimize these loss which are happening
at the surface. So in this video, I have talked to you about some unique features about this
SunPower cell. In the next video I’ll discuss the process
technology, and how SunPower was able to drive the cost of this of this IBC design
down, and produce it on a mass scale. So see you in the next video.

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