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.