In the last video I was talking about
this band engineering for making these multi-junction solar cells.
And I was particularly looking at the case of solar cell which had three functions.
And fortuitously we came across this material
system, which gives us these three semiconductors.
Germanium gallium oxide, and indium gallium phosphide, which are lattice
matched to each other. So I can grow them on top of each other.
Without creating defects.
And I can make a solar cell and in fact this solar cell it gives me efficiencies
of approximately 400, approximately 40% at, at a concentration of 500 suns.
And further looking deeper into it we found that this efficiency was
limited by.
Essentially this bottom cell, which had a band gap,
which was, less than what was desired, so it had
a band gap of 0.7 electron volt, which resulted
in a, in a high short-circuit current for this junction.
But, that was not really useful, because.
The overall current of the cell which
is constant throughout the three, three junctions was
limited by the top and the middle junction which
had much lower current as compared to this germanium cell.
And also since this had lower band gap this
resulted in lower open circuit voltage for the cell.
And since the.
Opens up a voltage of my, my overall cell.
It essentially related to the open circuit voltage of this individual cell.
It's it's given by the sum of these three.
So my open or all open circuit voltage was also, you know,
slightly lower because of Band gap of germanium being less than optimum.
So, now what do I do about it?
There are two ways I can essentially you know, get around this problem.
One is to lower the band gap of these two materials.
So essentially, lower the band gap
of my my top cell and my intermediate cell.
So essentially they also will now absorb more of the spectrum.
So the short circuit current of these two cells, which
are the top cell and intermediate cell, will hopefully increase.
And you know it will hopefully become closer to what germanium gives.
And my short circuit
current of my overall cell will improve.
So that's one way I can improve my overall efficiency.
The other way I can I can think about it, you know, I
can replace the germanium with something else which has a higher band gap.
So in that way the short circuit current
of this germanium, of this bottom cell will decrease.
But its open circuit voltage will increase.
And hopefully again I'll get some increase in the efficiency.
So now let's look at how you know, how I can,
how can, how I can go about realizing both these approaches.
So let me start with the first approach, where what I want to do is I
want to reduce the bandgap of these two
material which I used to make the top cell.
So there's indium, indium gallium phosphide which is used for making
the top cell.
And then there's gallium arsenide which is used for making the intermediate set.
So to reduce the bandgap you know, I looked at bandgap which is the
largest constant, and I see that if I essentially mix this gallium arsenide.
With the indium arsenite, essentially I form this, indium gallium arsenite.
Then it has, reduced bandgap as compared
to gallium arsenite, but it lies on the right-hand side of,
this gallium arsenite and germanium, so it has a higher lattice constant.
So, if I go to higher lattice constant and mix my gallium arsenide with indium then
I get a lower bandgap of this resulting material which is indium gallium arsenide.
Another way I can reduce the bandgap of,
of this top material which is indium gallium
phosphide so I just stated it to be indium gallium.
Outside, this is indium gallium phosphide.
So similarly for this top material indium gallium phosfide.
If I essentially increase my inputs in here, I move closer to indium phosfide.
So I'll have a lattice constant which is higher.
But I'll have a band gap which will be reduced as
compared to with indium gallium phosphide with 50% of gallium and
50% of indium.
So if I increase the indium percentage more than 50 I
essentially get a lower band gap but a slightly higher lattice constant.
So if I increase both the lattice constant of this indium gallium arsenide.
And the lattice constant of this indium gallium phosphide
by increasing the indium percentage, I'll essentially reduce the bandgap.
And this, this, you know, as is
shown more zoomed out over here.
So, if I look over here, I see that my germanium is lattice
matched to gallium arsenide and this
indium gallium phosphide with the 50% indium.
If I increase the indium percentage over to 65% I see that I'm reducing my bandgap
from this value of, you know, 1.8 to I'm reducing it further.
Similarly I'm
reducing the bandgap of gallium arsenide when I'm mixing it with indium.
So, if I'm mixing it with 17% indium.
reduce its band, its band gap.
But at the same time I see a problem with this approach too
that there's a, I have increased the lattice constant of these two materials.
So now these two materials are no longer lattice
match to germanium and the lattice mismatch over here.
So when I start growing
this material on top of the germanium. I expect defects to form between the
germanium and this indium-galium oxide, sub, indium-galium oxide layer.
And direction over here, so if I do, in fact, do take this travel down this spot.
When I start growing this, a reduction
indium-galium oxide layer on top of germanium.
There'll be defects which we created over here because
of the lattice constant mismatch between these two materials.
And these defects would, you know, further
propagate throughout the rest of the cells.
So, this, these defects will also
propagate into the indium gallium phosphide
layer which is grown on top of this indium gallium arsenide layer.
So I see this is very nicely shown in this in
this transmission electron microscope picture over here.
So over here I have germanium and germanium substrate.
And then I'm starting to grow this indium
gallium arsenide with 17% indium on top of it.
I see that these defects these threading dislocations.
And these stacking files are forming in this material.
And they, they essentially
continue to creep into all other layers which are grown on on
top of this material as well. So approach this is used is
essentially magnus metamorphic layer or magnus buffer
of Put a buffer of this indium gallium arsenide material and it's
usually a thick buffer you know, a few microns or less than or
greater than a micron in, in thickness.
And it essentially, the purpose here is
to absorb this side's mismatch and hopefully terminate.
A lot of these stacking files and
these trading dislocations within the sub buffer layer.
So this is a metamorphic technique of of growing growing this cell.
I can essentially realize this cell
which is germanium at the bottom.
And then I have indium galium arsenide with approximately 17% indium.
And then I have this Indian Gallium Phosphide with high percentage of Indium
and So I can see that my short circuit current of my top cell has
increased compared to 14, it has now increased it has now increased to close to
16 or at maximum 16.5 and.
In the short circuit, the current of my intermediate cell also increased.
At the same time, the short circuit current of my bottom cell which was 25
before is now substantially reduced because a lot more of the photons, or
a lot more of my spectrum is being now. Absorbed by the stop and
intermediate cell because now they have lower band gaps
as even less spectrum is available for gemanium and I
see that now approximately the currents of these three
cells of these three junctions are matched with each other.
But at the same time since I reduced the
band gap of this top cell, its Voc will decrease.
Similarly, I reduced the band gap of this intermediate cell so its open circuit
voltage will also reduce.
So, and also I have this problem of
these defects, these these defects in the metamorphic layer
which propagate And reduce the efficiency of both
this intermediate cell as well as this top cell.
As a result of that I get efficiency improvements as compared to
a lattice made system which has the efficiency, in this case, of 40.1.
I can improve my efficiency to
40.7 but it's not you know, it's not,
the gain is not substantial because of all these.
All these bad things, which are coming on with the,
going to, metamorphic system, which has, large amount of lattice mismatch.
So now let's look at the second matter which
involves replacing the geranium which had a lower band gap.
And what's causing all the problems with
some other material, which has A higher bandgap as compared to germanium.
So, I look for material which might have higher bandgap as compared to germanium.
And I realize that I can use something like indium gallium arsenide.
I can mix this gallium arsenide over here with
the indium arsenide which has a very high lattice constant.
And I can realize, you know, I can make
a material that has a higher band gap as compared to germanium.
But now I need to grow it on top of, on top of something.
So I, immediately if I look up and down, below this gallium
arsenide, I don't see any native
semiconductors such as silicon or germanium.
So, the only way to grow this indium gallium arsenide
would be to grow it on top of, let's say,
germanium or gallium arsenide.
But if I start with the, let's say, germanium and grow
this indium gallium arsenide, there'll be a lot of defects which will
be, which should be created because this indium gallium arsenide has
a larger lattice constant so there's a large lattice mismatch over here.
And all the subsequent layers such as my gallium
arsenide or my aluminum gallium phosphide, which would be grown
on top, would have these defects as well.
Because these defects will propagate into whatever is grown
on top of this indium gallium arsenide as well.
So we used this approach which is called inverse metamorphic approach.
So what is done in this approach is that. You start with gallium arsenide, so you
start with gallium arsenide, and then you grow the material which has the highest
band gap, which should usually be on the top, so you, which
should be facing the sunlight, so you grow this indium gallium phosphide first.
And then you grow galium oxide again on
top, which is your intermediate band gap material.
And then finally, at the very end of the growth, you grow this indium-galium
oxide material, which is the material you need for your for your bottom cells.
So the defects are in fact, created when you
grow this indium-galium oxide on top of galium oxide.
But at least they're contained in this, in this
cell which is made out of ilium galium oxide only.
So, the defects which were created here, they're contained in this cell.
And now what is done is you essentially
peel off these layers which you have grown.
So you.
peel off these
layers on on top of, which were, you grew on top of gallium arsenide.
And then you flip the cell over.
And the indium gallium phosphide layer which you grown it
in the start of your epitaxy it becomes the top layer.
And this gallium arsenide which you grew in
the, in between, it becomes your middle cell.
And this indium gallium arsenide layer which has most of the defects it
becomes your bottom cell.
So you can see over here that the defects are contained in this
bottom cell, and they at least do not affect these top two cells.
And this approach is called inverse, metamorphic buffer.
Because, you're, this metamorphic layer is located, essentially towards the bottom.
Instead of, propagating throughout the cell .
So these are some of the engineering techniques we can use
to essentially limit the pitfalls of the, this, three junction system.
And, improve its, efficiencies.
There are other techniques as well.
More recently there is, technique which involves,
wafer bonding that is, you know, growing, two
of these at junctions at once, and
growing the other junction, at, in separate epitaxial
processes, and then bonding them, together.
This is also being, researched by several groups
and, being pursued by quite a few start-ups.
There's also a, a lot of, Buzz around use of these
dilute nitride system which have you know, many of these materials.
They are also to your band gap, which is intermediate between,
between gallium oxide and germanium.
So, you can replace your bottom junction with this dilute nitride system as well.
So hopefully I have given you some idea of how this lattice
matching, it essentially limits the more of band engineering that you can
do, and some of the tricks people play to achieve the band gaps they want.