When people talk about thin film based solar
cells, they become very comfortable or very accustomed to
representing these different layers, which, going to this thin
film solar cells by this kind of a box.
And then representing each layer by drawing a, drawing a line.
For example over here is this cadmium terlite based
solar cell and in this diagram I have represented
the different layers. For example glass then this TCo
layer, then the Cds o5 anti-contact layer then
the cadmium terlite layer. It's all the different layers which Make
up this solar cell by using you know, these lines to distinguish between them.
Similarly over here, on this side I have a this micromorph set, which is a tandem
cell made up of amorphous silicon and microcrystalline silicon.
So over here.
The way it's drawn is the light is coming from the bottom.
And you have glass over here.
Then this tin oxide TCo.
Then amorphous silicon, which has a higher band
gap, which has a band gap of approximately 1.7.
And then micro-
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silicon below it. So, people are very comfortable when you
talk to them about solar cells by drawing them or depicting them this way.
But what is missing in this diagram is that
these thin films are essentially poly crysalin or multi
crysalin or amorphous materials. And penetrating
inside these cells or you know inside
these cells the huge density of these grain.
And these grain boundaries, which are essentially
running throughout the cell so essentially they'll be.
Grain boundaries, which are, you know, traversing like this inside the cell.
And this is never depicted whenever you
draw these diagrams of these thin film technologies.
But a very quick ACM
picture taken of any of these cells, there
is these these grains in these grain boundaries.
In in all its glory.
So shown here are again those same cells.
This is the same cadmium terlite cell.
And this is the cell, which is
a tandem between amorphous silicon and micro-crystalline.
But now you get you know, a very good sense of how the grain and
the grain bond rays are penetrating throughout the cell.
So you can that, you know, there are these plethora of grain boundaries,
which are, which are essentially running throughout the cross section of this cell.
Similarly over here you see this.
Way way large density of essentially grain boundaries, these stacking files
and all these, other things, which are which are running through the cell.
So one might wonder, you know, looking at this picture that how does even
this cell you know, produce any electricity
or you know, how does it convert?
It's able to extract any of these electron and hole pairs.
But, you know, one in the situation is not as bad, you know, as
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as one might presume by looking at this picture.
And these cells do operate and, you know,
level descent inefficiencies, such that they're used commercially.
And one of the reasons why this is even possible is because
this transport is essentially parallel to the direction of the grain boundaries.
And here's what I mean by it.
So, assume that the cell might and firm-based solar cell.
And these yellow lines are essentially, you know, the grain boundaries, which are.
Running throughout the cell and this is the direction of the grain boundaries.
Now the way this solar cell works is that this
electric field is essentially in this direction so, you know.
I have a my Eletima Emitter contact over here and then
my current is essentially or you know, my electrons then hold there.
Being collected or
they're flowing in this particular direction.
And this direction is, if you can see over here, it
lies parallel to the direction of most of these grain boundaries.
So if I have a, if I generated the electron and hole pair and it
basically gets collected at this contact over here, it might not encounter
this grain Although it might not encounter this recombination
due to the grain boundary at all.
Because the grain is essentially running parallel
to this, direction of this, current flow.
Versus if my cell was in, you know, of such
a kind, such that I had my contacts placed over here.
So, let's say I had my N-type and P-type contact placed over here.
And I was generally, I was collecting my electron
in holes, in this, you know, along the in
this direction, which is perpendicular to my grain boundaries.
So there's a very large chance that you know this, this
electron might recombine when it when it traverses through this grain boundry.
Maybe it missed out on this one.
Maybe this grain boundry missed out.
But, you know, it will soon encounter another grain boundary.
And it's very likely that it will get recombined
before it's gets collected.
So one of the reasons why, you know, why these these grain boundaries are you know.
They are they are still a major concern but they could be even
a bigger concern if the, if the current flow direction was like this.
But since the the flow of the current and the
flow of this electric field is parallel to the grain boundary.
It helps in,
alleviating some of the impacts of some of these grains.
So that brings me to the next thing I want to talk about.
That is how do these grain boundaries affect the performance of the cell?
And I've essentially drawn this diagram over here.
So, let me consider the case where I have just
one grain boundary in my, in my whole solar cell.
And I want to figure out, you know, how essentially it will effect effect
the device performance.
So this grain boundry essentially it means
that I'll have a higher recombination over here.
Now I can
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represent this thing by distributed models.
So, you know, I can depict this situation by this these distributed set of diodes.
So, you know, I've taken a 2D graph section of of this of this cell.
And, you know, I have each of these small regions, I'm depicting them as a diode.
So this is a diode.
It's forward biased and it's conducting current in this direction.
So one might wonder, you know, if it's forward
biased, how it's, it's conducting the current in the direction.
But you know, you can easily recall that's how a solar cell works.
So you have a volt, voltage, which is positive.
And you have a current flow in the opposite direction.
So this would be the IV characteristics of the cell.
So I'll have this is my VOC.
This is my short-circuit current over here.
And I'm operating in.
Let me use the same color. Let me use blue for my axis.
So I'm operating in this region, where essentially I am applying
a positive voltage and I'm getting a current in this direction.
And that's how I'm generating power.
Now, essentially, I can represent the region,
which is affected by this grain boundary.
So, you know, this grain
boundary maybe it ran through this region. And this region, you know, is the.
Is the spoiled orange among these different different diodes.
And it essentially will have a very
high recombination because all the electrons are whole.
Which, you know, which essentially come to
this grain boundary can very quickly recombine.
So the IV characteristic of this rotten cell or
you know, this bad cell, which has affected Due
to these grain boundaries would be essentially it
would have a much lower open circuit voltage.
Because you know I have a, a lot more recombination, more recombination
means reduced open circuit voltage, so its open circuit voltage will reduce.
And also its current short circuit current will reduce because essentially.
Now, you know, a lot of these electron hold.
They recombine before they're collected out.
So this, for this
biased cell, which essentially represents the
region through, which my grain boundary passes.
I can represent its IV characteristics by, you know, essentially, like this.
Where I have a reduced VOC.
So, let me call this VOC. One JAC one.
So I have, instead I get VOC two. Which is lower.
And also a short-circuit current. Which is much lower.
So now how does this affect, you know, the rest of the set?
So if you think about the rest of these cells which are connected in parallel
to the cell, so they all want to operate at this high open circuit voltage.
And they want to operate at this high short-circuit
you know, they want to supply this high short-circuit current.
But this diode, you know, this essentially cannot deliver you
know, current in nitrogen.
So now, since these, all these cells, the good
ones and the rotten ones, they're all connected in parallel.
They have to operate at essentially the same voltage.
So the only way this, this this
rotten diode can support this higher operating voltage.
Let's say, you know, all the other cells, they
operate at this voltage where, you know, they all
they all essentially generate par.
Now the only way this other bites it will lower VLC.
It can support that same open circuit voltages by going into this
going into this regime where it's now generate where it's forward by.
What it's doing now is that the current
flow in this rotten cell is essentially reversed.
So the current is now flowing in this direction.
And that current is being supplied
by all these other cells, which are in the vicinity of these rotten cells.
So are these good cells, which are in the vicinity of these rotten cell.
There are essentially supplying this current, which
is now getting synced into this rotten cell.
So in effect, what has happened is that because of this green
boundary, essentially all of these diodes, which are all around the region.
Which is present in the vicinity of this
this green boundary is essentially you are
not contributing to too much solar cell operation.
But in fact all the current which is being generated
over here is getting recombined into this into this grain band.
And it can be shown that the, you know, the number of these diodes effected.
So, you know, they like it say the length
of this region, which gets effected due to this
bio-diode or this region, which will get
effected is essentially this ratio of the length.
Or the number of the number of the diode, which
are affected, it's essentially depends on a lot of things.
Depends on the sheet resistance of the semiconductor and so on.
But it can be shown that
it's approximately proportional to its proportional to.
The exponential of
the difference between the exponential of the difference between.
The good cell, the exponential of the difference in the voltage,
open circuit voltage between the good cell and the bad cell.
So, if I have a very severe grain boundary and, you know, a very high
recombination of this grain boundary, this VOC is
essentially very reduced, this VOC2 is very low.
So it will effect a
much larger area around this grain boundry.
So even this one grain is essentially it's creating this sink,
which can the effect is very similar to having this small hole.
So you have you know about which is favored water.
But a small hole can essentially, you know, affect a lot of region.
And it can drain out the water from a large
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So a grain boundary essentially behaves in the same way.
And the more serial the recombination in this grain boundary is.
The lower is the VOC for that particular area
and higher is the region that it affects around.
So in general, we want to avoid having green boundary
and we want to, essentially, even if we have green boundary.
We want to try to pass it
in such that the recombination over there is not that severe.