Hi folks, in this video, I want to talk
how to optimize this, TCO, this, Transparent Conducting Oxide.
Which as we discussed, is often used in these thin film based solar cell.
So to understand how to optimize it, and
you know, what should be the optimal feature
of this TCO material, it's important to understand
how these thin film solar cell are are made.
And I find that quite fascinating, you know,
how these thin film solar cells are made.
So let me give you one example.
And I'll give you an example of a case where
these Solar cells are made in a super straight configuration.
So in those cases you start with essentially a piece of glass.
So this is a large piece of glass drawn over here.
So this glass of course is not a conducting material.
So, you can't use the glass itself as a electrode.
So what is done is that you first deposit the wafers.
Film
deposit is a film of this TCO material, which forms one of your electrodes.
So this is your TCO material.
Now, a very fascinate, thing I find very fascinating and that is unique to these
thin film based solar design, is that the way you form these different cells.
So this is a big piece of glass, it could be, you know,
all the way from one meter to a couple of meters long.
So how do you form these individual cells in this in
this, in this panel So one way would be to, of course you know, of course
to essentially break this glass into multiple multiple
multiple pieces and you know, make individual cells.
But you can see that that wouldn't very efficient because, you know, it's not very
first of all, it won't be very easy to cut this glass into very small small pieces.
Plus your yield will go down and there are
some other reasons where it would become difficult as well.
So you know there's a neat, very neat way to
essentially make different cells on this single piece of glass.
And the process which
is, used to do so is, laser scribing.
So what is done is that you essentially scribe this, scribe this,
TCO layer into, into, using a laser, and you form
these, individual, individual islands of this, TCO material, and this
is what essentially forms your, different cells and these cells are series-connected
again in a very intelligent fashion, that I'll just describe.
So the very first thing you do is essentially scribe this TCO layer.
Now, the next thing which is done is you deposit your absorber material, so you
essentially deposit your absorber material which is
your thin film, which will absorb the light.
So it will deposit everywhere. It will deposit on top
of your on top of your TCO material.
It will essentially also deposit like over here, in
between the scribes that you made in your TCO material.
So this is your this your absorber layer over here.
Now this could be different, depending upon different thin-film technologies.
For example, if you're using amorphous silicon based solar
cell, this would be made out of amorphous silicon.
If you're using cattail technology this absorber
material would be cadmium terylene and so on.
But I'm just denoting it as a generate absorber material over here.
Now the next thing you do is essentially scribe this absorber material again.
So you bring on your laser and essentially scribe this
scribe this scribe this absorber material using a laser as well.
And these again you form islands of these of these, of these
absorber materials. And they are separated from each other.
So now you can see that you know, each of these is an individual cell.
So this is my cell one over here which has this this electrode at the bottom
and then it has this absorber material. And then I have this other material over
here which is which, I have this other cell over here which has this electrode.
And this absorber material over here.
But you can see these cells are so far not connected.
Electrodes are you know still separated from each other.
So now, what is done is that you essentially deposit a second layer of
TCO material, so you come back and deposit this second layer
of TCO material.
And, of course this will deposit everywhere as well, so
it would, actually let me stripe the other way around.
So now this will deposit every where as well.
It will deposit not only on top of this absorber material but it
will also deposit in these trenches which were formed by these laser stripes.
And as you might have guessed, again, the thing I'm going to do next is essentially
scribe this us, scribe this top TCO material as well.
So this was my top TCO material and so this would be TCO2
and let me call it the first one TCO1. So now what I'll do is essentially
scribe this TCO2 as well and So
now essentially I can see my, my structure or my skeleton of my cell forming.
And let me show you how.
So now I have, I've, I've scribed this top TCO material as well.
And they are now essentially separated using the laser scribe as well.
But in the process another thing I've
done is that I've series-connected these these cells.
So let me show you how.
So this is my first cell, so this is my first cell.
This is my bottom electrode, this is my absorber material,
and this is my this is my top electrode over here.
Now, since I deposited this TCO everywhere, it is now essentially, you
can see it's Connecting to this bottom of this other cell over here.
So if this cell
was off a, let's say a PN junction over here, where
the bottom part is P and the top part is N.
Now this N layer is connecting to the P layer of this second cell.
So, now you can see that since this top TCO, this TCO 2 is touching this TCO 1.
Now you see that there's a serious
connection which is forming between these cells.
Similarly over here you se
that this is my second cell, so this is the P and N.
And then the end region of this cell is now connected.
Via this via this this laser scribe opening over here.
It's connected to this third cell, at the, TCO of the third cell over here.
So what I've essentially now found is
these PN injunctions which are connected in
in series with each other, so I can think of each of
these cells essentially this is PN cell and this is now connected to
this second cell over here, through this connection, which is now connected to this
third cell over here using this connection between these two TCO materials.
So this is how a thin film based solar
cell is made.
And I find it, you know, a very neat way to essentially.
Make this, series connect all these solar cells.
But it comes with some challenges.
And one of the main challenges over here is that since I'm using a multiple of
these laser scribes, so I'm using three of
these laser scribes as you can see over here.
So, this Was my first laser scribe,
this was my second laser scribe, and this was my third laser scribe.
So I'm using three laser scribe process over here.
And these three laser scribe processes, they're only very roughly aligned to
each other, so there's no lithographic patterning or anything going on over here.
Only way these, these laser scribes are aligned is optical alignment.
And that essentially you know is, is requires
you to leave quite a bit of
spacing between each of these laser, laser scribes.
So essentially if I think about this
region which in between these these three laser
scribe, that essentially is the width of this
region Is you know, typically hundreds of microns.
in usual cells. So, and also this, this region is not
contributing to any solar energy conversion, or this
is not essentially being part of a cell.
So this constitute what is the dead region, which
is called as often the dead region in my cell.
And its width is denoted by I'm denoting over here by this symbol, Wd over here.
So, if you are a solar cell
manufacturer, you essentially, you know, want to, you want to
minimize the fraction of this dead region.
So, let's say this is my dead region, and this is my actual cell.
So I want to minimize the fraction of this width of the dead region.
Versus the width of my cell. So, essentially I want to minimize I
want to minimize or I want to essentially minimize this fraction which is wd by w.
So, wd would be, you know, fixed by
how precisely I can align these different scribes.
So, essentially, a way to reduce this fraction would
be to essentially maximize my Maximize my cell rate.
So if I can make each of these cells very wide the contribution due
to this, or the no contribution due
to this dead region, it becomes insignificant.