Hello folks.
So this is the first of a playlist of videos that I want
to do on this very important topic of this topic of transparent conductive oxides.
Which are most commonly abbreviated or
most commonly referred to as TCO materials.
And these TCO materials I'm sure you encounter them in your everyday life.
But it's hard to notice
them especially since there's the word transparent over
here so you can't really notice their presence.
But these TCO materials are
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used in all touch screen based devices.
All touch enabled devices such as your iPad, your iPhone, your tablet.
They all use a coating of this of this TCO material.
And in fact the cost of some of these TCO materials has
gone up significantly because of their
extensive use in these touch-enabled devices.
The other application of these TCO
materials, and the one which I'm particularly
interested in talking about, in this playlist,
is the use of these TCO materials.
These TCO materials are also used for making these solar cells.
And in particular when we make the solar cells using any of these thin
film based technologies, such as amorphous silicon, cadmium telluride, or
CIGS based solar cells. We use at least one coating, or usually
two coating, and in some cases three coating of these TCO materials as well.
Now some of these crystalline based solar cells such
as this HIT design from Sanyo which stands out.
They also use this TCO material.
So these TCO materials, they enable a wide variety, or
you know, they are essential for enabling these applications and touch enabled
devices, as well as for making these thin films based solar cells.
Now, one of the main, one of the main
challenges with these TCO materials, lies in the first two
letters or the first two adjectives or the two adjectives
that I have essentially used to describe this TCO material.
So,
these TCO materials they are essentially oxide of they're
metallic oxides typically. And the problem or essentially
the challenge is these two adjectives that I have used to describe this oxide.
That is transparent and conductive are usually
not associated with the same same noun.
So these two adjectives essentially, transparent
and conductive usually don't go hand in hand.
And let me explain why.
So for something to be called transparent it has to essentially
have a band gap which does not absorb the solar spectrum.
So most of the solar spectrum, at least the visible part of the solar spectrum, it
lies within the energy range of 1.8 to 3.3 electron volt.
So for something to be a good transparent material, the band gap
of this TCO material has to be essentially greater than three electron volt.
Or the band gap of these TCO materials is
typically between 3 to 3.8 electron volt, electron volts.
Such that when you have this TCO material and you
have your, light incident on it, you have your green photons,
your red photons and your blue photons. So most of them can essentially you
know, they, they, they are transparent to this oxide material.
And they can be absorbed in the solar cell which is placed beneath this TCO material.
So that is one of the, one of the
requirements why it needs to have a higher band gap.
Now, when we think of this other adjective which is used to describe
this this oxide, it says that this oxide has to be essentially conductive.
And if, if you know of you know, if you think about conductive material,
this first thing that come to your mind is metals, which have a band gap of 0.
Or semiconductors such as germanium or silicon, which have a lower band gap.
So essentially, when you think
about increasing conductivity, you want the band gap to be lower.
or if you don't have a lower band gap, you essentially try to increase
the conductivity by doping the material heavily
or by increasing the mobility of the material.
And these things usually don't go hand in hand.
These things are usually don't sit very well with this first objective that says
that my material should be transparent.
So, if I may reduce the band gap of
course the, it will start absorbing these different photons.
If I increase the doping you'll again see in a different video
that it starts to essentially increase
what is called the free carrier absorption.
And that also decreases the decreases the transparency of the, this material.
So these this, this is the main challenge with these TCO materials.
So what are some of these common TCO material?
Let me give you some examples.
So far I've been just saying TCO, TCO, but let
me give you some example of these common TCO materials.
Some very common TCO material which is very widely used is zinc oxide.
And many times aluminum is added as a doping to this zinc oxide.
So it makes it aluminum doped zinc oxide which is many times also
abbreviated as AZO. Another another very common
TCO material is this indium oxide, and many times
it doped with a tin, so it's doped
would tend to make it more conductive. And it's this
tin doped indium oxide is also abbreviated as ITO or
indium oxide with tin doping. another commonly used
TCO material is tin oxide itself, and it's typically doped with fluorine.
So these are some of the, some of the common ITO materials.
Oh, I should also mention there's another material called cadmium cadmium
stannate, which has this particular this particular formula.
And this is used
specifically for these cattail based sources.
So these are some of the common common TCO materials.
ITO in particular is used is used very widely for these
touch-enabled displays that you use in your iPhone and your iPad.
And specifically these since the last five years, these the production
of these touch-enabled devices has essentially skyrocketed.
And the price of ITO has has shot up quite a lot, as well.
And a lot of that has to do with this indium which is used in this ITO material.
And the indium happens to be a pretty rare
earth material, and there's only a limited supply of indium,
which is produced every year.
So, that is one of the reasons why ITO has if you look
at the spiked prices of ITO, they have gone up quite a bit.
So these are some of the common common TCO materials.
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