You have been tasked with a very important job, and that
is to select this material set for making thin-film solar cells.
So you have been asked to you know, go figure out what materials
would be the optimum materials for
the manufacturing of these thin-film solar cells.
And furthermore, you know, at this point, you're not
supposed to worry about technological issue such as processing,
defects, you know things like that, you know.
You're assuming that you can hire a team of PhDs and
material science and electrical engineering to figure that out for you.
Right now, you just want to look at all
the materials which are available in in nature.
And figure out what would be the best
for you know making these thin-film solar cells.
So I'll give you a set of guidelines, or
you know at least a few things you should look for,
while choosing the right materials for making these thin-film solar sets.
So, first, you know?
A first starting point.
A good place to start, is to
look at the absorption coefficient of, different materials.
So I'm taking the side, over here.
Where I've, plotted out these, absorption
coefficient for a variety of, semiconductor
materials.
Such as, gallium arsenide, germanium, silicon,
cadmium telluride, cadmium sulfide, amorphous silicon.
So you see that, you know, absorption coefficient they
vary over a large range and they all, they all,
you know, seem to be very high for low
wavelength, or, you know, for photons which have high frequency.
But what I should probably look for is
how the absorption coefficient very close to the tail, or, you know, how, how
steep is the absorption coefficient near the band gap of the materials.
So for example, the band gap of cadmium sulfide, this
material over here, is somewhere around you know, 2.1 maybe.
The band gap of cadmium telluride is somewhere
close to, you know, 1.12 EV.
So, I see that my absorption coefficient is very, very high over here, and it's,
it's rising very quickly, at least near the band gap, and that's a good sign.
So, having a high absorption coefficient, essentially what it corresponds
to is that, if you have a high absorption coefficient.
That means a lot of, and especially if
you have high absorption coefficient near the binding cap.
That means you can absorb
most of your light in a very short distance.
And that would give, you know, that would give
you the ability to use less amount of material.
And make your solar cell using a very thin film of this
[UNKNOWN]
material.
So that's the reason why absorption coefficient is important.
And I'm showing in this chart over here, this chart I mean
this is the absorption coefficient for the function of energy, in this case.
And it's plotted for these you know, these common suspect of materials
which are used for making thin-film solar cells, such as amorphous silicon,
these two organic materials, which are used for making organic cells, then
cadmium telluride, CIGS, and you see that all of them have very, you know, they have
high absorption quotient, so absorption quotient of 10 to the power of 5.
per centimeter would mean that you would require your one divided by that
or you'll require you'll, you'll require essentially 10
micron of material.
So you, this would be centimeter minus, so it would be around 10 microns of material.
To observe, you know a lot of the
slide which lies at this corresponding leaf photon energy.
So a high absorption coefficient essentially it
leads to less amount of the material required.
And that is clearly seen in this other chart over here as well so you see
that if you use So this is plotting the short circuit
current as a function of the thickness of my solar cell.
So I'm making a solar cell but I'm varying the thickness.
And see you that materials which are strong absorbers such as this red color
on here, which corresponds to, this red color, which corresponds to CIGS.
Or this green color over here,
which corresponds to CdTe.
So you see that the short circuit current, it
[INAUDIBLE]
in each of the other functional thickness.
But it saturates very quickly, so you know if I have one
micron of this material, it sufficiently absorbed most of the light, or
the most of the light which I can extract in the form of electrons and
holes which contribute, which contributes to the short-circuit current anyway.
And you see that using, you know, a very thin amount
of this material I can, you know, make a good solar cell.
And in case of CIGS, I can, you know, achieve a very
high short circuit current using a very thin film of this material.
Worse is if I compare it to your, the
case of the case of say nano crystalline silicon.
You see that over here this, this, this essentially this short circuit
current is coming to be it's continuing to increase all the way to,
you know, up to more than 10 microns.
So typically the range we want to operate in is around this 1 micron
range or you know, maximum around 2 or 3 or 5 micron of material.
And.
So, it's, the first thing to look at is, essentially, absorption coefficient,
and how steeply it rises near the band gap of this material.
The other criteria, other guideline, which should
you should keep in mind is the band gap of your material.
So, shown here is the maximum efficiency possible, as a function
of a band gap so this is what is called a sharply
graph of efficiency, and you see that this has a broad optimum efficiency.
It can reach all the way up to you know high.
It can reach all the way up to 32 to 35% in an ideal world, you know
in an ideal world where there are no defect
centers and no green boundaries for these thin-film materials.
You know if you have assumed just an ideal
case these efficiencies can reach you know up to
up to lower thirties, but your band gap in which it happens,
is a, a broad peak around this all the
way from 1.1 to around 1.5 or 1.6 electron volt.
So you should choose your materials as that
you're buying gap of this material, it lies
close to this close to this optimum value. So
I showed you over here is that essentially if you have Your
your CdTe material, or your CIGS material. In CIGS
material, in CdTe for example, it has a band gap of around 1.4.
And that's very nice. For the CIGS material,
you can tune the band gap all the way from 1.1
to, you know, all the way up to 1.5 or 1.6.
Depending upon what percentage of gallium you have.
So if you have no gallium you are just have a CIS cell.
Then your band gap would be around 1.1.
If you have a complete 100% gallium and no indium, then your band gap would be high.
So you can tune this band gap within this range for the CIGS material.
The other material, the CZTS, which essentially
replaces the indium and gallium by this
zinc and tin which are more readily
available, and we'll talk about that just now.
It's also has this optimum band gap.
While some of these other materials for
example, amorphous silicon, it lies somewhere over here.
So its band gap is not
optimum, but it's still okay in terms of efficiency.
If you go even, further up so you, if
you consider materials such as cadmium sulfide or zinc oxide.
The band gaps are way too high to give you a high
efficiency, so if you choose a material like zinc oxide, it has
a very high band gap, so a lot of your photons, all
your photons, which corresponds to energy less than the band gap of
this of this material, will essentially just pass
by this material without getting converted into electron NO.
So choosing this optimum band gap is, again, very important.
The third and the final criteria, which I would recommend for for choosing this
material, is look at what is the
availability, what is the availability of these materials.
In the, you know, in the crust of the rock.
And how easily can they be extracted? For example, cadmium telluride is in
a very pervasively, is very actively used for making these thin-film solar cells.
But if you look over here, tellurium is one the
rarest available material in the cur-, crust of the Earth.
So, of course,
that is not a good sign. Similarly, for CIGS
. These I over here, it stands for indium.
And again, you see that indium is you
know, it's starting to appear in red over here.
Where it has, again, very low concentration
present in the crust of the Earth.
Then another thing you should look at is, you know, what is the
availability of you know, or how, what
is the availiability of buying these materials
from a market.
For example, you know, what is the production capacity of these materials.
So example, things like copper, you know, they are
readily mined for other industries, as well, such as
you know, construction and making pots and pans, so
there's a large amount of availability of these materials.
Whereas if you, let's say, consider silicon, it's also
available for making other chips and, you know, other things
that are made with silicon, but
again, this was something that people miscalculated.
even for silicon, there was a big scarcity of silicon
around 2007 to 2010 range, because, suddenly the demand for
[INAUDIBLE]
ranged so high that there was not enough production capacity.
So it's, it's important to take these things into account as well.
And that, especially true for tellurium which has
a production of only around 220 tons per year.
And a large fraction of that is currently being used for making these
[UNKNOWN]
based solar cells.
So these are some of the other things, you know, you should keep in to mind.