Material selection for thin film solar cells

Material selection for thin film solar cells


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 [INAUDIBLE] 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 [UNKNOWN] .
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.

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