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.