So in one of our previous videos we talked about Shockley Queisser limit for single

junctions based for decent. And this kind of sets a ceiling on what is the maximum efficiency I can get out of my

solar set. And this maximum efficiency turns out to

be in the range of 33 to 34% depending on

whether [UNKNOWN] the black body radiation. Or m 1.5 kind of spectrum to calculate

this efficiency. But let’s go into a little more

granularity and let’s care about what is happening to this individual

photons in my solar spectrum. So, if I, if I look at this individual photons and let me consider the case of

silicon. So, silicon has a band gap of 1.12 eV. And all the photons which are converted

into electron and hole pair. They are extracted with this maximum

energy, which could be equivalent or less than the band

gap. Or let’s say in the best case it’s equal

to the band gap. So all these photons are essentially

extracted with this energy which is equal to the

band gap of silicon. Of course photons which have wavelengths

which lie above this cut off cut off wavelength for

silicon. So for this silicon, this cut off wavelength is somewhere around 1,100

nanometer. So all these photons which have all these part of the spectrum which has wavelength

greater than the band gap of silicon.

This essentially just leaks through. Our this slab of silicon is completely

transparent to into all these photons. But that is something we already know. But you know I am concerned about these

photons which have energy which is higher. Or you know which have wavelength which is

lower than this cutoff of frequency. So, let’s consider the case of this photon which has a

wavelength of let’s say 500 nanometer. So the energy associated with this photon,

is given by given by this equation. So this has a much higher energy as

compared to the band gap of silicon. So if I look into more granularity, and if

I think about the efficiency Efficiency of converting this photon into the maximum energy of the electron and whole

pair that I’m able to extract. So now this would be essentially given by this divided by the band gap of my

silicone. So I see that for all these photons which are located at a much lower wavelength The

energy of this shorter conversation process is a

much lower. In fact, if I think about a photon which is uh,wavelength it will have even

higher energy. And it will have even lower efficiency of

a converting this photon into electron, and a hole pair.

So, first you know of talk that comes to my mind, is why don’t you, you know use a

material which has the higher band gap? So let’s say I use a material that has

twice the band gap of Silicon. So I’m using this material, which has

twice the band gap of silicon. So of course it will increase this energy

of efficiency of this photon to electron whole pair conversion

for all these photons. Which are located at lower wavelengths or higher energy. But in the process I’ll essentially lose

out or I’ll lose out on these on this part of the

spectrum. Which which lies at wavelengths which are essentially greater than the new cut off

wavelength. Which would be now, which would now be

half of what was the case for silicon. So you see the [UNKNOWN] that I’m using, a the material with a

higher and higher band gap. I’m able to convert these higher energy

photons into electronic whole pairs which are

higher energy. But at the same time losing all a larger part of the spectrum which corresponds to

wave lengths. Which have which have energy lower than

the band gap of this newer material. So to solve that conundrum, the idea that is used or the concept that is used,

is to use multiples of these junctions. So now what I can do is I can split my

spectrum in this way. So I have I split my spectrum into these

three blue and green and red. So these lower energy lower energy photons

are what belong in this in this red spectrum. The medium energy photons are what belong

in this green spectrum, and the highest energy, which is

the lowest wavelength photons. They belong in this blue spectrum. And now what I do is I absolve this, blue

spectrum using using you know, a higher bind gap material.

And then I absolve this green spectrum using this

intermediate bind gap material. And I absorb this red spectrum further

using the lowest band gap material. So what I’ve done by doing or using these

three materials at these three junctions. Is having, instead of having one cut off

wavelength, I now have, I now have three cut off

wavelength. One cut off wavelength corresponding to

the band gap of this blue material. Then another cut off wavelength

corresponding to the band gap of this green material. And yet another cut off wavelength

corresponding to the band gap of this red material. So if I think of a photon, which is located right

at this cut off wavelength for the blue material It is now converted into electron

in whole pair at a conversion efficiency of

100%. Similarly, this another photon which is

located at this cut off wavelength of the green material, it’s also converted

into electron in whole pair. with a, conversion efficiency of 100%.

Similarly, that is true for this wavelength which is located at the cut off

frequency of the red materilhere. So instead of having that single triangle, which was determining my conversion

efficiency of, of, of photon to electron and whole pair, now essentially I have this three

triangle. One corresponding to here, another one corresponding to this green

material. And another one corresponding to this another one corresponding to this blue

material. So there are more than, one ways I can achieve this achieve this

multi-junction configuration. I am just showing a couple of them over

here. So one way I can achieve this

multi-junction configuration is to have this what’s called as this

spectrally, spectrally sensitive mirrors.

Or spectrally sensitive filters, which What they do is they take this

sunlight, and they in this case they allow all the other

components to go through. Besides this blue component of my

sunlight. And that essentially is showing up on this

cell 1, which will have a band gap which is optimized for a

converting these high energy photons. And they would be converting to electron

in whole pairs over here. The green and the red components are

essentially they pass through this filter. And then there then there’s a second

filter over here. Which filters out the green component. Which are then subsequently converted into

by this second cell. And then finally the red components are

converted by this third cell. And there might be still some other

spectrum which is remaining which will leak out of this

system. Another way to do that would be to place these cells on on your essentially on one on top of each

other. Where my highest band gap material would

be placed there first and it would, it would absorb all

this blue photons. But these, green and red photons they have

energy which is less than the band gap this material so this

material is transferring to them. So they essentially pass through it.

And now they are absorbed by this cell number 2 which is optimized for optimizing

or absorbing this green photons. And finally you have this third material

which can absorb these red photons. And so both these schemes are you know can be used to realize this multi junction

configuration. And they achieve efficiency which are

higher than what Shockley and Queisser has report, had

reported for this single-junction cells. In fact, let me, know you, give you some numbers of how high of efficiency we can

achieve. So a one one junction cell can achieve a efficiency

of 32%, given by this Shockley Queisser limit.

If you think about a two junction cell. It can give you a efficiency of 40 2%. And these numbers are all reported at a

concentration of one [INAUDIBLE] . If you increase the concentration, these

efficiency numbers will go up. similarly a three junction cell, can give

you a efficiency of 48%. And four junction cell can give you a

efficiency of 52%. And that brings you to, you know, our the

topic for the next video. That if I keep on increasing the number of numbers of this cell, what

is the maximum efficiency I can get? So let’s say I have a 10 or 12 junction

cell, or even, let’s say I have a hundred, you know, hundreds

of junctions which I am using. What is the maximum efficiency you can

get? And the number turns out to be not 100,

unfortunately. It turns out to be close to 85% and that

is something we will discuss in the

subsequent videos using that number to make an

argument. So see you in the next video.

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