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
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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
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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
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.
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.