# Need for multijunction solar cells and efficieny improvement 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.

## One thought on “Need for multijunction solar cells and efficieny improvement”

1. Asmaa Montaser says:

Please , tell me if these videos are written on a site or not , as quick as you can