Multijunction based, solar cells, they have the highest
efficiencey of solar energy conversion among all uh,
[COUGH]
all solar cell devices.
So depending upon how many junctions you use, whether you use two
junctions, three junctions, or four junctions,
the efficiency of these multijunction cells.
They can be, you know, up for 40% for this energy conversion easily.
And also, another thing I know is that these multijunction cell, they
work best when when you operate them at high concentration, or you operate
them at a concentration of 500x or 1000x or even higher.
So in this video, I want to understand this very important question.
Is that, how does the temperature coefficient of
the efficiencies for these multijunction cell looks like.
Or how does the efficiencies, of these multijunction cells degrade as I increase,
or as there is an increase in the temperature of the ambient, or increase
in the temperature of the device. And this a very important question.
It's a very relevant question for these multijunction cells
especially since I operate them at this very high concentration.
And these are often placed in regions
which have a very high direct incident sunlight,
and they're usually placed in regions which
have high temperature profiles, such as deserts, and
so on.
This is where you expect high concentration of, direct sunlight.
So the ambient temperature, in those places of
course is, is supposed to be high as well.
So I want to know whether, this efficiency the high efficiency that these solar cells
have, will essentially this evaporate if I
increase the temperature, or what will happen?
So the first thing, I will look at is the open circuit voltage.
And, in one
of the previous videos, we studied that this open
circuit voltage, degrades as you increase temperature of your device.
And this is what causes essentially a decrease, in the efficiency of the cell.
And I can represent that decrease in the VOC by a set of mathematical equation.
So, if I represent my, if I can represent my current duty generation and
recombination by this equation.
Where the temperature dependence, is given by
this power dependence, and also it occurs due
to this exponential term over here, and
then I can express my open circuit voltage.
I can express it as as this formula. Where it depends
upon the log, of my short circuit current divided by this
[UNKNOWN],
which is given by this equation. So I can take a derivative of that, I can
take a derivative of that, and essentially calculate my Temperature dependent of Voc.
And it usually comes out to be this
formula, where this Voc, it has this negative term.
This temperature dependence of Voc, it has a negative term.
So this Voc degrades, as I increase
my temperature.
And also I see that this declaration of VoC.
It depends on the difference between, it depends on the
difference between my band gap, and my open circuit voltage.
So the higher is my open circuit voltage.
The smaller is the difference between, the band gap, and the open circuit voltage.
And the floor is the rate of degradation of this open circuit voltage.
So a good thing about these multijunction cells, is that they
are operated at these high concentrations at this 500x or 1000 x.
As a result of that, I know that when I increase the concentration of my light,
my Voc increases, because my short circuit current
it accordingly increases by 500 times or 1000 times.
As a result of that, my Voc essentially it has this logarithmic co dependence on
this short circuit current. So it increases, and that boards' a good
news for this VoC you know this VoC temperature coefficient of VoC.
Because now my VoC is higher, and there is a
delta that this difference between my band gap, and VoC lower.
So my VoC essentially degrades, at a much slower rate.
And this can be seen nicely
in this table over here.
Which tabulates these different for these, different kind of sets.
The VoC, the rate of change of the VoC.
And, you know, the rate of the change of VoC as a percentage of VoC.
So this tell me, how much percentage of degradation,
I have in my open circuit voltage for every
[INAUDIBLE]
increase in my temperature.
So I notice that, for this multijunction sense.
For example for this 3 junction cell, which
is very prevalent, which is Germanium, Indium, Helium, Phosphate.
I see that my VOC degrades by 0.23%. when I'm operating
the device, as a concentration of 1 sun. But now, when I increase
the concentration to 500 sun, and when I'm operating this as a concentration of 500x,
and I see that this a rate to degradation of usc, it reduces from this 0.23 to 2.15.
So if I am operating my device, at higher concentration
this rate of degradation of VLC it essentially slows down.
Now the other thing I am interested in,
is how does my short circuit current, or my JSC.
Behaves as a function of temperature.
And this is very interesting, because I have mentioned this,
what happens in the case of a single junction cell?
Let me repeat it here.
So if I have a single junction cell, if I need this one material
system, when I simply increase the temperature,
the band gap of the cell decreases.
As a result of that,
it can now absorb more of this polar spectrum.
So, for example, this this cell was only marginally absorbing the green light.
But now since its band gap has decreased, it can absorb more of this green light.
And as a result of that this
Jsc, definitely increases as you increase your temperature.
But this becomes way interesting, if you have if you have for
example a multijunction cell, let's say I had you know, another cell below it.
So I add, another cell below it.
And let's say I have one more cell below it.
So, among these 3 cells, it's usually one cell which
limits the overall short circuit current
of my overall circuit. So let's say, you know,
in this case, it was the top cell
which was limiting the, overall short circuit current.
So now when I heat up you know when I heat up the cell, or when the ambient
temperature rise, the band gap of all 3, all
these 3 cells will decrease as their temperature increases.
So as the band gap of this stops,
or decreases it's JSC will it's JSC will rise.
And, you would expect that for these bottom
cells, you know, the Jsc should rise as well.
But there's a
[INAUDIBLE]
interesting relationship over here.
So the J SC on this side is rising, because the band gap
of this stop cell is decreasing, so, it is absorbing more of these photons.
So that means these cells which are located below it.
They now, now see less of the spectrum, because more
of it is being absorbed by absorbed by this top cell.
So as a result of that, which of these says is Limiting
the short circuit current.
Can change as a function of our temperature.
For example in this case maybe, it was the top cell which was limiting the current.
But now since the Jsc of it stops, and rises
it, maybe the middle cell which limits my overall current.
And this is shown way nicely over here.
So this shows that the, it shows the charts are get current
of this individual sets in a, in a 2 cell configuration.
So when, I was at a lower temperature, my essentially my
top cell had lower current, so it was limiting the current.
When I, when I increase the temperature, I see there's a, a there's a crossover here.
And now my top cell, has more current as compared to my bottom cell.
And my bottom cell, becomes a cell which is
limiting the overall current in my in my device.
So that, gives you know a, a,
a way for interesting function dependence of the short circuit current.
And this short circuit current increases a function of temperature.
But since each of these cells is
limiting the, limiting the overall current can change.
So this temperature dependence of VoC, you know it can essentially have this
[INAUDIBLE].
When this crossover happens. But none the, nonetheless, the JSC on a
whole it increases as you increase your increase your temperature.
So the efficiency degradation of these these
multijunction solar cells, is often touted to
be, you know, one of the best among all the different solar cells, so these
CPV cells or these multijunction cells, which are used under these
concentration of, a very high concentration of 1,000x or 500x.u.
The VOC essentially it it reduces at a much
lower pace, as compared to all the other cells.
As a result of that, the efficiency degradation of these cells,
happens only between, if ranges between, you know to be way optimistic,
between 0.05 to around 0.15 percentage
decrease in the efficiency. With the first 1%
with every with every centigrade increase in the temperature.
So, if I had this cell and it was operated, operating at 100% of it's of its
plate capacity at at at normal temperature, when I
increase temperature by 20C, or if I increase the temperature
by 40C, even if I increase the temperature by 40C.
It gives, still gives me 96% of this of
this efficiency that it was delivering at normal temperature.
Worse if I compare this to modal silicon, or you know
single crystall silicon, it would only deliver 84% of that of that
nameplate capacity at with the, for we seen
temperature.
[INAUDIBLE]
is even worse. It it delivers only 80%.
Some of the thin film technology actually fair better.
And we'll talk about that in a later video.
But they have a temperature coefficient,
which are better than single crystal silicon.
But nonetheless, the, these multijunction solar cells,
which are used under a concentration of light.
They have the best temperature
coefficient of efficiency.
And that's a good news, because these cells
are the ones which have higher highest efficiency.
And they're usually placed in conditions where you have the.
Largest eh, change in the operating temperature.
So it's good to know, that the efficiency does not degrade as rapidly.