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In the last video I was talking about this phenomenon of light induced degradation.
And it gives a lot of sleepless nights to manufacturers of amorphous
silicon based solar panels especially because that's where this light induced
degradation is is most you know most severely experienced.
And it can reduce the efficiency of these panels you
know, very significantly, you know, to up to 30 or 40% in the very
first few hours of operating the panel. So this is of course not a good thing.
You want to avoid this light induced degradation.
And then I was explaining in the
last video how this light induced degradation occurs.
And I gave you two of these models.
This hydrogen bond switching model and the other model
which involves two of these hydrogen atoms.
To explain, you know explain how, how this light induced degradation occurs.
And we saw that you know, no matter which model you believe in, in fact you
know, no matter it's not a, you know, it's not universally accepted
like which model. is the one which which is the true
representation of this of this site in new degradation.
In fact, if you go to these thin frame solar technology conferences, in
the RAM session you know, when people are the researchers have some wine and then.
And there's at the evening of the
conference there's very heated debate which takes
place you know, debating which of these two models can is the true representation
of this light induced degradation.
But nonetheless, no matter which model you you know,
you put your money on both of these models they're.
Result in creation of these dangling bond states.
Now you might have already, you know, guessed that this
creation of these dangling bond states is not a good thing.
So what it does in terms of the solar
cell is that I can represent the density of state
in my solar cell.
So for these amorphous silicon I can essentially,
you know, I have a conduction band state, and
a conduction band tail, and so this is my
conduction band and this is my conduction band tail.
And similarly are the valence band and the valence band tail.
So this would be my valence band and this would be my valence band tail.
And this is my
band gap, which for amorphous silicone is you
know close to one point seven eight electron volt.
So now what these dangling bonds do is that they creates these states
within this forbidden region,so within this region
between your valence band and conduction band.
They create these states which lie right in the
middle of your band gap, so they would be representing
that in a different colour, that means blue.
So they're creating the resulting creation these dangling bond states.
And, they, they lie right in the middle of these connection
degradation and binding states that makes them a excellent recombination centre.
So what happens as a result of result of these light and degradation.
Is that you create a large density or you increase the density
of these dangling bond states, and that results
in a large increased recombination in your cell.
So, so now, essentially, your, your electron
[INAUDIBLE]
which are produced. Instead of getting connected.
They can easily recombine.
So they can make use of these make use of these dangling bond states.
And they can use it as a
recombination center to essentially recombine with this hole.
So this is of course, not a good thing.
So you want to avoid this light induced degradation.
And people try a variety of passivation mechanisms.
So you know, they do different
treatments so that these bonds between
these silicon and silicon they become good.
Or you know, these bonds are strong enough so
that they're not broken when electron and hole pair recombine.
So one might wonder, you know, why does this light induced degradation
does not happen in this way inside inside you know a crystalline silicon.
The reason is that, for crystal silicon
these bonds are essentially, these bonds between
these two silicon atoms they are more stronger.
So when an electron and hole pair recombine it's not, it's
not have sufficient energy to essentially to essentially break this bond.
So that's why this light induced degradation or this dangling
bond creation, is not as prevalent in let's say crystal silicon.
Nonetheless, this people will have tried to at least quantify
how these dangling bonds are created and you know, how they evolve over time.
So the researchers who work in this field such
as you know, these people who published this paper,
they very meticulously tried to measure the number of
these dangling bonds as a function of the illumination time.
Or the function of the time in which you keep the cell exposed to sunlight.
And it, it clearly shows that as you, you know as you, as you keep the
cell exposed to sunlight, you increase, you see
increase in the number of CR dangling bonds.
And this scale over here, it's plotting the data in log-log scale.
So what this linear, linear slope in the log-log scale, it means
that you have a power law dependent.
So you have a power law dependence of
this number of dangling bonds on the elimination time.
And then you can measure the slope of this line and that gives you the exponent.
That gives you the coefficient for that power law.
And over here you see that this number of dangling
bonds, it it increases as a, as a function of time.
With this cube
root dependence on the on the illumination time.
Similarly you see over here that, if you increase
your illumination or if increase the intensity of your light.
You see that if you increase the intensity of
the light, the number of these dangling bond, as
you increase the intensity from 50 to 100 to
200, again the number of dangling bond is increasing.
And in fact,
if you measure this functional dependence on this,
on this illumination rate or the generation rate.
It again comes out to be this power law dependence.
With the with the dependence of G to the power
of this generation rate to the power of two by three.
So these things, you know, these things
this power law dependence could be very easily
derived by you know, by using these daunting set of equations.
And in fact, you know, there is nothing to be worried about.
There's nothing to be afraid about these equation.
Let me explain the different terms to you.
So this you know this first equation, what it's saying is that, it's
it's trying to relate increase the number of dangling bonds.
So what I have to eliminate in respect to the dangling bonds.
So it's saying that, you know, the rate of increase
of the dangling bonds it's essentially proportional to this forward process.
Which creates this timing point that is you know, this, is
proportional to the to the generation rate of your electron and all.
That of course makes sense.
And there's a rewards process which essentially,
the rise in removal of these dangling bonds.
Out of, if you have dangling bonds present, and you have
your hydrogen atom present, this hydrogen
can essentially passivate this dangling bond.
So this is a reverse process, and so then the difference between these
two, it gives me the increase in the number of my dangling bonds.
[INAUDIBLE].
Similarly I can write an equation which delays
the increase in the number of my hydrogen atoms.
And it would be again proportional to the number.
It would be essentially equal to the number of dangling bonds created.
Because every time you create a dangling bond you create a hydrogen as well.
And it would be proportional to the worst process over here as well.
And that is that if you have two of these hydrogen atoms present,
they can essentially you know form a meta stable state with each other.
So that it was process which is proportional to
the square of the concentration of these hydrogen atoms.
So now I have these two differential equations.
So I have the system of differential equations and I'm
you know, how to solve a system of differential equation.
But,
you know, to and actually you know, it's not assuming
you know how to solve, a system of differential equation.
In fact, you know, it's a very difficult set of, if
you've forgotten solving differential equations it might take you a while.
So, you know, let me write the final formula.
So, let me write the final expression that is, you know, if
you feel so inclined you can solve these system of differential equation.
But I want to just give you a feel of
you know, where this where this power law dependence come from.
So if you solve this system of equation you'll
get a formula for your number of dangling bond.
And it's related to all these different coefficients over here.
So it's related to this this forward coefficient.
You know, it's related to the generation rate.
It relates to the reverse coefficient.
But the two most important things which stand out is this 2 3rd
power dependence on the, on the generation rate or on the intensity of the light.
And, this cube root dependence on the time.
So, of course now, now you have some understanding of how these dangling bonds
are generated and as their, as their density increases my
efficiency falls.
So my efficiency if I plot it as a function of time.
So, and let's say at, at t equal to zero, I at my maximum efficiency.
Now as I keep on as I keep
this panel exposed to light, the efficiency will degrade.
In fact, you know, initially I, let's say I have no dangling bond,
so immediately when I start to clear the dangling bond, efficiency will degrade.
But after
a while, you know, you already have a lot of these dangling bonds so
addition of these further dangling bonds, you
know, all the damage has already been created.
So the addition of these further dangling bonds it will there coefficient still keep
on reducing, but the rate of this decrease of the efficiency would be quite low.
And we want to measure, you know, at least when you
are by these panels, you should essentially ask for this efficiency
which is measured at this point in time.
And I just call it the stabilized efficiency.
So organizations such as NREL and you know,
other other organizations which new benchmarking of these cells.
So, for example, NREL has this chart which I'm sure you're all familiar with.
Which
[UNKNOWN],
efficiency of a champion set for different technology.
In looking at this amorphous silicon you know, let me look at amorphous silicon.
Amorphous silicon is the word here.
So you can see that for amorphous silicon, they always mention the term stabilized.
So the efficiency level of amorphous silicon says
they've been reported after they've been exposed to light.
And their efficiency you know, the.
It had the degradation efficiency
has stabilized and that's where you, you
should report the efficiency of these amorphous silicon.
But there are many times you read these
you know, these journal or conference paper and they'll
[INAUDIBLE]
very high efficiencies for these amorphous silicon
or your, the other thin film technologies.
So you should always ask them, you know if you meet
those people, you should ask them that did you measure those
efficiencies the night you made those solar cells or did you
measure it after it was exposed to sunlight for quite some time.
So you should always take those number with a grain of salt.
And you should always look for these stabilized efficiencies.