Light Induced Degradation in amorphous silicon solar cells: Part 2

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..

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