Passivation of the solar cell surface is one of the most
important topics as far as efficiencies of these solar cells are concerned.
So in this video I'm going to introduce why we require this surface passivation.
And I want to also introduce this, terminology and introduce this new term,
which is surface recombination, velocity, and how that is, used as a measure
to find out how good the surface passivation is.
So first of all let's, let's, you know let's understand a
little bit of what's this all this fuss, about the surface.
So we know that, in our semiconductor, in
this case, silicon, we have these periodic atoms,
periodic lattice structure.
And, that, this lattice in the formation of, these,
very beautifully placed, conduction and valence band,
and this bandgap, which separates
or, which distinguishes between these connection and, valence band states .
So this is, you know
this is, all fine and true when you have a large lattice
or, you know, you have these large number of these
silicon atoms,
but things begin to falter as I'm [INAUDIBLE] at this surface.
So when I reach this
surface over here, I see these silicon
atoms, which have not fulfilled all their bonding requirements,
so they, all of these silicon atoms have one or more dangling bonds
which essentially, ideally they would like another silicon
atom to be present on the other side.
But since it's not, it breaks the symmetry of this lattice and
it results in this formation of these states, which appear in this
forbidden region, or in this region of the bandgap of my, of my semiconductor.
And these are typically what is called as the surface states.
And these surface states act as a very good recombination centers.
So, for example,
if I have a, a electron in the conduction band and this hole in the valence band,
so when they reach near the surface they can utilize this surface state
as a trap center and use that to recombine with this hole present in the valence band.
So now, why is this problematic for a solar cell?
So let me explain that using a band diagram of of
a p-n solar cell.
So I have this let's say this this p-n junction over here.
And I'm I'm let me draw the band diagram over here as well.
So I have this p-type over here and n-type semiconductor over here.
And this is my depletion region.
So now if I shine the light on this, if I shine light on this, this will
generate these electron and hole pairs throughout this solar cell.
It will result in generation of these
electron and hole pairs throughout this solar cell.
And now I want to collect these electrons
at this particular electrode, on the n-type electrode.
And I want to collect these holes at the, at the, at the p-type electrode.
But when I'm forming these electrodes or I'll
usually have a surface associated with over there.
And now let me consider the case where I have these surface states present at
near to the electrode where I'm collecting these
electrons or I'm collecting these electrons and holes.
So now when this electron reaches close to this n-type contact, what it
sees is a large number of surface states present over here.
So instead of getting collected, it's can essentially
now use this surface state or this trap center
and it can recombine with this hole present in this in this valence band.
Similarly, this hole present in this valence band over here,
or this electron present in this conduction band over
here, they can also recombine using this surface states.
So instead of getting collected, now this electron and hole pair are just
annihilated due to this recombination which happening due to this
due to this surface state. So it's very important to reduce
the, reduce the density of these surface states and
a good metric to measure this density
and the effect of this surface recombination is to use this
term, surface recombination velocity.
So let me, let me explain how this surface recombination velocity is defined.
So let's say I have this unpassivated surface,
and I have this semiconductor over here. And I have this unpassivated surface.
And I'm shining light on it so, I'm i'm going to generate some you know,
some excess number of electrons and holes.
And let me consider the case of these excess electrons in the system.
So when I am far away from this surface, then I have
this excess carrier of electron present which I denote by delta n.
Now, because of this source of recombination present
in the surface when I approach the surface this
this excess
number of electrons will essentially reduce and I'll expect
a profile something like this when I approach the surface.
So, I can, I can represent this diffusion
current which [INAuDIBLE] because of this gradient in this
in this concentration profile of of my electrons.
And I can essentially represent this by this diffusion constant multiplied
by this gradient in the concentration of these excess electrons.
And I can equate this term at the surface.
And I can define this as equivalent to having a surface recombination velocity.
at the surface.
And this [INAUDIBLE] is going to multiply by
this same excess number of, excess number of electrons.
And that will essentially give me
the same diffusion current.
So if I do a dimensional analysis
now the unit of this surface recombination velocity
comes out to be centimeter per second
and that's why it's called surface recombination velocity.
And let me consider two cases.
So, let me consider the case in which the surface recombination velocity is zero.
So in that case, what it means is that is
there's a well passivated surface.
And there's no recombination happening over here.
And in that case there'll be no gradient in
this in this carrier concentration as it reaches the surface.
On the other hand, if the surface
recombination velocity is infinite, then according to this
equation, this delta n has to be equal
to zero to maintain this finite diffusion current.
And what it means is that for surface recombination velocity equal to infinite.
this this essentially this excess current concentration would
reduce to zero as I approach my surface.
So this is for surface recombination velocity equal to infinity
and this would be the profile when surface recombination velocity
is equal to zero. So I can denote this excess, excess number
of electrons by this quasi-Fermi level as well.
So I can denote it by this quasi-Fermi level.
So, in this case where this surface recombination velocity is zero,
this quasi-Fermi level remains flat as I approach this surface.
Worse in the case when this surface recombination velocity
is infinite then this surface recombination, this quasi-Fermi level,
It approaches the value of this intrinsic energy level of the semiconductor
in that case.
So usually, let me give you some values of
the, what these surface recombination velocity numbers look like.
For the case of, for the case of crystalline silicon based solar cell,
the best surface recombination velocity,
again, best meaning
the lowest surface recombination velocity,
bowever being reported around between one to ten centimeters per second.
But on a typical day on a normal day you know when the sun is shining, the
birds are chirping, the surface recombination velocity in these
most of these silicon solar cells which are manufactured
tend to be around this range where they are between
100 to 1000 centimeter per second, typically when
this semiconductor surface is passivated with a
metal or there is this metal semiconductor interface
then this surface recombination velocity tends to be
you know, in this higher around 1000 range.
If I have a completely completely unpassivated
surface, this surface recombination velocity could be
as high as 10 to power of, 10 to power of 6 centimeter per second.
That would mean that all these all these carriers that
come you know, in proximity to the surface, they are essentially,
recombine using these surface states. So, it's
desirable to essentially you know, have this, as far as this as far as these
solar cell are concerned, to have this surface recombination velocity as
low as possible,
and there are multiple ways that people try to achieve that.
You passivate your surface with thermally grown
silicone oxide, and that passivates a lot
of these dangling bonds, and it leads to
a reduction in your surface recombination velocity.
Similarly, there are other approaches such as
you, the surface recombination velocity is maximum
when this, you have equal number of electrons and holes.
So you have, both electron and holes present close to the surface
so both of them can recombine with each other.
So this surface recombination is maximized when you have n is equal to p.
So usually there are techniques applied such that you
repel these one kind of carrier away from the surface.
And two such techniques
are use of this approach called back surface field and
another is use of fixed charge present in the oxide.
This will be a topic of the next video on on this topic.